JohnF150

I hope one of the forum member can help me !

My 1998 F150, V6 4.2 Automatic has been acting up.
When standing still at traffic lights (with the transmission in Drive) etc the engine is not running at a steady rpm but fluctuates constantly and results in the engine stalling.
The car bucks when this happens (due to increased RPM).
If I put the car in neutral the rpms sort of stay at the same level.
Driving the car there seem to be no problem.

The mechanic who looked at this problem advised me that the Mass Air Flow Sensor was broken (as the OBD code was pointing in that direction) and replaced it but then told me that the problem was not gone so he thinks it is the PCM that is faulty.
Not to thrilled to spend another bundle on a guess.

So I have been checking out this forum and found some other problems mentioned which seemed similar with some advice on how to deal with it.

This afternoon I checked the IAC , removed it but is seemed pretty clean.
Then I disconnected the battery for 10 minutes, left the car idle for 10 and drove with it for another 10 so the PCM would relearn. Problem is still there after that.

Again removed the IAC and sprayed some W40 in it, reinstalled and started up. For a moment the engine , while in drive, seemed steady but then it started changing RPMs again.

I hope someone has an idea what causes this or advice on how to further troubleshoot it.

Many thanks !

John

blizzard63

Check your TPS sensor and Egr passages
had similar problem and did both now it runs great

hellrazor

i seem to have the same problem but mine dosent stall, the rpm's just go up and then back down.

torkum

Check for a vacuum leak starting with the PCV elbows.

67countryboy

Its your idle vaule my truck did the same thing i put a new one on and it was good

mctwist77

Hey man change your IACV and while your at it clean your throttle body and your MAF should help. Also make sure you dissconnect the battery so that the ECU can reset. Let me know how it goes.

JohnF150

Thanks for the feedback but the truck is still at the shop.

Problem is that the MAF sensor doesn't get any current from the PCM and that should be the case.
That is why the logic was that the PCM was not working properly.

We ordered a new PCM, that took a while to arrive.
But then still no improvement.
The shop & other experts decided the PCM may be faulty (as there is no current coming out for the MAF sensor) and another one was shipped in (again that took time). So the second PCM also did not help !

So we are back to square 1 and this is very frustrating.
I discussed the suggested items (TPS ect) with the mechanic but he said he tested these aswell, the TPS showed a nice even change etc.

Question now is :
Why does the PCM doesn't provide the current for the MAF sensor?
(the wire is checked and there is no current just after the PCM connector).
Normally the moment the engine is started the MAF should have it's current.

Anyone knows why the current from the PCM would not be provided?

Is there a safe way to bypass the PCM for current to the MAF ?

Thanks

John

Good old Bill

Get the color code off the wire in question. Am seeing four wires in my 02 4.2L diagram. tan/lt blu, red, lt blu/red, and blk/wht.

Good old Bill

Another thought came to mind as I posted. As I am not a tune up guru, I am wondering how the engine runs otherwise. Would think the MAF would control more than just idle issues.

JohnF150

Thanks Bill, I will check the wire asap and let you know.

The engine run pretty good when I am just driving. Problem is only when at a stop with the gear in drive.

My mechanic made a remark that the engine doesn't run smooth though.
But I am not an expert !

John