Mi16 On Motronic Idle Problem

[mmark118 0]

Hi,

 

i have a 88' 309 GTi with Mi16 conversion...

 

it's running on MOTRONIC ECU and the problem is that car idles too high when warm.

so, when cold, it idles a bit high just arround 1500RPM(it's ok for cold engine), and then it warms up to about 60°C and idle settles on just under 1000RPM and everything works fine, in a short drive revolutions drop quite fast to about 1500RPM and then a bit slower to 1000 and that's fine i suppose, but after it warms up to about 85°C idlling is still ok, but in a drive, after stopping at the lights, for instance, revolutions drop quickly to 2000RPM, and then it take about a minute to drop to 1000RPM, its quite annoying, and mpg-s are suffering(not that it's my main concern)

 

i have read a lot of similar topics around here but haven't found one that gives a solution to this problem...

 

so this is the thing

i have tested few things:

 

TPS is ok, and it switches on no throttle and on full throttle, not only it clicks, but also i tested it with multimeter, so the tps works properly

 

ICAV was thought to be the source of problems, but it seems to work fine, there is just one thing, it has a zerop position adjustment screw on top, and it was setup so that it had a little opening when not connected... it seemed wrong to me an that screw was tampered with from the previous owner, so i turned it back so it has no opening when not conected

don't have a spare one to try if it would make a difference...

 

ECT- engine coolant temperature sensor reads about 2kohm at 20°C and about 250ohms at 80°C

 

since i adjusted the ICAV not to be open when disconnected cold idle is much better, but when it warms up in traffic it's the same story...

 

please help...

[Guest jammy205]

sounds like its the way the ecuwas programmed

[mmark118 0]

i also suspect that the injectors might be dirty and the fuel cut-off isn't propper...

 

but on the othe hand that should cause problems for the revs dropping to 2000rpm, not from 2K down(that should be to the ICAV) ?

[mmark118 0]

i also suspect that the injectors might be dirty and the fuel cut-off isn't propper...

 

but on the othe hand that should cause problems for the revs dropping to 2000rpm, not from 2K down(that should be to the ICAV) ?

[matyasg 0]

ICAV was thought to be the source of problems, but it seems to work fine, there is just one thing, it has a zerop position adjustment screw on top, and it was setup so that it had a little opening when not connected... it seemed wrong to me an that screw was tampered with from the previous owner, so i turned it back so it has no opening when not conected

don't have a spare one to try if it would make a difference...

It's normal to have opening when the ICAV is not connected! So it was not the best idea the adjust that. With a disconnected ICAV you should have about 1500RPM idle when the engine is warm.

[mmark118 0]

after i closed the icav with this screw, things just got better...

 

to me it isn't logical to limit the operation of the icav with that screw, now ecu can drop the revs faster by fully closing the icav.... the icav is just a simple DC motor moving a sprung flap, flap is closed by spring and opened by motor, so the larger the torqe of this little dc motor the larger the opening is, and torque is directly controlled by the pulse-width-modulated current from the ecu...

 

i think that this screw is just some kind of limiter preventing the flap to fully close maybe to help with cold starts...

 

oh, and another thing i forgot to mention, when the screw on the icav was set so that the flap had just a small opening, after i started the engine, and when it reached working temp. i could disconnect the icav, and the car would be idling at about 1000rpm, and revs were dropping quite nicely... so everything was perfect with the icav disconnected, except i couldn't start the car when engine was cold...

[matyasg 0]

to me it isn't logical to limit the operation of the icav with that screw, now ecu can drop the revs faster by fully closing the icav.... the icav is just a simple DC motor moving a sprung flap, flap is closed by spring and opened by motor, so the larger the torqe of this little dc motor the larger the opening is, and torque is directly controlled by the pulse-width-modulated current from the ecu...

 

i think that this screw is just some kind of limiter preventing the flap to fully close maybe to help with cold starts...

It's not a logical limit, it just means that with 0 duty cycle the ICAV won't be fully closed. It's a factory design to help cold start when the ICAV (or the ICAV relay, wire, ...) is faulty. The ECU closes the ICAV fully under normal operation at a certain duty cycle (25dc if I remember well). So when you changed the default position of the valve you changed it's characteristic as well. I think it won't work as it should with this adjustment.

[kyepan 291]

perhaps you have a split hose after the afm, allowing more air in, also the afm has idle adjustment that you can set when warm.

[mmark118 0]

It's a factory design to help cold start when the ICAV (or the ICAV relay, wire, ...) is faulty.

 

i agree with you, i also supposed that this opening is a fail-safe when icav or wiring or ecu port driver is malfunctioning...

 

i was also sceptic about tampering with this screw, but it was evident that somebody was playing with it already, so i gave it a try, and things got a bit better when i did... that's the thing that confuses me...

 

also before i changed flap offset to none, when i disconnected the icav the car would be running perfectly on 1000 rpm, but wouldn't cold start.... perhaps the opening then was too small also...?

 

but i'm not sure how can the ecu close it fully if the screw is preventing it from doing that?

 

what about this AFM theory, there is only one screw on my afm, and i thought that was for adjusting the air/fuel ratio a bit?

 

any thoughts, anyone?

some examples would help, this seems to be pretty common problem

[JRL 14 1 Cars]

I once knocked my airflow meter whilst refitting the airfilter it was idling very badly I replaced it and straight away back to normal.

I would leave the icv alone unless you try it with another but that should be fine,

cleaned Injectors made a difference on my car but theres got to be something else causing your idle problems

have a look at the throttle body, all pipes secure and in good condition? also check the throttle cable isnt too tight or slack.

also standard mi16s are not known for great idle anyway.

[kyepan 291]

on the mi-16 it is only idle adjustment on the afm..

[Alan_M 66 1 Cars]

A stab in the dark, but what about the TB? The spindles wear and on mine, it caused loads of weird problems. I Junked it and fitted a GTi6 inlet and TB.

[Baz 421]

Yep, echoing Alan, if it's totally random then probably a worn TB. If it's always the same characteristics, more likely the ICV or similar.