help with 4.0

[BACRAKR]

I'm working on my friends 4.0 for him. Its in a 1996 EXPLODER. 136000 miles

Its got a rough idle and every now and then it will start stumbling on idle. If I rev it up the problem goes away. It has plenty of power, once you get it off idle it does fine.

If I rev it while its idling and missing it doesn't stumble at all. It picks right up.


Its got me stumped.

1. new plugs
2. new wires
3. no stink from exhaust or weird noises
4. no DTC codes or MIL light
5. new fuel filter
6. new fuel pump
7. air filter is clean
8. good fuel pressure

I was thinking the cat might be clogged but it doesnt make any noises at all and the exhaust doesn't smell funny at all.


Any suggestions would be great. I'm hoping these motors are known for something I don't know.

 

[MikeWild]

maybe try cleaning the MAF sensor.

 

[DCarey515]

Does it happen at first start up? Or does it happen when you/he starts it after driving for awhile, say 45 minutes driving,turn off, then stumble after starting back up?

 

[BACRAKR]

The times I have got it to mess up it has been after it has been driven. Like last night I put plugs in it, started it up and it ran good. I was hoping it was fixed. We went to walmart and when I started it back up from a 10 minute drive it started missing again. I revved it a little bit it clears up and idles fine.

 

[Kylan]

Try checking the IAC

 

[BACRAKR]

Today we started it up cold and it started doing it again. So it doesn't matter if its warm or cold.


I'm going to try to hook up a fuel gauge to it while it malfunctions later. It smells like its really rich while its doing it. I'm thinking it might be a fuel pressure regulator.


How do I test the idle air control valve?

 

[98blownranger]

test... I just replace them. and it was alway the fix

 

[dapper15]

what is the iac?

 

[jasonl]

Idle Air Control valve.

You can pull the IAC off and clean it with carb spray. Spray it and let it soak and spray it some more until it is clean. Sounds like maybe you have an injector leaking and loading up a cylinder. When it starts idleing rough and you rev it, do you get any black smoke out the exhaust like there was raw fuel? How did the plugs look in relation to each other? Next time it starts idleing bad, tap on the IAC with a screwdriver handle and see if it changes it. I also agree with the cleaning of the MAF sensor first with some MAF cleaner spray. Good luck

 

[BACRAKR]

I've been driving the truck the past few days hoping it would mess up for me so I could get a fuel pressure reading at the time of malfunction but I have had no luck.

At idle this things holds 38lb's of pressure. Somewhere on here I read that they should have 30lb's? Is this true?

I did not notice an excess amount of black smoke, or any smoke when I rev it after it misses.


All the plugs looked great. Just a bit worn, but nothing out of the ordinary.

I will grab some maf cleaner and clean or buy a new IAC valve tonight or tomorrow.

 

[woodyedmiston]

REPLACE THE IAC. Its on the side of the air plenum just behind the TPS and the Throtle body,

 

[BACRAKR]

Cleaned the MAF and replaced the maf but it didn't fix the problem.

 

[AKBroncoII]

Check the clutch fan. If it's bad, it will act as a miss at idle.

 

[RangerKid25]

try putting sea foam in it, it will get rid of carbon buildup and clean all the sensors you can get it at pepboys and auto zone and probably any othe car place