Stalliing/hesitation problems

I have a 96 XJ 4.0L, AW4 auto, 231 TC. It drives fine all around town. After driving at highway speeds for a while then deaccelerating it will stall or hesitate/stumble upon accelerating. This behavior ( hestiating on acceleration) will then continue. If it stalls it will restart, but with the continued hesitation problem. I have replaced the TPS, but no difference. Any suggestions?? MAP sensor, CPS ??

Thanks

DanDXJ

It sounds like a bad Idle Air controller (IAC). When go into deacceleration mode it becomes active. You could have a dirty throttle body, that why you are getting the strange reaction. Have your throttle body professionally cleaned.

Try that for a while but if it continue then change the IAC. I had a similar problem happen when the engine was less than 5 miles from home. It was finally fixed by a new IAC, but then I had 150,000 miles on the engine.

Additional info:

When it stalls/hesitates the check engine light does not come on and no DRB ll codes are stored. The Throttle body was cleaned when the TPS was changed.

DanDxj

i would check for corrosion under the plug wires at the distributor cap especially the coil wire, sounds like a secondary ignition problem to me.

I checked/ cleaned cap and rotor - slight carbon buildup not bad though. Plugs all looked fine, checked wires with ohm meter -- all within spec. Ohm meter check of coil at cold conditions showed within spec.

DanDXJ

Dano, if the idle air controller is messing up it won't set a code cause the computer things it is in the right place even though the engine dies. It took me a year to figure out that I HAD TO change the Idle air controller (IAC) to fix my engine stop running problem.

Just hate to see someone chase the wrong thing, by the way how many miles on your engine? One more thing not all problems set codes...

It has 130,000 miles. Thanks for the responses.

DanDXJ

Dan, it sounds like my problem and very close to my mileage. The IAC moves in response to the engine computer. The IAC, spark, and injector pulse width are about the only controls, every other "sensor" provides data so the computer can decide how the engine should run.

The IAC is a stepper motor. Every time you deaccelerate/coast it becomes active, it is also active when you are sitting at a stop light in gear or idling in your driving way. That little plunger moves in and out all the time. They just become worn out over time. I am sure if you did an electrical measurement on the IAC it would test good since the coil had not shorted.

On my 88 XJ I changed EVERY sensor which has an influence on the engine computer. I even installed new injectors, a rebuilt computer, and a new coil module assy. The one part I over looked was the IAC. I had even changed the Throttle position sensor. So everything has been replaced in the past 3 years but the IAC.

My XJ would die, I gave up on what to do and had the dealer mechanic look at it. He applied a Tech service bulliten to modify the IAC so it would not shut off the air completely. That worked for a year then it would die when I was going down the road and had let off the gas. My XJ would flood and not restart, I had to use the old carb trick of pushing the throttle to the floor to clear the flood.

I know how un-nerving your problem is, makes you start doubting your XJ. Go install a new IAC, take it for a drive. You will be amazed how much better the vehicle idle is and the pesky dieing problem will be gone. When changing the IAC inspect the throttle body, see if there is much build up in the IAC cavity.

Well it is day four with the new IAC motor and no problems so far. Martin thanks for your help. Now for the next problem. My 1990 XJ (4.0L AW4 150K miles ) idles around 300 RPM. The tach has only three lines for the 0 to 1000 range. The needle sits just between the first and second lines. Upon first starting - cold- it idles around 600 RPM but drops when warmed up. It does not stall, but there is a slight hesitation upon acceleration. I have tried a slight adjustment to the "do not adjust" screw, but no diffetrence so I put it back to the original position. Another IAC problem ???

With a 1990 and earlier 4.0 engine you have an EGR. When I have had a low idle on my 1990 4.0, it has been an EGR stuck part way open (it is supposed to close at low RPM). There is an easy test to see if its a failed EGR: unplug the vacuum hose going between the valve thing and the body of the EGR, and plug the vacuum tube (golf tee, screw, or similar). If the problem goes away, you need a new EGR. If you remove the air filter box, then they are easy to get at and change. Put some antiseize on the exhaust tube fittings (connecting to the exhaust pipe) if you have to take it apart. The EGR (Exhaust Gas Recirculation valve) is immediately below the throttle body, and faces toward the driver's side fender.
Edit: the EGR mounting bolts go into the aluminum intake manifold- do not overtighten, easy does it!

If the EGR doesn´t help or a IAC cleaning. Jneary gave me the hint to set my TPS to 12-14% on the three wire side, really smoothed out my idle and raised it a bit. O2 sensor and/or a vacume leak can also affect warm motor idle, tends to fluctuate up and down some. My motor had multiple problems, every one I corrected seem to help the idle some. Also noticed the hotter the motor, the lower it wants to idle on mine. If I idle a lot and the motor gets a little too hot, the idle will fall some.