timinclyde

GM 454 Backfires

12 posts in this topic

I have a 95 Bounder 28 with GM 454. I bought it used and discovered too late that it back fires under load during a hill climb. If I back off it stops but I also end up dropping down to 40-45 mph at times.

I have had several shops look at it and can't find the problem.

Any suggestions???

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Tim,

Welcome to the FMCA forum.

Good old 454's. Start by checking the vacuum advance unit mounted on the distributor. One needs to remover the Cap and the rotor to get a good look at at things, just be real careful not to pull off any of the plug wires if your not real familiar with the firing order of the cap connections.

Then connect a section of vacuum hose to the vacuum assemble, located on the passengers side, right side of the distributor. Pull a vacuum with your mouth, does the rod inside the distributor move in and out?

If not then the vacuum advance will not work. should everything work, then check the mechanical advance system. That is the system with the 2 springs one on each side are good and that the arms the springs are connected to mover freely in one direction and back again when released.

A drop of oil at each pivot point is good job while the rotor is off.

Post back your findings !

Should you not be familiar with this type system, seek some help.

Rich.

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could also be caused by cracked or broken vacum hose.What condition are plug wires in ?

they eat plug wires due to the way plugs are nestled close to exhaust. good luck

dave

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All of the above are good. My first was a Southwind with the 454, my last resort was to go a high performance shop and purchase a very expensive set of plug wires, that was my final solution, and it work for me.

BTW, I had replaced with factory replacements three times before, with no good results.

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This sounds like a need for an old Sun Diagnostics Machine. Scope and all, also a timing light. Find a shop that has a much older mechanic. He or she will have a vast knowledge older engines.

Herman

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I had the same problem, the shop told me it was the "stator". I do not now what that is, but that fixed the problem.

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Herman, Your making me feel old ! Not that I'm not, just got here allot faster then I thought I would !! :D :D

Rich. B)

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After you have checked that the vacuum advance is working and set right look for vacuum leaks under load. A lean mixture will cause backfiring and missing. Have you checked the ignition module? Hear are a couple of links that might help.

http://www.summitracing.com/search/part-type/ignition-modules/make/gmc/engine-size/7-4l-454

https://www.youtube.com/watch?v=OKCQPlr2k6c

Bill

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Tim I have the same problem with my 1993 Southwind on a P-30 chassis. My 454 has 78xxx miles and has run very well up until it started doing, I believe, what yours is doing.

I have tested the MAP sensor, replaced the TPS (throttle position sensor) - I replaced that because the OBD1 system on mine said the TPS had low voltage. Replacing it did no good.

Engine has the DI type distributor. It is not an HEI like its predecessors, it has the ignition coil mounted off to the left on the intake manifold (changed that with a spare coil I had) no change there.

The plug wires are the MSD street Fire 8mm ones and the spark plugs seem to be firing. I installed headers last year on the engine. Good thing about headers is that you can test the temperature on each one of the tubes while the engine is running to see if they are all the same temp and if not then likely one of the spark plugs are bad.

I have not changed the coolant temp sensor on the front of the intake manifold but the engine idles like it should, high when cold and then idles down when warm.

I tested the EGR valve and the vacuum ports on the throttle body and they appear to be good. I replaced the in tank fuel pump last month thinking it was fuel starvation while going up hills at WOT(wide open throttle). The fuel pressure with the old pump was low showing around 8 psi on the pressure gage that I permanently mounted 3 inches from the throttle body on the pressure line.

I have a timing light that I bought just for this engine and checked the ignition timing ( with the brown wire disconnected on the distributor) and set it to around 10 btdc. One thing I have not checked yet is the timing advance with the brown wire connect while revving the engine to around 2500 rpm. Will need to get help from my better half for that to press the gas pedal (or rather the air control pedal).

I plan on sending the two fuel injectors to a place called "Mr. Injector" to be cleaned and flow matched.

I'm not really sure about your distributor but mine does not have a vacuum advance or the mechanical advance on it. I did remove it from the engine last night to check the shaft for any play and it seems to be in good condition.

I am at a loss here. I really don't want to take the motorhome to a shop just to hear that they cannot find the problem and then be charged for the time they spent looking at it. Being a vet on disability I can't go out and keep throwing money at this thing anymore.

We have not been able to use the coach much this year because of this problem. Its very frustrating not being able to enjoy the coach because I can't find the problem to what is causing it to fall on its face while going up hills at wot or close to wide open throttle. Like Tim when it starts to cut out and run bad if I back off the pedal it will resume acceptable running but go above that spot and, well, it is not good. Especially if towing the Mini Cooper, its even worse. As long as there are no hills or other vehicles to pass the engine seems to run very well.

This is my first time posting on here guys.

Hope I did it right and made sense to all. Thanks.

John MSGT Ret USAF

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John,

Welcome to the Forum.

I don't know if Tim got his problem solved. I had another thought. Does his engine have 2 4 barrel carburater or a throttle body Fuel Injection. If a 4 barrel it might be the accelerator pump.

Herman

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Herman,

I'mpretty sure Tim's is a throttle body injection like mine. These look like a carb but are far from it. For that period it was GM's answer to a carb and I think they worked better than a carb, but they came with their own problems. I'm having problems with my engine which is a gen 5 454 tbi 220 model made in 1992. It has the OBD1 system which was the predecessor to the OBD2 system that all cars 1998 I think and newer have now. With the OBD1 system you don't need a code reader to get your codes. A paper clip works when inserted into the proper holes and the service engine light flashes in a way to give the code that is stored in the system. Mine showed code 22 which is the Throttle position sensor voltage low. I replaced the tps but the problem still exists.

Today I ordered a pickup coil for the distributor in hopes that this will fix the problem. If not then back to scratching my head again......

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Replaced the pickup coil on the distributor two days ago. Didn't fix the problem.

Getting ready to remove both fuel injectors and send them off to " Mr Injector" to have them cleaned and flow tested and flow matched. Even if this doesn't fix the problem at least both injectors will be like new.

So, here I am scratching my head again.

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