Jeep won't idle when cold

[DaveB]

One of my Jeep herd has started having issues running at idle when it is cold. It runs fine once it is warmed up but will die when you let off the gas when cold. I have done quite a bit of reading on this symptom and from what I've read here are some of the most likely culprits:
The idle air controller and throttle body: both are clean
Battery: brand new
Vacuum leak: Possible but haven't found it yet and it actually runs better if a create a small leak.
Crank sensor: Haven't swapped it yet
O2 sensor: Fairly new
Intake manifold gasket: This is one thing that I am thinking may be an issue. The jeep has an aftermarket header on it that was a pain to get it fitting correctly with my 99+ intake manifold and required some massaging to clear the intake. The header output flange also had to be ground off a bit since it hit the block. The header has a slight exhaust tick that you can hear when it is cold but goes away when warm. I am thinking about replacing the header with a stock manifold or possibly spending more on one of the expensive headers. I have read that folks still have problems with the expensive headers cracking so I can't decide on what way to go.
I have a wide band sensor on it to tune it and the wide band shows that is goes very lean when it dies. The motor is a 4.6 stroker.
Any thoughts ????

 

[DaveB]

Well I did some more checking and found that some of the lower manifold bolts were a bit loose (they are a pain to get to with the header) so I tightened them up. It may have helped a bit but it still has issues idling when cold. Another thing it is not doing is the higher idle speed that drops off quickly after start up. My next tests will be the idle air circuit and TPS. Hopefully something will show up. The engine codes that came up were battery disconnected within the last 50 starts, which is expected, and MAP sensor voltage low or high. I don't know if my MAP voltage adjuster would trigger that code. I bypassed the MAP voltage adjuster and it still had the problem. I have spares for the sensors and will be trying them out.

 

[Greg]

I was going to say that it sounded like a vacuum leak, somewhere. Hard to diagnose without seeing it in person!

 

[Jinx]

Sounds like a bad idle air control motor if it was a chev 350.

I had very similar issues but didn’t notice how bad it was because it was a gradual change. Finally changed it out and it is a completely animal starting cold.

 

[RogueJeepr]

X2 idle air control motor or tps .

 

[DaveB]

Well I swapped out the TPS, IAC and it helped a bit. But the real problem was a bit more involved. The motor is a 4.6 stroker and it was pinging so I added an adjustable fuel pressure regulator, and a MAP voltage adjuster. Once I got the thing to where it wouldn't ping it worked great when it was warm but the next time I tried to start cold it was too rich. Then the fuel pressure started acting up and I found the short piece of hose between the fuel pump and the sending unit was rotten and leaking. When I swapped out the hose I looked closely at the custom sending unit and realized it had a constriction in the line so I re-plumbed it for better flow. Once that was done the motor was getting way too much at idle and WOT. So I backed off the MAP voltage and lowered the fuel pressure and test drove it. My air ratio gauge showed it too lean at WOT so I bumped the pressure up a bit at a time. Now it runs much better. I may be able to eek out a bit more power since most places I read suggested that 12.5-13 was good for power at WOT. I am now sitting at 13.7. I want to make sure it will start well cold before I bump it up again.

 

[Greg]

Nice find, Dave!