99 XJ has Intermittent stumble

Hello all, this is my first post but I've been a member of the site for a little over a year. I have always found my answers through searching and reading the posts. Tons of great info...

I have a 99 sport with 140K miles. It's been running great until about a month ago. When I first start it up after it sits for a while, it runs great for about a minute or so then it stumbles and bucks or almost stalls. If I let off the throttle, and then hit it again after a second and it runs fine for the rest of the trip. It only happens after it sits for a while. It runs great through all RPMs for the rest of the trip. Even if I shut it down and run into a store, when I fire it back up it doesn't stumble at all.

After searching here:
Replaced TPS, O2 Sensor (upstream), plug wires, cap and rotor.
I checked all sensor connections on the throttle body and traced all vacuum lines - no leaks that I could tell. Oh, I did reset the computer by removing the battery terminal for 1/2 hour or so.

I noticed an improvement it my MPG and it seems a little more responsive but the stumble is still there. No change.

I just can't figure it out. Any Ideas?

I have read about an EGR valve that may cause rough idle and stumbling. I believe my 99 doesn't have this is this correct?

Thanks in advance,

tlilley

I reall don't know that much about the later XJs, as my MJ is an '87, so take my advice with a grain of salt. As I understand it, the later model XJs are not equipped with an EGR valve, so that would eliminate that as a problem. I'm wondering if you have tried cleaning the throttle body out? My MJ has been suffering from similar issues, and they've mostly dissappeared after I gave a cursory cleaning of the TB. It's certainly worth a shot, and as I understand, these 4.0s tend to be a bit finicky when it comes to TB cleanliness.

Hope this helps,

Pat

If you are not using top tier gas (QT, Shell, Shevron), I'd use an injector cleaner. Can't hurt, BG44 is my choice.
Did you try to see if there are any codes? I've had malfunctions that are stored and no light.
One thing I noticed is that a lot of people had to replace their CPS.
I would at least disconnect the CPS, spray it with WD-40 and reconnect it.
Sorry I can't help.

If you don't let up like you say, but give it more gas instead, do you get a backfire from the intake? Mine's been doing that for so long, I know within about 10 feet where on the road home from work it will happen.

I'm 99% sure that my MAP sensor is responsible for the same symptoms. If I unplug/plug it and fiddle with the wires it will work normally for a couple weeks before I notice it again.

The wire routing is pretty rough, with a 180 deg. bend right before the connector, so that's my primary suspect. Gonna splice in some new wire one of these days. Been running like this for years though.

I guess the next thing to try is the MAP sensor. I havn't been able to lock down the exact location of it or the CPS. I believe the MAP is on the throttle body, on the drivers side. I think it is the rectanglular sensor closest to the air box. I'm not sure though. Can anyone shade some light here for me?

I have a spare throttle body with all the sensors. I bought a spare a complete intake manifold from a 97 XJ at a junk yard. Before I start swapping sensors from the spare onto my throttle body, I'd like to be sure what I'm changing.

As far as cleaning the TB, what do you all recomend as far as a solvent. Soap, carb cleaner? I just don't know what to use.

As far as codes, I didn't see any codes listed when I had the obdII on it. I don't have a reader, I just paid my local mechanic to read it for me for $5.00. It didn't show anything that I can recall.

Any help would be great.

tlilley said:

I guess the next thing to try is the MAP sensor. I havn't been able to lock down the exact location of it or the CPS. I believe the MAP is on the throttle body, on the drivers side. I think it is the rectanglular sensor closest to the air box. I'm not sure though. Can anyone shade some light here for me?

Click to expand...

That's the one. It has a little rubber elbow from the bottom of the sensor that goes to a nipple on the TB.

As far as cleaning the TB, what do you all recomend as far as a solvent. Soap, carb cleaner? I just don't know what to use.

Click to expand...

Throttle Body Cleaner. On the shelf next to the carb cleaner. Carb Cleaner may be just fine, but I don't know if it's sensor safe or not. You may want to check the can for a notice of this. Also, Deep Creep, if you can find it, works too.

It's apparently a relatively common problem, and I don't think the symptoms always result from the same cause. There used to be a sticky on the subject over at JU ... although I've found it by accident before, I can't seem to find it now.

x2 on the map sensor. Clean your IAC sensor up a little to help with the stalling symptoms on start up.

Thanks for all the help guys!

I know what I'll be doing tomorow. I'll be removing my TB and scrubbing it down. I'll pick up some TB cleaner in the AM. Thanks for the confirmation on the MAP sensor location. That really helped.

