GM Auto Mechanics?? I need to ask a question....

[Dennis Peacock]

OK....I just KNOW that SMC has a few automotive mechanics hanging around. Here's my delima:

1994 GMC Suburban
Standard 350, V-8 engine
223,000 miles +/- a vew hundred miles
Always been maintained by a GMC dealership
Automatic tranny with an Overdrive
Throttle body and not fuel injected

Cranks up and idles very nicely. Does everything very well and smoothly...until you put it under a load, like going up a small hill or pulling a trailer...then it "backfires back through the throttle body (carb)". If you ease off the accellerator just a "tad" then it will smooth out and run smoothly. Ease down on it while it's going up a small hill or pulling a load and it will slap and kick like it wants to up and die...ease off the throttle a tad and it will run smoothly.

It's been at the GMC dealership since last week and it has every shop mechanic "stumped" on how to fix it.

What are YOUR thoughts? I sure need my wood hauler back. ;)

[John Shuk]

Painful suggestion but have you tried super unleaded in it? Or perhaps it is just a bad tank of gas?

[Chuck Wintle]

Dennis,
My initial thought might be some bad gas or water in the gas.
Do you know if any codes were set in the computer?
That said i would look at a possible lean out condition in the fuel delivery system.
Is there any black smoke coming out of the tailpipe when you press the throttle?
What about the spark plugs? Are they in good condition and properly gapped?

What about the timing chain? The condition you describe can be caused by a stretched timing chain.

I am only an amateur mechanic and I am just drawing on what little knowledge i know. :D

[Joe Pelonio]

I agree on the timing chain, I had a '94 Blazer that did that, basically the same engine but V6 (4.3) and that did it. Those timing chains rarely last as long as yours has, and the timing is thrown off.

The dealer wanted $1,100 because they had to jack up the engine, but my clever independent mechanic was able to change it without having to remove the engine, by snipping off the tips of the lower ears of the cover.

[Steve Clardy]

Has anyone changed the fuel filter lately?

[bill kiss]

Check the exhaust for excessive backpressure, sounds like a clogged cat

[Lee DeRaud]

Has anyone changed the fuel filter lately?What he said. Might also check that the fuel pump itself is delivering normal pressure.

This definitely sounds like a fuel-flow issue. I had a similar deal with an older Pontiac: got so it would literally run out of gas going up long hills. Turns out the filter in the carb wasn't flowing enough gas to keep the float bowl filled under continuous heavy load, but no problem in normal driving.

[Mitchell Andrus]

Throttle body and not fuel injected



I'm not sure, but.... in 1994,
Throttle body injection isn't a carb. It's an injector(s) in the manifold rather than in the cylinder.

[Dennis Peacock]

I'm not sure, but.... in 1994,
Throttle body injection isn't a carb. It's an injector(s) in the manifold rather than in the cylinder.

OK...I have to go with what you say....because I don't know the difference. I do know a carb and I do know fuel systems that are totally fuel injected.

Live and learn...

[Dennis Peacock]

Fuel filter has been changed by the local dealership.
The dealership put a new distributor in it and it's still doing the same thing.
The dealership is now saying that it's a "firing" problem.
I asked them about the timing chain and they said that it was fine. The timing chain is the 1st thing I thought of since the car has so many miles on it.

No it doesn't blow black smoke out the back.
I always get gas at the same gas station that I've been using for the last 8 years....Exxon/Mobile.
I've always ran Super Unleaded fuel in all my vehicles, to include my chainsaws. :D

The dealership that is working on it now, has been servicing it and repairing it for the last 8 years. They have never done me wrong...but there's always a 1st time I guess.

I'll ask them about all the other suggestions that y'all brought up.

[Lee DeRaud]

OK...I have to go with what you say....because I don't know the difference. I do know a carb and I do know fuel systems that are totally fuel injected.Easiest way is to think of it as a single-barrel carb with a fuel injector instead of a main jet.

And yes, that really is just a big a compromise as it sounds. :p

[Dennis Peacock]

Easiest way is to think of it as a single-barrel carb with a fuel injector instead of a main jet.

