1998 Chevrolet S-10



July, 29, 2008 AT 10:58 PM

Engine Performance problem
1998 Chevy S-10 6 cyl Four Wheel Drive Automatic 171000 miles

Everything I've read about code P0171 talks about how the engine is running lean, but I have this code, and my engine is running rich. I have the pungent exhaust fumes similar to a bad cat (which mine is not, as this was just replaced a year ago.)

The symptoms I have are at idle, in-gear, at operating temp, the truck runs very rough - almost dying. Once I start moving, however, it runs just fine. And as I mentioned above, it has a very pungent -rich- exhaust odor. (First noticed the problem while in rush-hour stop-and-go traffic. When I would stop, it would idle rough with the pungent exhaust, but would driver fine once I accellerated again.)

I've cleaned the MAF and IAT sensors with MAF sensor cleaner, but it didn't do anything. I disconnected the MAF sensor to see how the truck would run, and it runs better with it disconnected.

I should also mention that I had these same symptoms about a month ago, and the code that came up at that time was for the O2 sensor. I replaced it, and that did the trick - until now.

Other places have mentioned testing the vacuum, but how would I go about doing that myself? I've heard reference to spraying starter fluid into the intake to see if it has any affect on the RPMs, but I'm not clear on why. Is that if there is a reaction, then there's no leak? Or vice versa.

My next thought is the MAF sensor is bad, but I don't want to spend all that money until I'm farily sure.

What are your thoughts and/or suggestions?


10 Answers



July, 30, 2008 AT 2:31 AM

You may have a vacuum leak, get a can of spray gumout, spray the vac lines and maniflod area, when the idle changes, youve found the leak! You want the engine running and spray on the outside but do direct the stream onto the hoses, if it is a vacuum leak, the engine idle will change speed, then you have detected a leak, repair the vacuum leak and see how it runs! If that doesnt do it, you may need a new idle air control valve, it's bolted to the throttlebody
Do this b4 you start tossing parts at it.



July, 31, 2008 AT 7:12 PM

Gonna test the vacuum tonight, and if that doesn't work, then off to the parts store I go.



July, 31, 2008 AT 8:27 PM


A lean fuel condition can be caused by: * Low fuel pressure due to a weak pump or leaky fuel pressure regulator. (Use a fuel pressure gauge to check fuel pressure at idle)

* Dirty fuel injectors. (Try cleaning the injectors)

* Vacuum leaks at the intake manifold, vacuum hose connections or throttle body. (Use a vacuum gauge to check for low intake vacuum)

* Leaky EGR valve. (Check operation of EGR valve)

* Leaky PCV Valve or hose. (Check valve and hose connections)

* Dirty or defective Mass Airflow Sensor (MAF). (Try cleaning the MAF sensor wires or filament with aerosol electronics cleaner. Do NOT use anything else to clean the sensor, and do not touch the sensor wires)



August, 3, 2008 AT 10:24 PM

Ok, so we checked for vacuum leaks - none, checked pcv valve, hoses, etc. - Nothing wrong. Replaced the IAC Valve, still not fixed. Was looking into EGR valve, but it just didn't feel right, so I checked the codes again, and now I've got P0171 and P0102. I immediately thought it's the MAF sensor, but I still have a hard time accepting that, because if it truly was a bad MAF sensor, wouldn't it run bad all the time? The truck runs great when it's cold. It's at operating temp that I have all the symptoms.
Rough idle - RPMs drop - NOT spike up, RICH spelling exhaust - quite the opposite of a lean system, isn't it, lagging accelleration (keep in mind this is only when at operating temp - accellerates fine when cold)
I popped the hood and checked the MAF sensor connection, and got a small click when I pushed on it. So the 102 may have been from it not being connected all the way. I'm going to clear the codes, run it as it is to work tomorrow, check and clear the codes again when I get there. (See if 102 comes up again.) I'm going to disconnect the MAF sensor on my way home, see how it runs and check the codes again when I get home. (See if 171 comes up again - and see how the truck runs.)

