Hey guys, yesterday I just bought a new (to me) 94' F250, 351W auto. It started great, albeit very loud due to the straight pipe coming right off of the collector, but sounded strong.

So I began my 100~ mile trip back home with her and immediately noticed it was very sluggish. Giving it any more that say 30% throttle just made it pop and backfire and struggle to stay running. But below 30% is was pretty smooth, just slow.

Got through 1st and 2nd gear, but about the time I made it to 3rd it was terrible. It took me several miles and a bit of help from gravity pulling me downhill to finally make it up to 55 mph but every time I hit even the smallest hill I would have to let it fall back to 25-30 and some times back completely to 1st gear. Basically with any load on the engine at all, it was refusing to rev about roughly 3k. In neutral it would got higher, but had a very bad miss and was hesitant.

Well after about 2 hours I made it home, fearing I had bought a giant money pit. I finally made some time this evening to take a look under the hood, and everything looks to be 100% original. Obviously the typical tune up items are first on the list, but I went ahead and ran the CEL codes.

I got;

Hard Codes
26 - MAF/VAF Out of Range (I have MAP)
66 - MAF/VAF Signal Low
62 - Converter Clutch Error

Memory
17 - Fuel Control System Fault or Rich/Lean
23 - TPS Out of Range
32 - EGR Not Responding

So instantly I'm thinking bad MAP sensor. In my experience these usually work fine or not at all, no in between. So I went out to fire it up and the hesitation in Neutral/Park was gone. Also I was able to put it in drive and break torque it quite hard so it had plenty of power at the moment. I didn't let it run long enough to heat up because it was so loud with the straight pipe and all, but I did disconnect the MAP vacuum hose. It immediately started running like crap again, until I plugged the MAP back in. So now I'm questioning the MAP sensor.

Any suggestions on how to proceed from here?

 

I would check the signals from the MAP and TPS sensors, check fuel pressure, and pull the SPOUT jumper and check your base ignition timing. Set it at 10 degrees for now til you get it running right, then you can bump it up a bit if you'd like some more power.

Converter clutch will not cause it to run like that. EGR valve, if stuck open, will cause it to idle like crap or not at all but it won't cause the backfiring like you have now.

 

A 1994 F250 w/5.8L engine and E4OD should have 3 digit codes. How did you get these? Count flashes? Code reader?

 

I counted flashes and was surprised as well. They're definitely not 3 digit codes, I triple checked.

 

Considering how poorly the truck runs I suspect someone installed an older/incorrect computer. Time to pull it out, note the numbers on it and post here. A photo would be very helpful. While you have it out, open it up then inspect it for signs of leaking capacitors or burned components. Quite common these days.

 

My code book indicates that our trucks have three digit codes. However, the book was very clear about fixing the KOEO codes before proceeding to fix the KOER codes. I suggest getting a code reader and retest your codes.

I believe all the trucks up to 95 were OBD1 while the F250 and F350 remained OBD1 until the model for 97 or 98. The changeover to OBD2 began during the 96 model year.

If I combine your numbers I get 266 662 memory 172 332. My book doesn't have a code 266. Code 622 is Shift Solenoid 2 circuit, Code 172 HEGO (HO2S) sensor fault/lean, Code 332 insufficient EGR flow detected.

Good luck with your diagnosis.

 

Ok, I'll rent a code reader and double check the codes this evening. I'll also get a pic of the ECU and make sure it's the proper one. Thanks

 

Alright first thing I did today was throw some new exhaust on since it was previously cut off about 1" behind the pre-cat O2 sensor, so I know it was running super lean. Also got a code reader, and you right Dday, I was mis-reading, my apologies.

These are the codes;

Hard Codes
626 - Trans Coast Clutch circuit fault (Should be irrelevant at the moment)

Memory
126 - MAP Out of Range
128 - MAP Vacuum no change
172 - 02 Sensor is or was lean
327 - EGR Feedback low
332 - EGR didn't open during test

The MAP codes were most likely my fault, I disconnected the plug and hose from the MAP yesterday while it was running. SO, tomorrow I'm going to reset the codes and take it for a drive, hopefully the 126, 128, and 172 won't reappear. I'm just not sure about the 327 and 332.

 

Reset the computer and took the truck for a drive this evening.

626 Came back, so I'll be looking into that eventually.

The only other code I'm getting is 172, O2 sensor so it looks like it's probably shot. Below is a video I took where you can hear the engine struggling at WOT. Could a bad O2 cause this big of an issue?

 

There should be only one O2 sensor on your truck. I would just replace it. I think this one should work and its only $18 with prime shipping when you clip the coupon. I actually just bought one for my bronco today. Should be here Friday. I am replacing it when I do the 6-litre tune up this weekend.


Check your vacuum lines. I had similar codes on my 1994 5.8 F350 and it ended up being a combination of a bad O2 sensor and vacuum lines. Once I replaced them it passed inspection no problem. If you replace buy silicon lines and wrap in wire loom and electrical tape wherever it touches metal.

 

After watching that video you have a major problem.


Do a 6-litre tune up, check timing, replace TPS, vacuum lines, and O2 sensor. See where you are at that point would be my suggestion. Why I love these trucks. Parts are cheap but of course some are not so much if you need to go OEM.

 

Replace your O2 sensor if it's age is unknown. I did on my truck and it cleared up my code 172. As the others have said, check for vacuum leaks. They can raise hell with our trucks. Especially check the "coffee can" reservoir on the passenger side fender liner. It can get a rust hole on the underside.

 

My best guess is fuel pressure.

 

Ok, thanks for all of the help fellas! I rented a pressure test kit today, fuel pressure was at 25 PSI KOEO, KOER, and at WOT. So in went a new pressure regulator, and bam! It runs like new, for the most part. lol It cured part of the issue. It will easily rev all the way up when you floor it now, even under power, but there's a pretty severe hesitation when hitting WOT all of a sudden.

I've already planned to replace the O2, I just can't get the old one out. I've tried everything from a wrench to an O2 socket on a 3' breaker bar which just spun around the sensor. SO, looks like I'm headed to the muffler shop tomorrow. I'll probably just have them weld in a new bung right next to the old sensor and just leave it there.

Also I've done the air filter, dizzy cap and rotor, and plugs so far. Coil and wires are next. Hopefully I can squeeze some decent mileage out of her.

 

Glad to see your truck is running better.