Cold start rough idle/lean problem

[SilverXJ]

I have a bit of a problem on cold start. It is after my XJ has sat for quite a while, like over night or when I am at work. For the first minute or so it will have a rough idle, seems like a lean misfire. At that time the AFR is around 17:1. Lean. After about a minute or so I can watch the AFR climb to 14.7:1 and the engine smooths out. At first I though it was the IAC and I replace that. No change. The MAP sensor and TPS sensor check out good. Adjusting the correction values on the APEXI seem to make it better, but only temporarily.

When this is happening the PCM isn't looking at the O2 sensors (maybe when it starts to go to 14.7:1). So for some reason the PCM is telling it to lean out a lot. Which would make some sense if the fuel trim tables were a lot on the negative % side, but the long term trim is only -6% or so. I tried to have the apexi to increase the AFR at idle and it helps, but only that once and then it seems that the PCM learns it and negates the change. I also tried to reduce it idle thinking that it will make the long term fuel trim closer to 0. It idles even worse when I did that and even leaner.. then of course it learns around that and brings it right back to lean 17:1 AFR.

I know I need to check the fuel pressure at a cold idle.

For those that don't know: 2000 XJ, 4.6L (or could 4.7L now) stroker, Apexi MAP adjuster, ~25 lb/hr injectors, usual crap.

Any recommendations?

[YJason]

Have you checked the temp switch in the thermostat housing?

[SilverXJ]

I have not, but it should be reading correctly as the dash gauge isn't doing anything odd. But I'll check it.

[gradon]

I'd monitor the fuel pressure when you turn it off, see how fast the pressure drops, leave the gauge connected, then see what it is at before you put the key in the ignition the next time you want to use it. You could have a bum check valve on the pump. You don't think it is heat soak related?

[SilverXJ]

No, not heat soak related... its when cold. I replaced the pump about a year or so ago, but it is an Airtex so who knows.

[Cheromaniac]

I think your problem might be one of these:

http://www.wjjeeps.com/tsb/tsb_wj_1400201.pdf
http://www.wjjeeps.com/tsb/tsb_wj_1801001.pdf
http://www.wjjeeps.com/tsb/tsb_wj_1801301.pdf

[SilverXJ]

Nope, thats not it. I already had the TSB for the cold start issue done. Besides it hasn't been below 50* recently.

I primed the fuel systems 3 times this morning prior to start and same thing. I'll still be checking the fuel pressure though.

I'll definitely check the sensors.. possibly throw a resistor in for the CTS as a test and see what it does then.

[SilverXJ]

I checked the CTS today and compared it to the two IAT sensors and the OHM specs. They checked out. Fuel pressure was also good. I have yet to check the MAP and TPS as today I was busy installing a rear addco sway bar, fixing my driver's side window regulator and modifying the rear bumpstops.

[amcinstaller]

have you moved your IAT sensor yet? maybe it could have somethin to do with that?

[SilverXJ]

Yes, I moved it but nothing changed. When it is cold both sensors will show the same values.

[SilverXJ]

Tested the TPS and Map sensors today. The TPS is .815v closed, 3.7v WOT. Seems slightly out of range there. IIRC it should be .7v closed and 4.4v WOT?

Map sensor was 4.41v w/ engine off, idle was 2.41V, and increasing engine speed resulted in voltage drop. The idle spec for stock is 1.5-2.1v. However, that 2.4v isn't the map sensor's fault. At idle the engine is pulling 13 inhg of vacuum, which is correct for 2.4v. While I don't think that is the problem 13" does seem a bit on the low side.

[92tank]

if the vac. is on the low side, could that be due to your cam choice? and could that be why the guys that run aftermarket cams need to do all that tuning to get the stroker to run right? if thats the case then we need to figure out how to "trick" the map/comp into thinking everything is normal.

[SilverXJ]

I am unsure of the 13 in hg now. I was watching the sensor check display on the apexi going to work and it looks like the MAP voltage was right around 2.1 at a hot idle, which would be around 16 inhg. Maybe it just wasn't hot enough. I'll definitely look into it though and check it again. Possible vac leak, but no high idle.

[92tank]

not all vac leaks make high idle, its got to ba a big leak. if you take one of the vac lines off its doesnt idle high mine acually idles lower for a few sec. and then back to normal

[Cheromaniac]

While I don't think that is the problem 13" does seem a bit on the low side.

It is indeed so you might have a small vacuum leak somewhere. Check all of the hoses coming off the intake manifold for cracks/breakage.