KV6 engine racing

[SD1too]

No I'm not talking about motorsport! I have a problem which is really getting me scratching my head. I've tried lots of things none of which has worked.

It's my idle speed. Start the engine from cold or warm and everything's fine, but as soon as I press the accelerator, even blipping it, the engine speed rapidly rises to 2,600 rpm and stays there. Switch off, restart and it's fine again, until the accelerator is pressed again.

It's been on T4 and there are no relevant fault codes.

When I seal the air intake to the idle speed control valve the engine speed steadily reduces to a reasonable idle. When I let go, it goes up to 2,600 rpm again.

• The stepper motor is referencing its position on power down (as described in RAVE).
• I have fitted a new stepper motor and a new throttle position sensor.
• There is wiring continuity to these from the ECM.
• The throttle butterfly is tightly closed.
• The crankcase breather tubes haven't fractured.
• When I manually close the traction control butterfly, the reducion in engine speed is minor.

Can anyone explain to me why the engine would start and idle normally until the accelerator pedal is blipped when it races uncontrollably?

I need a solution because the car is undriveable at the moment.

Simon

[Typhoon190]

Potentially wrong, but I'm sure I've seen a similar thread and the solution was item 5 here.

https://rimmerbros.com/Item--i-GRID001951

[SD1too]

That is the gasket between the two halves of the idle air control valve. You're absolutely right that if damaged, air could by-pass the stepper motor setting and be drawn directly into the manifold chamber.

I renewed that for the second time yesterday afternoon. It made no difference.

Thanks for your interest Ben.

Simon

[vitesse]

Throttle linkage comes to mind as you seem to have covered everything else. I'd disconnect the cable and manually operate the linkage observing how smoothly it operates. If that fails then I'd fit a used throttle body and accept temporary defeat.

Good luck

[SD1too]

Originally Posted by vitesse

Throttle linkage comes to mind as you seem to have covered everything else.

Hi Mike,

My apologies for not including this in my opening post. I tried to keep it short and readable.

The first thing I did was to remove the throttle body from the manifold chamber. The screws seemed rather loose and the rubber seal was compressed so I thought I'd found the cause. Sadly not. A new seal provided no improvement.

I took the opportunity to clean and lubricate the whole mechanism, crucially checking that the throttle butterfly makes a good seal using an application of carburettor cleaner to one side. It was perfect.
Upon reassembly I adjusted both the accelerator and traction control cables correctly, ensuring that they rested upon their stops and moved through their full arc. I could not identify any fault with them.

Further tests performed today

Brake servo fault ruled out by disconnecting and sealing its vacuum supply.

With the engine idling normally, all connections and joints on the throttle body and manifold chamber sprayed with carburettor cleaner to identify suction by means of a raised idle. There was none. The only action which induces the fault is rotating the throttle linkage but strangely when it is released the engine speed remains high.

Simon

[Teflon]

I suspect it's something to do with the throttle body, but no idea what. Many years ago, in my quest to clean up my engine bay, I got a 2nd hand throttle body from Phil T4. I polished it up to within an inch of it's life, and fitted it. It had a life of its own, revving away as soon as the throttle pedal was touched. I teated it to a new gasket set, and all was well.

Fast forward a good few years, and I decided I would have a painted throttle body, so I cleaned up my original (known good), body, and painted it Lagoon. Fitted it, and it refused to idle properly, this time dropping the revs whenever the throttle pedal was used. A new gasket set and the same issue. A new Idle Control Valve and another new gasket set, still no change. Replaced it with the other throttle body and every thing is fine.

I still have a nice, Lagoon coloured throttle body, which looks terrific but refuses to work properly. Makes a great paperweight though.

So in summary, no real help , other than to confirm these throttle bodies can be awkward little swines when they want to be.

Cliff

[Arctic]

Hi Simon.
Check the out sheath of the throttle cable to see if there is any kinks in it along it's length, if so if maybe snagging? same goes for the half can the cable fits into any sharp edges.

[Synchromesh]

Is there any play in the throttle butterfly spindle?

[KWIL]

Simon.


I had a very similar situation some time ago. In the end it boiled down to wear on one of the components on the throttle spindle (where they rely upon the parallel section).


Opening the throttle would move it but upon closure there was insufficient effort available to actually close the disc.


I had another throttle body to strip for parts and the parallel fit can be improved by judicious hammering to tighten the fit.

[SD1too]

Thank you everyone for your thoughts and suggestions. Here's an update on today's investigations.

Originally Posted by Teflon

... I got a 2nd hand throttle body ... and fitted it. It had a life of its own, revving away as soon as the throttle pedal was touched. I teated it to a new gasket set, and all was well.

Those are exactly my symptoms Cliff but I too have renewed all gaskets (idle control valve body, stepper motor 'O' ring, seal to manifold chamber) but without the luck you had.

Originally Posted by Arctic

Check the out sheath of the throttle cable to see if there is any kinks in it along it's length, if so if maybe snagging?

Thanks Steve, that has been checked and it's running smoothly. I get the same high revs if I move the throttle linkage directly, i.e. not using the accelerator pedal so I think that rules out a cable fault.

Originally Posted by Synchromesh

Is there any play in the throttle butterfly spindle?

That's a question which was asked a year ago when the idle was hunting (which it had been doing for many years, off and on). When I had the throttle body off the car no plays was apparent but it's not easy to judge due to the effect of the strong return springs. However today I think I've ruled it out.

I found a rubber bung which fits the idle control valve air intake perfectly. With the engine racing I first closed off the main air intake by manually closing the traction control butterfly which precedes the throttle butterfly. The engine speed didn't change. I next inserted the bung in the idle control valve intake and the engine quickly stalled. This appears to prove that the high air flow is all through the idle control valve.

I next refitted the original stepper motor whose plunger adopted a position 5mm shorter than its new replacement. Consequently when the engine was started and the throttle blipped, the revs rose even higher to an alarming 3,000 rpm. This appears to add to the evidence that the idle air control system is not working properly but why?

Originally Posted by KWIL

I had a very similar situation some time ago. In the end it boiled down to wear on one of the components on the throttle spindle (where they rely upon the parallel section).
Opening the throttle would move it but upon closure there was insufficient effort available to actually close the disc.
I had another throttle body to strip for parts and the parallel fit can be improved by judicious hammering to tighten the fit.

Thanks Ken, this sounds interesting but could explain what "the parallel section" is please?

Simon