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Slight intermittent miss at idle and low RPM #951557 06/28/09 11:07 PM
Joined: Aug 2003
Posts: 712
john_luttrell Offline OP
Rock Warrior
Hey guys, I've got a question to run by ya on my 1991 YJ. I completely rebuilt the engine about 10k miles ago and it has purred like a kitten until the last month or so. I have checked all the usual tune up items and it all still looks new.

I next went and completely disassembled the throttle body and cleaned it and the IAC valve real good; throttle response is much better and it runs better, but I still detect a slight miss at idle intermittently. I have checked fuel pressure in the fuel rail and it is with-in specs; I replaced all the fuel injectors when I rebuilt the engine, so I don't suspect any of them to be the problem either.

I have gone through and retorqued all the intake/exhaust bolts to make sure they had not backed off on me; all were still good and tight. Have checked all the vacuum hoses, even though I replaced all of them along with the PCV lines when I did the engine. I have also cleaned and inspected all common grounds, connectors and harnesses for any problems and came up empty handed there also.

Today I replaced the ignition coil on it; even though the primary resistance check was dead on by the book. It's about the only part I didn't change on the jeep when I rebuilt it, so there is a good chance it's the original coil anyway. I was not too surprised when I noticed that the slight intermittent miss is still with me.

The jeep still runs really good and there is no fear of it leaving me stranded anywhere; it's just one of those gear head deals that I know there is something just not quite right and can't find it so far. I'd appreciate any ideas you may have on anything I may have over looked.

I also should mention that it sounds more like the IAC valve is causing the idle to drop for a second than an ignition type of miss. Anyone have a technique to trouble shoot the IAC Valve on this thing? As I mentioned above, I have already removed and cleaned the IAC valve and it was very carboned up, but cleaned up nicely. Thx.........

Note:
- There is no smoke from the exhaust when it happens
- The check engine light does not come on and there is no stored codes in the 'puter

Last edited by john_luttrell; 06/28/09 11:13 PM.

John Luttrell
2001 4x4 short cab, 3 inch lift with 33 inch mudders and Powertrax no-slip in the rear


1991 Jeep YJ, 4 inch suspension lift on 33" BFG's
From the Off-Road World
Re: Slight intermittent miss at idle and low RPM [Re: john_luttrell] #951558 06/29/09 01:27 PM
Joined: Sep 2005
Posts: 1,087
BobRowe Offline
Body Damage is Cool
John, in reading all that you have done in tuning and troubleshooting your YJ, it sounds like you might have some contaminant in your fuel that is causing an intermittent problem.

I'd suggest checking and/or replacing all your fuel filters. You also might have a slight amount of water from condensation in your fuel tank. You could try dumping a couple of the small bottles of "dry gas" in the tank and see how it runs after the present tankful has been used and refilled.


1977 CJ-7, fiberglass body, AMC 360 w/ headers, DUI ignition, Edelbrock intake and Holley 4150 carb, TF999, Dana 300, 4.56 gears lockers, York air comp, 4" susp lift, 2" body lift, BFG 35" M-T tires, Megashifter, AGR pump & box, REP8000 winch.
Re: Slight intermittent miss at idle and low RPM [Re: BobRowe] #951559 06/29/09 01:45 PM
Joined: Aug 2003
Posts: 712
john_luttrell Offline OP
Rock Warrior
Thanks Bob, it's possible as it doesn't get driven very much at all; I'll change the filter and add some dry gas to it. I'll report back with the results on it.


John Luttrell
2001 4x4 short cab, 3 inch lift with 33 inch mudders and Powertrax no-slip in the rear


1991 Jeep YJ, 4 inch suspension lift on 33" BFG's
Re: Slight intermittent miss at idle and low RPM [Re: BobRowe] #951560 06/29/09 01:46 PM
Joined: May 2009
Posts: 306
M
MartySoCal Offline
Mudrunner
*****
I'd suspect a spark plug before the IAC or a fuel filter. What brand of plugs did you use? Cheap "Bosch" Platinums usually fail at around 10k miles, the center "platinum" tip falls out and they cause a misfire, in my experience! Regular cheap Champions work very well in a 4.0l! I'd stay away from "Trick" plugs in any event! A diagnostic Scope check will tell you right away if it's a plug ot other ignition system problem.

You can eliminate the IAC by simply unplugging it, it will set a code, but it won't be able to move causing any surge.

A bad filter would show up under a heavy load, or with low fuel pressure.

