671Guam

Hey everyone,
I have a 91 f150 4.9 EFI straight 6 that starts perfect when it's cold, but starts really rough once it's warmed up.

I haven't pulled codes yet and I know that's the first thing I should have done, but I don't have an obd scanner. I'll pull codes this weekend.

Every single time I start the truck while it's still warm, I have to keep on the accelerator for about 20-30 seconds or idle will drop and truck will die. I also have black smoke and sometimes a mix of black and white every time it starts warm.

Cold starts, No smoke, no rough idle, one time start, runs good.

The truck also feels a bit weak on take off. I have to give alot of gas when I take off to get the truck moving or it'll die and when I start it again, I have to again wait 20-30secs until the smoke clears and the idle levels out. I don't want to do this in the middle of the highway. Not fun.

I read somewhere it could be the ECT sensor, If that's it, I think it's near the water pump. I don't know but I need some help please.

I've double-checked the firing order & I'm good there. I threw a timing light on just to verify my timing, but I'm not sure exactly which mark to use. Am I suppose to be 10 degrees, 14 advanced? I'm not really sure with this motor so I adjusted the distributor until the camshaft mark lined up with the larger V notch on the timing cover. Still starts rough. I played with the distributor a bit as I thought timing might be slightly off, but after I set timing I forgot to TDC again to double check. I'll do that after work today.

So far this is what I've done:

Check for vacuum leaks
IAC - new
MAF - new
TPS - new
Cap - new
rotor - new
plugs - new
wires - new
Ignition Module - new
Intake/Exhaust gaskets - new

I've read similar issues here on the forum but I've had no luck.

Thanks

Matthew Burr

sounds like u threw a lot of money at it already but if all thats new i would say you have a cracked or warped intake thats sealed when cold and open when warmed up causing the poor mixture. if it was a carb vehicle i would say the same and or poor choke adjustment, advance is generally around 10 degrees, but varies with the age of the engine, use a vacuum guage for best adjustment, should be around 18in of pull.

get a can of carb cleaner and spray it around the intake while its running and see if it helps, please dont hit a hot exhaust manifold with it

Chris_1

iTrader: (1)

Might be the temp sensor assuming your motor has one, (I'm more familiar with the v8's).
Pretty sure you should have one - that's what a faulty temp sensor will do, tells the computer the engine is cold, so it feeds more gas and floods the motor.
Worth a try.

Matthew Burr

O2 sensor could throw things off too, and running it too rich for too long can ruin it

Chris_1

iTrader: (1)

Also - you adjusted the new tps when you put it on ?

Matthew Burr

has it ever overheated?

Whaleytruckguy

If i pull fuel pump relay it will start

A pause for the COZ

Chasing the same problem. I think. Mine is a 89.

Still chasing mine. What happens with mine. When cold, it starts fantastic. Runs and steps down on the throttle as it should. As long as its cold you can turn it off and restart. No issues. The issue rears its head at a restart on a warm engine. It sputters and dies. Try it again and 75% chance it will stutter and die. If it does stay running. You can tell the computer is searching for the right mix. Third start when warm. Starts and runs normal.
Have a OBD1 tester= No fault codes. Now here is my list of fixes.
Idle air controller is 3 years old. Cleaned it up.
Throttle body gasket has been replaced. Was bad.
Throttle position sensor= new.
Coolant temp sensor= new
New MAP sensor= new
Computer = new ( had it on hand)
New 02 sensor.
EGR valve= Removed and cleaned to insure no sticking ect.

Thats about it for my fixes. Its has had a complete tune up with new ignition system. Truck only has 108,000 miles When I got it. The throttle body gasket had been bad for some time. To compensate, they just increased the idle speed to get past the air leak. Not sure how long they ran it like that. Was a muni truck so i have complete maint records. They had replaced the idle speed controller twice. So mostly I have been fixing their fixes. I think i will swing back to the Idle speed controller. I have another 4.9 in a F250 that runs flawless. Maybe swap it out to see.
I need a new hat R&R man!!

A pause for the COZ

Good things to try. Thanks

A pause for the COZ

Interesting. When i turn the key I hear a bzzzzzed click sound.
I will look into that.