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OEM temp gauge readings [message #357738]

Sun, 16 August 2020 10:11

lw8000

Messages: 201
Registered: July 2012
Location: S.E. Michigan

Karma: 1

Senior Member

Hello all:
Not too long ago our fan clutch completely locked up and we needed a replacement. I am not entirely sure which clutch it had. We ended up putting on an AC Delco 15-4947 after reading on it here, there was a thread by Ken Burton where he did some research and found this seemed to be the best replacement regarding the specs of the fan and setup on the GMC, and also after reading success and failures of Hayden clutches here too.

I haven't put many miles on the coach with this new clutch but this past week we were in temps up to about 88 degrees out. (In MI, that's what we call HOT Smile

Smile

). With the old clutch in this temp range , the needle would barely pass the quarter mark on the gauge, but it would also kick in quite often even on the highway and I'd hear the fan. Now, with the 15-4947 clutch, I noticed the gauge has gone as high as just between the quarter and half mark while on the highway, and I never hear the fan roar like the old one. But, when getting off the highway or slowing to 55 MPH or so, the needle settles back to the quarter mark.

I believe I've read that once it hits the half mark on the gauge, it's already overheating. Should I be concerned with the readings I'm getting?

Thanks in advance as always!
Chris

Chris S. - 77 Kingsley, 3.70 FD, mostly OEM - S.E. Michigan

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Re: OEM temp gauge readings [message #357740 is a reply to message #357738]

Sun, 16 August 2020 10:59

Matt Colie

Messages: 8547
Registered: March 2007
Location: S.E. Michigan

Karma: 7

Senior Member

lw8000 wrote on Sun, 16 August 2020 11:11

Hello all:
Not too long ago our fan clutch completely locked up and we needed a replacement. I am not entirely sure which clutch it had. We ended up putting on an AC Delco 15-4947 after reading on it here, there was a thread by Ken Burton where he did some research and found this seemed to be the best replacement regarding the specs of the fan and setup on the GMC, and also after reading success and failures of Hayden clutches here too.

I haven't put many miles on the coach with this new clutch but this past week we were in temps up to about 88 degrees out. (In MI, that's what we call HOT ). With the old clutch in this temp range , the needle would barely pass the quarter mark on the gauge, but it would also kick in quite often even on the highway and I'd hear the fan. Now, with the 15-4947 clutch, I noticed the gauge has gone as high as just between the quarter and half mark while on the highway, and I never hear the fan roar like the old one. But, when getting off the highway or slowing to 55 MPH or so, the needle settles back to the quarter mark.

I believe I've read that once it hits the half mark on the gauge, it's already overheating. Should I be concerned with the readings I'm getting?

Thanks in advance as always!
Chris

Chris,

Assuming that the coach still has the factory temperature sensor, then yes, half scale is a worry. The other problem with the long standard thermostatic fan clutch is that it has little or no relation to the actual coolant temperature. There is also the issue that most of those clutches are now produced off-shore and the calibration is problematic at best.

Do you own an IR temperature gun? If not, grab one on sale for ~20$us and measure the temperature at the thermostat housing.

Matt

Matt & Mary Colie - Chaumière -'73 Glacier 23 - Members GMCMI, GMCGL, GMCES
Electronically Controlled Quiet Engine Cooling Fan with OE Rear Drum Brakes with Applied Control Arms
SE Michigan - Near DTW - Twixt A2 and Detroit

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Re: [GMCnet] OEM temp gauge readings [message #357743 is a reply to message #357740]

Sun, 16 August 2020 11:13

James Hupy

Messages: 6806
Registered: May 2010

Karma: -62

Senior Member

One person's idea of overheated coolant, is another person's idea of proper
operating temperature. 241° degrees farenheit is DEFINITELY overheated,
where 205° might be considered "Normal". 165° - 180° might be considered
not fully warmed up by others. The correct temperature is that temperature
at which all of the air/fuel mixture possible is controlled burned with no
preignition or detonation. Just sayin'
Jim Hupy
Salem, Oregon

On Sun, Aug 16, 2020, 9:01 AM Matt Colie via Gmclist <
gmclist@list.gmcnet.org> wrote:

