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HELP!!! Start/Rough Idle After 45-60 Seconds

Here's my problem:

I am bringing an old '89 Cherokee Pioneer 4.0L Automatic back to life while I rebuild my '87 Laredo 4.0L engine (got about 250,000 mile on it before it died). The '89 Pioneer was very clean inside and body straight, but the engine compartment was a horrible dirty mess.

The problem is that, when I cold start, it runs fine (about 1200 rpm), then after about 46-60 seconds, rpm's drop badly to near stall (200 rmp's or so) and runs rough. After it warms up, runs fine, smooth acceleration and has plenty of power AND just passed California Smog Test.

Also, after a good run when warm, if I stop and shut down engine for

10-15 minutes or so and restart, it runs super rough (like on half the cylinders) until warm and reving the engine again.

Here's what I've done:

Thoroughly cleaned all connectors (TPS, IAC, CPS, O2 Sensor, Relays) and anything I could see and get my hands on.

Thoroughly cleaned Throttle Body, IAC and IAC Port, Idle Air Port, and re-installed TBS with new gasket.

Tightened exhaust/manifold bolts. They were a bit loose.

Replaced MAP sensor with new part.

Have tried two different used but cleaned IAC's. Same result.

Compression is 140-150 cold on all cylinders.

Set TPS to spec.

New Distributor Cap, Wires, Plugs.

Brakes and Booster seem OK (though the fluid was, still is, mocka brown. Full flush is next on the list of to do's). Though when in neutral, pumping the breaks causes about a 300 rpm increase. Is that normal? hmmm...

O2 sensor test reads 6 ohm. Dead center on spec.

EGR valve appears to be working OK. By the way, the 89 has a smaller EGR than my 87 and it has a Regulator upstream of the EGR. Is this the correct configuration? My Vacuum diagragm (chiltons) does not show the Regulator but the Jeep Dealer says its fine. hmmm...

Any ideas what could be causing this problem? I sure could used your help with this one.

One other aggrevation, the Fuel Gage at 1/8th tank still has about 6 gallons in it. Any way to adjust the gage in the dash?

Thanks in advance...

Mark

PS: First time on the list.. Thanks guys (and gals), it's a great help!!!

Reply to
CADman
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Did you check the Coolant Temperature Sensor? Are you sure your heated 02 sensor is getting it's 12volts and ground? This is what it sounds like to me, or maybe it is not working. I think the dealer is still doing recalls on these 02s. I found out about that after finding a stash of them for mine or for sale. :-(

I think that valve for the EGR is called a transducer. I had to clean the carbon out of mine to get it to vent properly. It would cause the EGR Valve to stay open and the engine would try to stall.

Reply to
Dave

If the EGR is plugged then no exhaust is sent to the carb.... this of course is not as per design but wouldn't cause his problem of stalling on a cold start.... are you saying the EGR was plugged but still had a clear flow path from exhaust to carb?

Again correct me guys if wrong here.... trying to learn.

After I cleaned my plugged EGR I could stall my YJ by forc>Did you check the Coolant Temperature Sensor?

Reply to
Jeepster

Sounds like the O2 sensor making it run rich but 30-45 seconds is a bit too soon for that to be the root problem. I think that vacuum leak in the brake booster may be the real culprit. I don't know if the computer reads anything from the heater on the O2 sensor, but that's another possibility. I fought cold idle problems with an 88 MJ

4.0 for m> Here's my problem:
Reply to
Will Honea

Reply to
Steve G

Hello -

Thanks for responding.

No, I didn't replace the Cooland Temperature Sensor. I'll check whether its up to spec. How would this Sensor effect operation?

As for the O2 Sensor, how would I go about checking whether it is getting the required 12 volts and ground?

I think I've read on this list somewhere that there is timing circuit that controls the period of "Open Loop and Closed Loop" operation. If the Heater on the Sensor is not functioning and the ECU switches to normal run (is that "Closed Loop"?) operation, would the Sensor not being properaly heated cause this problem until the engine and exhaust has heated it to proper operating temperature, after which it runs great?

As for the responses regarding Brake Booster, after warm up, it idles and accelerates fine. No Problems at all. Could it still be a problem?

Thanks for your responses,

Mark

Reply to
CADman

Follow up re O2 sensor:

Thinking that it may be the O2 sensor, as a test, I removed to O2 Sensor from my '87 Laredo and swapped it into the '89 Pioneeer. This seemed to substantially reduce the problem at start up, but still have a small drop in RPM at the same 45-60 second zone. Additionally, when warmed up, it seems to have lost "snap" when hitting the accelerator and may have developed a small bit of hesitation.

Problem definately seems to be in the O2 sensor, now to try and figure out which one (or a new one) gives the best blend of eliminating the startup problem and running performance.

Any thoughts?

Thanks, the BOARD is great!!!

Mark

Reply to
CADman

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