Bouncing idle on deceleration

[UNCC Grad]

98 XJ 4.0 with AW4, 240k miles. I've been having some issues with a new to me XJ. Mainly longer crank times and idle bounce from 1600-1000 and back on deceleration. Idle in neutral or park while sitting is normal, around 750 and consistent. I've also been getting a P0700 and P0740 which is likely the TCC solenoid. It works occasionally when cold but won't lock up when warm. I haven't verified fuel pressure after sitting or while cranking yet. I do notice if I prime the pump a couple times before cranking it seems to start a little easier. I have replaced the TPS, cleaned and regressed the NSS, new plugs cap and rotor, cleaned the throttle body and IAC and replaced the strainer in the fuel tank. The a previous owner had allowed trash to get in the tank so I replaced it with a freshly cleaned used tank, using the old pump which looked to have been a replacement at some point.

Could it be the trans downshifting causing the rpms to bounce? It does it while cruising at 55mph when you let off the gas when the trans should remain in the same gear. Leaking injectors? Vacuum leak? Weak compression due to worn rings?

Wondering where I should start with this.

 

[98NWCherokee]

Did you replace the TPS with a Jeep brand sensor? I recently replaced the TPS in my 98 with a NAPA unit and it lasted about 6 months before my idle starting fluctuating and I had all kinds of drivability problems. Even from day one, the aftermarket sensor did not operate as smoothly with jerky performance and even a delayed throttle response. At 6 months it finally failed and threw a DTC. I popped for a Jeep unit and it's been perfect ever since. If you have an analog meter you can pull the TPS and look for any flat or dead spots in voltage along its range.

Do you have a vacuum gauge you can install and take a reading? I'd be suspicious of vacuum leaks as well given the nature of your problem. You could also do a leak down and compression test to rule out rings or valves given your mileage.

 

[UNCC Grad]

The TPS was a parts store brand but for the price and lifetime warranty I thought I'd try it. On the commute home today (40+ miles one way) it quit hunting idle but I did get one severe case of death wobble that I haven't experienced yet. I hope that's the end of the idle issue but now I need to look at the track bar and castor to see about the death wobble.....not cool going 50+ down a country road and having the front end shaking like you lost a tie rod!

If the idle issue come back I will look at vacuum and compression/leak down

 

[Tim_MN]

A new genuine Jeep Idle Air Controller took care of the bouncing idle and uneven drop in rpms when decelerating for my 99.