Hello again,

I must sound like a broken record with my Idle Speed issues on my '92 XJ6 4-liter.

I spent a good portion of a day two weeks ago taking the entire intake plumbing off, cleaning everything religiously, cleaning the mating surfaces of the TB components (trumpet, adapter plate, etc), resealing everything, adjusting the throttle butterfly, adjusting the TPS, etc, and setting the Base Idle Speed.

I've performed the Idle Baseline Learning Procedure (Car warms at idle from cold, drive over 50 yards at 3+ mph).

I've checked every vacuum hose I could find, I've temporarily plugged the EGR solenoid temporarily while I sort this issue out. I've scoured just about every inch of intake, it's plumbing and vacuum lines with Propane gas and starting fluid to check for leaks. No luck.

The car has a proper idle when cold starting, 800 or so RPMs, which then eases down to 600 or so when the car is warm. The problem starts after driving away. When slowing to a stop, the engine speed very often hovers around 900-1000 RPM. This causes the brakes to fight the engine at stop lights.

If, when stopped with the high idle, I put the transmission in neutral, the RPM will jump to a high of 1500 RPM, usually surging from 1200-1500 (the ECM has a 1500 RPM rev limiter at idle...). The surging usually stops after a couple seconds, and will then slowly lower to 700 in neutral, and in gear, 600, where it stays until the next stoplight.

Other times, in the same driving conditions/speeds, I come to a stop and the engine speeds drops correctly to 600 with no problem at all. I don't remember the idle ever being high with the engine cold. This only seems to happen when the engine is fully warmed.

The IACV is a new identical replacement unit labeled for Renault (cheaper) that I installed about a year and a half ago. I still have the original and I'm thinking about reinstalling it. The gaskets were replaced when I had the IACV housing off to clean the carbon buildup. I have not touched it since then, and it may need a cleaning.

MAF is clean and responds quickly and correctly to engine speeds changes via the PDU. The TPS responds correctly to changes in throttle position. ECT is correctly reporting engine temperature.

Injectors are brand new, replaced about a year ago. Spark plugs were replaced 1 month ago. Wires, Coil, Cap and Rotor were all replaced.

I'm at the point where I'm thinking about getting rid of the XJ40 and getting another X300, but XJ6. I love the car a lot, but I just can't seem to track down this idle issue and I've been searching for the problem for over 2 years. Every time I drive it I'm *hoping* for a Check Engine light so at least I can have a clue!

Nick

 

Nick, I feel for you, especially the 50 years at 3+ mph

Seriously though, one thing to try would be a hard reset. It can't hurt!

I know after a hard reset the idle has to be re-learned and will be haywire on first startup after the procedure, but it should settle.

To perform said procedure:

Disconnect Neg battery terminal. Touch Neg cable to Positive terminal and hold it there for about 30 secs. This will clear all the stored fuel maps so your driving habits will have to be re-learned by the ECU.

Might be an idea to do an intake and breather muckout first so at least you're starting with a fighting chance

I've done one of these a couple of times over the years and I believe the car ran better afterwards ..certainly gas mileage improved!!

No danger btw, no need to worry, perfectly safe.

good luck and don't go to the ugly x300's just yet, ok?


Larry

 

Hello Larry,

After I took the TB and Intake Plumbing off to clean, I disconnected the battery for 30 seconds to reset any fuel maps and the base idle correction figure. I didn't touch the Neg cable to the positive terminal, however, if that changes anything...

I also cleaned the breather hoses and heating element as well.

Here's a video of coming to a stop in gear. Notice how high the idle speed is staying. When stopped, I put the transmission in Neutral, the engine speed surges up and down. It's important to note that my foot is off the gas the entire time. If the light didn't turn green, given about 5-10 more seconds, the engine would've dropped to the correct idle.


Here's a video of my driving down a level, smooth highway at about 70MPH. With my foot off the gas, I put the car in Neutral. See how the engine keeps surging up and down, over and over until I put the car back in Drive?


-Nick

 

You have to touch the terminals together for 30 seconds to reset the fuel maps completely!!

A simple neg terminal disconnect doesn't have the same effect, I've tried it and I know it works.

