Z71456

Hi everyone. I'm pretty new to the site, but thought I'd see if anyone had any thoughts on the problem I'm having with the Cherokee I just got a few weeks ago. It's a 1999 with only 35k mi. on it and it's been running great except for the last few days. I noticed on Monday that with consistent pressure on the gas peddle, the power seemed to go on and off a slightly. This morning and yesterday morning, the problem was way more apparent. It would almost cut out and then go again. Once it warmed up, it wasn't as obvious or noticeable. I took it to the Jeep dealer this morning and they haven't been able to duplicate it.

Any thoughts or ideas? The dealer is going to run it in the morning to see if they can duplicate the problem when it is dead cold.

Thanks very much,

Chris

COSXJFAN

A 1999 with that few miles on it likely has not had a tune up. I would suspect that the plugs, wires and what not are still from the factory. You mentioned a Stealership. If it was purchased from a dealership then they probably did not do anything to it but clean it up and sell it to you. Dealerships are not known to put much into a vehicle to get it ready to sell. It decreases the bottom line, and they don't like that!! Change all the plug wires, plugs, cap (if it has one), rotor (again, if it has one), air filter, fuel filter, and run a good bottle of fuel system cleaner through it. Being as old as it is and that milage, the miles were put on slow. That is not the best thing for a vehicle, they are built to be driven!! Hope my $.02 worth helps ya out, later!!

okcjeeper

O2 sensor and if memory serves a 99 has 2 of them one on the down tube just below where the down tube connects to the manifold and one after the catalytic converter...If you have a bad sensor you can get hesitations till the system warms a bit..Take it to somewhere like Autozone and have em hook their OBDI scanner up to it and check error codes as well.. Also do a tune up like COSXJFAN said.

BlueXJ

Is the CEL coming on at all? He may have no codes for the reader w/o a CEL.

Z71456

Thanks for all of the helpful advice, it's much appreciated. I actually didn't buy it from a dealership. I drove about 300 miles to AZ to get it from a couple that lived about 40 miles east of Yuma. The car has definitely been driven. They received it about 3 years ago from a relative and at that point it had only 7000 miles on it. They have been using it to commute to work about 100 miles per day roundtrip.
I checked the air filter and dist cap and rotor and they are like new. I haven't checked the plugs though. I will do that when i get it back from the dealer.

I'm definitely going to get one of those OBD readers to check the codes. That would definitely make sense about the sensors. If the dealer doesn't catch it, I'm going to check those first.

Thanks again for all of the help and tips, it's greatly appreciated.
-Chris

dobedyje

My '99 does the same thing (90,000 miles). About 30 seconds after a cold start it begins to loose power & miss. It never stalls, but after ~30 seconds of missing it clears up and will be fine for as long as you drive it. I have even had some pops back thru the intake. If parked for even 10 minutes it will usually have the same issues after a re-start.
Today I checked the first oxy-sensor. I suspected the sensor heater had failed. I disconnected the sensor and measured the resistance of the circuit created by the 2 white wires. I found a on-line article that claims the resistance should be 4.7 ohm, mine read 5.2 so the heater is not an open circuit. I did not check to see if the sensor is receiving the proper current will try that later this week.

What else could create this problem ???

BlueXJ

Iac

Z71456

Ok....so the dealership couldn't figure out what was wrong with it. Luckily they didn't charge me. However, when I picked it up, the guy that drove it up said it threw a Check Engine Light and the code said it was an O2 sensor. So from the dealership, I drove to Pep Boys and got two new ones. I just installed them and it is still doing the same thing. It seems to run a little better, but it still cuts out a little, mostly when it's cold.

The dealer suggested doing a pressure check on the injectors, but I told them no because they wanted to charge me $240 just to check it. Ridiculous. So I'm not sure if I should check that myself or if there are any other things anyone can think of? I'm stumped. I thought for sure the O2 sensors were going to cure the problem.

Thanks for all of the help.

-Chris

BlueXJ

Does it still have the CEL? If it does then you need to disconnect the battery for awhile then reconnect to clear the codes. That will cause the 'puter to re-learn everything. Might clear your problem. If not then clean your IAC and recalibrate your TPS. See if that helped. Then get back with us on CF.

Orangeamc13

X2

Z71456

sorry for the dumb questions, but what is the IAC and TPS? I just changed the dist. cap and rotor...Jeep runs better, but still has the hesitation/cutting out problem. Someone mentioned changing the fuel filter, but when I went to Auto Zone, they said it comes as a complete unit with the fuel pump. Does that make sense? Sorry I'm just not familiar with Jeeps.

Thanks guys....I'm stumped. I can't figure it out....the dealer can't figure it out....I'm not sure what to do? There are no Check engine lights coming on at all.

BlueXJ

IAC = Idle air controller TPS = throttle position sensor. Both are located on the throttle body. 99s are a little new for my expertise but that and the O2 sensors are what I would check. The TPS is adjustable so you can't just change it out for a new one but you will have to adjust it.
IAC is a little motor with a needle like valve that controls the air that bypasses the throttle plate to achieve a smooth idle. These can get a carbon build up and not flow the air that is required.
The O2 sensors are in the exhaust and there are two of them. One on the exhaust downpipe and one near the cat. They can cause a poor signal to the computer which thinks your air/fuel ratio needs to be rich so it gives the engine extra fuel as it always does when the engine is cold. This is called open loop. Once the engine and exhaust reach operating temp the O2 sensors change the signal to the computer and the air/fuel ratio is adjusted to give more air and less fuel which is called closed loop.

All of this will show up in the codes that a CEL throws. CEL = Check Engine Light.

okcjeeper

Ya beat me to it Blue....darn good explanations I must say...Also you can pull the IAC and clean it...I had to when I did the motor swap and helped smooth out the idle..Those suckers can get some carbon buildup for sure..

BlueXJ

Yeah the IAC motor can be cleaned and re-installed.

I am surprised I beat anyone as I only have one finger that knows how to type. All the others are illiterate.
Blue

Z71456

Although my idle isn't rough at all, could it still be the IAC? It does happen at lower rpms, but it's far from idle. Where is it located? I can try to pull it and clean it out.

Thanks again,

Chris