Home » Public Forums » GMCnet » Peripheral Question - 455 Woes
| Peripheral Question - 455 Woes [message #298429] |
Thu, 31 March 2016 20:46  |
Paul Kirsch
Messages: 21
Registered: September 2015
Location: Tucson
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Hello everyone! I have a question peripheral to our GMC motorhome experience, but perhaps some of you will have ideas.
In addition to my 1976 Glenbrook with 455 engine, I also own a 1976 LeSabre with a 455 engine. Since I have owned it, it has missed at idle and mid-throttle, but seems smooth at full throttle (both in neutral and under load). I have replaced all the vacuum lines, thermal switches and EGR valve, and checked for vacuum leaks by plugging/pinching lines; replaced the entire HEI distributer and verified a steady voltage into the distributor; installed a rebuilt Quadrajet and fuel pump and blew air through the feed and return lines to the tank (and tried running with the cap removed). After measuring uneven (107-127 PSI) compression, I took the car to a local shop for an opinion; they recommended the heads be rebuilt. I got the car back today and...it still misses at idle and part throttle.
Anyone experience something similar with their Olds 455?
Tucson, AZ
1976 Glenbrook with MAC dash, Alcoa wheels, Patterson carb, headers and A&E awning.
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| Re: Peripheral Question - 455 Woes [message #298431 is a reply to message #298429] |
Thu, 31 March 2016 22:22  |
Ken Burton
Messages: 10030
Registered: January 2004
Location: Hebron, Indiana
Karma: 10
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You have not determined whether it is ignition or fuel / air delivery. Lot of things to guess at here.
If you have not replaced the distributor cap and rotor I would do that. I would also suspect the coil. Replace it and keep the old one as a spare.
I would find the darkest place possible and start the engine. If you live is a city this might be difficult to find. Then go over all of the plug wires in the dark looking for a spark jumping to ground or from a boot along the side of the plug to ground. You could also mist the wires and plugs with water is a spray bottle. Look closely at any place where the wires are touching each other or in any of the wire mounting brackets.
Try running a wire from the base of the distributor to the engine block. A temporary wire on a pair of alligator clips would do for a test.
Those are my initial guesses. I'm thinking more towards ignition than fuel delivery on this one. I have been wrong before.
Ken Burton - N9KB
76 Palm Beach
Hebron, Indiana
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| Re: Peripheral Question - 455 Woes [message #298434 is a reply to message #298429] |
Thu, 31 March 2016 23:02 |
Gadabout
Messages: 124
Registered: March 2013
Location: Edmonton
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I fully agree with Ken on the possible ignition issue, also consider the distributor module and all wiring in the distributor. Make sure the vacuum advance is operating and the distributor advance weights are not sticking.
If the problem is not found in the ignition system, my guess is the low speed fuel mixture, this needs to be set, even with a rebuilt carb.
The APT (applied part throttle) setting is also a remote possibility, it's a bit trickier to adjust. I posted some information about that here:
http://gmc.mybirdfeeder.net/GMCforum/index.php?t=msg&goto=297637&rid=0&srch=Apt#msg_297637
Good luck, let us know what you find out.
Carl Harr : Driver NASCAR Pro Series #2
1978 Gadabout-Restoring
1978 Palm Beach
1976 Glenbrook
Prevost Featherlite H3-45
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| Re: Peripheral Question - 455 Woes [message #298437 is a reply to message #298434] |
Thu, 31 March 2016 23:51 |
Ken Burton
Messages: 10030
Registered: January 2004
Location: Hebron, Indiana
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One other really remote possibility if you have a tach, is the tach screwing up the HEI. It is simple to disconnect the one wire labeled "tach" in The HEI plug as a test.
As I said, this is really a remote possibility.
Ken Burton - N9KB
76 Palm Beach
Hebron, Indiana
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| Re: Peripheral Question - 455 Woes [message #298446 is a reply to message #298429] |
Fri, 01 April 2016 11:20 |
Paul Kirsch
Messages: 21
Registered: September 2015
Location: Tucson
Karma: 0
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Junior Member |
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Gents,
Thanks for the great suggestions!
