I think I’ve read almost every related post I could find on rough idling and such. Surely I’m over looking something so I’m am reaching out for some help.

i have a 90’ XJ 4.0l. The issue is it will start normally, it will idle normally for about 15-20 seconds or so the go to crap, idle down to almost nothing, I have to feather the throttle to keep it running or it will die. Starts right back up the same, the repeats. Plugs are black and Sooty. Eyes burn it runs so rich. You get the idea I hope.

It was running just fine until I replaced the steering gear box a few days ago. I can not figure out anything I could have screwed up in that area to make this happen suddenly.

Motor was completely rebuilt by a my friend and
from what I am told, stock other than a larger cam. It only has about 30 miles on the rebuilt motor. All the sensors are new with the motor.

In addition to all the threads I’ve scoured I’ve gotten pretty intimate with Cruisers suggestions. A lot was delt with the new motor but, I still went back again and verified.

So far I’ve checked all the grounds, clean shine unto metal. Upgraded the “big 7” a few weeks ago. Testing the sensor grounds all at 0-1ohms. Splices in the harness were soldiered. New vacuum harness, went through every inch of vacuum line and found the vac to FPR clogged and since replaced. No change, and I’m getting 31 psi at the rail, unplug the vac and it goes up to 38psi. Tested the MAP sensor, hooked up to a vac pump, checked voltage as I added vacuum and it appears to sweep as it should. EGR is new and tested according to Cruisers page. O2 bench tested good but replaced with a Bosch just in case. IAC, unplug no change. Of course the TPS is set and tested the CPS as well all test good. Edit: Also tested the fuel pressure at the manifold, found a clogged vac line to the new FPR but prior it was at 38psi. New vac line now reads 31-32 psi.

Crusier numbers 1,3,5,7,8 and 21 were done.

Testing the vacuum seems a little low (10-12 )but then again the idle is for crap but when I bring up the RPMs it goes up to 17 in.

Only other work I’ve done to the truck prior was reinstalling the original transmission. But it ran tops after that. Will try to upload a video if I can. Any help is greatly appreciated, I’m starting to go a little mAd!

Clean and new

Crimps soldered

Clean!

 

 

I've had nothing but bad luck with Bosh 02 sensors , and have you check the fuel presure reulator and fuel presure ?

 

10-4. original O2 texted ok, replaced twice with Bosch O2’s. Total of 3 now on the O2’s.

Fuel pressure is exactly 32 psi after I found the vac line to the FPR clogged. Funny thing is that FPR vac line has been cogged the whole time, defective from the factory. Was getting a steady 38psi at the rail till I found out it was to high


Engine running ( like crap but running )


FPR vacuum line

 

One more thing is the coolant temp sensor working right ?

 

I haven’t tested it. Didn’t make sense to me that a cold engine start would do what it’s doing from that but, I will get on that test directly.


Edit: tested and based on what I’ve read thus far. I am showing 1400 ohms on a cold motor. Which subsequently I assume that should read being cold. I can warm it up and test it but Does that make sense?

 

It may read cold like -20 or so , if it reads that all the time it will run the engine rich all the time .

 

well then let me try and warm this fool up. Last few times I tried to drive it wasn’t pretty, I’ll give it a shot!

 

But if your coolant sensor is bad even when warm it will still read -20

 

Although a crude test. I blasted it for a good 10 min. Hesitating a bunch in acceleration. Got it up to 55 plus twice and around the block several times. Parked it and measure 303 ohms at what ever temp it’s at. It’s surely a warm motor. Sorry I don’t have exact temps.

CURIOUS. Would the knock sensor cause these type of issues? If I’m looking at what I think is the knock sensor just below the CTS.. it looks rough, cracked and the connector is loose and busted

 

If the knock sensor isn't working you would get a ping when driving , is your Distributor in right if it was off just alittle bit it mite be the problem .

 

How can the motor run correct for a little bit if anything spark, distributor or timing related?

 

Ya that's true , did you check the tps voltage . just trying to think of things that could make it run bad .

 

I have a 90 XJ that was giving me all kinds of problems like that, did all of cruisers tips before the idle problem came along. So I know I was good there, tested and retested the sensors (all were new) all were looking like they were working. Getting ready to drive the XJ off a cliff it was driving me crazy. Then I found Nick who makes a Renix Engine Monitor (REM) with a built in check engine light. Got it, plugged it in and the check engine light came on and show I had a bad MAP sensor that was new, replaced the sensor again and all was good. New parts nowadays are not as good as they used to be. Hope that helps.

 

How is the vacuum at the MAP sensor?

In the process of putting a trans in, the line from the throttle body to MAP coulda gotten damaged.