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[GMCnet] Poor Mileage solved [message #324615] Wed, 04 October 2017 19:56 Go to next message
emerystora is currently offline  emerystora   United States
Messages: 4442
Registered: January 2004
Karma: 13
Senior Member
I put in a rebuilt 455 engine last January. I have about 6000 miles on it and have only used 1/2 quart of oil and it is running great.

However after installing it I was only getting about 7 miles per gallon.

Upon going and returning from the GMCMI Convention at Elkhart I was getting closer to 6 miles per gallon.

Yesterday while on I-80 in Iowa my engine stalled so I pulled over to the side of the highway.
I knew that I had fuel but it would not start so I lifted the engine cover and checked for spark.
There was none. I took off the distributor cap and it looked good and the rotor looked good so I replaced the distributor module.
It still wouldn’t start. I also carry a spare coil with me so I took off the coil cover and found that the coil was so hot that I could not touch it.
I got gloves and replaced it and reassembled the distributor and the engine started right up.

For the next two tanks of fuel I have gotten close to 9 miles per gallon even towing my Tracker and going about 70 mph on the Interstate. The motorhome also accelerated better and idled faster.

Apparently I had a dying coil and it likely was not firing the plugs all the time which gave me the poor mileage. Funny that the engine seemed to run smoothly though and I couldn’t detect a miss.

I carry spares for the distributor including cap, rotor, module, coil and pole piece (coil beneath the module). I have used all of the spare parts except the coil in the 36 years that I have had the GMC but I am sure glad that I had a coil with me. It is usually the module that goes bad but my rotor, cap, and pole piece have also failed in the past while on a trip.

I would have been laid up for many hours on this trip and probably would have had to be towed for over 100 miles if I had not carried the spare parts and knew how to install them.

Emery Stora
77 Kingsley
Frederick, CO
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Re: [GMCnet] Poor Mileage solved [message #324616 is a reply to message #324615] Wed, 04 October 2017 20:25 Go to previous messageGo to next message
Ken Henderson is currently offline  Ken Henderson   United States
Messages: 8726
Registered: March 2004
Location: Americus, GA
Karma: 9
Senior Member
Emery,

We tend to think of ignition coils as indestructible, don't we? Your
experience now, my temperature-sensitive one in CO in 2017, and too many
others prove that's not true.

I had a call and subsequent photos from a GMCer this evening with something
I've never seen/heard of before: He's having surging, etc, on a new-to-him
78 Royale. Weird finding: Inside the distributor, connected between the W
(Ign) terminal and the adjacent ground screw is a diode! Have you -- or
anyone here -- ever seen/heard of that mod? I can't imagine the purpose
since about the only thing it could possibly do is blow if the distributor
was somehow subjected to reversed voltage on the Ign lead --might 'nigh
impossible.

Ken H.


On Wed, Oct 4, 2017 at 8:56 PM, Emery Stora wrote:

> I put in a rebuilt 455 engine last January. I have about 6000 miles on it
> and have only used 1/2 quart of oil and it is running great.
>
> However after installing it I was only getting about 7 miles per gallon.
>
> Upon going and returning from the GMCMI Convention at Elkhart I was
> getting closer to 6 miles per gallon.
>
> Yesterday while on I-80 in Iowa my engine stalled so I pulled over to the
> side of the highway.
> I knew that I had fuel but it would not start so I lifted the engine cover
> and checked for spark.
> There was none. I took off the distributor cap and it looked good and the
> rotor looked good so I replaced the distributor module.
> It still wouldn’t start. I also carry a spare coil with me so I took off
> the coil cover and found that the coil was so hot that I could not touch it.
> I got gloves and replaced it and reassembled the distributor and the
> engine started right up.
>
> For the next two tanks of fuel I have gotten close to 9 miles per gallon
> even towing my Tracker and going about 70 mph on the Interstate. The
> motorhome also accelerated better and idled faster.
>
> Apparently I had a dying coil and it likely was not firing the plugs all
> the time which gave me the poor mileage. Funny that the engine seemed to
> run smoothly though and I couldn’t detect a miss.
>
> I carry spares for the distributor including cap, rotor, module, coil and
> pole piece (coil beneath the module). I have used all of the spare parts
> except the coil in the 36 years that I have had the GMC but I am sure glad
> that I had a coil with me. It is usually the module that goes bad but my
> rotor, cap, and pole piece have also failed in the past while on a trip.
>
> I would have been laid up for many hours on this trip and probably would
> have had to be towed for over 100 miles if I had not carried the spare
> parts and knew how to install them.
>
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Ken Henderson
Americus, GA
www.gmcwipersetc.com
Large Wiring Diagrams
76 X-Birchaven
76 X-Palm Beach
Re: [GMCnet] Poor Mileage solved [message #324617 is a reply to message #324615] Wed, 04 October 2017 20:51 Go to previous messageGo to next message
RF_Burns is currently offline  RF_Burns   Canada
Messages: 2276
Registered: June 2008
Location: S. Ontario, Canada
Karma: 3
Senior Member
Ken,
I'm thinking the module may have a built-in reversed biased diode in parallel with its output switching transistor. It would be needed for the same reason a diode is put across a relay coil powered by any electronic device... to protect the output transistor from the reverse EMF as the coil field collapses.