The Distributor, HEI module, cap, wires and plugs are all new; ran it in the dark but didn't see any arcing. There's no tach. Timing is at factory spec (12 deg BTDC) and the vacuum advance and weights move normally. I tried changing the APT and adjusting the idle mixture needles, but the stumbling/missing is not affected, except when almost seated (as expected). I've checked for vacuum leaks on everything connected to the carb, and have also pulled each line and put my finger over the ports while running, with no change in smoothness.
One thing I failed to mention - it runs fairly smoothly when cold, all through the choke cycle. Throttle tip-in is smooth and there is no missing from idle to WOT. But once the engine is fully warm, the idle goes lumpy and it seems to load-up at throttle tip in - also, the random missing/stumbling gets steadily worse at all throttle settings, from idle to WOT. Could there be something changing in the lifters as temps rise? There's no ticking, but maybe one of them partially collapses as the oil heats?
Again, thanks for the suggestions...like shooting in the dark w/o the engine in front of you 
Paul
Tucson, AZ
1976 Glenbrook with MAC dash, Alcoa wheels, Patterson carb, headers and A&E awning.
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| Re: Peripheral Question - 455 Woes [message #298452 is a reply to message #298446] |
Fri, 01 April 2016 12:36 |
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Matt Colie
Messages: 8547
Registered: March 2007
Location: S.E. Michigan
Karma: 7
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Paul Kirsch wrote on Fri, 01 April 2016 12:20Gents,
Thanks for the great suggestions!
<snip>
One thing I failed to mention - it runs fairly smoothly when cold, all through the choke cycle. Throttle tip-in is smooth and there is no missing from idle to WOT. But once the engine is fully warm, the idle goes lumpy and it seems to load-up at throttle tip in - also, the random missing/stumbling gets steadily worse at all throttle settings, from idle to WOT. Could there be something changing in the lifters as temps rise? There's no ticking, but maybe one of them partially collapses as the oil heats?
Again, thanks for the suggestions...like shooting in the dark w/o the engine in front of you
Paul
Paul,
Your other scenarios are unlikely. Possibly the only thing Smokey Yunick said that I agree with was the most carburetor problems start with the ignition. This may be an exception, but lets go.
That information tends to indicate that the idle circuit is lean. If this has been the case all along, it could quite possibly never have been right. Did anybody adjust the idle mixture after the rebuild carburetor was installed? If not, that is a very high probability. Unfortunately, it may take the special tool (no joke here) to do that. The idle jets are underneath and impossible to get at with normal tools.
The manifold crack as mentioned is another good possibility.
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: Peripheral Question - 455 Woes [message #298453 is a reply to message #298446] |
Fri, 01 April 2016 12:52 |
Ken Burton
Messages: 10030
Registered: January 2004
Location: Hebron, Indiana
Karma: 10
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Well, Paul
Everything I think of as possibility has already been replaced or checked by you. I thought about a sticking valve but you already re-did the heads. I thought about ignition but you have replaced everything. I thought about fuel delivery like a fuel pump or filters, or sucking air on the input side of the pump but those usually fail as fuel starvation on heavy acceleration or WOT. The chance of replacing something an having the new one fail in the exact same way are not very likely.
Since it is temperature sensitive I would think of coil, ignition module, vapor lock, a cracked intake manifold, automatic choke, carb base gasket. Have you looked at the hot air feed to the air cleaner or the heat riser if it has one? How about a sticking EGR valve.
This is a stretch but you might consider installing a wide band O2 sensor and watching it when it is failing. That might help determine if the issue is fuel/air mixture or something else. I realize that this is a lot of work and expense to install one.
I'm out of guesses at the moment.
Ken Burton - N9KB
76 Palm Beach
Hebron, Indiana
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| Re: Peripheral Question - 455 Woes [message #298460 is a reply to message #298454] |
Fri, 01 April 2016 15:27 |
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Matt Colie
Messages: 8547
Registered: March 2007
Location: S.E. Michigan
Karma: 7
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Senior Member |
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A Hamilto wrote on Fri, 01 April 2016 13:59Matt Colie wrote on Fri, 01 April 2016 12:36...The manifold crack as mentioned is another good possibility. ... It seems like normally the crack is closed when cold, and opens up hot. So it would idle and run fine until warmed up.
