I have a 1999 Sunfire with the 2.2L OHV that I rebuilt a few months back. The break-in went great & it drove fine for the 1st 100-200 miles or so. It had a bad MAP sensor & was throwing that code. I replaced the MAP with one from a working car & then it started trowing a P0171 (lean) code. It does not seem to be running rich so I think the code is correct & this is the only code. The most common cause of this code I have found is a vacuum leak. I then replaced the intake gasket & Injector O-Rings. No help. I took the intake manifold back off & inspected it thinking it may be cracked or the gasket was not in right. Both the intake & throttlebody (TB) look good as well as the gaskets. No leaks that I can find. While the manifold was off I inspected & blew in the three vacuum line coming out of the back of the TB. All held vacuum. The brake booster will hold vacuum for several days & its line is good so there is no leak there.

Before I rebuilt the engine I replaced the fuel pump & filter. That was the original issue before I found the engine was "rode hard & put up wet".

I have another running 1999 2.2L OHV in a Sunfire so I started swapping parts. I swapped the TB including its TPS & IAC & also the MAP sensor all from the good car. I still have the code. I also used my ScanGauge II to test those parts on the running (no code) car & the readings were about the same. I also swapped out the O2 sensor which again made no difference.

I had cut the muffler off to do some exhaust work but the Inspection Station wanted it back on. The one that came with the car was a straight threw type but I put on an original muffler from a 2005 Cavalier. I did not test with the muffler off before I put it on to see if there were any codes.

Im now at a loss as to what could be causing the issue.

On the engine rebuild the valve train was not replaced or modified. The cam was warn but in usable condition. The rockers were OK & the valves did not leak when tested with a pressure washer. The timing chain & tensioner need to be replaced which I was going to do at 1000 miles after the rebuild when I changed the oil & made sure the engine was in good working order. Could a worn cam ot timing gear cause this issue?

While on road test to track the issue & have a ScanGuage II connected. At idle & steady cruze the Short Term Fuel Trim (STFT) will creep up to 49, I assume this is the max, in about 20 seconds. When staring off from a stop or changing the accelerator position the STFT will go down to <10, usually zero. Note that I am not flooring it which would take it out of Closed Loop. This seems backwards to me because if the fuel pump could not keep up or the injectors or fuel filter were dirty it seems to me that the P0171 would show up during hard acceleration not at idle or cruse.

One other thing to note is its sometimes hard to get the engine up to normal operating temperature. Engine temp on a 50 degree day will normally run in the 130 range which is too cold to set the code. I think 150 was what the computer wanted. It may be a bad thermostat but I dont think this could cause the issue could it?

Please ask questions or give suggestions as Im stumped.

Grasping at straws but...

You may want to try replacing (or swapping with your good Sunfire) the temperature sensor (not the thermostat). If the temperature sensor is out-of-range then it feeds bad information to the computer and the computer makes bad decisions about fuel richness. If the computer thinks the engine is hotter than it really is, then it may send a leaner fuel mixture-- especially before the engine warms up to full operating temperature. Though once the engine warms up to full running temperature, I might expect the fuel mixture to be about right.

I had a difficult-to-pinpoint problem on my 2.4L 1996 Cavalier where the temperature sensor was sending bad (too-cold) information to the computer, and so the computer provided a too-rich fuel mixture (like setting the choke on an old-style carbureted engine until it warms up). That was fine when the car was cold, but then when it warmed up, it would stall from the too-rich fuel mixture.

If you figure it out, please let us know, as it may help somebody else.

Good luck!

Not to hijack the thread but could a bad temp sensor throw a p0507 on an ecotec? Reason I ask is I had a p0171 a few times but all I seem to have going on is bogging(like trans is slipping)/idle problems when warmed up and sometimes even when cold.

First I forgot to mention that I did replace the temp sender with the one from the other car. I also put them side by side in the car one installed & one sitting on the block next to it & they were within 2 degrees of each other on a cold car. It was in the 60s.

A P0507 is a high idle speed. 200 RPM on GM from what I found. I would not think the temp sensor would cause that. If it were a bad sensor the PCM would ignore it & run without it but keep at least the idle speed correct. Since you had a P0171 before & now a high idle speed would think you have a vacuum leak or the Idle Air Control (IAC) valve is gunked up & cant close enough to slow the idle. Id take off the throttle body & clean it out & check the IAC. Might as well polish the bore & remove some of the sharp edges while its off.

I have an 05 Eco & I think the temp sensor tells the PCM the temp which in turn tells the dash gauge. So if you gauge is reading OK thats probably not the issue.

Then I am asking for help with a P0171 code sooo...