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99 XJ 4.0: Power dip at 2300-2700 rpm

M

mdmbkr

NAXJA Forum User
Location
Pembroke Pines, FL
Hi,

My 99 XJ has 225k miles. It makes great power all through the power band, except when I floor it, there's a weak spot around 2500 rpm. It feels as though I lifted the accelerator about halfway and then floored it again. Above the flat spot it pulls hard to redline.

Other potentially related symptoms: For about 30 seconds after startup, the motor wants to die and makes little power. If I give it enough throttle, it will "snap" to life (you can feel a little kick in the pedal) and run fine afterward. This is most pronounced when the motor is hot soaked. Also I can occasionally smell overly-rich gas fumes when I drive around - not all the time though.

The whole Jeep is stock. No codes. Plugs/wires and TPS are pretty new; air filter is clean. MAP and coolant temp sensors test good. TB recently thoroughly cleaned. Cracked exhaust manifold.

At this point I sort of suspect the O2 sensors are not working right, but the fact that there are no codes makes me hesitant to spend the $120 for two new sensors that I may not need. Any advice?
 
mdmbkr said:
..... Cracked exhaust manifold......
Click to expand...

Start with obvious. Fix it. A leaky manifold will cause outside air to enter the exhaust and the O2 sensors to output incorrectly. Test the TPS for flat spots. Test the Coolant Temperature Sensor (CTS) for slow response or incorrect output readings

O2 sensors, TPS, and CTS are all normal maintenance items. They are replaced when they get lazy to respond or fail. O2 Sensor effective service life is about 100 - 150,000 miles.
 
Thanks for the input. I have tested the CTS, the MAP, and the TPS: they are all good.

I agree that it would be nice to replace the manifold. However, the leak is small (mainly manifests as a ticking sound that disappears once it's warmed up). I don't think it's contributing to the problem, although I could be wrong.

I've seen other posters with similar issues but none have ever seemed to arrive at a definitive conclusion.

To be completely clear, it's not a total power loss. It's most obvious at WOT. It feels the same as momentarily lifting the accelerator about halfway for a second or two. Then it picks right up and keeps going.

I have no idea if it's related to the hesitation immediately after a hot start but it seems worth mentioning.

Thanks!
 
Did you test the throttle position with an "analog" meter? If you have a flat spot at a certain rpm, that can correlate with a faulty spot on the sweep of the throttle position sensor.
 
How is your compression? Does it idle smoothly?
 
Leakdown test completed:

1 - 12%
2 - 12%
3 - 20%
4 - 20%
5 - 10%
6 - 7%

It idles OK, not like a new engine, but more or less what you'd expect for a 225k engine. I ran a bottle of Techron through it with no noticeable results.

The TPS checks good.
 
It's really obvious at about 50% throttle, 2nd gear (AW4), right at 2700 rpm. I bet sometimes that dip/sag causes the transmission to upshift, masking the problem.

I cleaned out some light corrosion in the distributor cap when I put it back together after the leakdown test. It runs a bit better but the power dip is still there.

I forgot something about the idle question: a few days ago it threw a code for #1 misfire. You can feel it lightly missing at idle after driving it hard .. fuel injectors?
 
If you're willing to try additives, how about a can of BG44K? I've used it a few times, and it smoothed a barely audible mis-fire, and for some reason helped it to start easier. Personally, I'm no fan of any 'miracle in a can' type of stuff, but it's about a $20 shot to see if it works. You'll have to look at their website to see where to get it.
 
Starting to think it's a combination problem: leaky manifold, worn injectors, old O2 sensors, etc. Agree with Tim_MN that the starting point is fixing the manifold. So I guess I'll post back in a few months!
 
I've had O2 sensors go bad on vehicles since they've been on vehicles, and have never felt anything but the very slightest dip in power when bad, something that no one would feel unless they were used to the vehicle, and could back to back test it. O2 sensors just don't cause a power dip as you've described. My guess would be dirty injectors or a faulty TPS. My '96 seems to have an appetite for TPS's, and units that tested OK on the bench, including a brand new one, felt crappy in the car, including surging, stalling, flat spots, funky tranny shifting, etc. Maybe grab a few from the junkyard and experiment?
 
I followed the BG44k recommendation and added half a can to half a tank of gas. Combined with the new O2 sensor and ECU reset, it has absolutely made an improvement. The misfire CEL cleared itself. Need to drive it more to see the MPG impact, but the driveability improvement was noticeable almost immediately after adding the BG44K.

I'm very surprised as I've never seen any changes using other additives such as Techron.
 
Blah, two tanks later and now it starts but dies unless I feed it throttle. Once it warms up for a couple of minutes it seems to do OK. Misfire is back as well. :-|
 
How dead? Jump every time to start? My '96 was acting up coming home from work once, battery gauge up and down, radio making whizzing noise, 'GEN' light on and off, even the tranny felt funny. I didn't even stop, bought new alternator on way home, threw the HEEP right in the garage, raise the hood to disco battery, and I find a cracked terminal, covered in arc flash burns. Oh well, the old one WAS dirty, anyway. Now we have a spare old one.

Sometimes, things aren't what they seem...
 
Not totally dead. I think one of the cells was internally shorted or something. The voltmeter showed 12 volts. The motor cranked slowly .. but I didn't realize that, because it had cranked exactly the same way since I bought it a year ago.

Once it finally kicked the bucket .. it would still crank the engine, very slowly, and then the motor would die right after it started. If you gave it gas it would keep running. After a couple minutes it ran and drove just fine.
 
My 99 was starting to run like a pig slowly after a few months until one day i couldnt get it to crank. The battery was a 1 year old autocraft gold and was charged and tested fine. And the starter was recently swapped a few months back. I went to the basics and refreshed all the battery cables with 4 gauge and baam it was like magic my engine had no more flat spots under acceleration etc. Look into the o2 and manifold. A cracked manifold is asking for trouble.

I also had trouble with the bogging after start up and all new injector o rings were my cure for that.
 
Thanks for the advice. Already put in new cables and a new O2 sensor .. all these things are making incremental differences. In fact I also replaced the alternator right after I bought the Jeep. The whole vehicle is in pretty tired shape, mainly from lots of miles with little routine maintenance. Biggest day to day issue is death wobble.

I'll look around and see if there are other ground wires that could use some help.

Any recommendations on a replacement part for the manifold .. I'm thinking a header, and I need to replace the entire exhaust.
 
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