Rough idling and smooth at RPM might imply the crack is open all the time. Shouldn't it burn some oil if the plenum is cracked?
A,
Your thinking is good, but you are leaving manifold pressure (aka inverse vacuum) and the turkey tray out of your thinking.
As said, your thinking is good, but the situation can be that there is little oil up there at lower crank shaft speeds, and when the speed is high and there could be lube oil (as mist) very available, then the manifold pressure is also high and there is less differential between that and the crankcase pressure to suck oil into the intake runner.
There are a large number of things I would like to check out if he were in striking range.
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: Peripheral Question - 455 Woes [message #298462 is a reply to message #298429] |
Fri, 01 April 2016 17:05 |
Gadabout
Messages: 124
Registered: March 2013
Location: Edmonton
Karma: 2
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Senior Member |
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Paul,
It may be a valve not fully closing is possible that as the engine heats up, you are losing the necessary rocker arm clearance. I have corrected this condition by installing shims under the affected rocker arm. If you get that far into it, pull each push rod and make sure it is not plugged, reinstall the push rod and rocker then rotate the engine until that valve is in the closed position. At that time you should have some clearance between the rocker and valve. If you can't move the rocker easily by hand then the valve is stuck open, and the cylinder is pushing some exhaust gas into to intake causing a stumble. In addition compression on the power stroke will suffer.
With regard to a possible carb problem,
What I understand from your last post, is while the choke is closed it runs fine, but when open, the problems start.
This suggests air/fuel mixture problems.
In 1968 my Dad's cure for a lean condition , was to restrict the primary carb inlet with a big rag shoved into the carb.
Did you have your carb rebuilt ? Or did you install a rebuilt carb.
If your carb was rebuilt, I expect you got back the same jets, metering rods and power valve spring. These are not changed as part of a typical rebuild.
Rochester's are also prone to leaking into the intake from the main body were caps are installed at the factory. As the carb gets hot, the leaks get worse and you develop a rich condition at idle.
If you have a known good carb on the other vehicle, swap the two and see if the problem goes away.
Carl Harr : Driver NASCAR Pro Series #2
1978 Gadabout-Restoring
1978 Palm Beach
1976 Glenbrook
Prevost Featherlite H3-45
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| Re: [GMCnet] Peripheral Question - 455 Woes [message #298463 is a reply to message #298462] |
Fri, 01 April 2016 17:36 |
James Hupy
Messages: 6806
Registered: May 2010
Karma: -62
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Carl, you had the problem before you replaced the carb, and also after you
replaced it. That would steer me away from the fuel system towards ignition
or compression. I would very carefully do a wet/dry compression test. I
would also do a high resistance wire ohms check on every secondary wire.
Some Buick HEI systems use a remote coil instead of an in cap coil. I would
check this as well as the HEI MODULE. I know you told us " It is all new",
but there is new but bad stuff out there. I have seen it many times. Not as
often in Genuine Goodwrench GM parts, but they do occur. If you have a
known good module and coil, try swapping them for the new stuff one piece
at a time until something changes. Lean misfires are hard to find, but are
affected by too wide a plug gap. Set them to .040". Your module and coil
will thank you. Also might be intake manifold gasket leak or crack. Lots of
possibilities.
Jim Hupy
Salem, Or
78 GMC ROYALE 403
On Apr 1, 2016 3:06 PM, "Carl" wrote:
> Paul,
>
> It may be a valve not fully closing is possible that as the engine heats
> up, you are losing the necessary rocker arm clearance. I have corrected this
> condition by installing shims under the affected rocker arm. If you get
> that far into it, pull each push rod and make sure it is not plugged,
> reinstall the push rod and rocker then rotate the engine until that valve
> is in the closed position. At that time you should have some clearance
> between the rocker and valve. If you can't move the rocker easily by hand
> then the valve is stuck open, and the cylinder is pushing some exhaust gas
> into to intake causing a stumble. In addition compression on the power
> stroke will suffer.
>
> With regard to a possible carb problem,
> What I understand from your last post, is while the choke is closed it
> runs fine, but when open, the problems start.
