Erratic engine behavior

DAKOTA BILLINGS

MEMBER
1996 PONTIAC FIREBIRD
3.8L
V6
2WD
MANUAL
180,000 MILES

Just replaced the MAF sensor due to confirmed inoperable sensor and now my engine started to rev by itself to approximately 3,000 rpm's for about 2 to 3 seconds then smooth out to a nice idle. Doesn't do it every time. Reset the ECMC by pulling fuses and battery and that helped immensely with other weird behavior including A/C compressor not kicking on. But still self revs from time to time. Could this be the ECM relearning the new sensor? Or indicative of something else such as TPS or MAP sensor?

Sunday, April 19th, 2020 AT 5:58 PM

3 Replies

ASEMASTER6371

MECHANIC
52,796 POSTS

Good morning,

What are the codes that were set? How did you confirm the MAP sensor was bad?

https://www.2carpros.com/articles/checking-a-service-engine-soon-or-check-engine-light-on-or-flashing

Resetting does nothing but make the ECM forget the code. It will come back until the proper repairs are made.

Did anyone check the EGR valve? If it is fluctuating open and closed, it will do this. You can confirm this with a scan tool to monitor the operation of the valve. the idle air control valve also controls the engine idle as well.

https://www.2carpros.com/articles/how-to-replace-an-idle-speed-control-motor-iac

https://www.2carpros.com/articles/engine-idles-too-high

https://www.2carpros.com/articles/engine-surges

Roy

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Monday, April 20th, 2020 AT 3:37 AM

DAKOTA BILLINGS

MEMBER
10 POSTS

Good morning to you as well.

There was only two codes set one for the EVAP system not holding pressure, I traced it back to the line that goes back to the gas tank. Also could be that the o-ring on the top of the fuel pump is bad too. And another for the MAF sensor. Low voltage. We tested the reference wire with a multi-meter and the voltage was where it should be. Also tested the MAF with it too and the multi-meter showed open circuit, but to be sure I also had it read with a live data computer and it confirmed a dead MAF sensor. No response. EGR valve is not hooked up on this engine. Could that cause conflicts? The EGR pipe broke so I was forced to bypass a while back. New IAC valve and no apparent vacuum leaks. Also I'm wondering if the ECU itself is bad. Because if you say resetting does nothing which I believe you then resetting my ECU shouldn't resolve other weird behavior unless the computer is glitchy. At least that's my logic. Your thoughts on this?

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Monday, April 20th, 2020 AT 4:16 AM

ASEMASTER6371

MECHANIC
52,796 POSTS

Okay, you indicated an open circuit in testing the mass air flow. What was open? That is a circuit issue not a component issue.

Again, what was the code number?

The EGR, is it still present on the engine? How did you bypass it?

Correct with the ECM. It does not resolve any issue just makes the ECM forget the fault until the issue happens again and fails the 3 trip rule set in the ECM.

It is possible but I doubt at this point it is the ECM.

Roy

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