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Beserkr

97 2.5l tj rough idle and slight miss under heavy load

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97 2.5l tj ax5 4.10 graers 2.5" lift and 31" at tires. 

Greetings, I'm a first time poster and also a master mechanic. I have owned the Jeep for 6 years, last year the engine lost oil pressure and I got a used rebuilt motor with only 1000 miles out of a 97 tj that got t boned a week after the engine was installed. After about 6 months of driving around the engine developed a miss on cylinder 1 I traced it down to a Crack in the intake runner for that cylinder and changed the the head with a rebuilt O'Reilly head for 300 bucks plus bolts and gaskets. 

The problem I have had with it all along even with the original engine is a rough idle slow Rev up and slight miss under heavy load and also pops on decel in gear out of the exhaust and also If i do a throttle snap in neutral it pops from the exhaust . I have replaced the muffler and cat, the front 02 sensor, cap, rotor, plugs, wires, coil, distributor, injector pig tails, head, exhaust manifold, lifters, fuel pump module, engine mounts, and radiator. Most of these parts were old and needed to be replaced I wasn't trying to fix the issues with them. I checked the timing chain it is tight and on time, distributor is on time, checked all the sensor readings and all seem within spec, fuel trims are 3% st and 6%lt at idle cruse it's closer to 0% for both and wot it is 0% (I live in Mesa AZ and it's hot as hell and the summer gas sucks here) I think the alcohol content has something to do with the trims being slightly positive. With a vacuum gauge hooked up at idle it is about 19" at 1300 feet above sea level but the needle vibrates about 1 to 2 inches at idle some times especially with the a/c on. at cruse it vibrates about a half an inch of vacuum. Checked exhaust back pressure and it's .5 psi at idle and about 1 to 2 psi at 2500rpm. Also checked the battery and charging system and at 110° ambiance air temp I'm charging at 13.8v and the battery is strong when tested also.

This is just driving me nuts any ideas that might help would be awesome! If any more info is needed please let me know? 

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Beserkr...You have been systematic with your troubleshooting and taken a professional tack.  Did you test the TPS?  Is the TPS an OE item or aftermarket?  I have experienced issues with aftermarket O2 sensors and TPS switches, typically with off-shore parts from popular parts stores.

The O2 sensor on my '99 XJ Cherokee was older and, as you would reason, I thought it was due for a replacement.  I purchased a generic O2 sensor from a well known parts source and immediately experienced mysteriously odd performance.  I bought an NTK equivalent to the OE Mopar O2 sensor and promptly resolved the issue.  I did the same thing with an aftermarket TPS switch and was amazed just how shitty an engine could run on a cheap TPS switch.  For any sensor that matters, I use OE replacement parts or higher quality labels like NTK or Bosch wherever possible.

Note:  Something as subtle as an aftermarket/Brand-X coolant temp sensor can be problematic.  Although the 4.0L Cherokee engine performs okay with an aftermarket CTS, the temperature reads 5-degrees F warmer than it did on the stock sensor—no other changes or causal factors, a surface temp infrared readout reveals that the thermostat housing is actually right at the OE sensor temp of 5-degrees F cooler...I'm just sayin'!

Try testing the TPS voltage at various throttle positions and throttle valve angles to see if there are glitches or faults anywhere along the voltage curve.  Check the O2 sensor for its part number.  Mopar used NTK sensors in our vehicle era, see whether the O2 sensor is NTK, and if so, compare the part number with the OE Mopar part.

Let's start here...

Moses

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Moses, I just replaced the front o2 sensor with an ntk direct fit seems to be functioning good, when I pull a vacuum line it jumps full lean and when I spray carb cleaner in the vacuum leak it goes full rich. I changed it thinking maybe it was biased one way or the other from the old head which had a Crack in it thus coating the o2 sensor with cumbusted coolant but guess it wasn't the problem. 

I checked the tips with obd2 graphing doing a slow sweep and no drop offs or spikes it looked nice and smooth. I have 3 tps sensors sitting around I could try, they aren't slotted so it should be easy to just change one out for another. I think I got from 14% to 75% calculated position on the sweep. I checked voltage at the connector (back probed) and I don't remember what it was but I remember it being in spec according to the service manual, maybe 1.3v to 4.3v or something like that. I could check it again when I try and swap it with one of the spare ones I have. I think they are all Mopar sensors. 

 

One more thing I noticed today we are in our monsoon season here in AZ and the humidity is at 60% this morning where as its usually at 10% or less and it seems to be running way better. Any thoughts on this? I can't imagine what would change in regards to moisture in the air that would be a positive, maybe lower combustion chamber temps? Maybe I should try running mid grade or super gas? 

 

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Beserkr...Sounds like the O2 sensor should be okay.  Compare the TPS sensors and install the new one carefully; as you note, the position of the TPS is preset on Chrysler MPI, not adjustable like G.M. and other TPS switches once were. 

The increased humidity increased the intake air stream density...I'm thinking you have a lean condition when the engine is acting up.  The O2 sensor and PCM compensate, so this shouldn't be confused with an ultra-lean air/fuel (A/F) ratio; A/F is carefully controlled on your engine.  If the TPS calibration is not en sync with the actual throttle valve angle, you could have an issue.  Another source of a lean condition (a vacuum leak, too) is a worn throttle shaft or shaft bores.  Spray a fine mist of carburetor cleaner at the throttle shaft ends of the throttle body with the engine idling.  Listen for a change in engine speed.  This is an air leak source that can cause havoc.  Rule out a throttle shaft or TB related leak before condemning the TPS and throttle valve synchronization.

Tip:  If you do suspect that the TPS is out of sync with the throttle valve and would like to test TPS voltage responses in a new way, take a TPS that you can sacrifice for this test and try elongating the mounting holes on the plastic TPS switch; remove the TPS's locating stub to make it possible to rotate/adjust the TPS.  Play with the TPS adjustment, noting the throttle valve angle carefully, and see if you get a change in engine performance.

The IAC can also create a lean condition.  The fact that the engine runs better with higher humidity hints that you are "normally" running lean—or have a throttle valve not reflecting the position of the TPS.  IAC would be more likely an idle issue, though it could contribute to deceleration pop, but IAC would be an unlikely source for your slight miss under load.

What do the spark plugs reflect?  See whether there's an indication of a lean mix.  Avoiding a a trip to the burn ward, carefully remove a spark plug after a heavy pull followed by immediate ignition shut-off with the clutch pedal depressed.  This is an old and often helpful diagnostic tool.  Typically, the plug insulation looks very light, often tan/white on these emission MPI/EFI engines.  See what turns up.  Take a cell phone photo of the spark plug, we'll talk about it.

Moses

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