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Slight miss at idle- barely passing emissions

Thought I'd give an update. I did a compression test today. Engine had been driven about 3 hours before, so it was getting cool. My numbers were...

123
115
123
120
127
123

Pretty consistent, but they seem a bit on the low side.

I then took off the cap to inspect it and notice that I could wiggle the rotor shaft around by hand, and the button was scored a bit. I think that might be the cause of my intermittent miss so I'll try replacing the distributor.

Question though: based on those compression numbers, is the engine getting pretty tired to keep fixing, or does it have a lot of life left? I might test the number one cylinder hot this week and see how much higher it goes, then up all my numbers by that percent.
 
I don't believe those pressures are right based on other statements you have made here in this thread.

It is possible you did not have a fully charged battery cranking, or the gauge connection was not tight and was leaking enough to give such low readings. Or you have gasoline in the oil, see my prior post on testing the FPR nipple for a fuel leak.....

What weight oil are you using? How many miles on the engine.

120 PSI is the factory spec for the lowest allowed pressure before it needs rebuilding. So the 115 PSI reading says the engine needs replacing or rebuilding, but, you MPGs are not possible with all cylinders at that low a pressure, so I highly suspect a testing error.

You can squirt some motor oil through the spark plug holes and rerun the tests, and make sure the threads on the gauge are sealed and make sure the battery is fully charge. I'd pull all the spark plugs during the compression test to get the engine turned over faster during the test.

On the distributor, mine did that 2 weeks ago, The helical gear sheer pin let go that holds the gear to the shaft in a fixed position, I replaced it. It made a huge difference. There is an indexing procedure you might need to try on www.cruiser54.com for the dizzy, but ask him first if the 1995 needs the special procedure or not.

I have no doubt you found the main problem, bad distributor.


Good luck. Oh, factory spec on new 4.0 engines is 170 or 175 psi, IIRC

@dam said:
Thought I'd give an update. I did a compression test today. Engine had been driven about 3 hours before, so it was getting cool. My numbers were...

123
115
123
120
127
123

Pretty consistent, but they seem a bit on the low side.

I then took off the cap to inspect it and notice that I could wiggle the rotor shaft around by hand, and the button was scored a bit. I think that might be the cause of my intermittent miss so I'll try replacing the distributor.

Question though: based on those compression numbers, is the engine getting pretty tired to keep fixing, or does it have a lot of life left? I might test the number one cylinder hot this week and see how much higher it goes, then up all my numbers by that percent.
Click to expand...
 
Hmmm...remarkably consistent numbers though. I downloaded fuelly when I started this thread and have averaged 20.8 mpg for the past few months, and that's with the miss and running AC

232k on the engine, 10W-30. Oil was changed at 3k intervals for most of its life but the past 35k or so I've gone up to 5-6k intervals (mostly highway miles)

I found it difficult to get my compression tester fittings threaded into the holes, and they can only go so tight when you're tightening them by the hose. Suggestions? All plugs were removed, but I did forget to open the throttle when cranking, but cranked for quite a while. I bled the gauge down to 100 psi between cylinders instead of 0 psi to save some cranking.
 
Last edited:
Is there an O'ring seal on the hose fitting? Did you lube it with oil?

I agree it is not easy to turn them by the hose, but I got mine turned enough to read as high 150 psi. I too forgot to open the throttle body. I never looked for a tool to make it easier.

I'd change the Dizzy and see if there are still any issues. If it ain't broke don't fix it (the low compression numbers are easy to get if you do not get a good seal on the hose fitting)
 
There are O-rings, but I didn't lube them. "Finger tight" is pretty loose when twisting by the hose.

My kit came with some long tubes, which I assumed I could use as extensions for the fittings, but you can't. Poor design.

Also, I replaced the injectors with 4-hole injectors. Quicker starting and throttle response, but no other changes.
 
I replaced the distributor (also did plugs and wires) and it seemed to help quite a bit...perhaps 70% better. I was hoping for balance-a-nickel-on-it smoothness though, and it's definitely not there. So far I've replaced....

