1996 Ford F-Series Super Duty (Ford dealer says it's an F-450), 2WD, 7.5 Liter EFI, automatic overdrive. Speed density EFI (no Mass Air Flow). Runs fine sitting still, idles fine, revs up fine. Driving down the road it intermittently (but severely) bogs down/hesitates/backfires out the intake. It backfires out the intake multiple times when it acts up, and has blown off vacuum lines several times. Could go 100 feet or 20 miles before doing it, but it always does it. It will bog down for a few seconds, clear up for a bit, bog, clear up, bog, etc. Whenever it starts bogging, you have no choice but to pull over, it will not accelerate. Several shops have already checked into it and found nothing. We have been throwing parts at it for several weeks with no luck. Computer is OBD1 and shows no codes. We have had the trans checked out, and have replaced oxygen sensor (only 1), MAP sensor, both coolant temp sensors, fuel pump module, distributor cap, rotor, plugs, plug wires, distributor, ignition coil, ignition module, and the idle air control valve. No change what-so-ever, still doing the same thing. Timing & firing order are dead-on and the fuel pressure is correct. No vacuum leaks that I have found. This truck does not have (and never had) a catalytic converter, and the rest of the exhaust system is bone-stock and in great shape. I have checked the EGR system, it seems to be functioning correctly/normally. This truck does have an A.I.R. Emissions system, but I highly doubt that is causing the problem. Was going to try a PCM, but they are obsolete/not available. I know you can have them rebuilt, but I'm in a time crunch, and it would be a 6-8 week turn-around. I'm out of aces, I don't know what to do next! Any ideas?
Ok so I don't see any mention about the fuel filter. Has it been replaced? Have you been able to drive it with a fuel pressure tester hooked up to see what the pressure does when the problem happens? Have you pulled the settement bowl (switch valve see pic included) to make sure there is no filter in it (have found there to be on in it from time to time)?
September, 26, 2012 AT 4:17 PM
Fuel filter is new. I have not checked pressure while driving, my tester is nowhere near long enough. At idle (and revving it up sitting still) the fuel pressure is correct. And I have not pulled the bowl on the switch valve, but I will try that as soon as I get a chance. Don't know if it makes a difference, but the fuel tanks were spotless on the inside, no dirt/corrosion.
September, 26, 2012 AT 4:57 PM
No that dont matter much, its good the tanks are clean. Now just be carefull not to mess up the oring that seals the bowl. I have now had 2 trucks (one of my own and one a customers) that have had filters in the bowl and there should not be. Now as for the fuel pressure test while running is there anyway you can put the guage under the wiper and shut the hood so you can drive it and see what the fuel pressure does when it acts up? Does the check engine light come on? If so have you been able to get codes?
September, 26, 2012 AT 5:54 PM
My gauge is not gonna reach. I do not see that switch valve/settlement bowl anywhere on the frame. I didn't mention this earlier, but this is an old phone company truck with a work body and a big hydraulic boom on it. It also has a gasoline generator on it. I noticed that the fuel line for the generator has been tee'd off of the return line for the front fuel pump. I don't think that would be a problem though, as the truck's been that way since it was new.
September, 26, 2012 AT 5:57 PM
The check engine light has only come on one time (out of about 25 test drives), and even then it recorded no codes and was only on for a very short period of time. The only codes it ever had were some old history codes for the A.I.R. Injection solenoids, nothing current.
September, 26, 2012 AT 8:51 PM
This truck has a splitter for the pressure line and one for the return, but it has no bowl on it. Any other ideas?
September, 27, 2012 AT 4:22 AM
Ok is this actually a super duty?
September, 27, 2012 AT 12:30 PM
The badge on the side of this truck says "F-Super Duty". I called the local Ford dealer with the V.I.N, they tell me it is an F-450. Remember this is a 1996, so it isn't the truck with the Dodge Ram-style grille (98-present), this one is an old body-style.
September, 27, 2012 AT 12:33 PM
I found a re-manufactured PCM for a reasonable price at AutoZone, it will be here today around lunch, I'm going to give it a try.
September, 27, 2012 AT 2:59 PM
Ok let me know how it goes. After doing some research with all that you have replaced it makes sense.