I have a 1998 ford f150 4.6L V8. When I go to start it, it has a really bad idle and then the idle gets slower and slower until it just shuts offs and sometimes it cranks but doesn't turn on. This started when I was at a stoplight and when the light turned green I accelerated and the whole truck just shut off. Mechanics have been telling me multiple things like the sparkplugs, the fuel pump, the timing, and the injectors. so I just need help to narrow it down. Thanks

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Have you pulled codes? We need you to pull codes FIRST, even if the light isn't on.

Knee jerk reaction is... sounds like an EGR issue to me.... but 8 seconds isn't much to go off of.

 

So when I put an obd2 it had no codes, zero. Sorry for the lack of information I don't have much to go off of either. Thank you

 

OK, if you have no codes, the next thing is to check for fuel pressure. You should have 37 to 39 psi with no vacuum applied to the FPR.

Can you hear the fuel pump?

 

IAC valve failure is also common and has the symptoms described.

Things to check and test
- vacuum leak(s); hoses, fittings, broken or missing; aftermarket intakes miss some
- IAC valve failing
- EVAP purge/vent solenoids failing
- throttle position sensor TPS failing
- low fuel pressure, failing pump or regulator
- MAF sensor elements dirty
- EGR sticking

Some if the above result in codes. Some do not.

 

Right, but I don't know. I'm hearing an engine really low on compression from here. Hard to tell, it's just a first thought.

EGR would be my first guess as well, like you, I would check Fuel Pressures.

I use different numbers with pressure tests though. 32 plus no vac and 42 psi w/vac. The 42psi # should be steady under all loads. Ford has it at 28 psi no vac minimum for the 4sixes, 29psi for the 5fours. However and over the years ime, the pumps fail way before those minimums.

 

Without going into detail, I have to disagree with 90% of this post lol.

 

Interesting... that seems too low to me. I've always found them at 36 to 40 when everything works properly. That's what the spec is on the Mustangs per my Ford EEC book, so when they all tested in that range - I put that together. The book also said to check without vacuum as well.

 

Yea, pretty sure it's 40 and a little above. Whatever it is, has to steady and can't deflect. I through the gauge behind the windshield wiper and take for a drive to monitor under load...make sure there is no deflection. It proves regulation with out a doubt.

Could be a little off on that number.. I can't recall it being in the 30's. I'd look it up, I have to take off...NOW I'M LATE!

 

LOL.... oops, didn't mean to make you late Brewster!