Random missfire at idle

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Feb 5, 2014 | 07:25 AM

#16

Roler

CF Veteran

Joined: Jan 2012

Posts: 3,170

Likes: 312

From: Australia

Year: 1997 (RHD)

Model: Cherokee (XJ)

Engine: 4.0 ltr

Quote:

Originally Posted by cherokeeyed

..It will also stumble when I give it gas from a stop or if I totally get off the gas pedal and press it again. As soon as the rpms rise above idle rpm, it has no miss at all.

Next, with this, I would also check the TPS.
If you have a little 'dead' spot in the beginning of the throttle moving more open, it could 'stall', and then once past that point (as you state the rpm's rise above idle rpm), gives no further issues...

And fuel pressure is a relative easy check. Simple things first.
I like TJ's "low fruit" expression.

Feb 5, 2014 | 12:39 PM

#17

cherokeeyed

Thread Starter

Member

Joined: Oct 2012

Posts: 145

Likes: 0

From: Springfield, Mo

Model: Cherokee

Quote:

Originally Posted by JackJare9455

sorry.. i misread and jumped to conclusions.. I'm the ***

anyway.. what roler suggested is a great start... i was feeling throttle body and IAC as well... if this was my jeep.. this would be my start..

And since it is only at idle.. but runs great with throttle applied.. its a good place to start

As was mentioned earlier.. cap and rotor are a must..

So... good wire contacts.. good plugs with proper gap, quality cap and rotor..

a bottle of strong injector cleaner couldnt hurt either.. i like BG44K... but its harder to get a hold of.. seafoam, berryman b-12, red line, chevron techron, all great products to get your hands on at any parts store..

the 4.0 has a "choppy" idle in my experience anyways.. mine runs smooth as silk down the road.. but kind of burps here and there at idle.. you can feel it when your sitting in the drivers seat.. nothing like a "cam" though.. as you mentioned.. thats not normal

a weak motor mount can also give you the sensation of a misfire is some instances.. but then again... doesnt really apply to a "cam" idle

do you have a fuel pressure tester to rule out the fuel feed as a problem?

I do have a fuel pressure tester, I will get a reading on the fuel pressure here in a little bit. I will also pull the throttle body and clean it and all the sensors.

Feb 5, 2014 | 08:05 PM

#18

Roler

CF Veteran

Joined: Jan 2012

Posts: 3,170

Likes: 312

From: Australia

Year: 1997 (RHD)

Model: Cherokee (XJ)

Engine: 4.0 ltr

Clean all your grounds while you're in there, its good maintenance and little effort.
But don't forget to get your scan for codes. It's one of the first steps in trouble shouting and can be a lead in the right direction if you have a code coming up.
Let us know how you go.