Ok, I just did my compression test on my 99' 4.0L. I have Alldata Pro, so I used thier testing methods. I have been getting a P0301 cyl 1 misfire code off and on. I have used Seafoam for the last 4 fillups and it is idling way better than when I bought it from my step father 3 months back. Replaced all of my ignition parts. I use 89 octane now. I think the Seafoam has had a lot to do with the idle. It was really bad a few months back like there were 3 cylinders not firing at all.

Now, here are my readings. I did each cylinder twice to make it more accurate. Alldata says to do 3 compression strokes. I did exactly that.

1- 135
2- 140
3- 140
4- 120
5- 150
6- 150

Why I get a lower reading on cyl 4, but a misfire on 1 is beyond me....

I did a 5 stroke compression also as they say in good old Haynes Maual. I got highter but avarage readings as with three compression stroke test.

1- 140
2- 160
3- 150
4- 130
5- 160
6- 160

I was thinking I had a bad valve in cyl 1, but it is within specs... Cyl 4 makes my worry. I think I will be yanking the head in the near future.

Any comments or suggestions?

 

Missfire could be bad injector bad spark plug or spart plug wire or bad connector on distributor cap. The way to check if the low compression is the piston or the valves is you squirt some oil in the piston if compression goes up then it's the piston if compression stays the same then it's the valves.

 

Actually, 1 and 4 concern me a little. You want to have all your cylinders within 10% of the highest. Since most of yours are at 160, that gives you a 16psi wiggle room. Both 1 and 4 are outside of that. 3 is 10psi down which is OK.

 

On Alldata the specs say it should be 120-150 PSI average. That is Chyryslers exact specs also, I called a friend in the service department at a Dodge dealer, his tech said 120-160 is what they call for. And the accepted MAXIMUM range of difference is 30PSI. Cyl 4 is right at that. 1 is almost there. I will do the oil test next.

 

Still getting intermittent crappy idle. What gets me is, why would it idle great one time, like crap the next??? Sensor? Sticking valves? Only code I ever get is P0301 cyl 1 misfire.

 

check for bad grounds or dirty connections. if its intermittent then its something dirty or loose

 

Has the engine ever overheated?

 

Not that I know of, and not for the 2 years my mother had it. Before then, that is a mystery. When she bought it it had a very minimal idle miss, not enough to complain about. It seems ober the last couple years she had it slowly got worse. It is way better now since I have had it in the last 3 months. New cap, rotor, wires, plugs, seafomed the intake, and a full can in the tank the last 4 fillups. That has greatly helped it smooth out. Yet it still has a miss sometimes more than others. Sometimes it idles so well I think I am in the wrong XJ... I think I am gonna get ready and yank that head off and replace it. I also did the paper to the tail pipe test and you can hear the rythmic miss, and it does suck the paper back to the pipe in almost a perfect rythm. I'm guessing vavle go bye-bye.

 

hmmm lets not think about replaceing a 70lb head just yet.. there are other " free checks " you can do...

1.) spark pulg gap and temp.. ez to do, make sher that you have the right pulgs with the gap spec.. this chould play a big role with a random miss

2.) intake, make sher theres nothing in the intake to stop or mess up the air going in.. may be silly to say but you never know..

3.) fuel injectors, theres 6 of them they dont all go bad at the same time. what ive been seeing is that they leak some times so fuel is going in at the worng times... or like on my sisers jeep her fuel lines were rusted out... and that was a train wreck

i wood try these things first befor takeing haf of the mortor apart..

aj

 

Plug gap is set at OEM, new plugs OEM Champions at that.

I have thought about the injectors too, possibly that cyl 1 had a bad injector.

My only concern is that cyl 1+4 have the lowest compression of the 6, not horribly low, but low.

 

how many miles do you have?

jeep 4.0 cylinder compression pressure

- pressure range 120-150 psi
- max. variation between cylinders 30 psi

( right outa the dealer book )

yes cyl 4 is not the greatest.. but your not have probs with that cyl?

for the po301.. look in to the injetor and get under and look at the lines form the tank all the way up..


anymore info to the back story?

 

they way to make sure your injector isnt bad is to swap the #1 and #2 injectors and see if the miss comes from the #2 cylinder.

Honestly i didn't want to say it and jump to the worst case but my old misfire problem sounds like your current issue. I overheated mine and about 6 months after that it started idleing different. I didn't worry about it at first. then it progressively got worse but every now and then it would idle fine. did a compression test and cylinder 1 had 90 psi while the rest were adveraging 150. So i removed the head and found cracks. one in between the two valve seats in cylinder 1 and another in cylinder 2. I told the machine shop to stop looking. The crack in the #2 just hasn't presented its problems yet.

One thing i did notice and i should have noticed before had was when i drained my coolant. There was a slight oil sheen in it.

So what you can do is removed some of your coolant and put it in a cup for a while and see if you can see evidence of oil.

But i removed and replaced the head and everything is good now

You could also do a wet compression test and a cylinder leak down test to try and narrow the issue down

 

I have Alldata Pro, it has the same specs as you posted 120-150 and a 30PSI difference is acceptable.

I have 137K miles.

I have had no codes at all from Cyl 4 at all, ever. I used to get a code for my o2 sensor, but replaced both of them, and never came back. I can clear the P0301 code, drive 50 miles, or 4-5 startups and the light stays off. Then I could start it up and it will come on within 30 seconds??? Crazy.

 

Isn't it a rule of thumb if you put a piece of paper to the tailpipe it's a bad exhaust valve if it sucks it to the tailpipe?

 

( warthog ) i see where your coming from.. it seems like there might two probs?

4cyl is not the greatest and the po301.. both the same? two differint probs? idk..

i think you should try this and see if it moves over... also the wet compression test and a cylinder leak down like warthog said..

let us know what happens