Rough Idle Problem

I have been trying to trace this idle problem for a while and cant find a solution. I am reposting about this problem because as time progresses i try different thing (after reading ppl replies and bc of searching), and i want to be able to show everything at once and get more suggestions.

The problem is my cold idle is wonderful its around 1k, its stable and smooth. As the jeep starts to warm up and it passes the first white temperature mark (about 150F) the idle becomes less about 700RPM and is very surgey and is far from smooth.

My initial thought was that the transition from closed cycle to warm cycle was triggering the bad idle from a bad sensor ? Well after that thought i changed the MAP,TPS, IAC, and O2 sensor and cleaned the throttle body. The warm idle did not improve at all. What i am wondeirng is what can cause a surgey rough warm idle ? Is it idleing too low warm, it idles fine cold (at around 900-1k)???

I have also changed my fuel filter, new fuel injectors ,checked fuel pressure and last month i got new plugs, and plug wires..Everything is almost new ??

Ground wires have been cleaned.
This is really starting to piss the crap out of me. I have spent way too much money and time and still havnt figured this out.

Pete
94 xj / 4.0 I6 / aw4 / 231 / d30 /8.25

I would hook up a vacuum gauge and see what the reading is at an idle. Watch to see if it varies. You are most likely correct about the transfer from open to closed loop.

Changing rpms can also be caused by poor grounds between block and the body.

I don't remember, is the O2 sensor heated on that model? If so, check the relay used to switch off and on the heater.

If it were an early model I would say EGR, but your year doesn't have an EGR IIRC.

good morning
pete

Well guys today got a can of carb spray hoping to find some vac leak that i didnt see before and i found nothing. In addition i tested the spark quality whihc seems fine. I disconnected the plug boot cables and had the spark ark on somethin metallic . The color was bluish and was able to extend a few mm above the boot. I even jolted my self somehow.

Disconnecting any of the spark plug wires made situations worse so i am not sure what that means.

Something to note was that i connected a fuel guage on the jeep again today and as the engine surged the gauges needle moved a bit.

I cleaned the wire from the distributor cap to the coil and nothing.

Pete

Is this something that strange ..No one has other opinions comments !! Please help...lol

Another thing i noticed today was that when i shift into nuetral (auto tranny) the RPMs drop very low and the jeep almost dies ?

Any ideas ?

pete

I hooked up a vacuum gauge today and at idle the needle fluctuates rather quickly from 15-17 in Hg. According to my chiltons manual this means a sticky lifter of ignition miss.

???
any ideas ?

pete

I'm not getting wether it's a cyclic RPM thing, or a stumble that kind of makes the motor do the rumba a bit.
Some things to look at, are a worn distributor shaft, if it has any side play, it makes the motor run rough. Almost like a miss, but a little more rythmic. Up and down play in the shaft, has never seemed to hurt much in my XJ's.
Most anytime I pick up a miss at idle the first thing I look at is the distributor cap. The inside gets a coating of oxidation which can cause arching. The gap between the rotor and the contacts, wears pretty fast. In my experience about twice as fast as the plug gap. Mine often work just fine with old plugs, but have problems, when the cap and rotor get more than 18 months old.
My XJ's have typically been sprark plug cable finicky. New doesn't really mean much, some work some don't. I've never had much trouble out of OEM, but have tried various sets of after market, that just didn't wrok well. My old OEM cables often worked better than the new cables.
Any vacuum leak has to be suspect. Some of the vacuum ports on the intake manifold are at one end of the manifold or the other and a leak causes a little more fresh air to one end of the motor or the other. Certainly can't help with a smooth idle and often causes the O2 sensor to hunt some for the right mix.
The two things that helped the most in stabilizing my idle (though it was actually many little things that just added up) was cleaning the connector at the TPS (mine got some oil in there somehow) and cleaning the connectors to the engine temp. sensor. A little squirt of a premium contact cleaner and a little scrubbing with a very fine nylon brush (used to clean between your teeth) and another squirt of clener, worked miracles on mine. It just took forever to stumble on the right one to clean.
I'm really proud of my idle now (in the 88), it is a little low, but thumps rather nicely. A partially plugged cat will also mess with your idle, I'm going through that right now with my 96.

Well today i replaced the distributor cap and rotor from dealer parts and the idle miss is still there. The IAC ,TPS and 02 are brand new units so i doubt the IAC or tps need any cleaning.

After changing the cap and rotor i also plugged in the vac gauge and it averaged 15 and was fluctauting.

pete

jeepsrock said:

Well today i replaced the distributor cap and rotor from dealer parts and the idle miss is still there. The IAC ,TPS and 02 are brand new units so i doubt the IAC or tps need any cleaning.

After changing the cap and rotor i also plugged in the vac gauge and it averaged 15 and was fluctauting.

pete

Click to expand...

