timinclyde

GM 454 Backfires

44 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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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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Resurrecting an old thread here. I have a '93 Allegro Bay with a 7.4l (454ci) motor and having the exact same problem the gentleman that started this thread did.  Under load she misfires but runs great otherwise.  I tried to find the vacuum advance components but am at a loss.  There is no vacuum advance on the distributor itself.  So it must be advanced in the ECM through some vacuum sensor that is located elsewhere.  I am stuck on where to look for it.  I don't hear any hissing so I don't think a hose is off or busted.  But then again that motor ain't really all that quiet.  Any help here would be appreciated!  

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Hey Herman, I resemble that remark. I'm sure Tim has a throttle body injection system. If he has had plug wire issues in the past, the first thing I would look at is the ignition coil. If you see any white stains around the ground bracket, the coil is breaking down due to being over worked ( too long at max voltage). Another thing to check is fuel pressure. The system is already a low pressure system to start with. Unfortunately you have to tee into the pressure feed line to get this info. I did have this problem on a customer truck and checked everything I just mentioned and it turned out to be a defective exhaust valve. The valve was starting to collapse. I am an OLD GM certified tech. of 53 years. You can PM me and I'll give you my phone number so we can talk.

Rick Ammerman

F442485

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11 hours ago, wrkey said:

Resurrecting an old thread here. I have a '93 Allegro Bay with a 7.4l (454ci) motor and having the exact same problem the gentleman that started this thread did.  Under load she misfires but runs great otherwise.  I tried to find the vacuum advance components but am at a loss.  There is no vacuum advance on the distributor itself.  So it must be advanced in the ECM through some vacuum sensor that is located elsewhere.  I am stuck on where to look for it.  I don't hear any hissing so I don't think a hose is off or busted.  But then again that motor ain't really all that quiet.  Any help here would be appreciated!  

wrkey, Welcome to the FMCA Forum !

There is No true ECM on the  1993 454. 

There is a mechanical advance spring set up under the distributor cap. Also, unless the engine system has been updated; there is a vacuum advance unit attached to the metal base under the cap. The top item is to replace ALL the vacuum lines. The age and the high temperatures dry out the rubber lines and they crack like an egg. 

Google -1993 454 distributor vacuum diagram-- some good pictures that might help.

 

Rich.

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wrkey! Where on the road and if I'm slow to respond - it is just the lack of WIFI. 

Page change so I need to learn how to Edit or add info to an existing post. LOL

Rich. 

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Rich, below is an example of the EDIT function, it only appears on your own post, simply click on Edit and you can modify or add. Very handy tool. Notice Edit in blue at the bottom of your post, next to Quote.

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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.

  •  
  •  
  • Quote
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  • Edit
  • _____________________________________________________________________________________________________________________________________

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Well whatever it is, it decided to get worse.  Thankfully I was in the storage lot where I keep her when she died and now won't start.  A quick look re sales she is getting fuel so it is an ignition problem.   I don't have the tools with me to dig farther right now.   But here are the photos of what she has.  First photo is the distributor with out the. So in place e, the second is a shot across the top of the motor and the third is under the distributor where a vacuum advance normally is.

KIMG1185.jpg

KIMG1188.jpg

KIMG1187.jpg

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Have you checked to make sure you don't have any water in your fuel?

 

Jim

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Well I would start with the basics. New cap and rotor new quality plug wires. Check/replace the fuel filters. It may just be the picture but the part on the upper right side in the second picture it is green on the bottom looks discolored. I  would check that. Make shure the air filter is in good shape/or replace. Look for any leaking vacuum hoses.

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

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

Bill 

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If the engine was running then shut off and would not start after that, I would change to Ignition Module in the distributor. Had this problem on several 454's. While at it, from the look of things I would defiantly change the Rotor and the Distributor Cap. They both look long over due.

Also as mentioned before, go ahead and change the Air Filter, and defiantly the fuel filter. It is on the passengers side of the frame Back to the rear close to the Tank.

Also a full tank of fresh fuel and a heavy dose of conditioner wouldn't hurt.

Good luck and let us know when and how you solved your problem.

Herman 

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If the backfiring only happens under a large load, like climbing a grade, and stops when he backs off the gas, its most likely fuel pressure related. (it's leaning out). If the backfires happening in the intake manifold, its low pressure/volume and if its happening in the exhaust, its most likely coil and/or plug wires. If all else fails, take it to the dealer. They are factory trained and have all the correct equipment to diagnose the issue. This is going to get real expensive if your just guessing what is wrong. Like I said in a previous post, I have 53 years experience in GM performance issues.

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Great suggestions all.  About a month ago I took her from Spring Tx (just North of Houston) to San Francisco and back and pulled a Nissan Sentra on the way there.  She had some issues with the miss-fire on the way there but they became more frequent about half way back.  So, I am pretty sure it's not bad/stagnant/water in fuel problem.  When I know she is going to sit for a while I always put Sea Foam in her tank.  As for the rotor and cap, well, I bought those new just before I made the 4,200 mile trip to California and back.  I didn't replace the plug wires at the time and they look fine, are not stiff and show no signs of cracking.  This is a miss-fire because it is happening in the exhaust and not back though the carb.

Since it has now decided not to run entirely and I get see / smell fuel in the TBI then I think this pretty much points to an ignition problem... at least to get it running again.  Herman may be on it with the ignition module in the distributor.  Is the 'brain' of the ignition system on this model or is there another ECM somewhere?  If the in distributor module was on it's way out then I suspect that would could have been giving me the miss-fire as timing advance under load portion of it wasn't working.  However, if I understand the purpose of this module correctly, it is only for fuel control and crank shaft position sensor.  (I may be completely wrong in this assumption.)  I read somewhere that this module can be removed and tested at a parts store (Autozone was mentioned), is this correct?  I see they sell at AZ for about $55 and that's not a big number but generally they won't let you return electronic components so if I buy and still no start then I guess I have an extra one.

Thanks for all the advice!  Keep it coming as I will continue to update as I go along and when resolved post here so the thread will be complete, at least for my problem.

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Check cam wear. Worn cam lobes will act the same way under load and run fine when not under a heavy load. I have seen this a few times

 

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I'm watching this thread with great interest. My 1989 southwind is occasionally "hiccuping", when under load.  Seems to happen mostly with a cold engine.  Probably needs a tune up and a good check of the vacuum hoses, but im reading all the suggestions here.

 Need to take care of air bags and alignment before I tackle the engine.

Chris G.

F3508s

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