IAC Motor
Fuel injectors
plugs/wires/cap/rotor
Distributor

I suppose next I'll check fuel pressure. Throttle response is great with the new injectors though, and now it starts on the first crank, so I doubt it's that. Perhaps I'll swing by an emissions bay and see how much further I have to go.
 
@dam said:
I replaced the distributor (also did plugs and wires) and it seemed to help quite a bit...perhaps 70% better. I was hoping for balance-a-nickel-on-it smoothness though, and it's definitely not there.
Click to expand...

Its a straight 6 truck motor thats design was already 30yrs old by the time yours was made. Some things just arent going to happen
 
@dam said:
Vehicle: 1995 XJ Sport, 4.0, 5-spd, 230k miles
Gets low 20's on the highway and runs smooth
At idle, there's a slight miss and you can smell it's running rich.

That sounds like an O2 sensor problem!!!! Is it an OEM or NGK 4 wire? Get a meter out and test it!!!!

http://www.naxja.org/forum/showthread.php?t=1012701&highlight=oxygen+sensor+test
Anyway, the miss has been there for ages, and emissions get a little worse every year. We got it to squeak by by revving it up to 1100 rpm for the idle portion.

Squeek?

I replaced the cap, rotor, wires and plugs (Champion Truck). I also replaced the idle air control motor. None of that made a difference. The tech said the timing must be off, but I believe that's not adjustable on these, correct?

Actually the mechanical timing of the rotor location in the distributor can be off a tiny bit. Check out www.cruiser54.com 's web site on distributor indexing. Email or post a question to if the that fix can work on a 95 also. It sure the hell worked on mine!!! And I had a new distributor!!! Seems Jeep had a small mechanical Cam-Dizzy gear timing error on many Cam shafts.

Any other ideas? The coil is the only ignition part left to replace. Not sure how to adjust timing. Perhaps it's a fuel injector whose spray is uneven at low power settings. Any suggestions?
Click to expand...


I would check the new plugs after 200 + miles and use the online color photo charts to see if any single plug is showing sings of too much or too little fuel, or oil contamination.

One more thing, what is the idle rpm? Is stable at 750 rpm in park or does it drop a bit or go up a bit from there, and cycle up and down?
 
Last edited:
Green XJ Jeep said:
Its a straight 6 truck motor thats design was already 30yrs old by the time yours was made. Some things just arent going to happen
Click to expand...

Speak for yourself, :laugh3: LOL, it took 13 years and 70,000 miles, and endless hours here, but my 292,000 mile 87 beast is quite as a mouse now, LOL, and yes it was running :laugh3:

:cheers:
 
Well, after driving for a few days I think my initial assessment was wishful thinking. It's definitely not 70% better. I suppose the only thing left on the ignition side of the house is the CPS and the coil, and on the fuel side it's the FPR and MAP sensor.
 
XSC9Q8


FullSizeRender 11 by Adamandshawn, on Flickr
 
If you can weld it up, that should get you by for a while. Or just buy a replacement manifold from 1aauto or your local parts store

Sent from my SM-N910T using Tapatalk
 
I forgot to mention I'm at 6000' altitude. After correcting for altitude, my compression numbers are...
147
138
147
144
152
147

Not bad for a cold engine- it still has life left. I'm thinking that low one is probably a sticking valve, so perhaps I'll run some top end cleaner through it. The thing starts on half a crank with those new injectors and distributer, although mpg dropped 1 mpg, probably due to me resetting the adaptive controls in the computer, so hopefully that gets better.

I'm trying to decide between an aftermarket manifold and a used one. I found a used one on craigslist for only $30. I'm sure fitment would be much easier so I'll probably get that.

Question though: while I have everything apart, is the crankshaft position sensor easy to reach? I hear that's a real bear, so if it's easy to reach I'd like to pick one of those up before I dig in.
 
Make sure the used one is not cracked, and that the inlets are all parallel and seal on flat surface. They tend to warp, especially if a crack is welded!!!
 
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