No power when warm/bucking/surging.

99XJ 4.0 AW4 stock everything 96k on the clock.

I was out taking my end of the work week cruise tonight and about 20 minutes into I was cruising along at about 40 mph and I gave it about 3/4 throttle and it the Jeep fell on it's face. I couldn't give it any more than about 30% throttle without it losing all power.

I've got a Scangauge II hard mounted on the dash so I starting monitoring some of the inputs. Throttle position was working as it's supposed to, MAP seemed normal, Timing seemed normal, Water temp stayed at 195-203, Intake air temp was around 105, it was running in closed loop(but went into open loop at full throttle), and I can't monitor fuel pressure with it but fuel flow seemed normal. It didn't and hasn't thrown any codes, and when I got to a friends house the cat wasn't any hotter than normal.

About 2 hours later, long enough for it to cool down, I hop in and drive off and everything seems fine. Once it got up to normal operating temp it started doing the same thing, and this time I'm almost positive I heard it backfire out of the intake. RPM doesn't seem to matter, but load does. If I keep it in a low gear I take slowly take it up to 4k or so with out any problems but as soon as I give it any amount of skinny pedal it falls on it's face.

I plan on tearing into it tomorrow but I figured I'd see if anyone has any advice on where to start.

My 2000 did this same thing when the oxygen sensors needed to be replaced. Under heavy throttle or when climbing a hill it would buck a little bit. Use a code reader somewhere and see what it says.

I vote for a clogged Cat. Either need to remove it and check or replace it. My FSM (89) says to check the O2 sensor as it may have a loose wire.

I agree it could be an 02 sensor. But it could just be cheap gas. Try a different gas station.

jeepboy381 said:

My 2000 did this same thing when the oxygen sensors needed to be replaced. Under heavy throttle or when climbing a hill it would buck a little bit. Use a code reader somewhere and see what it says.

Click to expand...

I do have a code reader plugged in and no codes. I will plug the laptop into it today and see what the O2 sensors are reading.

j99xj said:

I agree it could be an 02 sensor. But it could just be cheap gas. Try a different gas station.

Click to expand...

I'm going to rule out gas, one of my other cars that is very picky about fuel was filled at the same pump about 20 minutes after I filled the Jeep and it's running fine.

mikeforte said:

I vote for a clogged Cat. Either need to remove it and check or replace it. My FSM (89) says to check the O2 sensor as it may have a loose wire.

Click to expand...

All of you are saying it could be an 02 sensor, but at WOT it goes into open loop and doesn't use the 02 sensors, when I mash the gas the problem gets worse. I will check them out and I appreciate everyone's input.

Took another drive this morning. Open loop/closed loop, O2 sensors plugged in, or disconnected it makes no difference. It still doesn't do it cold, but the problem gets worse the warmer it gets.

I measured the primay resistance on the coil and it was a little high at about 1.6 ohms(spec is .950-1.2), and the secondary resistance was 15.2K(spec is 11.3-13.3) Picked up a new coil, going to throw it on and go try it out. Ordered cap rotor and wires but they won't be in until this afternoon.

New coil seemed to help at first, but after about 20 minutes of driving it started doing it again.

Any other ideas?

Well, do the easiest and cheapest things first.

-clean your throttle body/lube the IAC
-clean your battery cable connections and ground connections
-check all the sensor connections while you are at it
-check your fuel pressure while driving (I actually rigged up a flashlight to shine upon the gauge under my hood and camera to record the gauge.)

j99xj said:

Well, do the easiest and cheapest things first.

-clean your throttle body/lube the IAC
-clean your battery cable connections and ground connections
-check all the sensor connections while you are at it
-check your fuel pressure while driving (I actually rigged up a flashlight to shine upon the gauge under my hood and camera to record the gauge.)

Click to expand...

Cleaned throttle body IAC
Cleaned all sensor connections, checked/cleaned all grounds and even installed a few new grounds.

