95 f150, 302, auto trans, backfires through the intake at 4000rpm under load while driving, it idles fine and runs good at lower rpm's, we have checked fuel pressure and for vacuum leaks both seem to be ok, any ideas what it could be,

 

Backfiring through the intake is usually over advanced ignition timing. Spark plug fires before the intake valve closes. Check ya timing

I understand you checked for vacuum leaks but did you test manifold vacuum?

95 truck. Mass Air Flow sensor or Speed Density? If MAF test that as well.

 

Backfires at 4000rpm? Sorry but that sounds more like a mechanical problem.. probably a broken or weak valve spring or a damaged valve. Do a compression and leakdown check.

 

I'm not familiar with 302s nor 4000RPMS haha. My godbrother has a 302, but no tach so I have no idea he revs his truck.

I shift my 300 at right around 2800 to 3K according to my semi-accurate tach.

I'm just wondering if either of those problems would rear their head at lower RPMs. Kinda easier to hear **** flying around at idle.

 

well for him to be hittin 4k RPMs hes got it wound out fully... I have 4.10 gears and E4OD and 255/70/15 tires and my truck shifts at 4500 RPM's.

and yeah theres a problem in the valve timing as in a valve spring or something along those lines..

 

Its a 302 5200 is no problem , check compression also , have a look inside the valve covers....Lew

 

Well I posted to see what kind of replies I would get, to see if the info we were given could be correct, so from what we have gathered I think it is quite possibly a valve spring problem, we talked to a guy at work before I posted on here and he brought up possibly a broken inner valve spring or something wrong with the valve train, but I didn't want to show my cards and just see what else yall thought about the symptoms, I think he is goin to check the timing again and maybe try adjusting it again, we were just hoping for some better news than a valve spring problem, thanks for all the info and I will post back when we figure it out, again thanks for the info

 

update- replaced plugs and wires and readjusted timing again, all seems well for now, thanks for the help