hesitations and backfires on my 99

I've had this problem for the last year and a half or so and can't seem to figure out what is going on.

When things are warm I'm having a problem hesitating and then a back fire. It only does it on warm days and only after the jeep has warmed up to temp. When it does it, it feels like I'm not getting any gas, it just kind of bogs down, and unless you let off the gas it just continues this way until it back fires, and then everything is fine.

Anyone have any ideas on this one...It's doing it on my 99

Thanks for the help

Matt

is it happening at highway speeds or just in town driving....can you make it duplicate the problem when you want it to?? Is it when you get on the gas to accelerate past someone or do you just let off the gas to coast then press it to accelerate then have the problem???

Guess is a matter of failing sensor

How old is the oxygen sensor?

Both oxygen sensors are original...so that makes them something around 4 years old or so.

It only happens around town....more specifically when I'm pulling into traffic, out of the driveway etc. Basically it only does it just after I've started up, and am giving some gas through 1st gear or so at about 15-25 MPH

Matt

My '98 does that occasionally-

We are gonna have to keep on top of this one cause i experience this very occasionally myself. This kind of problem is the type that can be so very hard to properly diagnose in my experience because could be so many things and is only intermittent. If you do happen to find the reason please let us all, if not me by email, know due to it's potential difficulties!! Thanks-

Oh, by the way, St Joseph, isn't that where is super hot here in Ut?? Well, does not seem to be heat or altitude related necessarily because i experienced this in Va at 1200 altitude early this spring when still cool, as well as here in Ogden Ut in real heat and 4500 altitude.

Yeah, St. George is hotter than Hell in the summer...Already made it up to 109 and we're not even through June yet. That's why I can't wait to get back to Summit county where the mountains are tall and cool, that's where I belong.

For sure I'll keep things posted when and if I figure this one out. Like I said in the original post it's been going on for about a year and a half now, and it has me stumped. I do think it has something to do with the heat as I can't seem to remember it ever doing it in the winter (or when I was living up North). I flushed the radiator a couple of weeks ago and I'm pretty sure I haven't had any problems as of yet. I'm wondering if the heat from a hot running engine was giving me some sort of vapor lock. I think this also because it usually does it after I've driven the Jeep, parked it and then got back in a little while later while it's still pretty warm.

I've checked all the computer codes and everything is coming up normal, but I'm still thinking it might be worth it to replace my 02 sensors and see what happens.

If I figure it out I'll let you know, or if anyone else out there has an idea I'm open to suggestions.

Matt

First of all....

Matt,

Sorry for mis-stating the city, i meant St George.... Excellent translation. Have St. Joseph on the mind because i stayed there on the trip over from Va 1.5 weeks ago. Is a particularly good stop and try to schedule a stop there every time.

As far as the hesitation, don't think is a vapor lock thing, unless is resulting from a getting-weak pump. Vapor lock is virtually a thing of the past since the higher pressures of fuel injection. A simple test gauge connection to the fuel rail would diagnose a problem there. Don't think could be the O2 sensor either. Does not usually result in that type of drivability problem. Also, if you do replace the pre cat sensor, do not get a Bosch or the such. Have had real problems from them. Only OEM are good enough for that job and they aren't cheap!! Might end up with more problems with the aftermarket O2 than had before-

With the OBD II system, if you have not seen a ck engine lt there will be no stored codes. It has never come on with mine. Sometimes i wished it would!! Might lead me in a direction. My mind is definately thinking on this one-

My 98 XJ did this right after I bought it at 48K miles, and the Stealership had it 3 times before they figured it out. Finally it was ruled that the heater in the O2 sensor (front) was bad. The computer only checks the exhaust levels 2 minutes after starting and then 2 minute intervals from there on. Mine would hesitate, backfire and do all kinds of crazy stuff, then all of a sudden it was perfect. Drove us nuts!

Judd, I've heard the same thing about bosch...thankfully I can get OEM jeep parts at wholesale through work so things won't be absolutely rediculous, but they're still pricey. When you replaced them did you replace both or just the one infront of the Cat?

XJdaddy, you said that replacing the 02 fixed your problems? What's funny is that the more I think about it, it started at about 50K on mine. When you replaced it, which one did you end up replacing?

Matt

Have to think about this one...

Daddy, that is way good you seem to have discovered the problem!! This Forum is definately an excellent resource!! I am not familiar with symptoms resulting from a malfunctioning O2 sensor heater....only the sensor output directly. Most interesting. Requires more thought on my part to come to grips with this one. I have not replaced either on mine that just turned 80k miles today. Does not often act up- actually not once in the last 10k miles. Would think it would have to do with the primary sensor- the rear is used only to determine cat efficiency, not normally anything having to do with drivability issues. I will rem this one!! Thanks big time!!