Stumbling/backfiring through throttle body after warm/hot start up

98 XJ 4.0 103k.

Here's what it does: After it is shut down from being fully hot and restarted with in 15 to 30 minutes, I'm experiencing a pretty bad stumble the first time I try to accelarate. If I'm leaving a store or something and idling through the parking lot, it's smooth as can be as long as I give it little or no gas. As soon as I start pulling out on the main road, it will stumble, fall on it's face, almost die, backfire for a second or two, and then it take off normally. It will then be fine and run normally for the rest of the time until the next time I shut it down, and restart it within about 15 to 30 minutes.

It's been doing it pretty consistently for about 2 months now. I didn't change anything around the time it started happening. It doesn't do it when started up cold or when started within 5 minutes or so of shut down.

It got a new upstream O2 sensor in November, new plugs,wires, cap, and rotor last June/July. I've run a couple double doses of fuel system cleaner through it lately with no change.(B-12 Chemtool, 2 cans per fill-up)

I'm starting to lean towards it being something ignition related...

Anything I might be overlooking here?? I'm kind of stumped...

TIA

Chad

You may want to pull the throttle body off and clean it really good with some Carb and Throttle Body cleaner. All the black stuff comes off very easily with the cleaner and a small wire or plastic brush (tooth brush works good for this). Remove the Idle/air stepper motor (round sensor on throttle body, points to firewall) and clean the plunger and seat really good. Get a new seal or use a little gasket maker and re-install the throttle body on the manifold. Then check the vaccuum system for any leaks as well.

HTH's
Andy

There is a TSB on that. It's somthing to do with heat soak on restart. I would do Andys suggestion, then if it dosn't help check out you local dealer to remidy the problem. My 2k does it on real hot days if I have been running around town in slow traffic. You might give the cooling system a good flushing too.

Sounds like a clogged cat or a tps.

Well, I took the throttle body all apart tonight and cleaned all the components. I didn't drive it though, yet. I have some errands to run tomorrow that will have the Jeep sitting for about the right amount of time to see if the problem still occurs.

I checked the TPS and output voltage rises smoothly from 0.78v-closed to 3.79v-open. (Spec is 0.8v to 3.8v, so thats good) If the problem continues to occur tomorrow, I'll check it's output about 15-20 minutes after I get home while it's in a heat soak condition to see if I get any erratic readings.

It can't be a clogged cat because.... well... there's nothing in there to clog. ;-)

Scooby: I think that TSB only applies to 2000's and 2001's but I may check and see if there's anything like that for 97's or 98's.

Thanks for the help so far guys...

Chad

I don't know the years it covers I just remember reading about it. It stuck out in my mind 'cause mine does it once in a while. I haven't had it looked into it because it's not oftan enough to be a problem for me yet.

I am of the opinion that it sounds very ignition related from what your describing . What it sounds like is that on acceleration that there is too much fuel and either not enough spark or no spark in a cylinder . This sounds like a timing issue I could be off base on this but I have experianced this problem in a couple different vehicles I have owned and both times it was bad timing and bad plugs. Good luck!

My 98 does it every once in a while, after pulling in, shutting it down, touching base with a customer and starting it back up, maybe 15-25 minutes. I pull back out on to a main road and accelerate hard to get into traffic. Loss of power for about 3 seconds then it comes back on like gangbusters. Been doing it every once in a while and not consistantly. As long as it stays like this I'll hold off till warmer weather then do my yearly off the engine TB cleaning but I don't think thats going to help, it has done it for about 4 years now even after TB cleaning.
I know it's not the plugs, cap, wires, rotor because it has done it after a tune up. To compensate I now allow a bigger gap in traffic and don't take chances, don't want to get rear ended.

Well, I had it in the right 'parameters' a couple of times today and it never missed a beat. I'll see how it does tomorrow. Maybe cleaning the TB was the trick...

RichP said:

My 98 does it every once in a while, after pulling in, shutting it down, touching base with a customer and starting it back up, maybe 15-25 minutes. I pull back out on to a main road and accelerate hard to get into traffic. Loss of power for about 3 seconds then it comes back on like gangbusters. Been doing it every once in a while and not consistantly. As long as it stays like this I'll hold off till warmer weather then do my yearly off the engine TB cleaning but I don't think thats going to help, it has done it for about 4 years now even after TB cleaning.
I know it's not the plugs, cap, wires, rotor because it has done it after a tune up. To compensate I now allow a bigger gap in traffic and don't take chances, don't want to get rear ended.

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Same issue the other day with me. I thought maybe it was cause i was low on gas. hmmmm, i'll have to keep my eye out on this.

forgive me for the 4 year old thread bump, i am trying to find out what people have done to fix this...

cleaning the TB has not fixed it for me. as well as a whole list of new sensors and parts

anyone have any idea on this? after days of searching this is still about the closest thing to the problem i am having.

getblown5.9 said:

anyone have any idea on this? after days of searching this is still about the closest thing to the problem i am having.

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Check your Ignition coil. This is a standard failure for a Coil. When it heats up it will go bad, or weaken enough to give you trouble.
Swap it with a known good one, or a new one.

Tall Shadow :spin1:

Thank you, I will look into that next.

getblown5.9 said:

Thank you, I will look into that next.

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Year/engine?

mine is 96 4.0, just swapped in a 99 long block due to a bad cam. problem became apparent after the engine swap.

Just for grins and giggles, try opening your hood when you shut it down and see if the problem lessens.

are you hinting at a heat soak issue? i've been wondering about it myself. seems that when i sit and idle at a long red light or park it for a few then start up again, the under hood heat affects something.

getblown5.9 said:

are you hinting at a heat soak issue? i've been wondering about it myself. seems that when i sit and idle at a long red light or park it for a few then start up again, the under hood heat affects something.

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Yes, heat soak is what I'm thinking, although I don't believe the 96 was part of the TSB.

If opening the hood helps, and you have an efan, I would recommend adding a timer so the fan will continue to run for a short time after shutdown.

Here is a link on a timer: http://www.at-fairfax.com/P1786-ELK-960.htm