4.0 ZJ rough cold idle

A guy I work with presented me with the following problem tonight, so I thought I'd see what you all had to say....

He has a '98 4.0 ZJ which has a cold idle problem. If the engine is cold, or has been sitting for over 30-40 minutes it will start just fine and idle normally for about 10-15 seconds. At that point, the idle speed drops to 500-ish, and it chugs and barely keeps running, and it sounds super-rich, although I didn't check to see if there was soot blowing out the exhaust. A couple of times, it dropped to nearly 0 rpm, then caught itself and surged up to 1500 rpms for a sec and then back down to 500.

This goes on for a minute or so, then it clears up and he's able to go on his way. If he tries to drive when it's acting up, the vehicle has no power, and it will barely move - once it clears up, no problems.

It seems like a temp sensor to me, with the way it just clears up, but it will do it whether the engine is stone cold or has just been sitting for a half hour or more, despite what the engine temp seems to be. Any suggestions?

Thanks!

Hello? Beuller? Beuller???

How long since it has had a good throttle body cleaning and the sensor checked, any codes thrown ?

Check engine light isn't flashing, and I tried the turn-the-key-on-three-times trick to get any codes, but there was nothing. Is this an OBDII car, being a '98? I thought the ignition key/engine code trick didn't work with OBDII vehicles...

This guy is very non-mechanical, but he claims to have had his ZJ at "the shop" several times for this issue, but they so far have not been able to figure out what the issue is. I would *think* that the shop would've done the basic things already - clean TB, check all electrical connectors, check vacuum lines, etc. (however, we all know what happens when you assume...)

I was just wondering if this was symptomatic of a particular problem, that's all.

Andy

C'mon everyone - I know that SOMEONE out there has had to have seen this before, either on a ZJ or their own XJ! :>)

I'm definitely not trying to downplay Rich's suggestion, but it just sounds too simple. I was trying to give the ZJ's owner a bit more technical info to bring to the shop when he goes back, that's all.

Thanks again!

This is a total shot in the dark so dont flame me here but what kind of shape is the exhaust in? My catyltic onverter was apparently partially clogged and caused my XJ to do that for a while. It would run rough for the first minute or so until it started to warm up. I got lucky and when I upgraded my exhaust I replaced the stock cat (since it was rattling when I first started it up and when I shut it off) with a universal high flow cat and the problem went away. Just a guess.

AndyS said:

C'mon everyone - I know that SOMEONE out there has had to have seen this before, either on a ZJ or their own XJ! :>)

I'm definitely not trying to downplay Rich's suggestion, but it just sounds too simple. I was trying to give the ZJ's owner a bit more technical info to bring to the shop when he goes back, that's all.

Thanks again!

Click to expand...

Sorry about the simple but being a long term troubleshooter in the service, then industry and now in my own business I have come across countless 'it won't work' that involved anywhere from 22 hours flying to 8 hours of driving only to find out it was not plugged in when I got there. You eliminate the simple things first and crud build up in the TB starts immeadiately after you clean and reinstall it. No, most shops do NOT clean the TB and sensors unless you specifically request it. With it being a ZJ that means it's atleast 6 years old and if it has never been done as part of normal maintenance its about time it was. Instead of wasting money on a general repair shop have him take it to an Injector fuels system specialty shop where they will remove and clean the TB and injectors, flow test them for pattern and replace any bad ones.
Most of those shops also have a dyno that they can put the vehicle on and get a pretty detailed read of the engine and drivetrain. If he has the bucks it is the way to go...

i would at least check to see if the throttle body has been cleaned. it also sounds like the iac motor is hanging up. it seems like it is stuck at the setting for a warmed up engine. also one thing to look for on the zj's is to check and see if the insulation on the gear selector cable is chaffing the wiring to the crank sensor.

My ZJ has been having a similar problem, though only when I come to a stop after driving. I'm almost positive that it's the IAC in mine, already replaced the TPS, and it sounds like your friends ZJ is doing near the same thing. A mechanic friend of mine told me to clean the IAC with throttle body cleaner first as they can get pretty "gummed" up. There's also a way to check if the IAC is hung up, but I would assume that if it was completely shut off the Jeep would die. Hope this helps some.

Alright - thanks guys. The owner definitely is not a "hands on" person, so either he'll talk me into doing it for him on a Saturday, or I'll have to print this out for him to bring to his "shop"....

Thanks for everyone's input!

Andy

hey, i was travelling to boston last weekend and my lincoln broke down. had to get roadside assistance tow it to the dealer in shrewsbury. very nice people there. lot of dunkin donuts though.

That dealer's only been there for a year or two, but they seem like a decent place. We looked at Mazda cars there before we decided on a Liberty for the wife.

The dealer's only a mile or so from me....you shoulda stopped by for some homebrew! ;>)

next time i break down at 7:00 on a sunday, i will make sure it happen there. i could have used a place to crash. i didnt get to the hotel in boston until 1:00 am. the tow truck driver got lost and went the wrong direction on route 9 from route 20. then i got the map and figured it out. is the zj fixed?

if tb cleaning and iac don't do it (and you already did tps), check cts and map

I dont comein here much,but.... if the guy is still having problems...

I had a similar problem with my ZJ ('98 SE 4.0) . Mine turned out to a the throttle position sensor. Got a new one at Autozone. HAvent had a problem since. I have had issues with the 4.6 Crown Vic at work doing that. The Idle air control (IAC) same thing... would cause the engine not to idle. But it was a more sever problem. The computer could not catch up to keep the engine running sonce the valve would stick. It would stall no matter what.
On the 4.6, I just located the IAC on the throttle body and gave it a generous rap with one of our aluminum Streamlights. Worked fine for a while, then would stick again requiring another rap. THis was solved at the shop by the mech. removing and cleaning it with solvent.

If a light tap (remember its aluminum) doesnt free the valve long enough to run the engine, then it aint the problem. Idle surging is definitely a potential TPS problem, though. HTH

Billy