Is the needle in the vacuum gage wagging quickly or just kind of going up and down some, kind of rythmicaly. A wildly wagging needle is a sign of a miss or valve trouble. A rythmic raising and lowering of the idle is often sensor, connector (bad values) or vacuum.
Mine had both going on at once, it had a miss and it was hunting at idle (kind of a cyclic, raising and lowering of the idle).
Try pugging off the vacumm outlets on the intake manifold and see if things change. Try replacing one spark plug cable at a time (with a known old good one) and see if there is any change.
Try squirting out some connectors with a good solvent.
I always look closely at the distributor cap, you can see the corrosion and the wear in the contacts and the tip of the rotor. I don't often swap things out, unless they are bad.
Many of the sensor voltages are pretty darned tiny and it doesn't take much resisitance in a connector or at a ground to mess with the values. Under nine volts and water stops being a good conductor and starts being a good resisitor. Any kind of spray oil is worse than water.
It took me a long time and a lot of trial and error to get a idle good. It was getting to be one of those anal things.
My 96 has a thing going on now, the idle isn't stable and it will occasionally stumble. I'm almost positive it's the cat. My vacuum falls of noticably, when I raise the RPM's from 2000 to 3000.
There is a bunch of stuff that can screw you up, the last one I found was corrosion (contacts) between the coil and the ignition control module (on my 88).
Personnaly, I haven't changed half a dozen sensors, on the XJ's I've owned with a total of near thirty (combined) years of driving them and a combined age of like 47 years for the three. I usually find the problem someplace else.

Some other problems that can cause idle problems.
A stretched timing chain.
A warn cam lobe.
A bent push rod.
A sticking valve or lifter (usually knocks)
Low cylinder compression.
A plugged cat or restricted exhaust.
Excessive blowby.

The money that you threw at this might of been better spent at a diagnostic center.

I have linked a recorded video file here of my cold idle with a vac gauge, maybe that will help- it seems my cold idle isnt so great either.

http://www.filefactory.com/get/f.php?f=792f7908d98eabff636b6923

And yes midnight krawler, i am aware of those things- i am hoping its something simple it really doesnt have that many miles yet.

pete

good morning...have a look at the above linked file, maybe you can make some suggestions or comments.

pete

Here is an problem update.

In addition to my vacuum being all over the place weather its warm or a cold idle (see video file above.)

I have rechecked my fuel pressure and its seems to wag around just like the vac gauge. It hovers around the correct values but it moves and fluctuates.

Something else i noticed was that their is a pronounced hissing noise coming from the fuel rail next to where the rubber fuel lines enter the rail. I cant find exactly where its coming from ? Their seems to be no fuel leeking anywhere , and additionally i checked the fuel regulator vac line and it has no leaks.

The next problem is that sometimes as i stop at a light or sign the rpm head pretty low and then go back up. I have noticed this also happens when i shift into park and nuetral the RPMs dip and go back up.

Any ideas would be helpful, i am just trying to cover all bases before i shoot the thing.

pete

hey there-

Just a thought, I am guessing that you replaced the o-rings along with the fuel injectors? I wonder if that hissing you hear is vacuum sucking past one of the fuel injector o-rings... did you lube them at all before inserting them into the intake manifold? Another prossibility (which happened to me) could be a bad fuel injector...#3 did not properly close, which caused my 90 to idle roughly, and my exhaust ran somewhat rich, which trashed the cc in short order....I hope you find it, can be most frustrating, I know..... You might check your plugs again to see what kind of burn pattern you are seeing.....I replaced my plugs simultaneously with the injectors, which obscured the problem for nearly a couple months......are you setting any codes at all??? I know this is a bit random, just throwing some ideas out for ya....maybe I can learn something here too.....P.S. does the fuel regulator actually HOLD vacuum?

Bret S Gray
1990 XJ
1992 Comanche

jeepsrock said:

Here is an problem update.

In addition to my vacuum being all over the place weather its warm or a cold idle (see video file above.)

I have rechecked my fuel pressure and its seems to wag around just like the vac gauge. It hovers around the correct values but it moves and fluctuates.

Something else i noticed was that their is a pronounced hissing noise coming from the fuel rail next to where the rubber fuel lines enter the rail. I cant find exactly where its coming from ? Their seems to be no fuel leeking anywhere , and additionally i checked the fuel regulator vac line and it has no leaks.

The next problem is that sometimes as i stop at a light or sign the rpm head pretty low and then go back up. I have noticed this also happens when i shift into park and nuetral the RPMs dip and go back up.

Any ideas would be helpful, i am just trying to cover all bases before i shoot the thing.

pete

Click to expand...

The reason your fuel pressure is "wagging" around is because your vacuum is "wagging" around. Vacuum affects the pressure. That's why there's a vacuum line connected to it. The greater the vacuum you have the lower the pressure will be. The fuel pressure should be steady at around 39 psi with the vacuum line disconnected from the regulator.
Like mentioned above. I would check spark plugs for signs of rich/lean burn. Maybe one or some of your injectors are plugged.

Kyung

jeepsrock said:

good morning
pete

Click to expand...

Good morning pete.

How are you today?

Just a thought is your vacuum canister still connected? If so I think I would do a compression check next.

I am ok Rticul8, just pissed off at this damn problem, if its not one thing its another.

Actually two months ago the canister was replaced due to it having a crack.

pete

Someone else has posted a similar suggestion already, but I have not read that you tried this; so.... disconnect and plug ALL vacuum connections on the intake manifold with the exception of the one that goes to the fuel regulator....
Check your vacuum to see if it is steady.... IF it is ... then reconnect one of the devices that has vacuum connected to it and see if the vacuum fluctuates...
I have heard of vacuum booster diaphrams leaking which will affect braking, engine idle and vacuum readings...

Charles

Is there any chance the vacuum line going to the MAP sensor is cracked, chaffed or broken with a small leak?