> lw8000 wrote on Sun, 16 August 2020 11:11
>> Hello all:
>> Not too long ago our fan clutch completely locked up and we needed a
> replacement. I am not entirely sure which clutch it had. We ended up
> putting
>> on an AC Delco 15-4947 after reading on it here, there was a thread by
> Ken Burton where he did some research and found this seemed to be the best
>> replacement regarding the specs of the fan and setup on the GMC, and
> also after reading success and failures of Hayden clutches here too.
>>
>> I haven't put many miles on the coach with this new clutch but this past
> week we were in temps up to about 88 degrees out. (In MI, that's what we
>> call HOT :) ). With the old clutch in this temp range , the needle
> would barely pass the quarter mark on the gauge, but it would also kick in
> quite
>> often even on the highway and I'd hear the fan. Now, with the 15-4947
> clutch, I noticed the gauge has gone as high as just between the quarter and
>> half mark while on the highway, and I never hear the fan roar like the
> old one. But, when getting off the highway or slowing to 55 MPH or so, the
>> needle settles back to the quarter mark.
>>
>> I believe I've read that once it hits the half mark on the gauge, it's
> already overheating. Should I be concerned with the readings I'm getting?
>>
>> Thanks in advance as always!
>> Chris
>
> Chris,
>
> Assuming that the coach still has the factory temperature sensor, then
> yes, half scale is a worry. The other problem with the long standard
> thermostatic fan clutch is that it has little or no relation to the actual
> coolant temperature. There is also the issue that most of those clutches
> are now produced off-shore and the calibration is problematic at best.
>
> Do you own an IR temperature gun? If not, grab one on sale for ~20$us and
> measure the temperature at the thermostat housing.
>
> Matt
> --
> Matt & Mary Colie - '73 Glacier 23 - Members GMCMI, GMCGL, GMCES
> Electronically Controlled Quiet Engine Cooling Fan
> OE Rear Drum Brakes with Applied Control Arms
> SE Michigan - Twixt A2 and Detroit
>
> _______________________________________________
> GMCnet mailing list
> Unsubscribe or Change List Options:
> http://list.gmcnet.org/mailman/listinfo/gmclist_list.gmcnet.org
>
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Re: [GMCnet] OEM temp gauge readings [message #357745 is a reply to message #357738]

Sun, 16 August 2020 11:17

powwerjon

Messages: 849
Registered: March 2013

Karma: -2

Senior Member

I have to ask what temperature sender your using. If you still have the stock unit then you need to change it out to a NAPA TS-6469 unit. Check out the following information.

Temperature Sender
This may be old news for most but doesn't hurt to be told. The original
temp. sender on all GMC Motorhomes at :
1/4 is 225 degrees,
1/2 is 250 degrees,
3/4 is 270 degrees, and
H is 280 degrees.
This has caused a lot of engine damage when engine starts to overheat. Go to NAPA and get a
TS-6469 sender and it will show:
1/4= 180 degrees,
1/2 = 215 degrees,
3/4 = 240 degrees, and
H = 255 degrees.
Since the standard pressure cap raises antifreeze solution boiling point to 250 degrees this will be more accurate readings. I don't know why they did this but it is a serious problem. Info from GMC Motorhome News published by Cinnabar, Dec. '95 issue. I did this and it agrees exactly with my VIP digital
readout.

If you are using the 7 blade metal fan then a 2747 ish is an acceptable fan clutch. I run one of the newer 10 blade fiberglass fans and I found that I needed to us a 2797 ish fan clutch as the 2747 doesn’t have enough clutch to turn it. I know this from personal experience.

EXAMPLE: I had installed one of John Biwersi 10 blade fiberglass units. We drove the coach out to AZ to spend the winter and of course the temps were cool and cold on the way out. In the spring when we were coming back we decided to go to Page, AZ to see the Glen Canyon Dam and sandstone caves and canyons. The trip to Flagstaff was uneventful as the temperature was in the low 70’s, Going north on 89 into the high desert the temps were in the High 80’ to low 90’s and we were motoring along when I noticed that water temp was rising to 1/2 on the gauge. Remember that I said that I use the TS-6489 temp sender. Well I said maybe I got a bad fan clutch, so I stopped at the next rest area that happened to appear about 2 miles down the road. I alway carry a spare fan clutch and proceeded to change it out and go on our way north on 89. About 15 minutes later the same temp rise happened again. Once again much was in our favor as we entered Cameron AZ and they had an AutoZone and they had a 2797 rated fan. 45 minutes later new fan clutch and we were on our way again to Page, AZ. I run a 180 thermostat and it was locked in the rest of the trip and I still have that fan clutch install and yes I do have a spare 2797 type fan clutch in storage. What I learned from this experience was that the 10 blade fan requires more fan clutch engagement than the 7 blade fan. I gave my 2747 type units to my traveling GMC friend. He had a clutch failure so he got 2 fan clutches, one to install and one for a on the road spare.