There's only a couple of things that would make it surge like that, air and fuel, yes?

How's your fuel pressure regulator and aux air pump solenoid circuits? Carbon canister purge? Stuck relay on aux air pump? Just guessing tho ...

 

Nick,

I can't remember whether you've checked the VCM for any stored codes. I seem to recall that some of those can trigger without the CEL illuminating. I assume you'd be able to see them via your PDU, but it might be worth checking via the VCM anyway.

I like Larry's suggestion about a stuck open carbon canister purge valve would behave like an intake air leak, but wouldn't react to your propane tests since the valve and canister are mounted ahead of the left front wheel behind the wheel well liner.

You don't mention the O2 sensor or Intake Air Temperature Sensor (IATS), but if you can read their signals via PDU it would be worth ruling them out, since their signals are also taken into account by the ECM to determine fueling via injector pulse duration (also involved are the MAFS and CTS/ECT, which you seem to have ruled out).

I would also assume that looseness of the intake manifold itself would have been found by your propane tests, but it would be worth checking the torque on the nuts and screws that secure the manifold, since the torque spec is fairly low and they are known to vibrate loose over time.

Cheers,

Don

 

I'm really in a bit of a shock here...

I decided to tinker around for an hour tonight. I disconnected the battery, turned the key on, shorted the two cables for 30 seconds, turned the key off, reconnected the cables, and then removed the idle speed control valve housing.

The housing was clean, and the paper gasket that I replaced with the IACV was still perfect. I found the original IACV and installed it. Started the car from cold, idle speed perfectly at 800RPM, it slowed to a perfect 700 as the car warmed to operating temp. I drove straight off, and like magic, no high idle, no stumble, no nothing.

I drove the car for about an hour. Highway and Main Streets with lots of lights. The RPMs have dropped to 580 every single time. I even stopped and reconnected the EGR hose. No stumble at all. On the highway at 70, I shifted into Neutral. The RPMs fell to 580 and stayed right there, no bouncing around.

And now, when cold, as soon as the engine turns over, the RPMs jump to a lively 1500 or so, and instantly drop go to 800 as it should. The past couple weeks, the initial engine speed at turn over was a slow 800RPM, and the engine actually stalled on startup more than a few times. No problem there, now.

I really can't believe this... I replaced the IACV because of the stumbling. I replaced two OEM EGR ($$$) valves because I thought they were sticking. I went through the entire ignition system. I literally spent the last 2 years trying to sort this issue out.

Every time I did some work, I unplugged the negative cable for 15 seconds and figured the computer was reset, the way it always had been for my other (more modern) cars. I remember reading somewhere about shorting the leads but never put much thought into it. I'm a believer now.

Thank You Larry and Don for all your help with this ongoing issue. You both have helped me try to track this down over the past couple years. I really am beaming tonight. I can't even tell you how frustrated I've been with this issue. Driving the car wasn't enjoyable for me, hesitating and misfiring at every light, and then recently, having to stand on the brakes coming to a stop. It was to the point that I was having my girlfriend drive the X305 daily because I didn't feel safe with her in the XJ40.

-Nick

 

Woo Hoo!!!!

Fantastic news!

Cheers mate and happy motoring!!

Larry

 

Awesome, Nick! I'm happily sharing your shock!

I wonder if the Renault stepper motor never calibrated properly? Or, is it possible that the tip of the plunger on that motor was not identical in shape to the tip of the plunger on the original stepper motor? If I recall, there were two different designs of the Renault motor, one of which had a plunger tip that was not quite the same shape as the plunger on the original XJ40 motor.

I'm really relieved for you, and just look at the amazing research your threads will provide for future owners who are trying to track down similar symptoms.

Cheers,

Don

 

Thanks again, Guys!

Don, I'll have to swap over the LOWE IACV that I had shipped from Latvia (!) to see if it was the IACV or actually the Hard Reset of the ECM.

IIRC, I ordered the specific one you mentioned in your post about a year and a half ago. It certainly could be the valve, but I replaced it due to the stumbling on deceleration. I don't remember the exact profile of the plunger, but I'm interested now.

I'm nervous to touch anything though... Haha!

Nick