Mechanics

CHEVY MISFIRES

2002 Chevrolet Suburban

Engine problem
2002 Chevy Suburban V8 Four Wheel Drive Automatic 87K miles

Getting diagnostic codes PO 171 and PO 174. Have replaces TPS, MAS, plugs and plug wires, all three O2 sensors, and fuel filter. Engine misfires noticeably when cold and then very slightly if at all when at temp. I did notice the output side of the fuel filter was extruding a black fuel residue. I did run a can of good quality fuel system cleaner thru the system but still having troubles. Thanks for any advice.

Dave
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Dbennett
January 14, 2008.



A p0171 and a p0174 code usually means that either your Mass Air Flow sensor (MAF) is dirty, or your fuel filter is dirty. Change your fuel filter and run it about 1000 miles. Then change the fuel filter again. When you change it again save the fuel filter you take out. Run water through it and catch the water in an old can or large jar. Check the water for any dirt or stuff like that. If there is a lot of dirt in the water then repeat the process.
Do not use an air filter that requires oil to be sprayed on the filter--like a K& N filter. The oil gets on the MAF sensor causing hesitation and misfires. Even if you do not have an air filter like this, your MAF sensor is probably dirty. To clean it take it out and spray electrical contact cleaner or brake cleaner on it. MAF sensors are rather expensive to replace, so I would clean it first.
There also could be a vacuum leak somewhere down stream from the MAF sensor. Check the round gasket on the throttle body where the induction unit seals up against.

Tiny
Pear69
Jan 14, 2008.
Also, your engine is not misfiring. A misfire will be accompanyed with a trouble code of P0300--P0301--P0302 -- etc. The last digit is the cylinder that is missing. A P0300 code is a general misfire.
Use a fuel cleaner with Tectron in it. Cheveron makes a cleaner with this in it. Tectron is the same chemical that the GM Mechanics run through your system and it seems to work the best.

Tiny
Pear69
Jan 14, 2008.
I ran two bottles of Techron thru the system and replaces the fuel filter again. I ran water thru the old one as described and the output was clean as a whistle. I checked all of the vaccuum line connections and lines themselves using visual inspection and a can of carb cleaner. I also checked the gasket between the MAF and throttle body. I'm stock on the air filter, and replaced the MAF just incase.

Still having the same error codes and rough running when cold. It's not as rough when the temp reaches 210 deg. Or so. Should I consider coolant temp sensor?

Tiny
Dbennett
Jan 18, 2008.
Check for a leak at the intake manifold gaskets. In my personal experience the number one cause of P0171, P0174 is because of a vacuum leak. If you have checked all the hoses, you need to inspect the intake manifold. You can spray carb cleaner around the intake while the engine is running. If you hear a change in the way the vehicle runs, while spraying, you have a leak. Let me know if you have any other questions.
Dave

Tiny
Flexy5
Jan 18, 2008.
I agree with flexy5. This may be the problem. If you have no sucess finding a leak, you should realize that an intake gasket can leak on the underside also. Have a scan tool hooked up, and see if the MAP sensor is reading below spec. Which is 4.0 to 9.0 grams per second. If it is the intake gasket may be leaking where you can't see or hear it.

Tiny
Pear69
Jan 18, 2008.
A huge thanks to Flexy5 for helping me with this. He was dead on with the intake manifold leak. New gaskets and a few hours work and it's running like new. This is a great site and service. I'll be telling my friends.

Dave

Tiny
Dbennett
Jan 20, 2008.