J.R. Wright
GMC Great Laker MHC
GMCGL Tech Editor
GMC Eastern States Charter Member
GMCMI
78 GMC Buskirk 29.5’ Stretch
75 GMC Avion (Under Reconstruction)
Michigan

> On Aug 16, 2020, at 11:11 AM, Chris S. via Gmclist wrote:
>
> Hello all:
> Not too long ago our fan clutch completely locked up and we needed a replacement. I am not entirely sure which clutch it had. We ended up putting on
> an AC Delco 15-4947 after reading on it here, there was a thread by Ken Burton where he did some research and found this seemed to be the best
> replacement regarding the specs of the fan and setup on the GMC, and also after reading success and failures of Hayden clutches here too.
>
> I haven't put many miles on the coach with this new clutch but this past week we were in temps up to about 88 degrees out. (In MI, that's what we
> call HOT :) ). With the old clutch in this temp range , the needle would barely pass the quarter mark on the gauge, but it would also kick in quite
> often even on the highway and I'd hear the fan. Now, with the 15-4947 clutch, I noticed the gauge has gone as high as just between the quarter and
> half mark while on the highway, and I never hear the fan roar like the old one. But, when getting off the highway or slowing to 55 MPH or so, the
> needle settles back to the quarter mark.
>
> I believe I've read that once it hits the half mark on the gauge, it's already overheating. Should I be concerned with the readings I'm getting?
>
> Thanks in advance as always!
> Chris
>
>
> --
> Chris S. -
> 77 Kingsley, 3.70 FD, mostly OEM -
> S.E. Michigan
>
> _______________________________________________
> GMCnet mailing list
> Unsubscribe or Change List Options:
> http://list.gmcnet.org/mailman/listinfo/gmclist_list.gmcnet.org

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Re: OEM temp gauge readings [message #357756 is a reply to message #357738]

Sun, 16 August 2020 15:39

lw8000

Messages: 201
Registered: July 2012
Location: S.E. Michigan

Karma: 1

Senior Member

This is great info all, thank you.

I am not 100% sure but guessing I have the OEM sending unit. Is there any way to tell for sure? Normally my gauge is right at the quarter mark, once things have run long enough to warm up, even when starting it and letting it idle for a bit. But, if that's 225 and above "normal" I need to confirm my sending unit and temp.

I think I will do as Matt suggested and measure the temp at the thermostat housing to see where I'm at to compare against my gauge reading and go from there. Gotta go get a temp reader but will post back findings soon.

Thank you!

Chris S. - 77 Kingsley, 3.70 FD, mostly OEM - S.E. Michigan

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Re: OEM temp gauge readings [message #357766 is a reply to message #357738]

Sun, 16 August 2020 21:36

JohnL455

Messages: 4447
Registered: October 2006
Location: Woodstock, IL

Karma: 12

Senior Member

Just buy and install the updated temp sender. All coaches should get this update. Not expensive. With a 195 Flowcooler stat, that puts the needle at about 1/3 when sitting on the stat throttling point.
A basic go/ no go clutch test is you should have fan roar for first 60 seconds at least, after sitting overnight. This is because the silicone fluid redistributes by gravity partly into the working chamber. With the temperature spring cold the valve will be set to have the rotation pump the fluid back to the non working chamber. This takes a minute or so. If no fan roar at cold start fast idle, clutch may have lost too much fluid or be defective.

John Lebetski
Woodstock, IL
77 Eleganza II

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Re: OEM temp gauge readings [message #357767 is a reply to message #357738]

Sun, 16 August 2020 21:46

Rick Staples

Messages: 126
Registered: May 2014
Location: Johnstown, Colorado, USA

Karma: -1

Senior Member

Chris wrote:
"... I noticed the gauge has gone as high as just between the quarter and half mark while on the highway, and I never hear the fan roar like the old one. But, when getting off the highway or slowing to 55 MPH or so, the needle settles back to the quarter mark."