How about the CPS. Is it difficult to get to?

the crank sensor is a pain, easier with 2 pair of hands and about 30 inches of socket extensions and wobble head. Mess with your TB first. Put some oil in your down the plunger on your IAC sensor when you take it apart.

make sure you orient the IAC in the same way it came out. Just try not to rotate the pintle at the end.
The CPS is located on the driver's side of the bellhousing, about 10-11 o' clock, looking from behind. It is a PITA top get at. If you take it off make sure you don't drop the bolts holding it down into the hole in the bellhousing. SOme grease in the socket usually helps hold them in there once you unscrew them.

Since you've covered most everything else, I'd venture to say that your fuel filter is plugged. Unfortunately, the fuel filter is integrated into the fuel pump, which is located in the fuel tank.

Check out this linky---> http://www.jeepforum.com/forum/showthread.php?t=138580&highlight=fuel

It'll show you what a plugged fuel filter looks like, and how to replace the entire fuel pump assembly...

tlilley said:

I have a 99 sport with 140K miles. It's been running great until about a month ago. When I first start it up after it sits for a while, it runs great for about a minute or so then it stumbles and bucks or almost stalls. If I let off the throttle, and then hit it again after a second and it runs fine for the rest of the trip. It only happens after it sits for a while. It runs great through all RPMs for the rest of the trip. Even if I shut it down and run into a store, when I fire it back up it doesn't stumble at all.

Click to expand...

Sounds like you've already started throwing parts at your jeep without really analyzing the problem.
You've already replaced the TPS but I could have told you that this wasn't the problem anyway. Your engine's symptoms just don't match those produced by a bad TPS. The same also applies to the MAP sensor and CPS so don't waste your money on them.
I will suggest a possible remedy that'll cost you nothing but time. Clean out the TB, IAC housing, and IAC motor thoroughly. A sticking IAC motor will cause the engine's idle speed to either drop too low or sometimes do the opposite.
If that doesn't work, I suggest you check the fuel pressure at the injector rail after the engine's been switched off for at least an hour. You could have a fuel delivery problem either because the filter's blocked as Ivan suggested, or the fuel pump check valve may be defective allowing fuel pressure to bleed off into the tank. Either way, you'll need to remove the fuel pump assembly to investigate but luckily it's mounted on the front of the tank so the removal procedure isn't too difficult.

Well, I took a few hours today and tore down my throttle body. I removed all the sensors and the IAC housing. I then gave everything metal a good cleaning with Throttle body cleaner. I used pipe cleaners for the real tight spots and Q-tips for other spots. Everything was nice and clean when I finished.

I put the TB back on with the same sensors and took it for a test drive. No change. I still have the stumble when it first starts from cold.

I replaced the MAP with the sensor from the spare TB and still no change. Dr. Dyno was correct. I was hoping for a fix that I could do but it's now looking like the fuel pump may be the problem after all.

I took a look at the link that Ivan was so nice to post and it looks like a fairly big job to rip the tank off just to get at the fuel pump.
Dr. Dyno; can I just leave the tank in place and get to the pump from the front of the tank or does it need to drop?

Also, How would I test the pressure on the fuel rail? Do I need a special guage? Would it pay me just to take it to a mechanic.

I would really like to fix this myself. I have already learned a ton about my jeep's engine management systems and would like to continue. I try to do most of my repairs myself like the front wheel bearing, front universals etc. In my experience, dealing with the tank is bad news! I worked for our local EMS as an EMT for 7 years and have been to several fire standbys because a guy was working on his tank and ended up torching his car, garage, entire house and almost himself in one instance. I guess my point is how difficult/dangerous is pulling the pump going to be?

I haven't done the fuel pump in an XJ personally, but I believe that it can be pulled with the tank in place, I think you just need to knock the collar the holds on the tank loose, and pull the whole assembley.

For checking fuel pressure, on my '87 there's a valve on the fuel rail that you can hook a pressure gauge up to. I've heard of some people on the board using the guages that are for AC with good results, but I haven't done it personally.

What I'd do is check the fuel pressure at the rail. if it's not where it's supposed to be, relive the pressure in the fuel system (I believe that you can do that by pulling the realy for the fuel pump and running the engine till it dies, but don't hold me to it), remove the the pump and strainers.

Only you can be the judge of whether it's within you capabilities. But in general if you use common sense (no open flame, no metal to metal contact when you hammer the collar loose, have a fire extinguisher close at hand and the like) and be careful, it isn't too bad of a job to do. If it's not something you comfortable doing there's no shame in taking it to a shop.

Hope this helps.

Pat

I just spent a few minutes lying under my jeep inspecting the tank. I will have to drop the tank to get to the fuel pump. The link that Ivan posted is a very accurate writeup. It looks to be what I'll be doing in the very near future.

Question:
I was digging around the post that Ivan left and it seems that many people have had this problem for a while, even "Years". I was just curious if this problem could be something that could strand me somewhere if I wait a while before replacing the pump?

Thanks

tlilley