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Help diagnose

marcusguy

NAXJA Forum User
Location
Jing Zhou, China
My 99 with 67,000 has a stumble at idle. It's bad enough that when the car is idleing in drive without the brakes on it will surge a little. It's worse in the heat and worse when the A/C is on in the heat. I keep getting CHECK ENGINE lights, but it is brief and is gone by the time I check the codes.

The confusing part: Every week or two, my cherokee freaks out when I try to start it. It will start, but the engine sounds really strange and it dies as soon as I let off the throttle. The weird part is that the engine is amazingly smooth, as is the exhaust note. The sound and smoothness reminds me of my brothers rotary engine in his RX-7. The exhaust smells pretty strong of gas. I usually have to reset the computer and let the car sit for a while, then it will start up and run like normal.

Any ideas?

Thanks,
marcus
 
I was going to suggest CPS, but when I got to the part about the strong smell of gas I changed that to oxygen sensor. The O2 sensor should be changed at approximately 80,000 miles, but that's like saying a light bulb is good for 2,000 hours -- it's an average, and some will go much longer while some burn out in a week.

Try the O2 sensor. The '99 has two -- the one that controls the fuel-air ratio for the engine is the one forward (upstream) of the catalytic converter. The one after the cat is just to check that the cat is working.
 
Thanks Eagle. Is there a way to test the sensor before I change it? I think I read that there isn't but I honestly can't remember, that could have been about another sensor? With the two on the newer models, does one usually fail before the other... at least I could start there and see if it solves my problem.

Thanks again,
marcus
 
If the O2 doesn't fix it, the TPS is next in line.
 
I don't know if this will help narrow it down, but I also have had a significant drop in gas mileage since these things started. Before, I would get around 20 on the fwy, now I get 14-15. Still sound like O2?

Any link with tips on the replacement that might be helpful? I hate making mistakes that I could avoid with a little reading.

Marcus
 
marcusguy said:
I don't know if this will help narrow it down, but I also have had a significant drop in gas mileage since these things started. Before, I would get around 20 on the fwy, now I get 14-15. Still sound like O2?

Any link with tips on the replacement that might be helpful? I hate making mistakes that I could avoid with a little reading.

Marcus
How's the temperature guage read? If it seems to climb up to normal temps (about 190 on the dial) slower than before (or doesn't get as high), you might have a funny thermostat or coolant temp sensor.

Either of these could cause the vehicle to stay in open-loop mode (also known as warmup mode) too long, which means the engine is running a richer fuel-air mix than it would at normal operating temperatures. That could be contributing to the drop in mileage, and if not corrected, can be detrimental to the catalytic converter and O2 sensors.

Might be worth checking these as well, especially the thermostat if you've never had it replaced before. I don't have my manual handy, so I can't offer too much guidance on replacing them, other than the following: If you don't have a filler neck and pressure cap on your radiator (I think '97 was when they switched back to the closed system, but I'm not sure), getting all of the air out of the cooling system is going to be very tricky, but absolutely doable.

HTH,

Rob
 
I actually just replaced my thermostat because it was runing a little warm (like 210 on a 80-85* day, while moving). I think it still has the stumble at idle, but I will watch the gas mileage and see what happens. Thanks for that tip, I wouldn't have thought of the cooling system as a possible cause. Thanks for all the help guys.

marcus
 
mine has similar problems...I just changed the o2 sensor cause it was runnin way rich, and the plugs but it still has a very rough idle. Just put on a new stainless exhaust manifold, and thought that it would help as well (old one cracked as usual) Otherwise it runs good at any rpm over about 500
Buy tps do you mean throttle body sensor? if so how do I check or change that? Mine is an 88 with 237k on the ol ticker..mabye I should just be happy it even runs.
 
Well, I replaced the pre-cat sensor since I heard that is the one that makes the biggest difference. I heard the sensor after that cat just monitors but the pre-cat one will decide the air/fuel mix and things like that.

I've gone through 2 tanks already and the best I've done so far is 16mpg. What the next guess?

Marcus

BTW-I've noticed that in along with the occasional stumble at idle, the engine will bog and surge a little with the thottle just barely pressed. Like when I'm cruising through a parking lot and I just touch the throttle. If I push on it, there is no problem, only if I keep my foot barely on it.
 
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marcusguy said:
I actually just replaced my thermostat because it was runing a little warm (like 210 on a 80-85* day, while moving). I think it still has the stumble at idle, but I will watch the gas mileage and see what happens. Thanks for that tip, I wouldn't have thought of the cooling system as a possible cause. Thanks for all the help guys.

marcus
Just noticed this post, or I would have responded to it sooner.

