• NAXJA is having its 18th annual March Membership Drive!!!
    Everyone who joins or renews during March will be entered into a drawing!
    More Information - Join/Renew
  • Welcome to the new NAXJA Forum! If your password does not work, please use "Forgot your password?" link on the log-in page. Please feel free to reach out to [email protected] if we can provide any assistance.

4.0L hesitation under high load

blistovmhz

NAXJA Forum User
Location
Vancouver, BC
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:
Back
Top