deceleration mode

i have a 90xj.. or 89 wiring seems to be 89. anyway on the highway if im going faster than 55mph and i let up on the gas it goes into decel mode fine. however anything under 55 it goes in and out every second. if you look at the rpm gauge it goes up and down and you can hear it from the motor also.
does it both when hot and cold.

now it has been doing this for 2 years now. i have replaced all the sensors except for the knock and the sync sensor(inside disco). i haven't replaced the temp senors but they seem to be working.


i hope this isn't to confusing.

Guessing AW4?

properly adjusted the TPS?

What do you mean by 'decel mode'?

JJacobs said:

What do you mean by 'decel mode'?

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WHen the motor is above 2K RPM and the throttle input is less than 5% the Renix ECU will defuel the motor. It makes engine braking with a 5 spd most awesome, I'm not sure if it really does anything for the auto guys.

Good to know that something as old as the Renix systems had that, thought it was limited to newer vehicles.

I don't think it has anything to do with what the guy's talking about, though.

JJacobs said:

Good to know that something as old as the Renix systems had that, thought it was limited to newer vehicles.

I don't think it has anything to do with what the guy's talking about, though.

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Probably not, FSM says "deceleration mode" is only entered under HARD deceleration. Thinking maybe a t/converter lock/unlock situation?

joe_peters said:

Probably not, FSM says "deceleration mode" is only entered under HARD deceleration. Thinking maybe a t/converter lock/unlock situation?

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thats what its supposed to do. but i dont have a FSM so im using what little resources i have.

it seems like a converter lockup unlock but it works fine when im above 55mph and the tps is adjusted. to the .83% thing. is there any other sensors that would affect it? it does it any where between 30-55 after that it works.

Yes. It is perfectly normal. My XJ does that especially when I hit the exit ramp above 70 MPH and the ramp goes down. I could actually see the RPM staying above 2,000 and goes up and down say about 200 RPM until the MPH is under 55 or I put my foot on the breaks. I think at that time the torque converter un-locks and the 4.0 drops to idle RPM +/- 700 RPM. It offers some engine breaking when descending long hills.

this is the problem i think that when the t/c unlocks it rolls fast enough to lockup again. but it seems overly sensitive. and sounds so bad you can hear it in the exhaust. but when i got my xj it did not do this.

relive old post...

so i got a mt 2500 with all the jeep goodies.
seeing my jeep still does the decel mode weirdness. after plugging it in the transmission electrical is not to blame as it does not change any solenoids when it is going in an out.

however the injector pulse goes for 0us to 5us and back. so the ecu cuts the injectors like its supposed to. then resumes flow and cuts them again.

here's what iv done

changed trans fluid and filter. shifts smoother now. no fix
replaced and set using MT2500. still same thing.
all sensors are newish and mt2500 clams there all in spec.

do you think voltage loss on the wiring could cause this?

How much is the RPM changing?

Do you have a loud exhaust on your XJ? I only noticed my XJ doing that after I replaced the OEM exhaust with a 2.5" from cat to tip with a FM-50 muffler.

Good you were able to run a scan on the XJ. It allow you to see exactly what is going on. I always thought it the AW4 shifting in and out of OD. Good observation.

Would this PCM energizing and de-energizing the injectors have anything to do with the anti pollution system? Would that be an attempt to minimize the amount of fuel used while keeping air to fuel ratio within specs and /or at the same time reducing the chances of drying the pistons and cylinder walls?

On carbeuretted engines, even when you take your foot completely off the gas pedal, fuel is still drawn into the cylinders via the idle jets when coasting. I would assume this had some positive benefits on the engine. :dunno:

techno1154 said:

Would this PCM energizing and de-energizing the injectors have anything to do with the anti pollution system?

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It probably does.

Would that be an attempt to minimize the amount of fuel used while keeping air to fuel ratio within specs and /or at the same time reducing the chances of drying the pistons and cylinder walls?

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If the cylinder walls are wet with fuel you have other problems. They are not lubed by gas, but the oil splash from the crank. In my experience when the engine is coasting and doing the RPM change the AFR will go as low as 19:1, a very lean condition. 14.7:1 is what it should be during cruise. I don't think this is a problem as the engine is under no load during this time.

Both my ZJ and XJ do this. The ZJ had a Dynomax, 62 mm throttle body and a K&N CAI. The XJ is too modify to even compare.

well from what i have seen and experienced on other cars. the pcm does or is supposed to de-energize the injectors. at a high rate of speed, when you let your foot off the gas the transmission shuts off all the shifting solenoids and locks the TC. im assuming this is locking the drivetrain with the engine. at this point the injectors are suppost to shut off the fuel flow thus improving fuel economy and giving engine braking. how ever the ECU(pcm) is reading a signal (which im still trying to figure out) or the transmission is slipping(electrical wise its not doing anything) and the injectors resume a 5us pulse which then goes back to 0us and 5us. however anything above 55mph every thing is working as should. injectors shut down.

between 50-40ish the ecu allows the injectors to go on and off in a one second on one second off. this is where the problem lies.

like i said before on the tcu side of things. during this (problem) the tcu sees the throttle as closed and the tcu is not changing anything. so i dont think this is an electrical problem with the tcu.

when this is happening the ecu also sees the tps in the closed throttle position.

so im ruling out tps as a problem as it was adjusted using the mt2500 and replaced also.

My 90 has these exact same symptoms and has since new. Everything is new again after the overhaul of the whole Jeep and it still does it.

I did finally fix my low idle though:yelclap:

Once again, you are chasing a non issue. Good luck.

the only reason im looking into this is it did not do it when i got it. it started doing it a little and has sense gotten worse.

so i have more information on this.

the edu is entering decel mode then its switching to open mode and back to decel. looking into this the ecu is seeing the coolant temperature sensor going from 183f to 179f thus the edu thinks the engine is not at operating temperature and resumes warmup mode and disables decel mode.

i think i have found my problem. now only to trick the ecu into thinking that its at 190f and seeing if all works fine.

What thermostat are you running?