This appears to be a case of a bad (clogged/scaled) radiator. The fact it cools off at lower speed to me rules out the fan as a problem, as you normally need the fan more at LOW speed than on the highway. Of course the execrable aerodynamics of our GMC engine compartment and grill make the fan necessary at all speeds. Of course the engine needs more cooling under load. If the radiator is clogged up or internally coated with scale or rust, it can't transfer enough heat to the passing air. In an extreme case the air might not even be warmed enough in passage at speed to get the cooling fan thermostat to engage. I suspect this may be your situation.

Many years a go I had a Citroen which I loved dearly, and a beautiful young lady I wished to impress. She wanted to go to the beach one hot summer's day, so off we went. It was over 90F, and any time I went over ~50mph, my temp gauge would climb, so we drove down and back with the heat on to prevent boiling over. Needless to say, she was not impressed. Next day, I pulled the radiator and took it to a shop to be cleaned ("rodded out"). Problem fixed, but too late for that "one that got away". Wink

Wink

Unless some other cause presents itself, (double-check the thermostat) I'd recommend removing the radiator and taking it to a radiator shop (if you can find one these days) for disassembly and cleaning. Or you could replace it. You may be able to see down into the tanks with a borescope or something to examine the core passages.

My 2¢,, HTH
Rick Staples

Rick Staples, '75 Eleganza, Johnstown, CO "Advice is a dangerous gift, even from the Wise to the Wise, and all paths may run ill." -Tolkien

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Re: OEM temp gauge readings [message #357784 is a reply to message #357738]

Mon, 17 August 2020 17:13

lw8000

Messages: 201
Registered: July 2012
Location: S.E. Michigan

Karma: 1

Senior Member

Got it on the new sending unit and radiator. I too was wondering if the radiator is OK as I thought the same thing (temp drops quickly after slowing down). I do have the Robertshaw/Flowkooler 195 thermostat.

Once I have a laser themometer to check this I will follow up. Thanks again!

Chris S. - 77 Kingsley, 3.70 FD, mostly OEM - S.E. Michigan

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Re: OEM temp gauge readings [message #357787 is a reply to message #357784]

Mon, 17 August 2020 20:06

Matt Colie

Messages: 8547
Registered: March 2007
Location: S.E. Michigan

Karma: 7

Senior Member

lw8000 wrote on Mon, 17 August 2020 18:13

Got it on the new sending unit and radiator. I too was wondering if the radiator is OK as I thought the same thing (temp drops quickly after slowing down). I do have the Robertshaw/Flowkooler 195 thermostat.

Once I have a laser themometer to check this I will follow up. Thanks again!

Chris,

I fear you are stuck under a preconceived misconception.....
The thermostat in the cooling system does not determine how hot the engine gets, but how cold it runs. If you think that way, thinks will go better. 195°F if the temperature when the coolant starts to go out to get cooled. When you stop putting heat into the system, it will (relatively) quickly return to that temperature.

If you have any reservations, I could come along on a ride-around (and bring my own IR gun) and we can check things. Your sig says SE-MI like mine, well we live in the shadow (really) of DTW and if it would quiet any nerves, I would love to help.

Matt

Matt & Mary Colie - Chaumière -'73 Glacier 23 - Members GMCMI, GMCGL, GMCES
Electronically Controlled Quiet Engine Cooling Fan with OE Rear Drum Brakes with Applied Control Arms
SE Michigan - Near DTW - Twixt A2 and Detroit

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Re: OEM temp gauge readings [message #357793 is a reply to message #357787]

Tue, 18 August 2020 06:50

lw8000

Messages: 201
Registered: July 2012
Location: S.E. Michigan

Karma: 1

Senior Member

Matt Colie wrote on Mon, 17 August 2020 20:06

lw8000 wrote on Mon, 17 August 2020 18:13
Got it on the new sending unit and radiator. I too was wondering if the radiator is OK as I thought the same thing (temp drops quickly after slowing down). I do have the Robertshaw/Flowkooler 195 thermostat.

Once I have a laser themometer to check this I will follow up. Thanks again!
Chris,

I fear you are stuck under a preconceived misconception.....
The thermostat in the cooling system does not determine how hot the engine gets, but how cold it runs. If you think that way, thinks will go better. 195°F if the temperature when the coolant starts to go out to get cooled. When you stop putting heat into the system, it will (relatively) quickly return to that temperature.

If you have any reservations, I could come along on a ride-around (and bring my own IR gun) and we can check things. Your sig says SE-MI like mine, well we live in the shadow (really) of DTW and if it would quiet any nerves, I would love to help.

