M84 - Idle Valve Issue

Discussion and support for MoTeC's previous generation of entry level ECUs

M84 - Idle Valve Issue

Postby DrIng on Wed Jan 20, 2021 3:03 am

Hi there,

I'm new to the forum and have a question about idle control that I've not been able to resolve myself or through search.

The engine is a Porsche 964 3.8L running on individual throttle bodies. The car is starting and idling and running nicely.

The car has a Bosch idle control valve and I note some peculiar issues when it is active. Basically, it is quite inclined to cause lean lambda but does not seem to do so on a consistent basis. It's also not wonderfully effective in terms of actually controlling idle. If I re-start the car warm (say ET 20~40c) it will idle well above the set aim of 1100~1000 - quite possibly 300 rpm above aim. Neither of these issues are of particular concern to me because I can correct the erratic lambda with ST trim (dual lambda) and the slightly high idle just isn't an issue because it settles down fine once the engine gets a little warmer.

Auxiliary Out Setting #6
Function 2 = PWM Idle Speed Control
Paramaters are set to 100htz, Polarity 0, Output Mode 0

Setup settings are as follows:-
Prop Gain 0.20
Integ Gain 0.050
Deriv Gain 0.00
Deadband 25
Ground Speed (n/a - no speed sensor)
Activate RPM 1500
RPM Filter 4

Idle fuel compensation has been set based in the valve position and basically runs from 0% to a max of 40% fuel against position 0 to 100. Basically it's actually set for +6% at position 40 and +40% at position 100.

When I look at logs of the engine idling warm I might see ...
RPM 925 @ Engine 60c, Idle Position 40.3, Idle Fuel Comp +10.4% and perfect lambda with no trims.

But five minutes later I might see ...
RPM 950 @ Engine 58c, Idle Position 40.7, Idle Fuel Comp + 11.1% and ST Trim is having to add a further 6% in order to maintain lambda.

Two minutes later I might see ...
RPM 920 @ Engine 59c, Idle Position 40.0, Idle Fuel Comp 9.9% and ST Trim is having to pull 3.6% to maintain lambda.

Also the warm re-start - if any one has any clues? Starting from cold is fine. Idle is fine on start (1100) and settles at circa 950 once fully warmed. It does not idle high as it passes through ET of 20~40c, only does this if it's re-started in that temperature range.

Neither of these issues are significant problems but if anyone has any ideas I would like to try and resolve.

Ciaran, Dublin.
Posts: 1
Joined: Thu Sep 24, 2020 9:31 am

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