Paolo928

Hi there,
I have a 1983 Porsche 928s 5sp that needed to pass smog this November.
It was running very badly so I got the following new parts:

- distributor cap and rotor
- spark plugs
- spark plugs wires
- oxygen sensor
- got oil and filter changed

car run much better after this tuneup: power is back and passed smog check.
I am also planning to replace all those rubber boots on the intake manifold so with that excuse I can give a good clean to the "Alien" under my hood

One problem is still there:

- When cold (in the morning) it starts okay but the idle sounds low, the OXS light stays on and the dash lights are very dim until I touch the gas pedal. At that point the OXS light goes off, the dash light become nice and bright and the idle normalizes.

- After a good run If I turn it off and restart it right the way (2-3 minutes) it starts super fast.

- If I let it seat for a longer time (like 10 min) even if still hot acts like in the morning. Some times gets worse (at start up) and it sounds like not all of the pistons are firing: I wait a little with the gas pedal slightly compressed and it fixes itself.

Could a Temp sensor be responsible for this behavior or I should put my hands on the air flow meter?

jpitman2

Assuming its L-jet (barn door afm), check your thermotime switch for correct resistances (in WSM, and here also) and connections to aux air valve, and cold start injector- I think you have one of these, but not sure - extra injector on front of spider body . Check that volts are getting to thermotime switch, and cold start injector . With car stone cold, pull front hose off aux air valve and check its at least 90% open - shine a torch in there, or poke a pencil through. Mine had a very low idle cold and occasionally stalled in the first few minutes, and I had to open the aux air valve and get it more open when cold. Very cold, you should see idle as high as 11-1200rpm, but it should drop under 1000rpm quite quickly, in my experience. By very cold I mean around 10C, 50F or lower.
jp 83 Euro S AT 54k

Paolo928

Hi jpitman2, thanks for the reply.
Yes it is L-jetronic (US 4.7L-16v M28/19). I'll check the thermotime switch as you suggest.
The idle is pretty low (~800) when cold and drops even more when hot (~600)... do I need to worry about that?

jpitman2

Your hot idle in neutral (no A/C) should be 750+/- 50 - ie 700-800rpm. Get it hot, adjust idle (big screw in front of spider), then see how it is on cold starts.
jp 83 Euro S AT 54k

WallyP

The oxygen sensor light has nothing whatsoever to do with the running of the engine. It is just a light turned on by a mileage counter.

The oxygen sensor light being on, and the dash lights being dim, sounds as if the alternator is not being turned fast enough to start charging.

Paolo928

Thanks, I'll worm it up and play with the screw tomorrow.

Paolo928

Hi WallyP, thanks for coming to the rescue...
we were typing at the same time apparently
Yes, that's sounds right: as we were discussing with jpitman2 my idle must be too low.

Strat_928

I have an 83S US, and while I was getting it into a more "reliable" state, I noticed the same as Wally mentioned. You need consistent juice flowing everywhere! So before I started chasing individual parts, I rewired to a zero gauge battery cable to the starter, replaced the battery with a Deep cycle yellow top, replaced the wire to the alternator to a bigger gauge, replaced the alternator from stock to a Bosch that is rated around 110 I think. These items helped immensely, but eventually I had to replace the 14 pin connector and pull the engine harness and repair all the broken cover spots and thin places. Of course I cleaned the grounds as well.
I can safely say that doing all that made the biggest difference in getting the car stable enough to then start working on independent issues. I had a little more confidence that electrical problems were not complicating the issues seen.

In addition to idling and response, I recommend either getting a BG injector service done. I did and it made a world of difference in quick throttle response. The 83's are coming up on 30 years old now.....
About time you brushed it's teeth... HA

Good Luck

jpitman2

I recently found a significant improvement in running when i replaced the ignition switch, and I mean really noticeable lift in response and smoothness all around. Turned out the switch didnt fix the original problem, but turned out it was well worth doing, along with cleaning up pod contacts etc.
jp 83 Euro S AT 54k, CIS

Paolo928

Hi Strat_928, thanks for the advice.
Injectors are new, battery is new, I checked and cleaned the grounds when I replaced battery and alternator... still some little problems remain, like the rpm gage works whenever it wants, if it wants to, almost never when I need it. Like today, when I was adjusting the idle it stopped working and still now doesn't. Is there a relay for it?

I checked the big connector in the front passenger side of the engine compartment and there was no sign of corrosion (little oxidation).

Checked those 3 connectors in front of the spider, under the fuel pressure regulator and they seemed okay too.

I raised a little the idle screw and no more dim effect (thank you WallyP and jpitman2) but it still takes too long to start after resting from hot (I turn the key and let it spin for a while before it starts).

I bought a DMM today and I'll test the thermotime switch resistance tomorrow.

I don't know which one is the 'aux air valve'... is there a diagram or a picture of it that I can look at somewhere? I want to try to do this...

Paolo928

Yeah, I need to do that soon too... it wiggles a lot and sometimes fails to activate the starter... maybe is the responsible for my problems ??

Paolo928

I found a funny video about the aux air valve... very self explainatory

jpitman2

Actual non-starting problem in mine was in the starter relay, not the ign switch, so start with the simple cheap things first.
Aux air valve -see this thread
https://rennlist.com/forums/928-foru...regulator.html
But dont belive the note that its job is to make your mixture rich, thats plain wrong . It provides extra MEASURED air to lift the idle when cold.Extra air can never make it rich, only lean if its not measured.

jp 83 Euro S AT 54k

Paolo928

Thanks jpitman2, I'll check the valve tomorrow.

Paolo928

I measured resistances of the temperature time switch for the cold start valve:

terminal G and ground
below 86 F should be 0 ohms measured 0 ohms
above 104 F should be 100 - 160 ohms measured 135 ohms

terminal W and ground
below 86 F should be 0 ohms measured 35 ohms
above 104 F should be 100 - 160 ohms measured 65 ohms

terminal G and W
below 86 F should be 25-40 ohms measured 35 ohms
above 104 F should be 50-80 ohms measured 70 ohms

I guess that the switch is bad because of the data of terminal W.

I reset the mileage switch to the right of the passenger side seat and no more OXS light

Didn't get to the aux air assembly but I think the problem is the switch.