Just picked up a 99 Cherokee Sport 99000 kilometres on it, pretty good shape.
Bad misfire when engine is warming up after about 5 minutes of driving, then goes away until under load and it comes again.
I have replaced 1) Plugs 2)wires 3)Dist. Cap 4)rotor Button 5)gas tank pick up filters, both of them, the sock and the one at the pump in the canister.
It still misfires quite bad.
It was obviously an around town jeep, drove it home on the highway and when booted it I could not see the car behind me due to the black exhaust fumes, this has now cleared.
I have not cleaned the throttle body or put injector cleaner through it yet.
Any one got any ideas?
Thanks Heaps!!!

 

1. Any check engine light? If so, codes must be retrieved from the computer for clues and guidance on what to look at. Post the exact codes here for comment (not just a description of the code). With OBD, the devil is definitely in the details!

2. Test fuel pressure. Has to be done with a fuel pressure gauge. For a 99, the fuel pressure spec. is 49 psi, plus or minus 5 psi.

3. While not a high mileage engine, for any "new to you" vehicle, running a simple compression test is a good snapshot in time of the internals of the engine. The compression spec. for the 4.0 engine is 120-150 psi, with no more than a 30 psi variation between cylinders.

4. Cleaning throttle body and idle air control never a bad idea, but I don't think that is the cause of your misfires.

Good luck and keep us updated!

 

Thanks for your reply! No, unfortunately it does not have any codes, probably more like a surge than a misfire, It starts after about 5 min of driving then goes and then comes back every now and then, just seems to almost die out.
I am thinking it is the throttle body, I will pull that next and clean it, might also try the camshaft positioning sensor, I read somewhere that it is a possibility.
Do you know where the ground is that is under the centre console between the seats?
A friend of mine told me I should clean that ground up but I don't know where it is under there.
Thanks

 

What spark plugs did you use? I made a mistake and used autolite on one spark plug change. I had the asme problem. Had a really rough idle especially after it was warmed up. Reaplced with the OEM Champion and the problem went away.

 

I ALWAYS start with cleaning the grounds.. one on the dipstick is especially important! Run an extra to the firewall. Check the connections and clean them at the coil. And of course... the battery. It'll mess up all your readings and cost almost nothing.