Dreaded 2.9 "Rough Idle"

[KJM1923]

Figured somebody here has some 2.9 wisdom for me. Got a '90 B2 for fairly cheap, primarily cause it doesn't idle all too well. Got all the vacuum leaks eliminated, replaced any sensors that have tested bad, still has a real bad idle, worse when cold, gets better when it warms up to temp, should note I hear an occasional "pop" in the exhaust, doesn't sound like a backfire, real subtle. I'm getting about 40psi fuel pressure, did a compression test and I got about 125-130 across the board. The motor runs like a champ driving down the road, just has a terrible idle. I've done almost everything I could think of, so anything to point me in the right direction would be great!

 

[franklin2]

I had the same battle. All that was left was the injectors. I meant to pull the injectors and get them tested but never did before I took it off the road.

 

[rusty ol ranger]

Figured somebody here has some 2.9 wisdom for me. Got a '90 B2 for fairly cheap, primarily cause it doesn't idle all too well. Got all the vacuum leaks eliminated, replaced any sensors that have tested bad, still has a real bad idle, worse when cold, gets better when it warms up to temp, should note I hear an occasional "pop" in the exhaust, doesn't sound like a backfire, real subtle. I'm getting about 40psi fuel pressure, did a compression test and I got about 125-130 across the board. The motor runs like a champ driving down the road, just has a terrible idle. I've done almost everything I could think of, so anything to point me in the right direction would be great!

At the top of the 2.9 forum is a sticky thread on how to adjust the idle on a 2.9. I had issues as well, did the adjustment and its been idleing good (athough not perfect) ever since.

These older fords never did seem to idle glass smooth...but they shouldnt be as bad as you describe either

 

[gaz]

@KJM1923 ,

Pull the codes first, you could have any of a variety of faults at work.
A complete tune up, may offer more precise direction with this issue.
• oil and filter ∆ (inspect for contamination)
• backflush the cooling system, (inspect for contamination) replace coolant
• air filter
• plugs, wires, distributor cap and rotor

All I've ever owned or driven are the Ford 2.3l I4 and 60°v6 2.9l, 4.0l engines. They do not typically have rough idle; as a matter of experience, I've only ever had 1 rough idle with one. It turned out that after sitting for a number of years, the lower intake manifold gasket gave, letting coolant trickle into the oil pan through the forward galley.

TRANSLATION: if you have a rough idle, I expect you will learn the fault, IT IS NOT NORMAL TO HAVE A ROUGH "popping" IDLE in the 2.9l

 

[rusty ol ranger]

@KJM1923 ,



TRANSLATION: if you have a rough idle, I expect you will learn the fault, IT IS NOT NORMAL TO HAVE A ROUGH "popping" IDLE in the 2.9l

No its not...but if you expect it to idle like a 2023 mercedes youre gonna be disappointed.

 

[JerryC]

Put a vacuum gauge on it.
Usually they will have diagnostic info on them, plus you can get help online how to use them.

Here is the one Harbor Freight sells;
HF Vacuum Gauge

In the days before ODBII a vacuum gauge was a very valued tool.