>
> This suggests air/fuel mixture problems.
>
> In 1968 my Dad's cure for a lean condition , was to restrict the primary
> carb inlet with a big rag shoved into the carb.
>
> Did you have your carb rebuilt ? Or did you install a rebuilt carb.
>
> If your carb was rebuilt, I expect you got back the same jets, metering
> rods and power valve spring. These are not changed as part of a typical
> rebuild.
>
> Rochester's are also prone to leaking into the intake from the main body
> were caps are installed at the factory. As the carb gets hot, the leaks get
> worse and you develop a rich condition at idle.
>
> If you have a known good carb on the other vehicle, swap the two and see
> if the problem goes away.
>
>
>
>
>
>
>
> --
> Carl Harr : Driver NASCAR Pro Series #2
> 1978 Gadabout-Restoring
> 1978 Palm Beach
> 1976 Glenbrook
> Prevost Featherlite H3-45
>
>
> _______________________________________________
> GMCnet mailing list
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| Re: [GMCnet] Peripheral Question - 455 Woes [message #298473 is a reply to message #298463] |
Sat, 02 April 2016 01:48 |
appie
Messages: 902
Registered: April 2013
Location: denmark
Karma: 2
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Also check the ingoing voltage to the distributer. That voltage can be low after going through al those 40 year old switches and connecters in the collum
Appie
eleganza 76 "Olga" now sadly sold
6 wheel discbrake
Quadrabags
Springfield stage 2 462 olds
Manny tranny
( pictures at http://www.gmcmhphotos.com/photos/g6489-olga.html
Fulltiming in Europe july 2014 til july 2016
Denmark
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| Re: [GMCnet] Peripheral Question - 455 Woes [message #298577 is a reply to message #298473] |
Sun, 03 April 2016 20:07 |
Paul Kirsch
Messages: 21
Registered: September 2015
Location: Tucson
Karma: 0
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Junior Member |
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Problem found!
In addition to all your great suggestions, I decided to re-look at temperature-related vacuum systems. With the engine warm and stumbling as usual, I individually disconnected vacuum lines - when I disconnected the (brand new) EGR valve, everything smoothed out and throttle tip-in became perfect. The EGR holds vacuum, works perfectly, and the gasket is new. So, why did disconecting it make such a difference? I re-read the emissions troubleshooting section of my vintage 1976 Buick Chassis Service Manual and noted this:
Condition: Engine runs rough on light throttle acceleration, poor part load performance and poor fuel economy.
Possible Cause: EGR flow unbalanced due to deposit accumulation in EGR passages or under caburetor.
Seems I've got some blockage in the EGR passages inside the manifold. As this car does not need to pass smog tests, due to being insured as a collector car, I will leave the EGR disconnected. Hopefully, this will not lead to other issues...?
Paul
Tucson, AZ
1976 Glenbrook with MAC dash, Alcoa wheels, Patterson carb, headers and A&E awning.
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| Re: Peripheral Question - 455 Woes [message #298581 is a reply to message #298429] |
Sun, 03 April 2016 22:08 |
Gadabout
Messages: 124
Registered: March 2013
Location: Edmonton
Karma: 2
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Thanks for the update Paul, good to know and glad you found the problem.
Carl Harr : Driver NASCAR Pro Series #2
1978 Gadabout-Restoring
1978 Palm Beach
1976 Glenbrook
Prevost Featherlite H3-45
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| Re: [GMCnet] Peripheral Question - 455 Woes [message #298582 is a reply to message #298577] |
Sun, 03 April 2016 22:16 |
Carl S.
Messages: 4186
Registered: January 2009
Location: Tucson, AZ.
Karma: 13
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Paul,
I had a truck with EGR issues and I plugged the vacuum line with a BB (or something that fit tight in the hose). I never had a problem with it and it looked like it was all properly hooked up.
Carl Stouffer
'75 ex Palm Beach
Tucson, AZ.
Chuck Aulgur Reaction Arm Disc Brakes, Quadrabags, 3.70 LSD final drive, Lenzi knuckles/hubs, Dodge Truck 16" X 8" front wheels, Rear American Eagles, Solar battery charging. GMCSJ and GMCMI member
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