New user. Tried to find specific related threads, no such luck. Just bought this car from a family member who had let it sit for 6mos-1yr.

2002 3.8L Automatic, 214k with ~46k on the Jasper reman.

Symptom 1: Shaky idle on cold start dropping as low as 200 RPM. Does not die, just struggles.
Symptom 2: Intermittent power loss/backfire/stall on acceleration, especially from low speed.

Possible culprits: Vacuum leak - particularly intake manifold gasket, Mass Air Flow (MAF) sensor, crankshaft position sensor, or distributor (?).

When driving, the car will sometimes refuse to accelerate. Process is as follows: Stop, go, repeated stalling, let off accelerator, try again, feather my way to speed. It will also happen when maintaining speed ~60-80 mph, solution again is to feather throttle.

I initially perceived the issue to be related to a transmission solenoid, but now I believe it is intake or spark related. I would like to diagnosis this proper, before spending hundreds of dollars on sensors or countless hours replacing the intake manifold gasket.

p.s - This may be unrelated, but there is an exhaust leak in the cat or muffler.

 

Check engine light on?

As long as the exhaust leak is after the Cat it should not be a problem. In a EFI engine a vacuum leaks should cause a rise in RPM not a drop. If everything is working right your O2 sensor will pick up the increased oxygen from the vacuum leak and it will add fuel. More air + more fuel = more RPMS. Intake gaskets failures are common for coolant flow issues but I can not remember any 3800 Vacuum issues with the intake. Since your engines has been replaced the original factory defective intake gasket is long gone. I have not heard of a aftermarket intake gasket failing in any way.

Try unplugging MAF, if the car runs better it does not mean the MAF is bad but it is a clue that closed loop is an issue.

Try lightly tapping on the MAF and wiggling the wire going to the MAF. LIGHTY with the handle of a screw driver.

Try tapping the EGR valve, you may want to check it for being stuck opened. Search youtube for leak testing EGR valve.

 

Thank you,, Gorn.

I also suspected that the intake manifold gasket was not the culprit, due to the recent reman of the whole motor (and I believe jasper is very reliable).

I tried to clean the MAF but did not receive any better results. I will try with the MAF removed, as well as testing the EGR valve.

Can you explain the closed loop issue? I will update this post shortly.

- Dolph

 

When you first start your car it is in open loop. This mean the computer runs on predefined settings for fuel and timing. Once that starts and runs it goes into closed loop. That means it is taking real time data from sensors and adjusting the fuel and timing mapping.

If something disrupts the sensor inputs the computer switched back to open loop. As long as you are not at extreme hot or cold high elevation or anything dramatic your car should run decent in open loop. If the MAF sensor is giving bad data unplugging will cause the computer to go into open loop. So if the car run better in open loop you can be pretty sure your issue is computer/sensor related. If the car runs about the same I would focus less on the computer and more on the ignition BUT, the computer does use some senors when in open loop and if the computer it self is the issue closed or open loop will not matter.

Checking codes is still your best bet.

 

Gorn - Thank you so much for your input. I first tested and pulled the EGR - not clogged or stuck and was receiving 12v. Tried the tapping - but didn't receive any feedback.

I disconnected and removed the MAF - PRESTO! The car runs smooth as butter again. Nice to get my power back. Reading your last comment, this means that the closed loop was running incorrectly due to the MAF sensor being faulty.

The next step then, I believe is to replace the MAF. Is there any possibility that the MAF issue was a symptom and not the culprit? Additionally, is there a possibility that a new MAF could fail quickly afterward, or that the closed loop running issue could again be spurred up even with a new MAF?

 

Could be the MAF Sensor! It could also be any sensor that is ignored while in open loop. It can also be the connector to the MAF sensor. I had to replace the connector in my 96 3800. I had the same symptom you had but it was intermittent. I am not saying your issue is the connector just letting you know either MAF or Connector can give the same symptoms. May way to plug it back in and play around with the connector and see if you can get the engine to change the way its running.

 

Thanks Gorn. I ran the voltmeter on the connection and was reading a clear 12v. going to get the OBD2 code and cross compare them.

What other sensors would be bypassed in open loop?

 

New MAF and no more driving issues. Thanks for the help.