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Skip/Misfire at Idle

Griffin

NAXJA Forum User
Location
Indiana
Hello everyone. New to the forum, but I've had my 1998 Jeep Cherokee Classic for a few years now. Ever since I got it, it's had a skip at idle. I didn't have the resources or knowledge to fix the issue at the time but I've been saving up cash and buying parts when I can.

Over the past few months, it has received new spark plugs gapped to 0.035, new spark plug wires, new brass cap + rotor, a new IAC, and a new ignition coil. I have also reset the ECU and ran some seafoam through the gas tank. It's still misfiring at idle and it's driving me crazy. I've checked all the vacuum hoses and did not find any cracks or breaks (my cruise control works so I didn't suspect there was a leak anyway). I'm not entirely sure what to do from this point onward, so any advice is appreciated. Thanks.

Here is a video on youtube that shows my exact symptoms. https://youtu.be/X5FRFsGpuTo
 
I'd pull the airbox and retorque the manifold bolts. Be real careful of the manifold end studs, I usually leave them be myself if possible. If you have a manifold leak it is often near the center anyway.

If you feel you have to mess with the end studs, clean the threads with a wire brush and repeatedly soak them with penetrating oil. Soak them overnight.
 
I pulled out the airbox tonight and retorqued the manifold bolts. No difference. I didn't mess with the end studs.

I think I am going to take it somewhere soon and have it smoke tested. I can't find any vac leaks but maybe someone else can, especially with the help of smoke. I'm just out of ideas at this point.
 
I pulled out the airbox tonight and retorqued the manifold bolts. No difference. I didn't mess with the end studs.

I think I am going to take it somewhere soon and have it smoke tested. I can't find any vac leaks but maybe someone else can, especially with the help of smoke. I'm just out of ideas at this point.
Id you care to You could make one probably cheaper.
 
What about a sticking injector?
You could unplug each one and
see if that indicates a dead cylinder.
 
That smoke machine looks a lot easier than I thought it would be. I'll head to Menards on Saturday and pick up some supplies to make one. I'll update you guys as soon as I can.

I'll test for a sticking injector this afternoon.
 
Update on injectors: I did not hear any irregularities in the rhythmic clicking. The #6 injector (one closed to the firewall) seemed to click rather harshly. Not only could I hear it through my stethoscope but I could feel it. Kinda odd.
 
That is indeed odd.
If the engine is still missing when
you hear/feel the harsh click, try
disconnecting the spark plug wire
to #6 and see what effect that has...

Or disconnect the #6 injector.
 
I disconnected the injector that leads into the #6 cylinder and I did notice a difference. I don't think the engine is fully misfiring. Is it possible the injector is partially clogged and not allowing the right amount of fuel in? Maybe the needle is not fully retracting? I am thinking about purchasing 4 hole injectors anyway so maybe I should kill 2 birds with 1 stone.

For kicks and giggles, I went ahead and tested terminals B and C on the CPS to see if it was bad. It read open so I assume not. Here is the thread I referenced https://www.naxja.org/forum/showthread.php?t=1070683
 
Since #6 injector appears to be working, I doubt if that's the bad one.
However, one of the other five might be sticking...? You could disconnect
each one to test.

You've already replaced the parts that would be suspect; wires, coil, etc.
You did replace the cap and rotor, what about the coil to distributor cap wire?
 
I build my smoke machine and filled the vacuum system up. Couldn't spot any leaks. Did the same for the exhaust system since I already had everything set up. No leaks there either.

I tested the other 5 injectors and did not hear any irregularities in their clicking.

That wire was replaced when I did all of the others.

I am definitely leaning towards replacing the injectors since I've done almost everything else I can think of. Might be a while before I can get injectors done because of coronavirus.
 
Injectors should be arriving tomorrow! I've been eyeing the 4 holes off of this website. I've heard some good things about them and they have a 10% off coupon going on right now, so I went ahead and pulled the trigger a few days ago. I'll keep you guys updated.
 
4 holes are in. Smoother under throttle but still idles rough. I'm not sure where to go from here. If anyone has any ideas - and I mean any - please chime in.
 
4 holes are in. Smoother under throttle but still idles rough. I'm not sure where to go from here. If anyone has any ideas - and I mean any - please chime in.

Try pulling your throttle body and giving it a good cleaning. Shouldn't cost anything more a new throttle body gasket (couple of bucks at the part store), some brake or carb cleaner, some Q-tips, and a couple of paper towels. Take your time, take it as far apart as you can, (or deel comfortable with) and pay particular attention to ensuring that your idle air control passage is clean.
 
I haven't as I do not have one. I am not allowed to leave the house as I cannot risk bringing back corona to my family so I will have to order one online.
 
I cleaned out the throttle body following your instructions. All passages are squeaky clean. New gasket as well. Unfortunately, there is no difference. I was able to use my vacuum gauge and hooked it up to the intake manifold. The needle is rapidly fluctuating around 15 inHv. I tried unplugging the vacuum line at the intake manifold that runs against the firewall to stuff like cruise control to see if it made a difference. It did not.
 
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