Since replacing my head gaskets I've had an issue with misfiring (causing a heatshield fire), revs sticking and a high idle.

The misfire cleared itself up - I never found what was causing it as when I came back to the car to take a look it was running fine.

The sticking revs was really bad straight after the rebuild. Initially they would stick/hold at whatever RPM I revved up to, occasionally continuing to rise slowly on their own. I'd have to turn the engine off to correct this.

After a little while the engine would rev up and the revs would drop on their own - only to get stuck at around 2K RPM. Again, the only way I found to stop this was to turn the engine off. Sometimes the RPM would drop quickly from whatever I revved up to down to 2K where it would "snag" and then drop more slowly.

When I was finally able to drive it I found that it was holding revs well enough on it's own to chug along at 30mph without needing the accelerator pedal. Not slowing down on it's own and not speeding up. Using the brakes would slow the car down and the engine revs seemed fine once I came to a stop.

Now, after having it on the road for a few days the rev holding seems to have settled down and it's not sticking at any RPM but I have noticed that my idle RPM seems high. It seems to vary between 1,100RPM and 800RPM. It's not bouncing between these values, it's just sitting smoothly at different points in this range at any given time I look at it. It never gets as low as the 660RPM that RAVE says it should be.

I've had a brief chat with Sloth about it as the symptoms seemed to match an old thread of his that I found online. He said he fixed his by moving the throttle cable routing. My cable seems to be routed well and as far as I can see the cable is moving freely. The throttle body seems to be closing fully when the throttle pedal is released - the cables are returning to rest against the stop properly. I tried to check the throttle body by removing the intake pipe with the engine running but the engine immediately died. I guess the sudden change in airflow was just too much for it cope with. I think my symptoms are now a bit different to his.

Given that I lack a Nanocom to investigate with, can anyone suggest things for me to try to diagnose this issue?

I've seen a few threads online (mostly Disco 2 related, in America) saying that they've reset the adaptive values and that's sorted high idle issues. Is this only possible with a Nanocom?

If you ever turned the ignition on with the throttle pot unplugged, there's a good chance it's learned an incorrect voltage for the idle setting.

What happens is, when the throttle is closed and the throttle pot voltage is around a certain threshold the idle stepper controls the idle speed as you'd expect. Off idle the stepper screws all the way out, and when you shut the throttle suddenly it'll wind slowly back in so it doesn't suddenly slam the air supply shut.

The ECU only seems to learn the lowest voltage it ever sees, so with no throttle pot on it'll see idle as being 0V, and when you reconnect the throttle the "real" 0.5-0.7V idle voltage will be seen as the throttle being somewhat open.

Unfortunately you will need some diagnostics (Nanocom does it) to clear the adaptives. You can bugger around with expanding the mounting holes for the throttle pot into slots to rotate it round a bit, that works too.

IIRC the adaptives on the Bosch/Thor are actually lost if the battery is disconnected - whereas on the GEMS it definitely needs resetting with diagnostics.

I haven't tested this theory to confirm though myself as I've not had the need.

Sorry, I've not come across the term throttle pot before - do you mean starting it up with electrical connector on the side of the throttle body (throttle position sensor?) disconnected?

If so, then I'm not sure.

I know I reconnected the battery briefly so I could open the boot while the engine was in pieces but I don't remember if I turned the ignition on. I can't think what reason I'd have had to switch the ignition on with the heads removed but it's possible that I did during the later stages of reassembly.

It did spend a few weeks with the battery completely disconnected. If that cleared the adaptives as Sloth suggested, (mine is a Thor) might it just be a case of the ECU needing a bit of time to figure things out?

Are the "adaptives" the same as the idle voltage Gordon mentioned?

Would the adaptives have any affect on MPG? Struggling to get above 13mpg at 50-60mph, definitely lower than it used to be.

The adaptive values are a number of things that the ECU learns while the car is being driven, so things like correction factors on the MAF sensor, long term fuel trims and most importantly in your case, the throttle position sensor idle voltage. It is a simple potentiometer, or pot to us electronic types, hence throttle pot. As Gordon says, if the stored voltage for a closed throttle is higher than the voltage from a closed throttle, it will adjust downwards but it doesn't, or at least the GEMS doesn't so no idea about the Thor, adjust upwards . So if the stored voltage for a closed throttle is lower than the actual voltage it will see that as the throttle being slightly open and raise the revs accordingly. As it uses the idle control valve to raise the revs when you give it a tiny bit of throttle, it isn't going to adjust upwards or giving it a crack of throttle to raise the revs slightly wouldn't do anything.

So it sounds like first port of call is resetting the value stored for idle RPM that can only be done with diagnostics?

Next question then, what, besides Nanocom, is capable of doing this?

Sloth did say that the TPS idle voltage can be cleared on a Thor by disconnecting the battery. If it can't you need Nanocom, Faultmate, Testbook or one of the other dedicated diagnostic units.

RAVE confirms that adaptive values are lost when battery power is removed. Presumably that includes TPS but it doesn't actually say.

Another potential issue is low or unstable supply voltage to the pot due to connection issues. Known to be problem on many other system but ours are usually pretty well behaved. Worth a squirt of cleaner on the connector contacts if the problem persists.

Clive

Gilbertd wrote:

Sloth did say that the TPS idle voltage can be cleared on a Thor by disconnecting the battery. If it can't you need Nanocom, Faultmate, Testbook or one of the other dedicated diagnostic units.

Sloth wasn't sure but I guess I was premature in saying it could only be done with diagnostics. I was figuring that diagnostics would be guaranteed to do it whereas disconnecting the battery wasn't :)

I'll try some cleaner in the TPS connector, see how that goes then try disconnecting the battery and see again.

Cheers guys :)

Disconnect both battery leads and touch them together. That will ensure there is no residual current stored in any of the electronics. Leave it like that for 20-30 seconds, then reconnect.