93 XJ Rough Idle

I bought this XJ about a month ago and it had a rough idle. I have replaced the plugs, cap/rotor, sea-foamed, plugged the pcv and removed the catalytic converter (it was bad). After all of this it ran the same but over time it got better. Yesterday it started acting up again. If I pull the plugs off the injectors one by one they all change the idle exept the back cylinder. Could it be an injector going bad or something else?

First thing I noticed about your post is that you didn't replace the spark plug "wires". Don't forget the wires. They go bad over time. Also install a new fuel filter if you don't know when the last time one was installed. Fuel filters for your vintage are inexpensive and easy to install; located right along the fuel rail.

Yes, it could be an injector problem. You could try a noid light, checing electrical resistance on the injector or swapping the suspect injector to another location and see if the symptoms follow. Another thing to try would be a good quality fuel injector cleaner (I like Chevron/Techron) in about 3/4 of a tank of gas.

If after eliminating all of the simple stuff above, you're still having problems, then it's time for a compression test. Do all 6 cylinders if you're going through the motions. Can't completely rule out something internal.

Good luck and post back what you find!

birchlakeXJ said:

Can't completely rule out something internal.

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Even though its intermitent?

My strategy for troubleshooting this wouldn't really be different for solid or intermittent problems. Eliminate the simple stuff first that I outlined, then a compression test. Compression testing is pretty simple, doesn't take too long and is always good to know.

Troubleshooting is a "process of elimination". Finding out what it isn't is a big part of finding out what it is.

Good Luck!

Ok. Well I was given 2 injectors tonight that are known good. Im going to replace it in the morning I will post my luck.

Ok, I replaced the injector and that didnt help. Also the pcv was disconnected and plugged so I removed the pcv and cleaned it out and reconnected the hose. That seemed to make alittle difference. It was doing it last night then I went out this morning to wash it and bam it didnt miss at all!!! WTF... This is an intermitent problem and its driving me nuts lol. Any more ideas?:helpme:

93 is known to have occasional bad splices in the injector wiring harness. Often intermittent, appearing when it heats up, especially after a hot shutdown and restart. Check ohms on the hot side (coming from coil) to the injector. It can be hard to trace because the resistance changes with heat, and the voltage may drop just enough to stop firing the injector, but still pass as OK with a "noid light" or injector tester.

Try swapping line from bad injector with its nearest neighbor, and do the changing idle test again. If the problem follows the plug, it's the harness. If it stays at the injector, it is, of course, the injector.

If it's a bad splice on the hot side, you can just splice in a piece to jump over it. All injectors run off a common positive feed, and are switched on the neg. by the PCU.

Try Matthew Currie's suggestion on the harness or get a NOID light so you can test the bad cylinder's FI connection for a signal.

Replaced the plug wires today... No change :banghead:

joe_peters said:

Try Matthew Currie's suggestion on the harness or get a NOID light so you can test the bad cylinder's FI connection for a signal.

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I agree with the above. Gotta make sure that there is a good "signal" going to the injector telling it to fire. If you have isolated this to one specific cylinder, you still haven't completely verified firing of that injector.

If after the injector for that cylinder is verified and you're still symptomatic, check compression.

I would expect a harness problem to throw a code (though this is not guaranteed), so a compression check seems a good step if swapping harness connectors leaves the problem at the same cylinder. Put a vacuum gauge on it. If it's a non-firing injector you won't see much of anything happening. If it's bad compression, and also probably if it's bad spark, the gauge will flap around.

Don't entirely discount the possibility of a defective spark plug too. It's rare but sometimes you get a bad one right out of the box.

Too few people these days use vacuum gauges.

The thing that gets me is that its better sometimes. Its not constantly the same and doesnt get better or worse with the temp outside or the temp of the engine. Its alot better right now. Could what I guess is the coil (what the 7th plug wire connects too) be bad? Now its just like an everyday spark miss :dunno:

A coil can be tested (primary and secondary resistances) with a manual and a meter if so desired.

Intermittent/variable idle issue are not uncommon. I think you really need to verify if all cylinders are FIRING. A noid light will help you with that. Don't be afraid to check compression...not that hard to do.

Test your way to a solution. Don't throw parts at it. It's expensive and terribly inefficient. If you get stuck, having a shop put it on an engine analyzer is an option.

If the problem is always at the same cylinder, it's not the coil. Check again if this is confined to one cylinder.

If you think it's spark related, then check the plug, the wire, and the distributor cap for faults. Swap in a different plug. Swap a couple of wires. Make sure that the connector at the plug end is actually engaged with the plug. The end wires on an XJ use an L-shaped connector, and sometimes you can push it on without having it actually lock the contact onto the plug. Open it up and look. When you reconnect it, make sure you feel the snap. Look inside the cap for carbon tracks or faults. The possibilities here are limited.