And yes, that really is just a big a compromise as it sounds. :p

LOL.!!! OK...understand now. Thanks Lee. :)

[Ken Fitzgerald]

Dennis,

I have seen 3 problems cause what you describe and they have all 3 been discussed here:

1. Low grade or cheap gas

2. Fuel filter clogged restricting gas flow

3. Timing chain. At a 120,000 miles I had to replace the timing chain on our Toyota 4-Runner. I did it myself.

IIRC....cheap gas or a restricted fuel filter result in the engine running lean. This causes the temperature to be a little hotter and thus gas arriving at the valve before it opens ignites due to the increased temps.

The timing chain is kinda self explanatory. The chain wears and stretches over time. This results in an intake valve being open at the time the spark plug fires which ignites the gas beyond the incorrectly opened valve.

If you have the original timing chain...UH....that would be my suspected component failure.....but...it would certainly be worth trying a higher grade and quality of gas first. In the case of the 4-Runner, I could actually hear the chain slapping the side of the timing chain cover. When I pulled it off, there were actual wear marks in the timing chain cover where the loose chain had been slapping it.

Good luck my friend!

[Joe Mioux]

Air up your tires. they might be low.

Joe

p.s. sorry, couldn't resist.

[Dennis Peacock]

I can't be running out of gas.............I put 75¢ worth in it just last week. :rolleyes: :p :p :D

[Steve Clardy]

I can't be running out of gas.............I put 75¢ worth in it just last week. :rolleyes: :p :D



75 cents. Well that isn't much..:rolleyes:



Couple weeks ago I was really broke, needed gas, and went in and asked them to ring up 50 cents worth and set the pump at that amount.
Price of gas these days, its hard to stop the pump at a certain amount.






Well.............










Wait...............




















The attendant farted, then handed me a receipt. :mad::D:D:D:D:D

[Bruce Page]

Dennis, I had an 82 Silverado 305 with a carb and it behaved exactly as you describe. It would drive fine if I had an egg between my foot and the gas pedal but if I stomped on the gas trying to pass someone it would kick & sputter. It turned out to be the fuel filter.

[Tom Godley]

Well I will put in my 2c :) Since we are all guessing anyway.

The dealer may not see that many 94's, now that it is almost 15 years old. Often as cars age they are less likely to see the dealers and the dealers can actually be less familiar with the problems of the older models.

I must be lucky but we have never had a problem that could be traced to "bad' or "low' grade gas. I can not see why you would need to put premium fuel in that engine - unless it has a buildup in it and pings with regular. Many two cycle engines tell you not to use premium :(

Anytime I have ever had a problem with a timing chain -- the idle has been affected. The old carburetor 350's had small fuel filters but the T bodies came with larger filters -- I never had to replace one.

We did used to have some problems with the fuel pumps around that time - and they can do strange things when they are weak and they are hard to pinpoint as the problem. We also had a lot of problems with the TPS (throttle position sensor) in the early 90's --they use to get dead spots just off of idle and would through the mixture control way off. I also remember some small vacuum valve that was at the throttle body -- it used to crack -- but that also would affect the idle.

My guess!

[Steve Schlumpf]

Dennis - had a 75 Chevy Silverado, 350 with a 4 barrel - same problem you explained and it turned out to be the fuel filter. Worked fine at all speeds until you wanted just a little more gas (like going uphill or passing) - then you had a bucking bronco to deal with until you let off the gas and it settled back down. Seriously - all it was - was the fuel filter!

