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Old June 16th, 2014, 17:07
blistovmhz blistovmhz is offline
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Join Date: Aug 2007
Location: Vancouver, BC
Posts: 1,269
4.0L hesitation under high load

98 4.0L HO, Ax15.
Engine mostly stock. Cowl intake, 700 injectors, AC bypass, exhaust headers, etc. Nothing special.

Last year I had to take it to air care, failed miserably, didn't think to check the CAT, replaced a tonne of stuff, discovered the CAT was hollowed out (previous owner), new CAT installed, passed air care with very good numbers.

During the troubleshooting, I figured I might as well install some exhaust headers as the stock one had the stock crack. The header flange was thicker than the intake flange, but I ground it down as well as I could to make sure it all bolted up nice. I'm PRETTY sure it's all sealed up, but I figured I'd mention it.

On hard acceleration, usually between 2800 and 3500, I get a little hesitation. If I continue accelerating through it, it goes away, and usually goes away once it's fully warmed up. If I let off the gas a little though and just maintain my speed right in the hesitation, it'll continue hesitating forever. Push through, all good.
Also, I've noticed a fairly serious loss of power throughout all RPMS. Some days the Jeepy screams and somedays it just bogs right out. I'll climb a hill (highway) in 5th one day, and have to gear down to 3rd another day.
I get the same hesitation for the first 5 minutes after cold start, as soon as I try to rev up quickly, even at low RPM. This goes away pretty quick.

I've checked everything I can think to check and everything looks good. All sensor values look right (when I'm testing, which isn't under load, but it doesn't hesitate when not under load).

All I can think is it's gotta be air/fuel related. My O2 sensor thinks everything is good, and I've graphed everything coming out of the ECU, and nothing looks suspicious. All I can think is that there's a small intake leak that comes and goes depending on temperature/humidity, and that under high load, it'll draw a lot of extra air in, while under all other conditions, it just leaks slowly.

Exhaust doesn't smell rich, and O2 claims my A/F is Stoich. Fuel trim is between -3 and +3, and levels out at 0 while idling and under constant load. It never goes past -/+3. I figure if i've got an intake leak, and all my sensors are working properly, my A/F and F/T would adjust fast enough to mask the leak.

Yes, already sprayed all sorts of stuff around the intake and no change in idle. I've inspected the harness, gave it a twist, tried removing all sensors one at a time, replaced TPS/O2's, plugs, wires, injectors, dist cap/rotor, changed the oil, verified my fuel pressure, etc.

Last time I had an issue that produced the same symptom, was caused by an intermittent short in the MAP leads, but this was on a Renix puter and it'd just reboot the ECU (which was handy, because that's how I figured it was a short). No such luck with the Chrysler ECU.

Any idea's/direction? This has been making me insane for about 2 months.

Other considerations:
* I'm not sure when this started. It may have been before or after installing the headers, or even before/after the 700 injectors.
* I'm not confident the 700 injectors can supply enough fuel. Since installing them, I definitely notice my F/T going up to around 25+ when above 3600RPM, but the problem starts around 2800, and seems to resolve itself after driving a bit.
* Engine idles a tiny bit rough. Most Jeep guys say "it's a Jeep" but I've owned 6 or 7, and every one of them has idled dead smooth, or I fixed it until it did. The rough idle is definitely "a thing" and may be relevant.
* Fuel economy is decent. 18-20mpg city/highway. Seems to be the same regardless of city/hw as when I'm in the city, I cruise around in 4th/5th at 60-80km/h and see 20-22mpg (minus acceleration), and highway I see around 18-19mpg at 110km/h.
* Fuel rail loses pressure after a few hours powered down. Suspect the check-valve, but haven't got around to fixing it because I can't imagine how it would cause any performance problems. (just a tiny bit annoying when I start first thing in the morning).
* The hesitation FEELS like a single cylinder misfire, lack of fuel delivery, or poor timing.
* Plugs all look nice and clean, but not TOO clean.

Last edited by blistovmhz; June 16th, 2014 at 17:48.
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