Erratic tickover RR 3.9EFI

The idle adjustment screw isn't really an idle adjustment.... all it does is set the base idle level.

In order to set it correctly you need to completely block the throttle bypass so that the only extra air getting around the throttle goes via the base idle adjuster. You then set the base tickover.

The best way to adjust it is by putting a blocked tube in circuit with the idle control's air pipe. Without this blockage the idle control will "compensate" for any changes you are making. You need to check that the small passage way that the base idle adjuster controls, is clear. You should be able to get enough air past the throttle to set the base idle as high as about 800 rpm. If you can't then you have a full or partial blockage.

Once the base tickover is set, the idle control can try to do it's thing.

The "blip" that you get when the aircon kicks in is supposed to be just that. It is there to stop the engine stalling before the idle control has enough time to respond. Once the blip has occurred the idle control will gradually return the idle to the correct level. It may well vary the idle a bit while it constantly adjusts but it sounds to me like the base idle may be too low.

Another thing to check is the T piece in the breather pipe at the front left of the plenum chamber (near the TPS). You will see a larger diameter pipe running from the rocker cover to the plenum chamber intake with a smaller pipe coming off it and going into the body of the plenum chamber.

The plastic (AND VERY FRAGILE !!!!!) T piece has a metal bush with a pin hole in it. The pin hole gets blocked with crud from the breather and can cause erratic idle and more crap ends up in the throttle housing and blocks up your base idle air way... Take great care when trying to remove the pipes from the T piece or you WILL snap the smaller pipe off ! The plastic seems to get very brittle due to the heat and all the fumes from the breather. Did I say, the T piece is VERY fragile ? To test it take the small pipe off the plenum chamber and try and blow down it. It should offer some resistance but not be blocked.

HTH

cheers

Dave W.

Aye - There are two screws really - one that sets the position of the throttle flap - that's set as per spec.

Done that - base idle is right.

With the pipe to the stepper motor blocked it does set to whatever I want - reconnect the pipe and it goes crazy again.

Set to 750rpm - should be about right...

All good thoughts (and ones I've had myself) - I've had all the piping to bits, cleaned as clean as an operating theatre and put back together. I did break the 'T' piece and machined myself a nice new one out of a block of aluminium :-) Maybe a little bit of an over kill, but it wont break again.

Hmmmm....

Well, you could (if you haven't done already) try the other hotwire "cure all" - replace the ECU coolant temperature sender....

Seems like you've tried everything else ! Have you tried a different stepper motor ?

cheers

Dave W.

I didwonder - but no fault light on on the dash... Unplugging the coolant sensor brings up a light, so it's detecting something... Any idea of what the sensor should be showing? A range of different resistances depending on temp I suspect...

I reckon the stepper is OK - it's winding in and out OK - starting the car with the stepper laying on the engine (connected) show's it opens to start then winds back down to try and control the tickover.

Starting the car with it installed but unplugged gives a tickover of about 1500rpm.

Mine failed a while ago, but it wasnt showing an error - its one of those thigns thats worth changing just to be sure.

Did you reprogram the motor after you had unplugged it?

Its something like ignition to pos 2 (dont start) wait for motor to stop winding, then unplug the motor and switch off ignition. Repeat 6 times.

Another potential is that the stepper motor, although working, has worn the "head" bit, and therefore isn't working properly. Another thing that my garage suggested was the bits that the motor activates inside can also wear (they're only aluminium after all).

Personally, I would start with the coolant sensor

Si

No... He confessed shamefacedly...

Hmmmm...

You could be right - although it could be the MAF or ECU :-(

Time for a beer and a ponder I think...

Cheers!

You won't get a fault light unless it's open circuit. It's a thermistor so gives a different reading depending on temperature. When they fail they seem to just go out of range so you end up running incorrect fueling levels. They seem to cause a whole range of problems when they do fail and only cost a few quid to replace so I usually change them as a matter of course when I come across any problems on hotwire systems.

Correct readings are:

-10 degrees C : 9100 - 9300 ohms

0 degrees C : 5700 - 5900 ohms 20 degrees C : 2400 - 2600 ohms 40 degrees C : 1100 - 1300 ohms 60 degrees C : 500 - 700 ohms 80 degrees C : 300 - 400 ohms 100 degrees C : 150 - 200 ohms

cheers

Dave W.

Cheers David - I might replace that as a matter of course. The anoying this is that the tickover is perfect without the aircon compressor running - it's only when the system calls for more revs at tickover that this hunting up and down starts.

With the temp gauge in the middle and the car idleing I got 340 Ohms on it.

Cheers.

Hmmmmm..... that looks about right then - have you seen these :

:-)

I'll have a think about it while I have a pint and see if anything comes to mind.

Where are you ?

cheers

Dave W.

For some reason I was very loth to click that link ;-)

Raunds - Northants. The problem is, it's not really a problem! Without the aircon on it's fine - it's just the aircon causing it!

I'm sure I'll get to the bottom of it eventually (or die trying)!

I've now got half the dash out to suss out why the heater wont blow cold (air con is OK but the heater floor and window vents only blow HOT)...

On or around Sun, 30 May 2004 22:22:48 +0100, "Simon Atkinson" enlightened us thusly:

sounds more likely to be the bypass air valve thing, mind.

I think I am fast coming to the conclusion it's "something". Perhaps I should leave it at that?

On or around Mon, 31 May 2004 10:22:18 +0100, "Simon Atkinson" enlightened us thusly:

hehe.

The bypass air valve (and its stepper motor) allow more air into the plenum (and hence more air through the AFM, and more fuel) when the engine, idling, is put under load so as to lower the revs (most likely by the aircon compressor cutting in).

so sayeth the book. I assume that it gets signals from the throttle position sensor (tells it that it's at idle) and the engine speed sensor, and instructs the stepper motor accordingly.

It's doing all that (sort of) - just not right. It doesn't hold the revs steady - they flucuate up and down all the time. Without the load (aircon) it's fine...

On or around Mon, 31 May 2004 18:08:14 +0100, "Simon Atkinson" enlightened us thusly:

mayhap the valve or stepper motor is sticky, causing it not to move and the move too far and overcompensate?

does it continually fluctuate, or does it eventually settle?

does it do this if you apply load some other way, such as putting it in gear and lifting the clutch slowly at idle? - it's just occurred to me that the aircon might be applying a fluctuating load for some reason.

Possible - although it looks and moves OK when I had it out.

Continually.

It's an auto - does it in neutral or in gear, but not without the aircon on.

It's a thought - but if it was that fluctuaty (if that's a word) then either the eircon wouldn't work, or it would make a noise or break the belt...?

On or around Mon, 31 May 2004 21:36:57 +0100, "Simon Atkinson" enlightened us thusly:

buggered if I know.

if it idles correctly in gear, I'd be inclined to suspect that the idle compensation is working, since putting it in gear puts load on it too.

you got several relays involved with the air con so it might be worth checking these!