Hi all, after finishing the 4.0 swap into my 1989, I've been driving the car every day since Sunday, and it's been running beautifully.

However, when it's first started after being shut off, the idle lopes up and down. It will not stop doing this untill you put the car in gear and hold it stationary for 5-10 seconds.

I'm suspecting the IAC stepper motor, from what I can see it doesn't appear to be moving when the key is on, and it's currently set at about halfway between it's cone seat, and the housing.
I should note I am using the 1994 IAC assembly, as the 1989 unit did not work with the 1994 Intake manifold. I swapped connectors, the wire colors were the same between the 1989 harness and 1994 harness.

I have checked for vacuum leaks, cleaned the IAC, throttle body, and the MAF sensor and all their connectors, with no change in the loping. The air bypass screw on the IAC housing only makes an idle difference after it stops loping. It is currently set all the way in, and once the idle settles down, idles smoothly at 700 rpm.

My next step will be to compare resistance values between the 1989 IAC motor and the 1994 IAC motor, but since I haven't gotten any check engine message relating to it, I don't suspect a compatibility problem.

As a side note, If I do have to replace the stepper motor, I noticed they are quite pricy. I had a look at a mid 90s General Motors IAC stepper motor and it looks almost identical, down to the connector. Perhaps the GM unit would work?

Cheers, David

 

I suppose you've already set the stepper motor as per the initializing procedure, right?

The renault megane stepper fits - I got mine on ebay for $12, but it too has to be set up - these things aren't necessarily plug and play!

here's the procedure (if you don't have it)

The bypass adjuster [air bleed screw] doesn't really do much in my opinion, but try it, maybe you can get it to do something.

PS - if you pull the stepper out and do the test - maybe you can see if your piston is actually moving?

Larry

 

David,

If I remember correctly, the Workshop Manual warns that it is possible for the IACV stepper motor plunger to screw all the way out of the motor if it is powered while not installed in the valve, so if you do check its operation that way, do it in short bursts.

To echo Larry's great info, I installed a Renault Megane stepper motor on our '93 with great results. Here's a link to some thumbnails (sorry the large photos aren't working):

XJ40 Idle Air Control Valve IACV Stepper Motor

Cheers,

Don

 

Thanks Larry and Don, you guys always have helpful information!

I can't remember where I found my initializing procedure, but it was very similar to what you listed Larry, only it did not include the 15 second wait before replacing the connector. I will re-try it tomorrow morning before I head off to work.

The Renault stepper will be something I look in to should I find my stepper is faulty.

The later half of today I've been noticing that sometimes it only lopes up and down a couple of times, then settles to a steady high idle, which drops down to 800rpm after 15 seconds (in park)

 

hello all, how are you ?
here, the IDLE procedure for my 3.6 Jag
SOVEJAG XJ40

 

Thank you all!

(I hope this one actually posts, as this is the 3rd attempt at typing this message out)

French, Thank you for your procedure. The mention of the 100 ohm resistor reminded me that the other day I saw a baggy in the glovebox with 2 resistors in it. I checked on my lunchbreak and sure enough it was 2 100 ohm resistors, a 7/32nds allen wrench, and a piece of paper with stepper motor initializing instructions.

I did the procedure on my lunchbreak, and on the first start up, it was idling at a steady 500 rpm, which after letting out the bleed screw some, I got to 800 rpm. I felt accomplished and went back to work.

However on the next startup, it was loping again. When I got home, I un bolted the IAC assembly from the intake, and propped it up on the valvecover so I could watch it from the drivers seat. With the resistor installed, the stepper motor goes all the way out with key on, and retracts when the key is turned off. With the ECT connected, the stepper motor goes in, but its still off the seat, I feel an excessive amount.


So heres what Im thinking. I know the temp sender for the gauge is accurate, as I checked it yesterday with an IR thermometer. Perhaps, the 2 pin temp sender (which I assume talks to the ECU) is faulty. Maybe I have a poor connection?

I will likely replace the connector for good measure (the plastic has a crack anyways). Could anyone provide me with a resistance value I should be seeing across that sensor at a certain temp? Once warm my car seems to sit at 90 degrees celcius.

Cheers, David

 

Well, my temp sender is reading accurately as per resistance values I found in the Jag lovers E-book for the XJ40.

For the meantime I got the stepper motor to wind all the way in and then I un-plugged it, and its idling smoothly, although a little high once its warm.

My current thought goes back to comparing the 1989 intake manifold to the 1994 intake manifold. The only difference between the two that I could find was where the IAC assembly mounts, the 1994 intake has one small hole (matching up to the air bleed screw), a larger hole, about 1.5" diameter going into the intake chamber (not sure what its for) and another smaller hole (matching up to the stepper motor chamber).

Whereas the 1989 intake did not have the larger hole in the middle, it is blanked off. So now my next shot in the dark is assuming that the 1994 ECU compensates for that larger hole, and the 1989 ECU does not. I will make a new gasket or perhaps a blanking plate to block off that larger hole, and see what happens.

After that, I guess Im taking a day to swap intake manifolds back over.

Cheers

 

It was only a matter of time before I hit my mark. Blocking the larger hole in the middle did the trick. The car idles as it should.

Cheers, David