Note: Normal slight fluctuations or change in exhaust tone will occur as the PCM varies the fuel pulse in response to the O2 sensor reading. The PCM wil drive the pulsewidth past the .5 volt switching point, a little rich to heat the cat, a little lean to give the Cat O2. This is normal.

Lastly, Keep that original coil in your console or glovebox as a trail spare, it's the most likely part to fail on you!

Re: Slight intermittent miss at idle and low RPM [Re: MartySoCal] #951561 06/30/09 12:42 AM
Joined: Aug 2003
Posts: 712
john_luttrell Offline OP
Rock Warrior
Ok guys, I may be getting somewhere now. I was driving it into town to get some fuel additive in case is was water in the fuel tank, when she started bucking really bad and almost dies on me. The good news is, that is exactly how a bad TPS switch will act and the check engine light came on for the first time since this all started. When I got her home, I was able to pull a single code from the computer; it was code 24, for high TPS voltage.

Tomorrow I'll be picking up a new TPS switch for her and see what happens.


John Luttrell
2001 4x4 short cab, 3 inch lift with 33 inch mudders and Powertrax no-slip in the rear


1991 Jeep YJ, 4 inch suspension lift on 33" BFG's
Re: Slight intermittent miss at idle and low RPM [Re: john_luttrell] #951562 06/30/09 08:09 AM
Joined: Sep 2005
Posts: 1,087
BobRowe Offline
Body Damage is Cool
I'm glad it crapped out long enough for you to troubleshoot it. Intermittent problems are a bear!


1977 CJ-7, fiberglass body, AMC 360 w/ headers, DUI ignition, Edelbrock intake and Holley 4150 carb, TF999, Dana 300, 4.56 gears lockers, York air comp, 4" susp lift, 2" body lift, BFG 35" M-T tires, Megashifter, AGR pump & box, REP8000 winch.
Re: Slight intermittent miss at idle and low RPM [Re: BobRowe] #951563 07/01/09 02:42 AM
Joined: Aug 2003
Posts: 712
john_luttrell Offline OP
Rock Warrior
I was able to "order" that TPS today and it should arrive at the parts store tomorrow morning by pony express; the joys of living in the sticks.

Well, I found myself caught up on projects and with out the TPS switch on hand, I decided to tear into the tilt steering column, to apply some loc tight and retorque the small bolts that will loosen up on ya every 10k miles. Some things never change, like the joy of disassembling a tilt steering column on a 100 degree day.

Tomorrow evening, I'll let y'all know if the TPS gets her back up and running good again.


John Luttrell
2001 4x4 short cab, 3 inch lift with 33 inch mudders and Powertrax no-slip in the rear


1991 Jeep YJ, 4 inch suspension lift on 33" BFG's
Re: Slight intermittent miss at idle and low RPM [Re: john_luttrell] #951564 07/01/09 09:26 PM
Joined: Aug 2003
Posts: 712
john_luttrell Offline OP
Rock Warrior
Fixed!! Installed the new TPS and she runs baby butt smooth again. I can't believe it took it over a month to finally set a trouble code, but I'm glad it finally did. Thanks for all the ideals guys, I was about to pull my hair out on this one.


John Luttrell
2001 4x4 short cab, 3 inch lift with 33 inch mudders and Powertrax no-slip in the rear


1991 Jeep YJ, 4 inch suspension lift on 33" BFG's
Re: Slight intermittent miss at idle and low RPM [Re: john_luttrell] #951565 07/01/09 11:30 PM
Joined: Sep 2005
Posts: 1,087
BobRowe Offline
Body Damage is Cool
Quote
I was about to pull my hair out on this one.


Don't do that! It will ome out soon enough by itself! <img src="/forums/images/graemlins/lol.gif" alt="" />


1977 CJ-7, fiberglass body, AMC 360 w/ headers, DUI ignition, Edelbrock intake and Holley 4150 carb, TF999, Dana 300, 4.56 gears lockers, York air comp, 4" susp lift, 2" body lift, BFG 35" M-T tires, Megashifter, AGR pump & box, REP8000 winch.
Re: Slight intermittent miss at idle and low RPM [Re: BobRowe] #951566 07/02/09 03:27 AM
Joined: Feb 2000
Posts: 6,246
4x4Wire Offline
Trail Leader
***
Quote
It will ome out soon


Is it true the hair follicles are connected to the fingers??

If it was meant to be a "c", perhaps there is an imbalance???

<img src="/forums/images/graemlins/lol.gif" alt="" />


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