75 d-jet 2.0 with hotspark module rather than points.
no vac leaks -
after looking at the idle A/F and the fact that my adjustment screw on the ECU had no effect on it, I was told the TPS was not set correctly or it would. so I did adjust the TPS and now as I adjust the ecu knob it does change the idle slightly and A/f- prior my idle was lower and it seemed to be more on the rich side as well.
- but after making this adjustment, I went back thru and due to the idle being a bit higher( it is now around 1200 warmed up), it had been about 900-950, so
I went back thru and rechecked the timing mark is at TDC, and was re-applied by the Porsche mechanic last year so- verified, then timed at 3500rpm at 27 degrees btdc.
also the distributor is clean, like new and advance plate is moving properly.

my airscrew is all the way in, and the AAR is closed. if I adjust the ecu setting knob to more rich, the idle comes down a bit but then its really too rich and feels like its not a smooth . - am I now looking at the MPS setting possibly not being correct?
its the only other item I have not touched, and the A/F info says the MPS is where it should be at WOT and part load. those numbers are really good and my gas milage has been good too.

thanks,

Are you reading the dash tach or a tach/dwell meter?

My dash tach reads about 300 rpm high at idle. so 1200 = 900....

That said, I don't think mine would run with the screw all the way in, are you sure the AAR is closed all the way?

I've chased idle issues for over a year - similar to what you described - EXCEPT you had a good idle at some point, where my idle was never steady at 900.

In my case, I tested for air leaks - none, AAR - tested OK, got a rebuilt TPS board - adjusted OK, had the MPS rebuilt, adjusted valves, timing, blah blah blah....

I noticed on my TB that the plate had a small hole in it - so even if the TB plate was shut, that hole was allowing air in. I plugged it - just to cut down on the airflow - that helped a bit. Most plates do not have that hole - some do. I never found out what that is.

I'm at the point that it's as good as it will get for now. I will replace the TB at some point as I think it's a bit worn out.

Good luck!

Len

yep I have the same issue , tach now reads 1500, so about 1200, which is what the tach reading on my timing light shows , bounces between 1150 and 1200.

yes the AAR is closed- verified.
newer throttle body(cleaned etc)
yes idle was less when the TPS was not set correctly and it was running too rich, and timing was actually off. .....

To me it seems it would be on the air side, not the fuel side.

So while you checked the usual areas for vacuum leak, have you checked the distributor diaphragm and hoses?

How about the crankcase vent that goes the the plenum (i.e. any and all hoses to the plenum)?

Adjust the tps like this.
Then set dwell with a meter
Then set timing

Yes, rich I had it set exactly like that but the ecu idle adjustment did not work at that setting. Contacts look like that

Possibly weak advance springs in your distributor not retarding your timing enough at idle.

Following this with same issue.
GREAT thought on the advance springs in the dizzy.

Hi,
it's quite easy to adjust THIS KIND (the newer ones) of TPS because they have an integrated stop.

Loosen the body-screws a liitle bit and turn the body clockwise until you feel the stop.
Then turn further on in same direction for half of the distance between the markings near the screws to pretension the 'following-contact'.
Then retighten the screws.

No need to take off the cap or measuring anything electrical.

Try it out - it works.

But don't do this at the old-kind-TPS with the cap mounted by screws. They don't have the integrated stop and cannot be adjusted this way.

Regards
Norbert

which kind is a new kind? not sure ive seen one other than the OEM.

The TPS for a 2.0L is calibrated with an ohmmeter to verify the circuitry of the two idle contacts, which open and close with throttle position. Use a meter, why guess?

Have you also looked for other 'free' air' sources? 4 manifold gaskets, throttle body gasket, leaks in the air plenum housing or at the intake manifolds to head gaskets? All intake connections verified as leak free? The entire intake manifold system must be air tight and the throttle body screw should have an effect, or something is not right

On the left: old type with the screws fixing the cap
On the right: newer type without screws

The procedure Jeff Bowlsby shows is necessary to control the electrical function.

It's not necessary to adjust the newer-type-TPS correctly.

Try it out...

Norbert

I thought the knob on the ECU was for mixture, not idle? I use the idle bypass screw on the throttle body to adjust the idle...

I replaced the TPS sensor board and adjusted it as per the illustration above (I built a small wiring harness and attach it to my Fluke meter to listen for a tone for a closed circuit). AAR is gone (some prior owner removed it) and it idles rock solid once it's warmed up.

Now to find someone with a CO sniffer to get the ECU adjusted...?

It is. The knob can be used to enhance or lean out the idle mixture, but that circuit is only in effect if the TPS is in the idle position. If the TPS is misadjusted the throttle may never be in that position, and the ECU knob can't have any effect.

Phil, since I'm in the middle of AAR issues, are you sure its closing completely at warm up?

Sadly I think that bowlsby is correct and you need to become really detailed to figure out where the air is getting in.

I start simple.
Then only hose you need connected to the plenum for the motor to start and idle is the mps .
I remove all other hoses and cap off all the ports with tape or rubber caps.

Done like this you will eliminate a few potential leaks and know if one of them has any affect on idle.

If you see a change to normal idle, at least when I find it, I put one hose back on at a time, then re test. Then you will know which system needs repair.

If there is no positive change then you have the plenum, runner hoses and intake gaskets to deal with.

Don't overlook poorly adjusted valves either.

I chased the same problem. Don't forget the injector seals. Worked for me.

Ditto here, I chased that same dam idle issue over a year as well. Dont forget the Cylinder head temp sensor. I read Pbanders trouble shooting guide and thought I needed a spacer for the CHT, installed but condition persisted, eventually figured out it was bad connection where the CHT plugs into that plastic spade housing connector, cut that section out and installed a new spade connector. Starts and idles at 900, then I re-installed the AAR and now on cold start ups it idles at 1200 until the AAR closes and goes back down to 900.

Good luck

I have seen several 2.0L plenums that were cracked and a few that had rusted through on the bottom, several small holes.

Not someplace you expect a vacuum leak.

Jim

This method will identify the problem area unless it's an injector seal. BTW, that is where I found the last of my leaks.

I got to do a bit more on this thursday, really set the tps, and idle is right at 1000. I do think the only other 2 things here would be to check the valves again, it's been a year,, other might be to check that cht connector. Those are great ideas. I have thought it might be an issue for a while too.
All these are valid comments. I have already, not too long ago, pulled the plenum, throttle body, injectors (last year) and put back together with all new sensors connectors and seals, after which the car ran great, but idle was actually a bit low. It was after we diagnosed a non starting issue as a failing mps, I found and replaced it with a rebuilt one that we have had a faster idle and I went back to recheck the tps, etc along with timing to then get a better idea of whether the mps was what might need to be tweeked.
I do think it may be the mps still, but my numbers at part and full load are really pretty good so other than adding a bit of resistance to cht and checking that connector I can deal with a nice steady idle of 1000
I have found in the past that if that idle circuit, cht is too low then you get that issue of when the car is hot and you park it for 5 minutes, cme back and restart, the idle is too low and doesn't want to star running for the first few minutes and right now I don't have that issue anymore.
But the next thing will be to try and get my 2 mps tuned that I just rebuilt, which st the moment neither one work (cAR won't run)