Hello & thanks to all the pros for your help and assistance.

I purchased a used 1999 F250 Super Duty 4x4. When I went to see it, it ran PERFECT and started perfect. The guy who i purchased it from offered to change the tranny seal as it was leaking for free before i took it home. I agreed.

When I went to pick it up to start it, it didnt start. It backfired through the throttlebody loud one big bang then stalled. I attempted to restart it over and over since then, nothing but stalling and backfiring.

If you go to turn it over, it starts, then you have to tap the pedal a little to keep it running and it backfires alot through the intake. It will not stay running and it will jump in RPMS very high (overrevving) then back to stalling. Smells like fuel at times, not all the time. Wont stay idling or running without backfiring and overreving, you have to tap the pedal to keep it going.

Its weird, a it ran perfect. I talked to the guy whom changed the tranny seal, he told me he lowered the tranny and tilted the engine to make it easier to get out, he didnt do anything else he mentioned. I did see him attempting to start it from the starter as he couldnt get it started from the key one time when the big backfire happened.

Here is what i have done so far from what I think i should be checking:

1. No codes whatsoever from the time i got it.

2. I used a friends Genisys Scanner (pathfinder 2004) and no codes, nothing, i recorded a small start/stop if someone knows how to interpret it at all i dont. But no codes, nothing from the scanner.

3. Checked vacuum lines near rear and replaced rubber in case on the rear of the PVC hose. Replaced PVC itself also.

4. Changed Cam sensor in case.

5. I see good fuel when i turn key on, didnt check pressure though, checked from fuel rail, good spray, pump working good it appears.

6. Didnt check plugs but no report from Genisys Scanner says no misfires or cylinder misses anyways.. COPS all had similar resistances, checked them all. Didnt inspect plugs as heard sometimes they strip.

7. Took out MASS and wiped it clean.

8. All fuses are fine engine/cab areas, reset is pressed in left firewall for pump too to be sure.


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thats about all I did, been trying to figure it out, no luck. It is obviously something he did while under there that he is unaware of. it hasnt started since he dropped the tranny, worked perfect before. I assumed it was a vacuum leak, i will have to double check that again.

If anyone has experienced this sort of problem before or might have some suggestions for me to check, then i would love to hear from you. I have a Genisys Scanner to use while figuring this out, not sure if this will help, but figured one of the expert mechanics here might have an idea why it is backfiring so much? It will start easy, just wont idle or anything.

I tried to explain it the best I can, hope i gave enough details. I am thinking ignition or vacuum or theres something behind the engine he did not plug in(?) or put together wrong?

I thought when he jumped the starter, that it might have done something to the PCM or something, but it seems to connect and work fine with the Genisys Scanner.

Would the severe backfiring be a vacuum issue or could he have crushed wiring or put the wrong plug on the wrong sensor or something to cause this? I figured looking under it, theres a limited number of sensors he could have unplugged. He said he didnt touch anything in the engine bay, but i did see him playing with the wiring near the side of the battery with a scrwdriver trying to jumpstart it for some reason as he said it wouldnt start.. then he went under and it gave a large backfire when he jumped it.. thats all i know about the process he did that i witnessed.

Hope someone can help out, i have been trying to figure this out with no luck at all.. i dont want to replace everything if i dont have to. The genisys seems helpful but i am not sure what everything is suppose to read, it logs everything but i would have to make an MPG or something as i dont know how to take it off the scanner.. if someone can interpret the results i started it and recorded all the sensors as the backfires happen, dont know if its useful at all.

.......... sorry for the long description, was trying to be thorough as possible so someone that is knowledgable or works on these trucks for a living might be able to give me some ideas or help get this to run properly again.

1999 Ford F250 SuperCab, 4x4, Auto, 5.4l, Gasoline
(the only work i assume was done was tranny seal changed, engine tilted to make it easier he said, thats it).

 

wow.. thought some mechanically inclined person would have had some suggestions, but i guess the truck is a little advanced for the average techs here but thanks for the people who reviewed it anyways..

it was a simple easy problem, when it backfired, the lower intake portion of the intake blew a crack in the front of it, therefor allowing a vaccum leak and causing the problem -- i fixed it myself, free of charge, it was so simple just hard to find..

so -- if any other person has this problem, please check your intake for cracks, from what i discovered, the backfire caused severe pressure with the intake backfire, and the pressure caused perhaps vibration and extreme pressure which fractured the lower part of the intake, hard to see unless disassembled. I can see this being a common problem.

I hope this discovery, from a non-mechanic, know-nothing-about-fords person has discovered (grin).

But i ended up buying the Genisys Professional Scanner for so cheap off the guy who lent it to me, and now i also have a very powerful scan tool way beyond the average code reader, does EVERYTHING and i am learning more about it, with other makes/models too..

So perhaps one day this info will assist someone and perhaps save them HOURS of diagnosis, like i went through. The fix was easy, but perhaps beyond the knowledge of the average ford mechanic generally, but i thought at least one would have suggested such a possible problem, as it seems from reading on google that this is a 'common' problem diagnosed by many professional mechanics on other makes/models of vehicles that had similar problems such as mine does... so it could happen to other vehicles too if you dont find any codes, leaks, etc.. check the intake... it worked for me!

Good luck.. and hope this helps someone one day Cheers.

 

Have you checked/cleaned your IAC? On the 2V engines it is usually the most likely cause of a vacuum leak/backfire issue.

Given the scenario, a leaving vacuum hose is another likely cause. Sometimes a torn hose is hard to find.