Matt

Thanks Matt! Yes I do understand that and was referencing that we have the 195 thermostat compared to the other replies. This should be simple enough but if I run in to any dilemma I can reach out. I've been meaning to pick up one of these laser temp readers for a long time, now I have an excuse to get one Smile

Smile

Thank you for the offer!

Take care,
Chris

Chris S. - 77 Kingsley, 3.70 FD, mostly OEM - S.E. Michigan

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Re: OEM temp gauge readings [message #358141 is a reply to message #357738]

Sat, 29 August 2020 16:22

lw8000

Messages: 201
Registered: July 2012
Location: S.E. Michigan

Karma: 1

Senior Member

Finally got a laser temp gun. Warmed up the engine for about 15 minutes to let it get nice and hot. Opened the hatch while engine running, and while the gauge is on the quarter mark, and I measured the temp at the outlet at about 193 degrees. This may not be the best test as I did not take it out on the road.

Next step will be to get the Napa sending unit and put that on to see where that one lies.

Chris S. - 77 Kingsley, 3.70 FD, mostly OEM - S.E. Michigan

[Updated on: Sat, 29 August 2020 16:24]

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Re: OEM temp gauge readings [message #358700 is a reply to message #358141]

Wed, 16 September 2020 18:35

lw8000

Messages: 201
Registered: July 2012
Location: S.E. Michigan

Karma: 1

Senior Member

Just dropped in the Napa sending unit today. In taking temp readings now, the Napa unit is spot on with what J.R. posted. I ran the engine and let it idle up to normal temp for a good 20 minutes. The highest the gauge showed was now just slightly above halfway between the quarter and half marks. My readings right at the sending unit on the manifold was about 201-202 degrees.

Now that it's a little more accurate I will have to watch the gauge very closely to see if it's running too hot. Based on the feedback, if this gauge now gets up around the 3/4 mark, I may be in trouble (~240 degrees)?

Thanks again!

Chris S. - 77 Kingsley, 3.70 FD, mostly OEM - S.E. Michigan

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Re: [GMCnet] OEM temp gauge readings [message #358714 is a reply to message #358700]

Thu, 17 September 2020 00:13

jimk

Messages: 6734
Registered: July 2006
Location: Belmont, CA

Karma: 9

Senior Member

Rule of thumb, what the gage shows do not go much more than 1/8 more than
normal.
Sometime one can try to go 1/4, but risky.

On Wed, Sep 16, 2020 at 4:36 PM Chris S. via Gmclist <
gmclist@list.gmcnet.org> wrote:

> Just dropped in the Napa sending unit today. In taking temp readings now,
> the Napa unit is spot on with what J.R. posted. I ran the engine and let
> it idle up to normal temp for a good 20 minutes. The highest the gauge
> showed was now just slightly above halfway between the quarter and half
> marks.
> My readings right at the sending unit on the manifold was about 201-202
> degrees.
>
> Now that it's a little more accurate I will have to watch the gauge very
> closely to see if it's running too hot. Based on the feedback, if this
> gauge
> now gets up around the 3/4 mark, I may be in trouble (~240 degrees)?
>
> Thanks again!
> --
> Chris S. -
> 77 Kingsley, 3.70 FD, mostly OEM -
> S.E. Michigan
>
> _______________________________________________
> GMCnet mailing list
> Unsubscribe or Change List Options:
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>

--
Jim Kanomata
Applied/GMC, Newark,CA
jimk@appliedairfilters.com
http://www.gmcrvparts.com
1-800-752-7502
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jimk@appliedairfilters.com
www.appliedgmc.com
1-800-752-7502

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Re: OEM temp gauge readings [message #366061 is a reply to message #357738]

Tue, 10 August 2021 19:12

lw8000

Messages: 201
Registered: July 2012
Location: S.E. Michigan

Karma: 1

Senior Member

We finally were able to put some miles on the coach with the Napa sending unit. I must say it's MUCH better than the OEM unit. Recently, while on the highway and climbing small hills, around 85 degrees ambient temps, the needle would get up to exactly the halfway point, but never above that. Based on the feedback that would be close to 215 degrees. When coasting or flat areas, the needle would sit just below the halfway mark consistently which would put it close to probably 205-210 or so. And, when getting off the highway and driving in traffic it would dip downward to close to the 1/4 mark when moving.

