ToothMechanic

1997 Cherokee has a slightly rough idle. Idling around 750rpms with a slight vibration/needle bouncing. I have been randomly getting the cylinder 1 misfire code. I will clear it and then a few days it comes back. Engine feels like it makes good power, drives smooth.

Maintenance that has been done before me:

2016: New Fuel pump, upstream O2 sensor
2017: New O2 sensor
2018: New distributor, crankshaft psotion sensor, spark plugs, ignition coil, service sheet doesn't say wires but they look new, maybe they included those with plugs charge.
2019: New throttle position sensor

Thing I have tried:

New spark plug in cyl 1 - no effect
Switching wires - no effect
Spark tester - sparking away
New cap/rotor - no effect
Injector circuit - good resistance
Injector solenoid - clicking away to a stethoscope
Seafoam spray into throttle body - no effect
Seafoam liquid into fuel tank - no effect
Dry compression test on cylinder 1 - 150psi.
Unhooked and inspected/cleaned IAC valve - no effect. Good circuit resistance. But I'm not sure how to evaluate whether this is working well or not.
Checked fuel trim with scanner - negative 4%


The next thing on my list is to swap injectors from cyl 1 to 2 to see if code still reads #1. After that my next ideas are to check all grounds, look for vacuum leaks, and maybe get a shop to professionally clean the injectors.

Only thing I am aware is wrong with the car mechanically: I am confident that there's a small exhaust leak or two (silent, but I can smell the exhaust in the car if I stay stopped with windows down).

Any thoughts?

dave1123

Try using a vacuum gauge to see if the intake valve is leaking. If the engine pulls good above an idle, there may be carbon on the intake seat causing a slight leak at low rpm. If you want to pull the valve cover, that can be verified by looking at the wear pattern on the valve stem under the rocker. If the wear shows circular rings, the valves are rotating. If the wear scratches are straight lines, it isn't rotating. Rotating valves will clean their own seats as the engine runs. This is a directive from Mopar to service techs. They have a product similar to Seafoam, but much stronger that will clean off that carbon. They say engines that run most of the time below 2500 rpm tend to build up carbon on the valves.

OR you can go wind the **** out of it and see if it cleans itself! (Old school mechanic) My opinion is that carbon is caused by oil vapors coming thru the CCV system from the crankcase and soaking the back of the valve heads and baking on them. On my worn WJ engine, I put an oil catch can on that CCV line and was catching about 1/2 cup of oil a week.

cruiser54

Possibly carbon under a valve in that cylinder.

Best way to de-carbon is with water.

bluejeep2001

He would not pull 150 on a compression test if he had an issue with the valve or seat...for the price of getting his injector cleaned or repaired he could buy a whole new set...run a timing light on plug 1.... make sure its a steady consistent flash and then check that it does not jump around the pulley marks

cruiser54

But see, that's the problem. And there was a Bulletin out on it. Valve springs were too weak and valves didn't seat good and hard. Carbon built up as a result.
At cranking speed a compression test shows good.

ToothMechanic

Ok so had more time to do some testing today. Since doing everything I said earlier, the CEL has not come back on with the P301 code. I'm still getting a rough idle. Car idles around 750 but the needles flutters up/down a bit (nothing severe, maybe +/- 75rpm).

Full compression test. All 6 cylinders reading around 150-160 dry and a little bit higher wet.

Vacuum test. I used the vacuum attachment on the back of the intake manifold. It was a steady 15. The needle was fluttering a little bit but I am assuming that's just how it reads. I'm saying flutters from like 14.8-15.2. Went up to about 20 and stayed there when held at 2k RPM. When the throttle is blipped the vacuum drops, pops back a bit higher, and quickly resettles at 15. From my research this is the appropriate behavior. I did a visual inspection of every vacuum line I could fine and did not see any obvious problems.

I inspected the air filter. It's a K&N and didn't seem dirty at all. I'll clean it next time I can get the special cleaner at the parts store.

The only damaged thing I found today is the the plug going into the front of the coil has a slightly damaged wire (green/orange) where the insulation appears to be cracked/broken off of it. It appears the wire inside is fine. I'm going to wrap it with some electrical tape tomorrow.


The exhaust definitely has a leak. I was noticing again today that when just listening to it idle I can hear random momentary increases in noise coming from it. It seems to basically just match up with my rough idle.

I decided to just replace the basic ignition components since it was cheap, easy, and would rule out a lot of stuff. So I put on new plugs, wires, distributor cap, and rotor. No difference at all.


Any further advice?

ProGunOne

I had the same problem on my 1992 Cherokee 4.0. While running I manipulated the injector wire and plug to injector 1, even turning the injector in the seat, and could hear the miss go away at different stages of my manipulating the injector wires, plug and injector. I'm now certain either the plug, plug pin, or one or both wires is broken, occasionally making connection, fixing the miss. I ordered a set of EV1 plugs, $25 for all six, From KSuspension out of Nampa, Idaho. They have great stuff. https://www.ksuspensionfab.com/store...igtails.html#/ After snipping the wires, then soldering, and shrink sealing them, the miss is gone. I will be doing the other 5 tomorrow. I am NOT affiliated with KSuspension in any way, other than now being an extremely satisfied customer.

kaliko

My 1997 Jeep 4.0 has a rough idle but mainly noticeable when I’m drive. The needle bounces a little not to much though. When I let of the brake and give it not gas it creeps froward but feels like stalling. I have a code for miss fire in cylinder 1 and my check engine light is constantly on never turns off or flickers just keeps glowing. I cut a hole in my cat so what I noticed which isn’t normally there when I’m at a light is very faint popping noises from it like back fires. Just replaced spark plugs and ignition coil. Nothing changed really still having miss fires and rough idling. My evap canister is replaced and working good hoping that could do something. Anyone have suggestions just want this misfire to go away.

dzywicki

What kind of plugs are you using? I have a '98 and a '99. I made the mistake of putting something other than the recommended champion coppers. I kept getting a miss at idle after the engine warmed up. Triple checked and replaced just about everything in the ignition system and couldn't get rid of it. Went back to champion coppers and the miss went away. Wouldn't have believed it unless I went through it.