Idle valve operation

[kabaman]

As part of my investigation here: http://www.pistonheads.com/gassing/topic.asp?h=0&a...

I've got Rovergauge up and running.

Three interesting things I note:

I see the idle valve go to a '1' and then back to midway at startup. Gradually over the time I drive the car the control goes further and further down towards '0' untill it looks like it's run out of adjustment.

I presume this is symptomatic of it opening further and further to get the fuel mixture down (trying ot get leaner) until it can't go any further. Am I making sense of this correctly? I've converted the 0 to 1 reading to 0 to 100 to make it easier to fit on the graph.

Also, I get an idle fault, that is the Rovergauge green LED for idle never comes on and a search of codes reveals this as a fault.

Also I notice that roadspeed is exactly half of the actual roadspeed.

Does anyone have any comparison data or graphs and or comments on this?

Thanks, Neil

[kabaman]

I should have added that the attached trace isn't from start up. If I capture data from start up, the vale goes up to 1 (100 on graph) down to 0.5 (50) and then gradually heads down.

N

[blitzracing]

The idle valve should start full open on a cold engine, then typically when warm it will be around a mid point, My engine is around 130 steps out of 255 available so around 50%. It should never be either 0 % or 100% or it will throw a fault code after a period. The stepper does not control the mixture, simply the amount of air the engine gets. If the stepper is closing off the air supply bit by bit, you have an air leak somewhere around the plenum thats letting in extra air as the engine gets hotter that the stepper is trying to limit, or the base idle screw is wound up too far in the first place.

The road speed on the TVR is fixed by the speedo calibration box under the dash, and does not represent a real road speed value. It simply goes from 0 to some speed (between 30 - 60 mph Ive seen so far) at 3 mph. Ive not seen it vary like that if Im reading your graph correctly- it seems to be following engine speed, but I assume you are changing gear....???

Edited by blitzracing on Wednesday 7th August 18:37

[kabaman]

Brilliant - there are no obvious leaks. Winding in the idle screw in the manifold has moved me to the point where the valve isn't running out of play (around 25% now when warm). The car is much better behaved now just off the throttle - has made me realise how bad it was before!

Thanks,

Neil

[FlipFlopGriff]

There is a rubber hose which goes from the stepper to the back of the plenum which is difficult to get at. Its about the same ID as the vac advance and only a few quid to replace.
FFG

[davep]

Blitz I don't think the LT77 Griffs have a speedo calibration box. Nice to see your RoverGauge at work Kabaman, neat graph as well. Did you use Excel?

[kabaman]

Is that hose around the back the likely culprit FFG?

And yep - the magic of excel :-)

[FlipFlopGriff]

kabaman said:

Is that hose around the back the likely culprit FFG?

And yep - the magic of excel :-)

Not sure but its one less thing to worry about and a few quid for a silicon replacement, befoe you go searching deeper. From memory its 4mmID and to do the vac advance and this you'll need about a metre.
FFG

[blitzracing]

davep said:

Blitz I don't think the LT77 Griffs have a speedo calibration box. Nice to see your RoverGauge at work Kabaman, neat graph as well. Did you use Excel?

Are thanks, I did not check the age of the car before posting.

[Steve_D]

FlipFlopGriff said:

kabaman said:

Is that hose around the back the likely culprit FFG?

And yep - the magic of excel :-)

Not sure but its one less thing to worry about and a few quid for a silicon replacement, befoe you go searching deeper. From memory its 4mmID and to do the vac advance and this you'll need about a metre.
FFG

The small hose round the back there is the vacuum feed to the fuel pressure regulator.
The feed to the idle stepper is the best part of half inch diameter.

Having said that the condition of any of those hoses should be checked as they could be a difficult to find air leak.

Steve

[kabaman]

I think a ruptured vac advance was the main culprit and possibly the seating of one or two of the inlet hoses.

I can now get the valve to operate within the middle of its range by adjusting the base idle (I know the correct way is to adjust the base idle first).

There's still debug to go, but this at least seems ok for now.

Cheers,

Neil