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Poor idle on cold start - if it starts


New member
Hello all. Need some help here. I've got a 90 XJ with the 4.0L and an automatic. On a cold start, the Jeep runs very rough, backfires, and smells very rich and stalls occasionally. After 5-10 minutes when it warms up, the idle increases then settles back down, evens out and runs just fine. Warm start there are no problems. Here's what I've done:
- New coil, ignition module, wires, and plugs - all replaced about a month ago. Crank position sensor is 3 months old. All these were done to solve a no start problem. I thought I solved the problem because everything ran fine until a couple of weeks after all these were added.
- Since this seems to be temp. related, I bought a new Coolant Temp sensor, plugged it in and stuck the end into a glass of warm water (didn't want to install it if I didn't have to). No changes.
- I have a dual TPS since it is an automatic. The rectangular connector is fine, reading 5V supply and 0.8V feedback. The square connector is reading 4.67V supply, but 3.75V feedback (blue and black wires) with throttle plate closed and decreases to 0.25V at WOT. Is this normal?
- It is very cold today (in the 30s) and now it doesn't start at all. No spark from the coil to the distributor, but it is getting gas.

Am I dealing with a bad computer? Any ideas will be appreciated.
remove the t-body and clean. remove the ias sensor (does renix use one?) if eqipped and clean the cone and the cavities in the t-body.

I would start here, cause its easy labor, and nothing to spend money on, except maybe a can or carb cleaner.
The super rich idle is usually caused by the MAP sensor vacuum line being disconnected; however, you also said that you don't have spark. The prime culprit for no spark is the CPS, not really the sensor but the connector. I think the TPS sounds okay. 3.75 is about 80% of the input voltage and the ideal setting is 83%. This isn't enough difference to be noticed. You can check for similar results on the TPS flat connector. You should see the voltage smoothly change to a lower voltage as you open the throttle. The ECU looks at the flat connector and the tranny looks at the square one. I've found that the ECU doesn't really care much about the TPS at idle. You can disconnect it and see for yourself.
Took off the throttle body and cleaned it thoroughly. Cleaned the stepper motor cone. Cleaned the MAP vacuum line. Voltage at MAP sensor appears o.k. (~5.05V). Still same problem. Runs like a piece of sh*t until it gets warm, then it runs like a gem. I'm running out of ideas....
What does it actually take to test the EGR. I was told to just test for vaccum from the lines but is there a different method to check it???
there's one vac connection on the power connect end, and two vac connects going sideways out the other end. with it idling see if that one connector has vac, if it does then to test correct operation pull the two hoses off the other end and see if while idling the one of those two closest to the power connect should not have vacuum, then disconnect that power connect and it should now have vacuum on that one.
CPU water temp sender.......

left side of block... under intake behind powersteering pump....

you can get at it underneith..... but have the newone ready for install as when you take the old one out, coolant will leak some...

Do it when its cool and make sure the cap is tight on the water tank (or radiator) so that it creates a vacume when you pull the sensor and that will keep more of the coolant in the engine while you hurry and get the new one in.

hope this helps..... its what I had to do on a cold day to get to work.