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severe misfire under load

sewerzuk

NAXJA Forum User
Location
Oregon
My wife's XJ has been acting up lately; I'm no stranger to the 4.0L but so far I haven't been able to figure this one out.

Specs on the engine: 150,000 miles. Well cared for, good compression, no noises. Mostly stock except for stainless header and airaid cold air intake. Has run well with this setup for years.

Symptoms: Idles perfectly. Drives perfect, as long as you stay under 1/2 throttle. Above 1/2 throttle, engine begins to miss, and if you go to WOT will eventually die. This happens whether it is parked in the driveway or on the highway. The misfire doesn't seem to be related to RPM level; just throttle position (it will run smoothly at 4500RPM, as long as I keep it under 1/2 throttle). Although it is a little difficult to tell for sure, it seems like I can hear a little bit of detonation through the misfires. After a few days of driving, the misfire codes begin to accumulate; PO301-306. No other codes.

What I have checked and tried:
I have checked fuel pressure at idle and under load while driving, and at WOT. Also did a volume test (passed) while monitoring the pressure. Always stays between 45-50 PSI with very little variance.
Using my scanner, I have tested the TPS, CTS, O2 and MAP sensors under driving conditions. All return expected values thoughout normal ranges, and operate smoothly.
The O2 and TPS sensors are about a year old; O2 was replaced because of mileage interval, TPS because the old one failed.
Cap, rotor, plugs, wires, ignition pickup, and coil were all replaced today. None had any effect on the symptoms.
Computer was reflashed by chrysler about a year ago due to a recall; don't remember the exact details.

I can't figure out what to try next. Any suggestions??
 
Forgot to put the year in my OP; it is a 1997

Here are a few pics:

XJ1.JPG


XJ3.JPG


XJ4.JPG
 
Restricted exhaust.
 
Restricted exhaust.

Good guess...forgot to add that to my OP. I pulled the downpipe (disconnected in front of the cat) and revved it in the driveway for a few minutes...other than being really loud, nothing changed. Same misfire still present
 
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Darn. Thought that could have been it.

Have you tried using an analog meter to test the TPS for dead spots?
 
No, all the sensor testing I have done is with my scanner. I'll try the analog meter method...but the range where the misfire happens is like 40% of the throttle travel. I'm assuming that I would have noticed that large of a dead spot with my scanner. I did slowly move the pedal while watching the scanner; it seems to move in about 2% increments smoothly throughout its full range of motion.
 
Yeah, but the digital meters have built in buffer circuits that can hide things from you.

BTW--what did your sparkplugs tell you? Were they all the same? What condition?
 
BTW--what did your sparkplugs tell you? Were they all the same? What condition?

They were all identical. All extremely clean; missing the brown haze I would have expected for plugs with over 10k miles on them. I would have read them as the engine running lean, but kind of dismissed that because most of the newer engines I tinker on seem to leave the plugs looking cleaner than I expect. If you would like, I can take a pic of the plugs and post it here...
 
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Yeah, but the digital meters have built in buffer circuits that can hide things from you.

Yes, but this was hooked up to the OBD2 port; it was reading what the computer was seeing, so I'm assuming that the reading I got was accurate. I will still try the analog method...only a few minutes tomorrow morning.
 
Used to be I would also sweat very white plugs looking for an intake leak, but I think some of the fuel changes--ethanol and other formulation changes--are resulting in a cleaner (hotter) burn.

Kinda hoping one or two would have shown the missfire and/or detonation.
 
The computer does eventually store misfire codes for all 6 cylinders, and the random misfire code. So it is something affecting the entire engine; that's why I focused on the fuel pump and coil/cap/rotor/pickup first. I hate shotgunning stuff...hoping something said here will help me take a step back and see what I'm overlooking...
 
might try this-

Ignition OFF: work the accelerator 15-20 times the FULL range. (Gently!) Then retest.

-This only "works" the mechanical parts of the TPS etc. and can temporary "fix" moving contacts enough to show a 'bad' one. (would start running fine)

P.S. -bad Fuel?

Good Luck,
O
 
might try this-

Ignition OFF: work the accelerator 15-20 times the FULL range. (Gently!) Then retest.

-This only "works" the mechanical parts of the TPS etc. and can temporary "fix" moving contacts enough to show a 'bad' one. (would start running fine)

P.S. -bad Fuel?

Good Luck,
O

I like the idea of testing the TPS this way; I'll work it as you suggest tomorrow. However...I'm doubtful that the TPS is bad because I used my scanner to graph throttle position (along with several other parameters) while I was driving it. There were no jerks or jumps in the TPS graph, and it indicated properly at WOT while the engine was misfiring.

As for the bad fuel; the Jeep has been running this way for about 3 weeks and about 5 tanks of gas from 3 different stations. It's a good suggestion...but I think that the gas isn't the problem.

To me, the symptoms indicated a fuel delivery problem (which I feel like I've eliminated with my flow and pressure tests), weak spark (which I've eliminated by replacing essentially the entire ignition system), or a bad MAP or TPS sensor (which I'm trusting my scanner to test). I will more thoroughly test the TPS as suggested...but I'm thinking I must be overlooking something else.
 
Thoughts:

---Disconect and Clean battery posts - both,
---this also resets the PCM so it relearns/resets settings.---

-Re-seat connectors on throttle body sensors.

-Freshen Grounds - Engine to firewall, Battery to fender.

-------------------------------
As I understand it - going to WOT or Acceleration drops back to Open loop.

Regards,
O
 
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