[mark page]

Been a few years since I was with GM but will try to remember a few things that come to mind. Early 90's trucks had a grounding problem at the intake manifold (this may have been a year or two earlier than your model). Have them check for a weak ignition coil and distributor pick-up coil problems. They also had a few problems with fuel pumps and pulsators. If the symptoms are bucking and jerking under a load, this inclines me to believe that it is more of an ignition or powertrain management type problem. You didn't say if any specific dtc trouble codes are setting. Also I do believe that with your mileage, intake manifold and throttle body base gasket vacuum leakages were common. This should be picked up with a simple propane test for vacuum leaks. My gut feeling is that the problem will actually lie with either the primary or secondary igniton system, and is pointing towards the ignition coil breaking down under loads or internal distributor/pick-up coil problems. Both of these problems will elude the pre-OBDII system codes. Usually fuel system failures and converter failures will cause a smoother bogging down under higher rpm's/loads. A plugged cat will cause superheated exhaust temps and will actually burn your hands at the tailpipe, plus at extremes superheating a converter to glowing red. Hope this helps. I checked my all-data information and nothing else comes to mind. Jim O'Dell may be able to access earlier tsb's for your vehicle at the dealership. Please let us know what the prognosis is when found, as I am very curious.

[David Sallee]

Hey Dennis

Well, I will throw my .02 cents in.

I am an ex-mechanic of 15 yrs (82-97) and was ASE Certified.... Everyone has some good advise but I'm seeing you have 2 different symptoms so here is my thoughts....


...."backfires back through the throttle body (carb)". If you ease off the accelerator just a "tad" then it will smooth out and run smoothly.



Ease down on it while it's going up a small hill or pulling a load and it will slap and kick like it wants to up and die...ease off the throttle a tad and it will run smoothly.When it does the "slap and kick" does it backfire too? and when it does backfire... does it go something like pop,pop,pop,pop or does it just do it once and act like it's dieing "slap and kick"?

If it's the pop, pop, pop, pop .... I would look at the following:

Cam lobe worn down slightly
Weak or cupped lifter
Worn or bent push rod and/or rocker arm
Crack in distributor cap
Bad plug wire or spark plug
Timing chain
Timing off

If it's the pop, slap, kick, slap, kick, pop .... I would look at the following:

Fuel filter
Fuel pressure
Cat converter
Ignition module
Crack in distributor cap or rotor
Bad plug wire or spark plug

These are just off the top of my head... and without hearing and seeing what it's doing... With that many miles and the word "backfire" I bolded the first place I would look....



It's been at the GMC dealership since last week and it has every shop mechanic "stumped" on how to fix it.For some reason this doesn't surprise me... :rolleyes:

Dave

[Denny Rice]

OK....I just KNOW that SMC has a few automotive mechanics hanging around. Here's my delima:

1994 GMC Suburban
Standard 350, V-8 engine
223,000 miles +/- a vew hundred miles
Always been maintained by a GMC dealership
Automatic tranny with an Overdrive
Throttle body and not fuel injected

Cranks up and idles very nicely. Does everything very well and smoothly...until you put it under a load, like going up a small hill or pulling a trailer...then it "backfires back through the throttle body (carb)". If you ease off the accellerator just a "tad" then it will smooth out and run smoothly. Ease down on it while it's going up a small hill or pulling a load and it will slap and kick like it wants to up and die...ease off the throttle a tad and it will run smoothly.

It's been at the GMC dealership since last week and it has every shop mechanic "stumped" on how to fix it.

What are YOUR thoughts? I sure need my wood hauler back. ;)

Dennis,

I have heard this story before. 223,000 plus miles......Make the mechanic remove timing chain cover and visualy check timing. A engine can jump timing by too much slop in the timing chain and it will jump one or two teeth and still run and idle fine. The problem comes in when you put the engine under a load, it needs the extra power and cannot get it because engine is not 100% timed correctly. When an engine is out of time you have intake and exhaust valves opening and closing too soon or too late and the spark plugs firing too soon or too late. All these thing happen because the cam and crank are not in sync with each other. Both of these problems can cause driveability issues and worse yet can cause damage to an engine. Also when reving engine listen for "Chain slap" there are times a timing chain will strech so much when its put under a load you can hear the chain hit the inside of timing chain cover. Back in the early 90's GM had real problems with the Quad 4 engine and chain slap at very low milage. Keep us informed.

[Dennis Peacock]

Well, here's what I'm going to do.

