Mechanics

98 350 STALLS, BOGS DOWN, HESITATES O

1998 GMC Suburban

Engine Performance problem
1998 GMC Suburban V8 Two Wheel Drive Automatic 239K miles (engine has 120K)

Here's what's been replaced or checked ok:
2 cats; 4 o2 sensors, muffler, fuel filter, air filter, expensive wires, plugs, cap rotor, pcv, fuel psi regulator, fuel pump, throttle position sensor, high quality gas (always used); regular gas treatment (lucas oil or sea foam); timing, fuel pump psi = 60 and holds, no vacuum leaks, cam position retard setting is good. NO CHECK ENGINE LIGHT, trans was rebuild in Dec. 2009 (still under warranty)

I've had 3 professional shops look at it and no one can give me a definite answer without ripping out my pockets. You're welcome to give me your.02 worth. Thanks.

I would donate with this question, but please review my current status. I just gave my last $426.00 to two certified mechanics today. Sorry. Better let this question entertain all the viewers. If you really care to help, please email me at jbatpbc@sbcglobal. Net.
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Sierrajb
May 8, 2010.




Get some choke cleaner and clean the throttle plate on both sides as well as shooting some in the IAC. That causes a lot of stalling/idle problems. Then if you can do it, check the ohms of the crank sensor and cam position sensor. The sensors soemtimes don't set a code but aren't the best. If they read higgh ohms then replace them. ( Sorry I don't know the ohm readins on what is good or bad off the top of my head). Also clean the throttle plate shaft where it is by the throttle position sensor. I fyou do the shaft thing while it's running then you may have a vacuum leak and the computer is trying to cure the leak while it's running then you get driveability problems like you describe. The can of cleaner is about $3-4. Get CYCLO brand if you can find it as it's about the best i've used for years.

Hmac300
May 11, 2010.
Failed to report that I did that, too. I've checked throtte position sensor, cam sensor and crank sensor. All three checked fine by a certified mechanic. Throttle body and MAF are all sparkling clean. Absolutely NO CARBON BUILD UP to speak of anywhere near or around these areas.

Also, I double checked for vacuum leaks by spraying some of that carb cleaner around intake, throttle body, everywhere. No change in rpms. Still idles like a dream.

One more thing, did I mention I replaced the coil, too? Well, I did and there's no change. My gut tells me it's the fuel pump, regardless of what the gauges tell me. Any more ideas?

Tiny
Sierrajb
May 11, 2010.
The only other thing that I can think of is that this truck has that plastic topped manifold. Other than a bad connection on top it may be teh injector lines inside the manifold that are giving the problem. Due to heat and time=mileage they tend to crack and leak. Another thing if you suspect the fuel pump it could have done two thing that I can think of the pump isn't on the line right or the wires to the pump in the tank are bad. One other thing I just thought of concerning your fuel pump your vehicle year has had fuel pump trouble as a problem. GM found that the harness oing to the pump was bad and when a delco pump was purchased in later years, they included a 11" piece of harness due to the original being screwed up and would give the problem of a bad fuel pump. As you have already replaced this pump and probably had it done at a repair shop you don't know what type it is. But if it was a delco there should have been a 11" harness installed due to the old one going bad. This could be your problem. If they installed the delco and not the harness then it's still screwed up and needs the harness as well. This happened in years1997-1999 in pickups/suburbans/tahoes etc. So check that. If it was installed then check to see if it is not instlled correctly. These are the only things that I can think of without haveing the vehicle to properly diagnose.

Hmac300
May 11, 2010.
Good insight. Yes, the last time we replaced the fuel pump it was with an Airtech (highly recommended). It came with all the latest upgrades on the pigtail connection issues. I had a reputable shop remove and replace the fuel pump to see if it fixed the problem, AND IT DID.

In spite of the pressure readings on the tester, there was apparently enough inconsistencies to produce the problems I was experiencing. Right now, it runs smoothly at all rpm's, and when you romp on it at any speed it runs like a scalded ape!

AH. Mechanics and technology. Thanks for all your input and insight. Great forum!
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Tiny
Sierrajb
May 12, 2010.

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