View Full Version : '95 is missing at idle- fails emissions. New ignition parts

April 16th, 2012, 15:17
I have a 95 4.0 Sport with 202,000 miles.

Recently, it has a little stumble at idle. It doesn't feel like it's going to die or anything...just a little rough, and you can see the engine move. You can also here a little "poof" sound at the tailpipe about once per second or so, and smell a little richness in the exhaust.

I replaced the cap, rotor, and wires. I inspected and gapped the plugs (Bosch platinum with 15k on them- they look fine). The cap and rotor were DONE.

I take it back in and idle emissions of HC went from 360 to 325 (180= passing). At 2400 rpm, it passes like a champ. This leads me to believe maybe it's a clogged fuel injector, and once it's opened up and spraying the tiny bit of clogging isn't much of a problem, but at idle it isn't always spraying a uniform mixture and doesn't always ignite. I put a bottle and a half of techron in the tank, but I'm not sure what else to do. Any ideas?

April 17th, 2012, 00:32
Just a few ideas, I've had a rough idle because of a vacuum leak. Most of the vacuum connections on the intake are on one end or the other, If anything is leaking you can get more air to some cylinders than others and at the same time the O2 sensor is trying to adjust for the extra air. It can make for a rough idle and rich at idle. Tends to smooth out as the RPM's come up. Try plugging off all the vacuum outlets at the intake for a test.

Intake bolts can do the same thing, they come loose. Surer won't hurt to double check the torque on the bolts

If I fail emissions the first thing I do it replace the O2 sensor. O2 sensors go bad by degrees, it may run OK at 2400 but not at idle or red line.

Have you done a key test and checked for your MIL codes? I found a bad TPS that way, it was messing with my idle. It should test between 1 volt throttle closed and 4 volts throttle open. If it is way out it could affect your timing and this can make for a crappy idle.

Might be time for a compression test, just to rule out some sort of major internal mechanical problem.

I always make a run up the interstate for 20 plus miles and make sure the engine is warm for the test. Cleaning the soot out of the motor seems to help. A hot motor always seems to do better than a cool motor. I flunked one test because it was 15 degrees out and the motor wouldn't get up near 200 degrees. A piece of cardboard in front of the radiator and I passed.:)

April 17th, 2012, 03:31
Might or might not help but... Get those platinum plugs outta there and put in some regular champion resister plugs. I and others have tried the platinum route and they gave all sorts of stumbles and issues. The jeep 4.0 in the distributed style set up never ran and was not designed to run on platinum plugs.

April 17th, 2012, 09:25
Thanks for the tips. How do I key-test for codes?

April 17th, 2012, 23:05
MIL Diagnostic Mode
1) Start engine (if possible). Move transmission shift lever
through all positions, ending in Park. Turn A/C switch on and then off
(if equipped).
2) Turn engine off. Without starting engine again, turn
ignition on, off, on, off and on within 5 seconds. Record 2-digit
fault codes as displayed by flashing MIL.
3) For example, fault code 23 is displayed as flash, flash,
4-second pause, flash, flash, flash. After a slightly longer pause,
other codes stored are displayed in numerical order. When MIL begins
to flash fault codes, it cannot be stopped. Start over if count is
lost. Code 55 indicates end of fault code display.
4) Refer to FAULT CODES/MESSAGE table to translate trouble
code number

April 21st, 2012, 12:33
No fault codes- I just got the 55.

I did discover the rear CCV hose was loose. I secured it with gorilla tape, but it didn't seem to help.

April 21st, 2012, 14:52
Those Bosch Platinums looked fine, but I just threw in $2 Champion OEM plugs and it seems to be idling a bit smoother now. Not perfect...but a significant difference.

April 21st, 2012, 15:32
does the 95 still use an EGR valve? a leaking one could cause those symptoms.

April 21st, 2012, 16:10
No EGR. Thanks for the suggestion.