Tomorrow morning, I will print this thread off onto paper and go to the GMC dealership and talk with the shop foreman and see if we can talk things over. Right now, I'm looking at almost $800 in shop fees, repair, and testing. Needless to say, this will HAVE to go on the old credit card.

I'm going to talk with them over all this to see what else I can "avoid" in them trying stuff just to try "something" to fix the problem.

I sure miss the old mechanicing days when all you needed were a few tools and a good long wooden handled screwdriver (used as a stethascope). Oh well, those days are long gone.

[Jude Tuliszewski]

I used to work on cars a lot in the past and in my experience back firing out the intake (carb/throttle body) was for the most part timing related. Sometimes as simple as adjusting the timing on up to cam replacement, but it does sound like it could be a worn timing chain. Take this info with a grain of salt as I don’t work on cars much since technology did for Detroit what they could not do themselves and weed out the shade tree grease monkey. Good luck with it.

[Dennis Peacock]

Ok thanks everyone....I've collected all the responses and I'll be headed out to the dealership in about 30 minutes. I'm sure hoping for a good peaceful talk with them today.

[Chuck Wintle]

Ok thanks everyone....I've collected all the responses and I'll be headed out to the dealership in about 30 minutes. I'm sure hoping for a good peaceful talk with them today.
let us know the outcome of your visit to the dealership. :D

[Dennis Peacock]

So far...here's that they've done.....

1. A LOT of "testing".
2. Computer time to diagnose the problem. Computer codes were "clean" and revealed No Problems. (stupid computer doesn't know a problem until it gets it little bitty processor bit. ;) )
3. Installed a new distributor ($400 Ouch!!!!)
4. Installed new distributor cap and rotor-button (free) (note: uh-hu..free (right!))
5. Ignition coil changed out.
6. New plugs and wires.
7. Checking the timing AGAIN this morning.
8. Checked fuel filter and flow and that was good.
9. I read them the list I took from here....all I got was "the look". (Like a calf looking at a new gate, in my book)
10. They will be checking the timing chain and other timing related "stuff" today.

I gotta sneaky suspicion that this ain't gonna be a cheap fix.!!!! :(

[mark page]

It does sound like it jumped time. They can check it all they want and it will look good from the timing marks on the crank, but it will be the cam timing that will be off. Have to pull the front cover to check that. One other very very remote possibility might be that a driver circuit in the ecm may be faultering, but I highly doubt it. Good luck and best wishes.

[Tom Godley]

It can get painful ($$) when they do not know what is causing the problem.

Many items can be borderline and it can be expensive before they find the one that is actually causing the problem.

Why did they replace the distributor?? They must have seen a timing problem! (or was it an internal problem?) - Did the new distributor fix that particular issue?

I thought they already indicated that the chain was OK ??

[Dennis Peacock]

They said it needed a new distributor, and no....that didn't fix the problem.

All that they've done / replaced has only improved the issue but has not resolved it. I'm supposed to hear from them this afternoon.

[Dennis Peacock]

Still no word from the GMC dealership. I'm beginning to get a tad irritated with them...but I'm willing to be patient...as long as it is at their expense. 4 days labor is a bit steep, plus parts that they felt should be replaced.

I don't know....can it be "that hard" to fix a car that is almost 15 years old? After all, they have been working on this exact vehicle for the past 8 years.

[mark page]

Dennis,
I hope that they've been upfront and honest with you on diagnostic charges, and have given you the proper courtesy calls beforehand. If not, I would tend to get a little upset. My next question would be to them if they call with further replacement parts needed is "what if this don't fix it, and are you guessing at it?"
Good luck.

[Dennis Peacock]

Mark,

To this very minute, they have NOT called me with hey, we need to replace; kind of a call. As a matter of fact? They never called ONCE since we put it in the shop with them last week....it's been US calling THEM.!!!! It's 20 til 5PM here and I'm about to call them "again". They told me that they would call me by early afternoon....well, it's almost close of business now. :mad:

[Dennis Peacock]

Ok....they said that they FOUND the problem and it will be ready to pickup by noon tomorrow.