Good suggestions but I've done it all but check fuel pressure(I can't find my gauge). It's still only doing it when fully warmed up after about 15-20 minutes of driving. Here is a Video of what it's doing. I was giving it about 1/2 throttle.
http://www.youtube.com/watch?v=sGhvnhAAxLQ

This test was not done on a closed circuit with a professional driver. :looney:

Geez that sucks, your foot was steady on the gas pedal when it was doing all of that?

Made another run fuel pressure stays right around 47 psi, occasionally it will dip to 45 or surge to about 51, but it stays fairly solid.

I ran up to the store, shut it off and was in the store for about 10 minutes. Came back out and started it up and when i was pulling out of the parking lot it fell on it's face right off of idle and backfired a couple times.

I think I'm going to run through the checks for the IAC in the Diag manual.

j99xj said:

Geez that sucks, your foot was steady on the gas pedal when it was doing all of that?

Click to expand...

I was at half throttle for most of the video, about the middle of the video I let off for a second or 2, and then gave it the go pedal again.

Strange.

The engine seems to be running very lean, and then it gets a healthy dose of gas and accelerates until it reaches a point where it gets lean again.

This seems to be a fuel problem, but it could very well be a CPS thats about to go. We really need Dr. Dyno or 5-90 for this one.

But until they can chime in I would just check the fuel injector connectors and run a can of fuel injector cleaner, BG 44k is the only stuff that really works.

j99xj said:

Strange.

The engine seems to be running very lean, and then it gets a healthy dose of gas and accelerates until it reaches a point where it gets lean again.

This seems to be a fuel problem, but it could very well be a CPS thats about to go. We really need Dr. Dyno or 5-90 for this one.

Click to expand...

I'm almost positive it's not a fuel problem. Fuel pressure stays right in spec the whole time, and fuel flow goes up with load and doesn't fluctuate when it's acting up. Also the spark plugs I just pulled out don't really indicate a lean condition.

It almost seems as if the ignition is cutting off during the times it's doing it. I just put some new plugs in and took it for another drive and I couldn't get it to act up, but I don't think I've seen the last of this problem.

I hate intermittant problems, hard breaks are so much easier to troubleshoot and fix. :wierd:

mine behaved in a similar fashion before the cps went out.

IslanderOffRoad said:

mine behaved in a similar fashion before the cps went out.

Click to expand...

Now are you talking about the Crank position sensor or the Cam position sensor?

I know most people refer to the Crank Sensor as the CPS and the Cam Sensor as the distributor pick-up but I just want to make sure that is what you are talking about.

I'm half tempted to pick up an extra set of all the sensors and keep with me.

Crank Sensor

mine failed last august and initially only had symptoms when it was hot out and I had been running for a while. the mechanic had trouble duplicating the issue because of this.

Ok, since none of the locals have a crank sensor on the shelf what do you guys think about splicing a toggle onto one of the crank sensors wires and seeing if I can duplicate the symptoms?

I've got to wait on the sensor to come in anyway, might as hell make sure it's the problem.

Just saw this , and wanted to tell ya what mine was a few months back .

Mine was doing the same thing as yours. It felt like the fuel was getting stopped / restricted , but only after getting warm . After I had the filter changed and the sock on the Fuel pump , it turned out to be a bad coil .Haven't had a problem sice, and if ya have one around , maybe worth a shot to hang it off the side and see what it does .

Good luck !

*EDIT*

Just found the post where ya said you already did this. Sorry .

Good luck man , let us know when ya find it .

Well I replaced the Crank Position sensor. That was a royal pain in the ass, I've R&R'd engines in less time than it takes to change that stupid sensor. Shouldn't take very long the next time as I now know the right combination of tools to get to it.

Went for a little test drive and guess what... still does it. Tried driving it again with the upstream sensor disconnected, then the downstream, and then both and it still does it.

What's next? It's really not that big of deal because it only does it after about 20+ minutes of driving and my daily commute is only 5 but I'd still like to fix it.

One other thing, while it's doing it if I pull over, shut it down and restart it it's fine for about 5-10 minutes.