Long time reader, first time posting. I knew nothing about trucks/engines when I bought this truck but I’ve learned a TON from these forums over the years but I’m really in some trouble now.

95 f150 5.0 4x4 has been running rough since I got it a few years ago. Truck would idle rough (especially the warmer the engine got) and sometimes stall at a stop sign or stop light.

Truck also would seem to sometimes “lose power” by failing to accelerate, sometimes make a “knocking sound” and then suddenly jump back to where it should be at.

Seller said it was bad MAF which checked out upon reading codes. I replaced that and was fine for a bit then issue came back and other codes came about for O2 and EGR. I replaced O2 sensors and tested EGR. I checked vacuum lines and replaced some that some that needed it.

the issue persisted and a shop told me to replace the PSOM. Speedo hadn’t worked since I bought it and odometer read ERROR since I had the truck.

i replaced it and it ran great! But only for about one tank of gas and now all issues are back, but no codes!! Took out PCM and inspected it, didn’t see any noticeable resistor leaks.

when I run the OBD1 test the CEL usually will not blink and if it does only 111 comes up on KOER. Nothing will come up on KOEO, it will just stay lit and not blink.

these codes were thrown however not all at once.
137 - O2 Rich
157 - MAF low/grounded
332 - EGR not responding


could it be a bad wire to the MAF from PCM?
clogged cat?
fuel regulator?
another vacuum leak?

Sorry for lengthy post but any help is GREATLY appreciated!

 

First thing I would check would be the fuel pressure. You should have 30psi at idle, and 40 psi with the key-on/engine-off. When you turn off the key, watch how fast you lose fuel pressure. It should be about 1 psi per minute between 40psi and 20psi.
Check your fuel pressure regulator by removing the vacuum tube while the engine is running and watch for fuel leaking out of the vacuum nipple on the regulator.

 

thanks! I’ll check it out this weekend

 

I would also add in a compression test. Recruit someone to do that if you must. Firing the infamous Random Parts Cannon like you have isn’t the way to do this.

30-year old engines don’t always have good health. Let’s check that first.

 

I checked the fuel pressure and that was ok. I finally got a chance to use a smoke machine to check for leaks and found two. A leak from the TPS and a leak from the IAC.

the IAC had a lot of smoke coming out. It looks like it was coming out of a “port” on the top that had a plastic cap on it.

why is that “port” there with a cap?

im going to try and clean it out before buying the new parts

 

Engines with vacuum leaks cause idle problems but they'll run fine at high power settings. Engines with weak fuel pumps will idle fine, but cause poor acceleration and back fires at high power settings.

 

Posts 10 and 11 at this thread describe the vented Idle Air Control valve https://www.fullsizebronco.com/threa...ur-iac.206960/