I have a 1998 Jeep Cherokee Sport, 4.0L, 4x4 manual transmission with 198,000 miles. The problem i'm having is my first cylinder. It's idling rough, different than before. I was getting a P0201 code, but i replaced the injectors (all 6) and injector plug on the first cylinder because of frayed wires and the code has gone away, but the rough idle still persists. Some other things i've done are the spark plugs, wires, distributor cap and rotor, crankshaft position sensor, and coil pack. I don't think the first cylinder is firing because i can pull the spark plug wire while running and it has no effect on the motor. I pull the injector plug on the first cylinder fuel injector and still no effect. I just took it to a mechanic when it threw the P0201 code and when he hooked up his OB2 sensor it was showing misfires in cylinder one. After the new injectors and new injector 1 plug this does not register any longer. It's not giving any error codes, so i could be wrong, but something isn't right. I've done a compression test, and it was showing 1) 112, 2) 115, 3) 115, 4) 115, 5) 127, 6) 120. I realize they are a little low, but nothing that would indicate that the cylinder is bad. Any help is much appreciated.

 

No check engine light, everything is ok then.

When the check engine light goes back on, read the codes, pull the plug(s) in question and post pictures.

Somebody will read them for you.

 

Around that year, there was an issue with valve springs.
Something to look into.
The result was the valves didn't rotate well and therefore didn't seat well. Carbon got between the valve face and valve seat.

It will show good on a compression test.

Here's my suggestion:

Pretty easy.

Get a cup of water, preferably something like a 24 to 32 oz. cup.
With the air cleaner off, hold the engine RPM at 2000 RPM or higher and dribble the water from the cup, which you've pinched to look like a spout on top, into the carb or throttle body. Go slowly and the engine will bog a bit. Stop and let it recover. Repeat until the cup is empty.

 

You mention everything except changing the spark plug.

If so, pull the wire and attach another plug (leave a plug in the cylinder) and lay that on the top of the valve cover and start the engine to verify that you have spark.

If you have a noid light attach it and see if the signal for the injector is there.
Consider taking the VC off and verifying operation of the valves on #1,,

 

I have a 98 also with the same issue. My compression is a little higher than yours even. It's the leak down test that will show you that a valve is not seating. My leakdown test was great except cylinder 1 which wouldn't even get to 15 psi. I haven't had time or money to correct my issue yet. I have replaced plugs and wires and ran water through the intake a few times and used many varieties of fuel system and combustion cleaners. All without a change. Last resort is going to be some BG44k. Likely it won't do much but worth a shot before I tear it down.

 

A compression leak down tester. Interesting. Im so old school I can't believe I never heard of this before. So you pump air into the cylinder and it reads out. got it. Nice.
Especially since it found your problem.
Had never seen one but I just googled it.
I learned something today.

 

Thanks for all the input. I'll try cleaning the carbon with water or SEA FOAM. If that doesn't work i'll try the leak down test. If it is a valve problem, will a rebuild fix this issue, or is there another solution?

 

Water.

 

The compression test doesn't indicate a valve sealing problem, the numbers posted are low but w/ that kind of mileage..

I read you changed plugs.

Your check engine light isn't on, if there was a valve problem there would be a lot of popping and misfire. Which would cause the cel to light for sure.

Your going to poor water into the intake w/ it running?

 

Yes. Water with it running.

The issue described here will pass a compression test, but the valve doesn't seat well enough while running causing low compression. No popping or backfiring.

This is a well documented issue.

 

I'd go with Cruiser. Steam clean it with water. And go from there.
I've seen a lot of 4.0L"s have misfiring issues right on point with what he said.
I use to use a valve cleaner from Dodge with great results but water works just as well and it's free.

 

So i've never used water before, how do you do it? Do you pour it into the throttle body slowly while the engine is hot?

 

I understand the concept, it'll knock some stuff loose.
But, "no popping or backfiring" maybe.

Not enough right now apparently to trigger a cel.

Op's engine has a lot of miles on it. It all sounds typical, I'm surprised the cel is'nt on.

It will be after he pours some water in there.

Use a real funnel though!!

 

Warm the engine all the way up before you do it. You need it hot to create the steam. Pull the tube off that goes from the air box to throttle body. Then use something like the end of a zip tie to wedge between the throttle stop and throttle arm to keep the rpm around 2000. After that, do exactly what Cruiser posted in post #3. Be sure to dribble the water and not pour it in. When it bogs down, stop pouring until it clears back up. Continue until your water bottle is empty.

 

What funnel?

Won't necessarily trigger a CEL.

Why don't you stand by the sidelines for a bit and let him get on with it.