The solution? Fix the problem with "too much voltage on the ignition circuit". That's their words and not mine. Timing was spot on according to the lead mechanic.

So...how can you get too much voltage on an ignition circuit when it's only 12V to begin with?

Total repair bill? Just shy of $900. :(

[mark page]

Dennis, after 22 yrs in the dealership auto service management field, please take my advice: Go to the dealership general manager and start there. Voice all concerns including you didn't even get a complimentary call on how your vehicle was doing. Go through everything step by step that happened. Either the gen man will get the service manager involved or not, it really doesn't matter at this point. Voice your concerns about the shoddy service and follow-up that was performed. Let them know that you are disgruntled with the service received and want some consolidation in the matter. Let them know that you are being nice about it, but there's something that needs to be amended in this situation. You have a butt load of parts on your vehicle that you didn't need in the first place. State that you feel the bill does not constitute the repairs that were actually needed. They may come to terms such as "you pay parts and dealer eats the labor" or what I would offer would be "I will pay the actual dealership cost of parts and labor". They don't lose anything except gross profit from something else the tech would be doing. Anyway if they meet you halfway you gain something. Don't take no for an answer, be nice, but be adamant about it. On a side note, high voltage on the ign circuit??--what caused it and what is the repair required to fix it??? I would be questioning this one, and have them show you what they did to fix it, actually open the hood or wherever the fix was performed. I will be very curious to find out this repair, let me know. High voltages can occur in newer vehicles that look for a certain reference signal on solid state technology, but your vehicle does not have all pcm, bcm, tcm, and other related control modules or can affect some sensors such as tps, map, maf, etc. The only thing that comes to mind for overvoltage would be an alternator/regulator problem ON THIS CIRCUIT, but has been a long day and am brain dead right now.Like I said, let me know as you have my curiosity peaked.

[Tom Godley]

Well I do hope that they have found the problem - I would be interested to know if this voltage "thing" took out the distributor or if it had just reached its mechanical life.

Cars manufactured in the early 90's had all kinds of issues -- the electronics were advancing at an unbelievable pace but were often attached to parts from the early 70's.

Always a little frustrating when you do not get reasonable updates -- I also would look over the shop time. Nothing wrong with pointing out that as a dealer they "should" be better able to find a problem.

Never fun to spend a lot on an old car -- but most of us have been in the same situation.

[David Sims]

My 90 Chevy van did the same thing only it got progressively worse. Turned out that it was the fuel pump. Had it replaced and it ran better than it had in a long time.

[Charlie Schultz]

Well, if they still don't have it fixed, you can try "Click and Clack, the Tappet Brothers" Saturday morning.

[Heather Thompson]

Ok....they said that they FOUND the problem and it will be ready to pickup by noon tomorrow.

The solution? Fix the problem with "too much voltage on the ignition circuit". That's their words and not mine. Timing was spot on according to the lead mechanic.

So...how can you get too much voltage on an ignition circuit when it's only 12V to begin with?

Total repair bill? Just shy of $900. :(

Dennis,

When I was nineteen years old I had a 75 Chevy van (350 four barrel), it started to run like crap, took it to the stealership for repairs. They called to advise that they put it on their computer testing system and determined that the carberator needed to be rebuilt, I gave them the go ahead and received a call the next day to pick up my truck. Went to the dealer, paid the bill, jumped in and TRIED to drive away. Went into the service department, grabbed the manager to take him for a test drive, he advised that he would have his best mechanic on it the next day. Next day I receive a call at work advising that the truck was repaired, ends up it was a bad plug wire, couple of bucks. When I picked the truck up I spoke with the manager, said I expected the charges for the carberator rebuild would be refunded, he advised no way (it needed to be rebuilt) BS. At that point I went out and bought the service manual for every vehical I have ever owned, if I can read it I can do it. I have rebuilt transmissions, engines, brakes, fuel pumps, power steering, motorcycles etc, I don't mind getting dirty. I would have a hard sit down with the service manager/owner, hope all works out well.

Heather