in the last 2 weeks the car has developed a rough idle at cold start.
idle will dip below 900 rpm on initial start and fight to keep going - a slight application of throttle will smooth it out. Once warm - it idles just as it should.

the car has new injectors, FD, FPR, TPS is not that old.

I have done some searching and it seams the culprit may be a failing ISV as this is directly responsible for idle control on cold start???

is there any way to troubleshoot the ISV without replacing it?

are there any other sensor that may contribute to rough idle on cold start?

can it be a failing AFM - barn door?

Can anyone outline the cold start sequence for these cars, i.e. what systems happen in what sequence so I can understand how it all works together?

thanks

 

DME temperature sensor. My 87 951 was doing the same thing.

 

good one - i'll just replace it.

 

Vacuum leaks at the TB, AFM,etc.
ISV can be cleaned and checked. Remove the valve and connect the leads to +/- to get the valve to open/close, reverse polarity and move the valve open/closed. Spray brake cleaner in the hoses and ISV to rid it of any oil grime. Some suggest carb cleaner but it will and does leave a film that does effect the AFM sensors, got this when doing my TB from a tech. Brake cleen leaves no residue.

I doubt it is the engine temp sender, not on a cold start, warm yes. My bet is a vacuum leak at the hoses near the AFM.

The AFM may also have a worn wiper track. As the cars warms the wiper will move on the board and perhaps the section on the wiper pickup is degredated where it is supposed to sense idle air flow but has good contact beyond idle so it runs fine once warm as it is making good contact on that section of the wiper board.

Dal

 

Vacuum leaks. Barn-door on AFM/MAF is dirty/sticking. Idle bipass valve malfunction (that's buried under the intake manifold, so a bit tricky to get at). Dirty contact on 4-pin connection at AFM/MAF (get's grimy fast).

Haynes manual describes in good detail how to diagnose your Bosch Ignition/Motronic system using an Ohm meter. Rview it, if you have a copy.

 

Where is the DME temp sensor?

 

i'm going after the DME temp sensor first - will post results shortly.

i have replaced and inspected all vacuum lines and hoses.

 

UPDATE

DME temp sensor changed - no real improvement and rough idle is somewhat intermitent.
Changed Plugs too.

Today on cold start - it idled below 900 and was somewhat stumbling.
I tapped the ISV with a screwdriver at the electical connection and the second I did it went
to normal smooth idle. So I'm thinking its the ISV now.

I didn't know they could be cleaned. I really dont want to pull the intake but i guess it's my next task.

Dano, i'll check the AFM connection too.
No vaccum leaks that I can sense.
I'll need an Ohm meter to do any of the electronic testing - i also have a spare AFM