924Board.org

[staticsan]

I've begun diagnosing why my '924 doesn't like starting. It's slow-going, partly because I'm basically learning what it means to be mechanical on a car. And I'm very much not as young as I used to be...

With the help of trying thoughts in friend's heads, it seems to be the classic thermo-switch problem. The WUR has correct resistance. The AAV is stuck closed - fortunately, a little accelerator whilst starting sorts that until I can replace or repair it. The secondary fuel solenoid's sensor seems to be correct when cold, but I haven't rigged a proper test of the solenoid yet. The CIS pressure test is awaiting purchase of a pressure meter...

The thermo-time-switch... ah yes. I did finally find this in the Haynes manual. And I wish Porsche hadn't put it where they did. There's no way I can get to it - I can't even find a way to see it!

Now, since I've worked with electronics a lot longer than with cars, I can understand the wiring diagram and think I can figure out a way to test it from the cold injector plug. So I'm interested in what other creative and/or inventive ways people have tested bits that are awkward to get to?

Wade.

[dpw928]

You can test the thermotime switch with an ohm meter by pulling the connection at the cold start valve and using the lead going to the switch. Also, note that regardless of the temp (below or above 100F) you will see a circuit but the higher temp circuit will be greatly reduced.

Dennis
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81 931 5 sp
78 928 5 sp Silver
78 928 AT Euro Black

[9XX Girl!]

[9XX Girl!]

[dpw928]

Agree that a heater in the thermotime switch should cut the ground after a few seconds but I was talking about a static ohm test with no volts.

Dennis
_________________
81 931 5 sp
78 928 5 sp Silver
78 928 AT Euro Black

[staticsan]

Yes, taking the head off will give me access to the TTS... (laugh? I almost started...)

The problem I have is that it is difficult to start, both when cold and when warm, but much harder when warm. When it's starting really well, it will turn over about four times and then catches and run. It tends to do this best when it's hot. When it's cold, it then usually runs for about 1 second and dies. But then if I crank it again straightaway, it will usually fire and run.

Once it's started cold, it stumbles for about a minute. This makes sense: the AAV is stuck closed and so there's no fast idle. That was easy to confirm. Likewise, I've confirmed the heater in the WUR is electrically whole; the heating action requires a pressure test, though.

But, I need to confirm that the TTS is working at all and for how long. Scouring the current flow diagram, I've figured out I can open the multi-pin plug in the harness behind the throttle body which will give me access to the TTS pins without the distraction of cranking the engine or the inconvenience of pulling the head off (!). I can use the same trick to confirm the secondary solenoids's connection. Aware that as the car ages, the CSV needs to open a shade longer when starting, I have designed a small mod to the wiring that can achieve that. But I want to get diagnosis complete, first.

I'll let you know how I go!

Wade.

[9XX Girl!]

Wade, it may pay you to read this http://www.924.org/techsection/hot_start_problem_fix.htm
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924 (84) N/A 2 ltr - 5 Speed - BLACK - Project (looking for 200BHP, any ideas!)
924 (81) N/A 2 ltr - 3 Speed Auto - SURINAM RED (Metallic) - Near Original Spec (sticking with originality)
BOTH ON THE ROAD, BOTH USED EVERY-OTHER DAY

[dpw928]

Try unplugging the cold start valve before starting. If still the same symptoms, the problem is probably elsewhere.

Dennis
_________________
81 931 5 sp
78 928 5 sp Silver
78 928 AT Euro Black

[9XX Girl!]

plus: are you following this thread
pressure seems ok still stumbles when cold
_________________
924 (84) N/A 2 ltr - 5 Speed - BLACK - Project (looking for 200BHP, any ideas!)
924 (81) N/A 2 ltr - 3 Speed Auto - SURINAM RED (Metallic) - Near Original Spec (sticking with originality)
BOTH ON THE ROAD, BOTH USED EVERY-OTHER DAY

[RC]

[staticsan]

[9XX Girl!]

hmm....CSV may be sticking open or dripping for a while after being closed, take it out and put it in a jar and fire it up! Then you will know
_________________
924 (84) N/A 2 ltr - 5 Speed - BLACK - Project (looking for 200BHP, any ideas!)
924 (81) N/A 2 ltr - 3 Speed Auto - SURINAM RED (Metallic) - Near Original Spec (sticking with originality)
BOTH ON THE ROAD, BOTH USED EVERY-OTHER DAY

[leadfoot]

The thing about these cars is there is quite alot of things that can prevent good starting.
But generally speaking they need ;
alot of fuel and air at startup
good grounding to the block , head and alternator
good connectiong between starter and battery
good connection between starter and alternator
proper system + cold and warm start fuel pressures (WUR)

A couple of things,
If your starter cranks over really well then it is likely the fuel / air
If it sounds a little sluggish check the resistance of the wiring and look for a problem between the starter and alternator - this is common and can be quite expensive to fix when you starter and alt get fried from a short.

I regularly check my spark plugs as this is a no brainer sometimes. Fouling can occur and once you pull one if it has black sooty/ powdery coating it's excess unburnt fuel. If you have been trying to start it for a while this is the result. Remove clean replace. Check gaps whilst your at it.
Likewise the spark leads can also break down when they're older and can definately cause hard starting.

The AAV does help, but it's a crude system.
Pull it and test it with a 12 V source and check it's opening correctly over time.

Once these are sorted the WUR is the next challenge.
You basically need a CIS tester for this...
good luck
Stu
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1981 ROW 924 Turbo -
carbon fiber GT mish mash
LS1 conversion in progress...