Rough idle after warm..runs ok in limp mode after MAF disconnected

[jonny4wd]

Hey all, long time lurker here and have been pretty lucky I haven't had to troubleshoot much in the past. My Dad purchased a '94 XLT Firecracker Red new and drove it for 10 yrs up in CLE, then I drove it 10 yrs in ATL, sat in a barn in NC for 4 yrs (well, drove about 1,500 miles in that time period)..and now it's been passed down to my son and it's his daily..

It still gets many compliments as it looks so good, I put a James Duff lift on in '05 and aside from the normal wear/tear it's been rock solid for us. Had the transmission rebuilt last summer and other than an alternator, maf and computer ~10yrs ago..everything else has been great. A few months ago it started running rough intermittently. Long story short, I knew it needed a LIM and had a rusty/leaky valve cover.

Started getting 172, 176 KOER. One day, it got so bad we had to disconnect the MAF to get it home. We decided to tackle the LIM, UIM, valve covers, O2 sensors etc..all at the same time. While I had everything apart I decided to do plugs, wires, fuel injectors and ended up touching/breaking EGR valve and line and the DPFE (installed new silicon hoses) while pulling off the harness and intakes. Joy that everything was original and breaking and cracking. Fast forward a week ago..got everything replaced, torqued and ready to roll. Fired right up and no leaks and purr'd like a kitten.
Took it out for a test drive and once it got warmed up..started rough idling, no power etc.. Same symptoms as before I did the LIM/UIM/Plugs/Wires etc.. I pulled over and unplugged MAF and drove home. I have a Actron scanner and pulled these KOER 157, 998.

Ended up taking it to a shop that would take a look..they did a smoke test, wiggle test, tried 2 PCM and 2 different MAF's. Still the same symptom. Runs rough after it warms up..push accelerator and it bogs down, rough idle. Same codes. Brought it back home to start back to the basics. (Honest local shop..no charge). Runs ok in limp mode w/ maf disconnected.

Heads spinning a little bit on where to start looking again. Just ordered the Charles Probst book..thinking it's something maybe with the EEC side of things, but wanted to reach out to anyone who has gone through this or point me in a direction i haven't tried. I'm able to pull live data feed..but have not found where i can compare the values.

Thanks in advance!

 

[Turdle]

Try a new engine coolant temperature sensor. Air fuel ratio can be way off if it is bad. Classic symptom



Not to be confused with the coolant temp gauge "sender". There are 2 coolant sensor things, I suspect the one with 2 wires for easy way to tell difference

 

[jonny4wd]

Thanks for the tip! I actually did replace it when i took the upper & lower manifolds off. Everything was caked w/ carbon. I was able to find motorcraft sensor and an air intake sensor as well. I was hoping the cracked/leaking egr tube was the culprit but i was able to get a new motorcraft one of those as well. The only thing non-motorcraft so far is the MAF. Thinking i need to dig into the wiring closer.

 

[mcfixstuff]

If you have the funds, I highly recommend the Bosch 1300 scan tool, as it lets you read the live data from the ECM. It's about $180

 

[jonny4wd]

I actually just picked up an Actron CP9690..which I think is made by Bosch and is the same model as the 1300. That’s how I got my codes. Got sick last few days and didn’t get a chance to get back out do more troubleshooting. But the cp9690 and 1300 look same and have same feature list/menus from looking at my scanner and the Bosch manual.

Is there a reference site/book that shows the correct value ranges for the live data?

 

[Wrenchtt]

Good looking vehicle ,
I’m new to the site but I have been on many others over the years now retired from the field due to disability after about 40 years Masters ASE Technician. I am just wondering if you have by chance connected a vacuum gage to your manifold vacuum port and monitor your engine vacuum when driving and see what you vacuum reads when you are having the issue. I would anticipate that if your vacuum drops low it could take you to a different path to diagnose. Do you have a OE converter? I see you have replaced the EGR valve how about the DPFE sensor? Possibly egr opening up? I see you have been spending a lot of time and money. 94 didn’t have that much information from your data stream supported. Sometimes you have to get back to the basics. Just a little thought,nothing to loose, I will be happy to help you thru this.just post update on your progress, Good luck buddy .

 

[jonny4wd]

@Wrenchtt I will def. take you up on that offer. Sorry to hear about the disability. But love to share your information out. This is a "heritage" 28yr old vehicle bought new by my dad for 10 yrs, then drove by me for 10+ years and just passed down to my son (just got out of HS..working on his EMT/Fire rescue cert's). So it'll be his daily again. Love to get this knowledge passed down to him (and anyone else on this forum).

So the cats..they were original up until ~6 months ago when his original muffler fell off. So lasted 28 years. We don't have emissions and the exhaust shop said a straight pipe was fine and would pass emissions as long as it was tuned properly. IDK..i was wondering if not having cat's would cause a problem. But it was running fine for ~4 months prior. (so i believe the answer is ..they are not clogged).

DPFE was replaced with EGR and egr tube. (the EGR tube was rusted and barely holding onto the manifold). Took me a full day getting that removed it was rusted so bad. The DPFE tubes were bad but i got a new set of silicone. I kept the original DPFE (and all other parts to bench test as i learn how to do it..so as to pass down to my son).

I just picked up a vacuum gauge but haven't tested vacuum yet. (shop i got it back from said they did a smoke test). But i'd still like to do a vacuum test. *i'll run a test and report back findings!

Thanks in advance!!

 

[jonny4wd]

Reset the PCM (unplugged battery..jumper wire between + and - cable).

Plugged my MAF back in and fired it up. Vacuum was 18-19. (this was engine cold). I was never able to warm it fully up as after about 45 seconds, the vacuum dropped to ~5-10 and it would barely idle.

I unplugged the MAF..and idle went back up and vacuum went up ~22.