Has the throttle body ever been cleaned. They gunk up and can cause this issue. (Common and cheap to clean)

I had an EGR fail on me that caused this issue. (Seems to be uncommon though)

The IACV is suspect. (Not uncommon)

It can be lazy oxygen sensors. (Should be done every 50k mikes anyways?)

It can be a dirty MAF (cheap to clean)

A dying TPS (expensive)

Corrosion on the ECU pins? (Free)

 

Thanks for the replies
ATF has always been on the mark.I recently dropped pan and changed filter/gasket.Measured what drained and replaced it.Too full so I adjusted it.Drove down the street it didn't shift.I had noticed filter (from NAPA been on my shelf awhile) didn't have extension pickup.I cleaned old screen and put it back in .and 3 gallons of Maxlife atf so level is still on the mark.

Yesterday I pulled off TB and replaced iacv. I had previously cleaned it a couple months ago chasing down this stall issue and had removed idle valve to clean it out.
I put it back on after reinstalling TB and 1 of the screws snapped so I knew I had to remove it to repair that.I used HF lh drill set.drilled pilot to correct depth and the next size bit pulled out what was left of the bolt although I lost a few of the top threads and replaced both with 2 M4-0.7 x16mm philips head ss machine screws (everbilt at lowes). IACV was $17 FDB942 ECCPP Idle Air Control ValveI wasn't sure about the ecm resetting new valve but i cycled the ign sw several times and it cranked up cold to about 1300 and immediately started stepping idle back down.Warm idle with a/c is 760 P,610 D
Now for a 60 mile test drive to see if stalling on decel. / stop is fixed

 

Throttle body screw? One of the 10mm ones? I would consider fixing permanently with a TimeSert? You'd be able to go back and use the original bolt again.


Just re-read your post....you snapped an IACV bolt. Happens all the time. I'm pretty sure I used a timesert for both of those bolts.

 

SEE post #9

 

Updating on my original post, as I had feared, all was not well, and the problem persists. In short, the car starts instantly hoy or cold, but the revs drop away until it stalls. If you catch it with the throttle pedal, it will idle quite happily, and it is driving faultlessly. No codes.
I finally managed to get my mongoose IDS to connect, and performed the throttle position adaptation, which the diagnostic reported I had “passed”. No difference.
when driving, the car will sometimes idle correctly, sometimes (mostly) not.I have tried tightening the throttle cable to see whether I could cause the throttle to remain open enough to tick over correctly, but she still dies. This last point has me wondering whether this is a fuel issue rather than an air issue, but in truth I may be clutching at straws. Any ideas?
Thanks guys
.

 

Come back around to the IAC

with your foot off the pedal the IAC should be open larger ( during crank ) then the settle down opening to maintain the target idle speed

the fuel pressure is behaving correctly to get you the initial start

May have a dragging injector under fueling a cylinder at the most critical area of idle speed

The ignition timing should be retarded towards 0 TDC during crank but resumes an advanced position after start so there is a event correlation but hard to test unless you go back to a old school timing light

This may accure at the same moment or triggered by the switch from the CMPS to the CKPS

You can remove the CMPS connector and the engine will start with the CKPS and once the engine is running you will not have the switch over from CMPS to CKPS

Your initial starter rotation count will suffer before light off % 50 of the times trying to start

 

disable the IACV in the warm idle position

see what happens

 

It’s be great if you had a spare MAF to test with….

It’s not a great idea to throw parts at a problem, but I’d swap out the IACV then MAF, if I was willing to throw parts at a problem. (Do the supercharged engines use a different MAF?)

Crossing off the TPS is a big deal. Glad you were able to connect the Mongoose.

 

The S/C does use a different MAF

To see if the MAF is alive the center wire on the connector will be 1.2 volts DC at the proper idle speed . on paper

The connector must be installed for the test so a needle behind the connector would work

Someone recently saw 1.3 to 1.4

The voltage will increase on throttle up with higher MAF

 

Once again guys, thanks for your help.
Having replaced the MAF a while ago when I was chasing down a high idle, I still have the original, which was not the culprit for the original fault, nor the solution to this one. I fitted it this morning and it made no difference.
The symptoms keep taking me back to the IACV, which was why I replaced it in the first place, but I am confused by the periods when it will idle more or less satisfactorily, and by the fact the idle will at times fluctuate of its own accord, suggesting the IACV is receiving some form of input from the ECU. I am wondering about the integrity of the wiring ‘ connectors, and dreading an ECU problem.

 

I noticed you have replaced the IAC valve a couple of times before

Unfortunately it doesn't have a follow-up position sensor to verify it is behaving correctly

The 4 IAC wires have no splices between the ECU and IAC

Q in Hong Kong found water migrating into the wire loom as it goes over the fuel rail and goes behind the firewall

This was from a bad seal on front windshield replacement

His system effected was starter enable

The IAC should waver in valve opening through your drive cycles and step through and settle down but it can still waver if one of the windings inside temporarily opens by heat expansion or long run wiring loom fault

The EVAP valve should be closed at idle and may be intermittent but I wouldn't cap that line risking the fuel tank collapsing

 

This shouldn't be that difficult to sort out with your diagnostic kit.
Your car is SC, one thing to remember is that you cant use cheapo IACV, hopefully you don't, I remember someone having issues with them until they swapped it with genuine Jag part with correct plug orientation, not an issue on NA cars.
You don't have to unplug the IACV to check its operation like on NA car, follow the loom and add a plug to it, hopefully that should be possible and you'll be able to check it that way.
What's the TPS value at idle, especially when its dying? Does it drop below 11.37%?
How are other values? CTS, o2 sensors ok? MAF showing anything?
I don't think this would be an issue with fuel as you wouldn't be able to rev it either but just to be sure you can always pinch return line to boost pressure, see if it does anything.
Also I know you said, MAF is ok but also just to be sure, start the car, disconnect MAF, see what happens. Should hopefully not die and default to basic map. You might not be able to start it without MAF connected though, depending on which year your car is but it should be running ok without it plugged in.
Also, as always worth swapping coils one by one if you have a spare one.

 

A long overdue update, and at the risk of tempting fate, I think I have found my missing idle!
Katar83s last post set me thinking, as my TPS reading at idle was only 11%. In my previous efforts to cure a high idle, I had always assumed that it was a case of the lower the better, but it hadn't occurred to me that If the TPS value is below the minimum threshold recognised by the ECU, it would explain why it did not attempt to regulate idle. With the throttle plate fully closed and the IACV closed, there would only be the air in the manifold for the engine to run on, hence the catching and immediate dying I was experiencing.
Much further head scratching regarding possible causes for this low reading prompted me to go through the pain of extracting the throttle body from the car (again), and to check everything over carefully. The throttle body was spotless, as it had been previously ( I am wondering whether the absence of EGR on Uk spec cars explains this?) and everything appeared to be in order, so I refitted it, paying particular attention to the tension of the throttle relaxer to throttle cable. I used my scan tool to check that at idle it was not opening the throttle, and I also fitted a stronger throttle return spring.
Fired her up, and she settled into a slightly fast idle. Repeated a few times, whereupon she settled into normal idle, where she has been ever since, without having had to perform a TPS adaptation procedure.
I am happy that she is back to running as sweetly as usual, and feel that I have further expanded my familiarity with the throttle body. I would be happier still if I had been able to find the smoking gun, but I guess that was not to be.
Thank you again to everyone for your help.