Stutter/Sputter After 60 sec of Run

Hey Guys, first post, this one really has me scratching my head. First let me introduce the vehicle, stock 1998 Cherokee Sport 4x4 4.0L, 42k on the O.D. (yes original), wicked clean.

The issue that I am having occurs after about 60 seconds of run time. Right about when it gets warm. The jeep drops rpms to about 500 then surges itself right before stalling up past 2 grand. It will cycle like this for about 30-45 seconds completely unresponsive to throttle and in any gear. Then everything will return to normal and the jeep will run fine no issues. Its been doing this for about a month and a half with no CEL. All gauges (volts temp and oil pressure) are reading fine during the occurrence.

I did my research and most frequently VAC leaks came up. I checked all lines on the intake manifold and sure enough I found and fixed one. Much to my dismay this did not fix my problem. After more research I decided to swap the IAC. Still no fix so I consulted a local mechanic who told me maybe it was the MAP sensor. Swapped that out and had him reset the air/fuel ratio. Problem still exists.

Before I go pouring money into every sensor/electrical component on the jeep I figured I would consult you guys for some help. Any suggestions??

Thanks.

Just guessing here, but eliminate the brake booster
by removing the vacuum hose and plugging it
(for test purposes).

There should be a temp sensor on or near the
thermostat housing. Check for a loose connection
there.

There's also an IAT sensor that might be a suspect.
(Intake Air Temp)

Thanks for the response R.O.E.

What should I expect to happen when plugging the brake booster?

The tstat sensor seemed to be okay but is there a way for me to check if that and the IAT are bad? Should I check for Volts on the plug side?

All in all i would assume my issue is either a VAC line or something electrical. Its safe to say this is not mechanical correct?

I don't know that it'd be the same case with your 88 as my 98 but I had a VERY similar issue.

I replaced the upstream O2 sensor and fixed the problem.

The PCM starts to read the O2 sensor about 60 seconds after start, or until the sensor gets up to temp.

My money is on a bad TPS or CPS if it does not end up being an o2, spark plugs, coil pack, fuel delivery issue.

Thanks for the feedback guys,

It sounds like the general consensus points to the upstream o2 sensor. I'll check it out this weekend but wouldn't it throw a code if the o2 sensor was bad?

Ignore my previous post, I now agree it's probably
the upstream O2 sensor.

It takes 2 consecutive faults for a code to be stored,
and apparently that sensor is still working somewhat.

Okay cool I will definitely look into the upstream o2 sensor this weekend. Does anyone have the p/n number for this one?

Thanks guys

Oh, wow. I see now that you said 1998. I thought it said 1988.

I'm about 120% confident that the O2 sensor will fix your issue.

Bosch - 13100

http://www.oreillyauto.com/site/c/d...gen+sensor_5014062_1490&keyword=oxygen+sensor

***Update***

Replaced the upstream 02 last weekend drove around with it the past week and had zero issues. Case closed. Good job guys thanks for the help.

Cool!