I would certainly remove it as I'm sure GM Engineers already installed the needed protection. If it failed shorted, it would certainly take out several modules before it was found.

Just my electronic technician and farmboy mechanic opinion.



Bruce Hislop
ON Canada
77PB, 455 Dick P. rebuilt, DynamicEFI EBL EFI & ESC.
1 ton front end
http://www.gmcmhphotos.com/photos/showphoto.php?photo=29001
My Staff says I never listen to them, or something like that
Re: [GMCnet] Poor Mileage solved [message #324618 is a reply to message #324617] Wed, 04 October 2017 22:39 Go to previous messageGo to next message
Ken Henderson is currently offline  Ken Henderson   United States
Messages: 8726
Registered: March 2004
Location: Americus, GA
Karma: 9
Senior Member
Bruce,

I agree with your analysis. I'm especially attuned to that application of
diodes. It's too long for here but remind me sometime to tell you the
story, related to protective diodes, of how USAF was, in effect, repaid for
sending me back to school for 3 years to get my BSEE & MSEE. It was the
ONLY time, during the balance of my 23 years in USAF that I used anything
from those 3 years. :-)

Ken H.


On Wed, Oct 4, 2017 at 9:51 PM, Bruce Hislop wrote:

> Ken,
> I'm thinking the module may have a built-in reversed biased diode in
> parallel with its output switching transistor. It would be needed for the
> same
> reason a diode is put across a relay coil powered by any electronic
> device... to protect the output transistor from the reverse EMF as the coil
> field
> collapses.
>
> I would certainly remove it as I'm sure GM Engineers already installed the
> needed protection. If it failed shorted, it would certainly take out
> several modules before it was found.
>
> Just my electronic technician and farmboy mechanic opinion.
>
>
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Ken Henderson
Americus, GA
www.gmcwipersetc.com
Large Wiring Diagrams
76 X-Birchaven
76 X-Palm Beach
Re: [GMCnet] Poor Mileage solved [message #324622 is a reply to message #324615] Thu, 05 October 2017 08:21 Go to previous message
Billy Massey is currently offline  Billy Massey   United States
Messages: 916
Registered: January 2004
Location: Central Texas
Karma: 1
Senior Member

Very good argument for carrying a spare, known good, distributor. Takes up
very little space, very light, and could save your bacon!
Thanks for sharing, Emery.
bdub

On Oct 4, 2017 7:57 PM, "Emery Stora" wrote:

> I put in a rebuilt 455 engine last January. I have about 6000 miles on it
> and have only used 1/2 quart of oil and it is running great.
>
> However after installing it I was only getting about 7 miles per gallon.
>
> Upon going and returning from the GMCMI Convention at Elkhart I was
> getting closer to 6 miles per gallon.
>
> Yesterday while on I-80 in Iowa my engine stalled so I pulled over to the
> side of the highway.
> I knew that I had fuel but it would not start so I lifted the engine cover
> and checked for spark.
> There was none. I took off the distributor cap and it looked good and the
> rotor looked good so I replaced the distributor module.
> It still wouldn’t start. I also carry a spare coil with me so I took off
> the coil cover and found that the coil was so hot that I could not touch it.
> I got gloves and replaced it and reassembled the distributor and the
> engine started right up.
>
> For the next two tanks of fuel I have gotten close to 9 miles per gallon
> even towing my Tracker and going about 70 mph on the Interstate. The
> motorhome also accelerated better and idled faster.
>
> Apparently I had a dying coil and it likely was not firing the plugs all
> the time which gave me the poor mileage. Funny that the engine seemed to
> run smoothly though and I couldn’t detect a miss.
>
> I carry spares for the distributor including cap, rotor, module, coil and
> pole piece (coil beneath the module). I have used all of the spare parts
> except the coil in the 36 years that I have had the GMC but I am sure glad
> that I had a coil with me. It is usually the module that goes bad but my
> rotor, cap, and pole piece have also failed in the past while on a trip.
>
> I would have been laid up for many hours on this trip and probably would
> have had to be towed for over 100 miles if I had not carried the spare
> parts and knew how to install them.
>
> Emery Stora
> 77 Kingsley
> Frederick, CO
> _______________________________________________
> GMCnet mailing list
> Unsubscribe or Change List Options:
> http://list.gmcnet.org/mailman/listinfo/gmclist_list.gmcnet.org
>
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bdub
bdub.net
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