XJ6 EFI cold start heavy hunting Up&Down
 

I had this problem from September until the begin of April and i solved it after a long search.........

IF you read the service manual on the fuel part you see that there is a overrun cut off at about 1470 RPM with closed throttle. Page 19-4 The part under; Vacuum Throttle switch (UK european only)
The microswitch detects closed throttle.

http://img186.imagevenue.com/loc68/t...9_123_68lo.JPG

What happens is that your RPM goes beyond this 1470 1500 or a bit more at coldstart.
This happens meanly in colder periodes of the year and not in the sommer. Due to the values given by coolanttemperaturesensor.

The fuelpump will be cut off and the motor RPM goes less and then passing 1400 RPM when the fuelpump becomes activated again.
In reality the RPM falls down to maybe 600 RPM etc. In worse cases less then 600 and the engine stops and has to be started a few times or the foot on the throttle. Depending on the state of maintenance of the car.

This problem stops when the coolant has a temperature that the coolantsensor gives the ECU the signal to decrease the enrichment so that the RPM becomes less then 1470 RPM and the engine slows down to normal idle of 800 rpm like it should.

In the summer the coolant has a temperature that the coldstart RPM won't get as high as 1470 RPM or beyond

How does this all happend?
Probably there is a leak somewhere;
- injection rubbers,
- behind the airflowmeter
- exhaust manifolds have cracks (which make the engine shake)
- Vacuum leaks
fuel pressure regulator ? leaking hotfuel valve cutoff.

These are problems that can cause the idlespeed to become less then 800 rpm. 700 RPM? 650?

A reason to turn up the idlescrew to get the idle back to 800 RPM when the engine is hot.

Due to the upscrewed idle with idlescrew from the airblock; at cold start in the colder periodes of the year, when you get an higher enrichment the engine comes beyond the 1470 RPM and the overrun cut off does it's job;
shutting down the fuel pomp.
When the engine falls below 1400 RPM the cut off is released.
In reality the RPM falls deeper.............as i said earlier.

Do you turn the idlescrew in to get the engine run less then 1470 RPM at cold start your idle RPM when the engine is hot will be less then 800, depending on the problem maybe 600 RPM.

You have to find the problem; ( i had all the mentioned problem due to poor maintenance)

The immediate solution is to remove one connection of the microswitch ...........and the overrun cut off does not work any more. In extremer colds the engine can run above the 1470 RPM in parked position of the gearbox. Minus 20 Celsius maybe 1700 RPM.


I have tried all other solutions mentioned on the internet; they didn't work.
aux air valve or tentioning the spring of the overrun in the airblock.

What happens; should happen....... overrun cut off fuel by the ECU beyond 1470 RPM.
The real reason for the pumped up idle screw needs to be found.

Driving without the microswitch is no problem what so ever.

A idle screw turn is not without consequences and not a propper way to deal with idle less then 800RPM..............

hi,

I have been told that the up-down hunting during idling is the classic symptom of a sticking Auxiliary Air Valve.

One solution I have heard about is a Techtron 'tune-up', which entails adding the Techtron stuff to each tank, driving around for 2 hours, accelerating to 90 mph, then shifting to 2nd gear. They say you should see a cloud of black smoke in the rear. All the crud was burnt off.

I've never done it, so I cannot swear by it. but others do. I just don't believe them.

Jose