Can a bad MAP sensor cause the engine to run lean?

**chrisrexmandie** · 08-02-2020

I have been battling a lean code for a while now. It is very strange with how it is behaving. I have checked for vacuum and exhaust leaks. The fuel pressure is good and I have a new filter. Fuel trims do not drop if I raise the RPM off of idle. The CEL sets the p0171 every so often. Seems to get set quicker if I am using the A/C? My question is would a faulty map sensor cause the ECU to thing the engine is at a different load and command the wrong amount of fuel and timing that in turn would create a lean exhaust mixture?

2004 GTP

**GTPpower** · 08-02-2020

I doubt it. Try unplugging the maf and see if anything changes.

**chrisrexmandie** · 08-02-2020

I tried that and the engine ran okay just extremely lean my wide band was maxed out at 20

**GTPpower** · 08-02-2020

Ok, what happens if the maf is plugged in and you unplug the map?

**chrisrexmandie** · 08-03-2020

The car continues to run. If I start the car with the MAF unplugged and plug it back in while running the engine will stall

**GTPpower** · 08-04-2020

Do you have a scanner that can read maf output?

**BillBoost37** · 08-04-2020

This is going to sound odd, it's experience talking though.

Look at your battery cables where they go into the ends by the battery and the other ends of them. Do you see the insulation pulled back slightly and greenish color instead of nice clean copper? If so.. consider replacing them.

I chased P0171, replaced the egr tube (they crack at intake manifold), checked grounds in the harness etc etc etc. Then had guy who had been a GM mechanic for years tell me to replace the cables. Since I had done everything else, I did. Fixed the issue and when I opened up the insulation on the old ones I found a decent amount of corrosion which messes with the voltage of the vehicle.

**chrisrexmandie** · 08-04-2020

Thats something I will have to check in the morning. Come to think of it sometimes when sitting idle with the door open I notice my dome lights would fluctuate and I didnt think anything of it. Just thinking it was the voltage being regulated by the alternator.

**chrisrexmandie** · 08-15-2020

I reflashed my ecu with a modified tune and the problem completely went away I didnt even adjust the fueling in the areas that was previously lean. Now im getting a weird P0403 code every time the car starts up. I monitor for misfires after startup and there is none. I tried another reflash to see if that would do anything and it didnt change anything im confused on why I would only get the misfire code for cylinder 3 on startup and see no symptoms.

**BillBoost37** · 08-16-2020

Might need a crank relearn after the flash. Also the flash will reset all your fuel trims and you need a certain amount of drive cycles for that to log etc if it's going to come back

**chrisrexmandie** · 08-16-2020

Ive never done a crank relearn I dont really know what a symptom of needing to do one is.
As for the fuel trims they have been stable and no longer lean like they were before the reflash

**BillBoost37** · 08-17-2020

Symptoms inlcude odd stuff that you can't explain.

Like getting a misfire code after doing nothing but a flash.
Odd misfires that you can't determine, no matter the amount of testing etc.

I've had a few that after a motor/trans swap etc that upon startup run like crap and after checking things over you are left with no leads. Crank learn is like waving a magic wand and all woes are fixed. The issue is basically that the pcm doesn't learn the exact crank position and therefore is not tossing the spark etc at the correct times.

BrandonHall recently had some odd behavior from his Regal and no matter what he tried nothing quite fixed it. Crank learn and it's a happy car once again.

**chrisrexmandie** · 08-17-2020

Ill have to try that. Ive never done one and there has been many many flashes and a lot of things I just cant find the cause for that would come and go.

**mark97564** · 07-17-2021

Bad EGR or bad wire connection between the EGR and pcm .. if the EGR isn't closing all the way the motor will run lean

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