Unicorn64

A cold/warm re-start problem

The 928 starts perfectly first thing when cold, but after a 5km journey to the shops. Turn 928 off, get milk and bread, back to car - won’t start. Turns over on the starter motor but won’t catch. Let it to sit for a longer time (say over 1-2 hours) starts perfectly, idles sweetly and runs as if everything is OK. When the 928 is up to operating temperature, restarts perfectly.

My solution to date is to leave the 928 idling, not a good solution for a security point of view. I have had this problem for 15 years and have replaced, fuel check valves, fuel pump, spark plugs and leads. – no difference.

Is there someone out there that has had a similar problem? Better still, does someone out there has a solution?

Cheers from Batemans Bay Australia

Porsche 928S4 1988 Auto

Kiln_Red

What would you say the approximate temp range is that presents the greatest restart issue? I would try it again and pull the pump fuse to shut it down. Replace the fuse when you get back to the car with the milk & bread.

If it fires right up, then you likely have some leaking injectors.

Petza914

How about the temp sensor?

peanut

yes Petza's suggestion of the engine temperature sensor should be your first check. its the most important sensor for starting.
if the ECU is not seeing the correct engine temperature it cannot deliver the correct air and fuel mixture.

If the Temperature sensor is faulty and tells the ECU that the engine is cold when it is actually hot then the ECU will increase fuel delivery and flood the engine.
When the engine restarts normally after 10 - 15 minutes after being left for the excess fuel to evaporate that is a good indication of a faulty engine sensor

FredR

The temp2 sensor is located on top of the water bridge on bank1/4 side of the bridge. The dash panel sensor is located sort of next to it but more towards the bank 5/8 side of the bridge.

The temp2 sensor has two channels- one each for LH and EZK and both should read the same resistance values. When the engine is cold measure the values by checking the resistance of each pin to earth and you should see a value in the region of 2k to 4k ohms depending on the actual temperature of the coolant. Warm the engine up to running temperature take the same measurements and this time the resistance should drop to around 200 ohms or so and report back what you find. I doubt this will be the issue but eliminate it anyway.

Similarly you can also start the engine, run it until it is warm and then when back on your driveway or whatever switch it off, open the bonnet, leave it for 5 minutes and see if it will then fire back up again.

Kiln_Red

Agreed. I wouldn't expect a 'warm' failure either, but ruling it out is easy enough.

peanut

I think many might be interested to know what you would ' expect ' if not a faulty engine temperature sensor? ...
Here is what we know .

1. Starts perfectly when engine cold ...
2. Will not re- start at all when the engine has warmed up .
3. Engine will consistently re-start later when engine is left for a short period or hot

Kiln_Red

Hi, peanut. Maybe I cut that sentence off prematurely. I meant that I don't think the Temp II circuit is our most likely culprit because these symptoms don't overlap well with what we normally see with a Temp II issue (whether wiring or the sensor itself).

This car restarts from cold and after reaching operating temperature, yes. Leads me to think the Temp II is probably fine. I would wager they could take an IR gun to the engine at any point of their no-start, then attempt to restart at comparable temps following a full run cycle. I wouldn't be surprised if it started easily in the latter case... suggesting the LH is doing its job well in all temp ranges.

peanut

thanks for the explanation.

its been a couple of decades since i last owned a 928 but my old service manuals show that the NTC ll sensor functions similarly to a 944 it is a thermistor .
It is named NTC ll to differentiate it from the Thermo -Time Switch.
The ECU applies a fixed voltage to the sensor and depending on the resistance of the sensor increases or decreases the fuel pulses. I read that the resistance ranges from approx 3000 ohms when the engine is cold to around 400 ohms when hot .

It might be useful the next time the car is warm and fails to start to measure the resistance across the terminals of the temp sensor

Unicorn64

Thankyou for the suggestions to date. Ambient tempertaure doesn't seem to make a difference as the warm start issues happens summer and winter. The problem I have is that it is a spasmodic problem that to date has been difficult to replicate. I will try and Kiln's fuel pump suggestion and then explore the temperture sensor route. Cheers