To recap so far, I've replaced the O2 sensor, IAC Valve, Fuel pump & filter recently replaced, Air filter recently cleaned/oiled (K & N), MAF sensor cleaned, checked pcv valve/hoses, no vacuum leak.
I've thought about the fuel injectors, but how would faulty injector(s) add up with a lean code, RICH smelling exhaust, and no obvious misfire (no misfire codes either)?

Any further suggestions given all this info, I would greatly appreciate it. (Please consider everything I've said, before jumping to conclusions. The symptoms, the timing of the symptoms, etc.)

Thank you - this forum has been a lot of help!



August, 3, 2008 AT 11:12 PM

Test the coolant temperature sensor might not be switching/inspect pressure regulator. Monitor fuel pressure before it warms up- also you need a scantool to catch the live data- problem with P0171

OBD2 P0171 indicates the engine is running lean. This means there is too much air and/or not enough fuel. You can confirm the engine is running lean by looking at Short Term Fuel Trim (STFT) and Long Term Fuel Trim (LTFT). Normally, STFT and LTFT should be plus or minus 8 from zero. If STFT and/or LTFT are more than about +10 or higher, it indicates the engine is running LEAN. Negative numbers (-10 or more) indicates the engine is running RICH.



August, 4, 2008 AT 2:36 AM

K & N air filter? Oil soaked? If so replace the MAF Chevy has a tech bulliten on this, and I have done a few over. Cleaning the MAF does not work with this condition, especially if it has caked on crap.



August, 8, 2008 AT 12:38 AM

To update, in case anyone else has the problems that I ran into, the truck is not completely fixed yet.

I ran the truck as it was with the MAF connected - ran very rough- and got codes p0300, p1133 and p0131. I figured these were as a result of the poor performance of the faulty MAF. I cleared the codes, then ran the truck with the MAF disconnected, and only got P102 - truck ran better, but not perfect. The system's standard settings were better than the incorrect readings from the faulty sensor.

I replaced the MAF, and the truck ran much better. Still not done though. I'm getting p0171 again, and am getting a much more "raw" gas smell from the exhaust at times, and my MPG got worse than the already dismal 17 average I was getting.

My next step now will be to check the O2 sensors again, and if they're fine, then I'm going into the shop for the fuel trim tests, tests on the egr valve, coolant temp sensor, and fuel pressure regulator. I'm still not convinced it's any of these other sensors, but I'm not ruling them out.

I'll check back in once I have more answers, so someone else may benefit from my situation and have all their potential answers in one thread. Hopefully for my sake the next post will be the last on this topic

I'm still open to advice. If anyone can think of anything else.



August, 8, 2008 AT 2:29 AM

Check the ECT(Engine coolant temp) sensor, and did you replace Both O2 sensors or just one? 171,000, means you re about 71,000 miles over the useful life of the sensor. The ECT may be calling for more fuel, might think the engine is cold.



August, 21, 2008 AT 10:00 PM

Well, the idle is fixed.
We pulled all three O2 sensors (bank 1, bank 2 and post catalyst). Bank 1 & post catalyst were completely black (carbon coated). Replaced all three since we already had them out.
This pretty much took care of it. I didn't have any of the symptoms as described in the previous posts.
I also replaced the ECT (Engine Coolant Temperature) sensor, since I had been planning on doing a coolant flush anyway.
Now my truck idles very smooth. Almost can't even tell it's running : )

I do have an unrelated question, and I can repost under a different area if necessary, but often when my truck is running at full temp, usually at high speeds, the footwells get really hot (hot enough that in the summer I don't wear sandals unless I have the AC on. Which haven't done this summer since I need the AC system recharged). Any suggestions on what might cause that? I was hoping that replacing the ECT sensor would make some of that go away, but it almost seems to have gotten worse. The thermostat was replaced about 40,000 miles ago, and that didn't do anything either.

Thanks for all the help!



August, 23, 2008 AT 6:28 AM

Check under the truck for the insulation pads, these keep the heat off the floor. If torn up or missing floor will get hot.

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