Hi All,
My friend and I did a complete tune-up on my 93 Cherokee sport,4.0,with only 332000+ miles on it,original engine and transmission.Occasionally it misfires after start-up,no matter if the engine is warm or cold.If I turn it off and restart it immediately it runs as if it just came out of the factory and doesn't misfire even once after the second start.Besides the usual tune-up stuff,we replaced the distributor including cap and rotor,the ignition coil,the map sensor and cleaned the O2 sensor.We also did some connector refreshing by unplugging almost every connector under the hood including the harness from the PCM and sprayed them with electrical contact cleaner.The problem still persists.Has anyone ever encountered anything similar?

 

Have you done a compression test, leakdown test, and a vacuum test? Worth doing to rule out the overall condition of the motor. If all checks out, you're left looking at bolt on parts.

 

You might have an injector getting stuck. Is there any codes coming up? can you pin point the cylinder that is miss firing?

 

I wouldn't clean o2 sensors. Maybe that's just me but they aren't made to be cleaned

 

Thank you all for the replies.The vacuum leak test checked out OK.We haven't done a compression test.Oil consumption is normal.The engine doesn't even smoke.I can still burn rubber,which is impressive all things considered.I tend to lean more towards a leaky injector as dans98xj suggested.I added fuel injector cleaner to the fuel.I'm waiting to see if that does the trick.

By the way,is it true those 4.0s can go as much as 400k miles or even more?What is the most mileage anyone has ever gotten out a 4.0?

 

You can pull the fuel rail with injectors installed. Prime the fuel pump a few times with the injectors on a towel. You should see leaking if they do.

You could also clamp the return line on your fuel lines going to the rail. If you lose pressure you know it's leaking out the injectors.

 

We will be doing the valve cover gasket within 2 weeks or so.We'll do the injector leak test at the same time as you described.Thanks for the suggestion.

 

The 4.0 can go plenty more I'm sure. Just like anything, take care of it and it will last a long time.

 

My '88 Cherokee with the 4-liter has 890,000 miles (that's miles, not kilometers). Trust me - that's a jawdropper. I bought it brand new on January 16, 1988. The engine was replaced at 599,000 miles. It still ran, but was losing a quart of oil every 150 miles. The rear seal had let go. I was told that if there was excessive play in the crankshaft, if would have pounded out a new seal in a matter of weeks. It's on its fourth clutch, third set of front brakes, fourth set of rear brakes, second muffler (the original lasted 26 years) and ninth or tenth windshield. The transfer case and rear axle are still untouched. It had transmission work (new third gear, new countershaft and new bearings) last year. The original synchronizers were still good. It has the Peugeot T-10 5-speed.


It's been as faithful as any spouse could ever be and it never argues or talks back.

 

The jeep runs then stars misfiring and shuts off after a 2000 to 98engine swap jeep Cherokee sport

 

I sure hope you have fixed your isyour know personally it is a doozie!

 

I sure hope you have fixed your issue I know personally it is a doozie!

 

Update: my '88 reached the million-mile mark on October 30, 2019. Seven zeroes on the odometer.

 

good morning,
my baby has 567K(orginal eng & tranny) on her and just started the misfire issue ..no codes and i changed everything you listed.
i watched a bunch of videos last night and now i am gonna try marvel mystery oil to see if that helps.
i will let you know ..

 

Had a similar misfire problem on a 1986 Dakota, 3.9L V6. One bad plug wire. Could also be a bad plug wire connection.

My 1996 XJ Sport has just over 371,000 miles right now. Recently replaced the valve cover gasket, other than thermostat and water pump, first time the inside of engine has been exposed since the factory.