Based on how this is fluctuating, would it be recommended for us to have the radiator flushed or checked? We do still have the OEM brass radiator. Appreciate the feedback on this!

Chris S. - 77 Kingsley, 3.70 FD, mostly OEM - S.E. Michigan

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Re: OEM temp gauge readings [message #366062 is a reply to message #366061]

Tue, 10 August 2021 19:32

6cuda6

Messages: 975
Registered: June 2019

Karma: -6

Senior Member

Were you running with the A/C on or off?

The one thing you might want to do is clean out the rad....perhaps the fines are full of crap.

lw8000 wrote on Tue, 10 August 2021 20:12

We finally were able to put some miles on the coach with the Napa sending unit. I must say it's MUCH better than the OEM unit. Recently, while on the highway and climbing small hills, around 85 degrees ambient temps, the needle would get up to exactly the halfway point, but never above that. Based on the feedback that would be close to 215 degrees. When coasting or flat areas, the needle would sit just below the halfway mark consistently which would put it close to probably 205-210 or so. And, when getting off the highway and driving in traffic it would dip downward to close to the 1/4 mark when moving.

Based on how this is fluctuating, would it be recommended for us to have the radiator flushed or checked? We do still have the OEM brass radiator. Appreciate the feedback on this!

Rich Mondor, Brockville, ON 77 Hughes 2600

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Re: OEM temp gauge readings [message #366063 is a reply to message #366062]

Tue, 10 August 2021 19:41

lw8000

Messages: 201
Registered: July 2012
Location: S.E. Michigan

Karma: 1

Senior Member

6cuda6 wrote on Tue, 10 August 2021 19:32

Were you running with the A/C on or off?

The one thing you might want to do is clean out the rad....perhaps the fines are full of crap.

AC was off.

I will double check that, but last time I looked it was pretty clean.

Chris S. - 77 Kingsley, 3.70 FD, mostly OEM - S.E. Michigan

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Re: OEM temp gauge readings [message #366066 is a reply to message #357738]

Tue, 10 August 2021 21:24

lqqkatjon

Messages: 2324
Registered: October 2010
Location: St. Cloud, MN

Karma: 5

Senior Member

You can shoot the IR gun at the radiator.

Recent experience tells me (and I remember many posts saying same). that not all fan clutch’s and thermostat works the same.

I put 4000 mile trip on my coach last summer. Super hot and big mountain climbs. One thing that was rock solid was my engine temp, oem guage and my ebl efi read out. 198-202 entire trip.

Then last sept, it warmed up in town. I first
Tossed a new thermostat in.(went from flowcooler
To a stant). That did not help,
So replaced the fan clutch. (2747 same as before. But now a napa. As the old one was hayden)

So now this summer. 210-212. I am going back to the flowcooler thermostat next chance i get. Some say that should not change my temp, but that is only different factor from last summer. (Always ran 198-202 even before my west coast trip).

You may think of trying a 2797 fan clutch. I had one for a while on old engine. Never seemed to be as bad as some claim. Pretty quick to
Change back and forth on a cold engine.

Jon Roche 75 palm beach EBL EFI, manny headers, Micro Level, rebuilt most of coach now. St. Cloud, MN http://lqqkatjon.blogspot.com/

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Re: OEM temp gauge readings [message #366069 is a reply to message #357738]

Wed, 11 August 2021 09:54

JohnL455

Messages: 4447
Registered: October 2006
Location: Woodstock, IL

Karma: 12

Senior Member

Understand that the fan clutch engage point needs to be at least 10 Deg above the thermostat rating to prevent nuisance engagement. This allows for a bit of creep up before the clutch “engages”. What is important is that above that point it DOES engage and stop the temp rise from creeping further. You should hear it and watch the gauge drop within a minute or 2. Usually the clutch will slightly overshoot on the down side then relax. In severe conditions the temp will then creep up and engage and cycle repeat. C4 Corvettes being bottom feeders would do this at highway cruise with AC on. The gauge would slowly creep up, then at about 220 you would get fan roar and feel the drag, run 60-90 seconds then relax with gauge about 190. This cycle would repeat every 4-5 mins or so as there was not enough ram air cooling on it’s own. On the GMC the theory is the engine room becomes pressurized by air getting in and under, which reduces natural ram air cooling. So one more thing we have in common with Corvette besides fuel separator, SMC body and washer nozzles.

John Lebetski
Woodstock, IL
77 Eleganza II

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