Normal operating temperature for a 4.0 is 210 - they like to run nice and warm. I mis-stated the temp gauge reading - it's supposed to sit right at the 210 mark if everything's working properly. I think I accidentally quoted the thermostat temp as the proper gauge reading. (sorry 'bout that)

What's the temp rating of the new t-stat? If it's lower than 190-195, you're running the engine too cold. I ran a 180 for a while, and when I did go back to the 195 I think I noticed a slight improvement in mileage. General consensus is that the thermostat and gasket from the dealer are the best ones to get.

If the drop in mpg appeared after the t-stat change, it's reasonable to assume that the two are connected. Additionally, when a t-stat fails, it tends to fail in the open position (safer for the engine). Folks have reported occasionally seeing t-stats fail that were either new or had only been installed a short time, so there's also the chance that your new thermostat may have been defective. Fortunately, thermostats aren't that expensive, and are fairly easy to replace on the 4.0L.

Rob
 
I put in a 180 because my temperature was staying over 210 most of the time, even when cruising on the fwy at 70 on a 55* night. Even with the 180* in it still hangs between 200-210* all the time, creaping over 210 when I slow down. I think I left some air in there when I changed it that I need to purge. I changed the t-stat because the low mileage problem, to see what difference it would make. No change in mileage since the t-stat.

Would the t-stat really make a 25% difference in mileage? I would always get at least 19 mpg if not 20 even with lots of mixed driving. When I prerun the MDR desert race course, I would drive about 120 miles to get there, prerun HARD for about 60-70 miles, off road about another 20 miles while watching the race and looking for camping, and drive home and still manage 17-18 mpg. Coming home from monterrey ca. last year, I pulled 23mpg cruising at 70mph the whole way. This is a huge drop for my jeep. All numbers are from my GPS BTW.

marcus
 
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I don't think that the 180 would account for that large a drop, even if it got stuck in the fully open position.

If your temps are rising as you slow down, that's an indicator that the fan clutch is going, but that wouldn't account for the mpg drop either.

Have you replaced the coolant? I've heard it said that the green stuff is only good for two or three years. Unpurged air would contribute to warmer running. Also, if the fins on the radiator are clogged, that would also reduce cooling efficiency.

I can only think of two other things to suggest checking for the mpg drop:

1. Air filter - is it clogged? When was last cleaning/replacement?

2. Brakes. Is it possible that one or more of the brakes are dragging?

Wish I could be more help.

Rob
 
Thanks for the ideas Rob, I'll check into those. The coolant was replaced last year and still looks pretty healthy to me.

I got another check engine light on the way home today, and this time it stayed on instead of flickering on and off like before, or disappearing when I shut the jeep down. I tried the directions for newer jeeps and I'm not sure what I found.

I heard that if you hold the trip reset and turn the key on it will do some kind of check. But this just checks the gauges on mine. But I did find that with the key in ON if I push the trip reset and immediately turn the key off (before it resets the trip odo) 2.3 would show up on the odometer for a few seconds after everything else is turn off. What does this mean? Could this mean code 23? That codes says there is a problem with the air temperature sensor. Ideas?

Thanks

marcus
 
I'm afraid I can't help with the codes - My rig's a '92, so I can do the ignition switch trick and get the codes blinked out through the Check Engine light. Other than having someone with an OBD-II scan tool pull a dump on your ECU, I don't know how to get in to check fault codes on a '99.

You can take a look around the allpar.com site and see if you can find any tricks there to do it.

From what I've heard around here, codes (at least on the OBD-1 models like mine) usually stay in the memory for five on-run-off cycles, so you have a small window of opportunity to get it to a code reader.

Got any friends who work at Chrysler dealers or major parts chains? Autozone has been known to read codes for people, but that probably varies from store to store.

HTH,

Rob
 
So I replaced my first o2 sensor and nothing changed. Should I replace the second? I had heard that the first is the one that makes the difference, but if the second is bad will it give me poor mileage? Any more ideas would be great.

marcus
 
I would definately replace the TPS (trottle position sensor). My XJ did the same thing. Smelly black smoke from the exhaust, rough idle along with stalling especially at light trottle, poor gas milage you name it. I went directly to the stealer ship and the old XJ went back to the her purring kitten self. It is easy to change with just a torx bit.
My $0.02.
 
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