Stormy

Dear all,

I'm working on a bad (cold) starter, and carrying out a number of checks.

When measuring CO% during (cold 20°C) starting I see that there is very little enrichment at cold running compaerd to warm idle. The A/F-ratio during cranking not measured yet, but expected to bee not rich enough, causing the bad starting. The engine runs fine when warm, with approx. correct CO% etc. For this reason fuel pressure is not measured yet.
First suspect: coolant sensor, but it reads approx. 2.5kOhm at the ECU connector at approx. 20°C; no mass connection, no wire problem. Does anybody has a specification (resistance vs. temp) of this sensor?All other ECU inputs outputs seem to be OK as well (after I corrected the throttle stop, so now the idle contact of the TPS works as well, unfortunately no influence on cold start)
CO% was generally a bit low, but re-adjustment did not have any influence on the bad starting next morning.

I have two issues that are not clear for me: 1) "vacuum" hose routing and 2) altitude switch.

1) "vacuum" hose routing (944NA, 1983, without canister purge system)

What puzzles me is the way the extra air for the AC is controlled. One side of the extra air solenoid valve is connected to the manifold, but the other side is not connected to anything! So when the AC is operated, extra air is dragged into the engine NOT PASSING THE MAF!! I doubt that the Porsche engineers wanted this to happen, so there might be an error in the hose routing. Unfortunately I couldn't find any diagrams of the "pre 1985/2" routing.
Does anybody have a 1983 diagram (OEM or home made), or does anybody can tell me how the AC air solenoid should be hooked up correctly?
2) altidude switch

From the wiring diagram I read that an altitude switch is connected to the ECU. The reason for this puzzles me as well, because the MAF used in this application actually is a mass air flow sensor, so a low density operation (mountains) would not lead to incorrect A/F-ratio (in theory). I also do not see any reason for changes in ignition mapping at altitude, but I was not involved in the system design, so I don't know the reason behind it.
Main question: should the sensor be "closed" (zero Ohm) at sea level?? (and open at 3000 ft??) This because the wiring diagram suggests that the contacts would be "normally open" and at the car I'm working on the contacts are closed (I live at, or even slightly under sea level;-)
Second question: where is this sensor/switch located?? I can't find it!!Thanks for all valuable tips!!

Toni

PS all suggestions for bad cold starting are very welcome, apart from my two main questions.
PPS I have access to (and read) the 944 WSM.

St3mpy

I've never heard of the altitude sensor, but it may be one of the sensors I unplugged during my engine removal that I didn't know the function of. But as you said, the car has a Mass air sensor that constantly reads the mass of air entering the engine, therefore rendering an "altitude" sensor redundant.

BTW, the main sensor that comes to mind when I say "sensor I don;t know the function of" is a sensor located under the running tubes of the intake manifold. It has 2 hoses that connect to th PCV system and an electrical connector. The 2 hoses connect parallel with the engine block, you probably won't be able to see the bottom hose. . Maybe this is your altitude sensor?

M758

These cars so not have a MAF. The have an airflow meter AFM. It means it measures air flow not mass flow.

The altitide sensor is right next the DME on the early cars and 924S. It is round brass hockey puck type device. At 1000 meters it operates to cut back fuel flow by electrical signals to the DME. It has no impact below 1000 meters and has only one setting so it does not cut fuel flow more at 2000 meters than it does at 1000.

You car will run rich at altitude without it.

Stormy

Dear Joe and dear ?

Thanks for your fast answers, however you give me hardly any help on my questions marked in bold.
I looked for a hockey puck type of switch near the ECE, but could not find it; might be removed or located elsewhere??

My main questions:

• routing vacuum hoses (<85 and no canister purge) especially the open end at the AC extra air solenoid can't be original..
• spec of the temp. (coolant) sensor (e.e. xx kOhm at yy °C)
• should the altitude sensor be open or closed (to ground) at sea level

In the mean time I'll test what happens if I "un-ground" the altitude sensor swith.

Thanks in advace for all your help.

Toni


PS the L-Jetronic air flow meter actualy is a MAF, like the K-Jetronic, since it uses the law of impulse (F = m*v) as a result change in air density (in theory) is included in te sensor reading. If the word AFM is preferred: fine!