Codajay

hello, my 99 4.6 has a very bad idle, it bounces from 200 rpms to 800 and when I hit the gas it stalls out sometimes killing the engine and it has a loud back fire coming out of the intake manifold when gas is half pressed. no codes are being thrown when the engine "barely" idles, the truck sat for 3 year but it was started every week the first year, I drained the gas tank because the gas was bad and I changed the fuel filter and while it sat I changed both coil packs and the wires and plugs. It has a new pcv valve and the IAC valve has been clean. After all this its still running bad any tips and if you might have any idea what might cause this please feel free to let me know, thanks in advance.

akdoggie

Ck your air filter, MAF sensor, your throttle body (clean it if dirty), check your fuel filter. Check your onboard diagnostic computer (OBDII) for codes.

That is where I would start. Good luck.

Codajay

I have done all that already, I have done all the basic stuff and still nothing, I have taken the top end off and cleaned it all up but still nothing

igotrockstars

By the sounds of the symptoms, it seems like your truck might be running rich and flooding itself. While your truck is running, can you smell gasoline fumes coming from the exhaust?

Czuber

You try the PASSENGER side PCV. The easiest way to find this elbow is to find that PCV and follow it all the way to the back of the engine. It was a 40.00 part for the valve and hose from Ford.

Codajay

I was thinking that I can smell fuel from the exhaust and I can cover half the air intake hole before the filter to restrict the air and it runs fine and I can rev the engine but when I take my hand off the hole and try to rev it drops idle and almost dies

Codajay

That was the first part I got for it when I started working on it but thanks czuber

Jbrew

I would have posted a few days ago , but you said you covered the basics.

That's either an EGR poppet valve problem or a plugged cat, but yea, these are the basics when the RPM's fluctuate that much. You probably already checked that out.

Come to think of it, perhaps if you posted what you have done already to determine the problem, you may get better answer's...sound good?

Jbrew

Slipped my mind. But always check fuel first as general rule...delivery.

Jbrew

Also, I'll just throw this in here,since you mentioned a like symptom - signs of a dysfunctional MAF. They rev way up and die. Check harness connector, all pins for contact.