3.2 won't start when engine at operating temp.

[Porsche 2]

Is an '87 3.2 in clean and good visual condition. Always starts and idles correctly through start up, then operates normally through all rpm range and idles smoothly at +800 rpm. We enjoy mild day time temperatures.
Recently will not start at normal engine operating temperature. Starter spins engine as always, but engine does not fire at all. After engine cools off to half way to cold and cooler, will then start and run normally. Thus, appears to be temperature related (?), and only with regard to starting the hot engine.
I have checked resistance on the three engine sensors; CHS, flywheel reference, and speed, and all check out correctly per Bentley instructions. The CHS looks fairly new (not sure) and the other two are old, possibly original with outside protection sleeve cracks. Recently, and before this new hot start problem, replaced; O2 sensor, spark plugs, distr. rotor and cap, and checked spark --(nice strong blue spark today). Engine had a valve job and related repair some 9,000 miles ago, and runs normally (125,000 miles on car). Since this hot starting problem began, I have replaced the fuel pump and pump relay, which did not eliminate this problem
I have studied the various related threads here, but now out of ideas. Any advice will be very welcome.

[Rodsrsr]

I had a very similar situation which caused me to troubleshoot just about every component in the motronic system. The car would start and run perfectly when cold, but as soon as it hit 180 degrees it would pop, backfire and die. Would not start again until it cooled down somewhat. Turned out to be the DME relay.

[yogi]

I had a similar problem several years ago and it turned out to be the fuse for the fuel pump.

[T77911S]

check fuel pressure.

[Drisump]

My guess would be the CHS if problem is purely temperature related. Since you've indicated that it is perfoming properly, you may try the DME relay....you should always have an extra relay anyway. It is my understanding that the flywheel sensor can be slightly out of adjustment giving the occasional no start condition. The fuel pump will only run if the DME see's that the engine is in motion. Good luck

[gomezoneill]

Did you check the resistance of the reference sensor when hot? If the insulation is cracked they need replacing even though they may not be the problem because they will fail at some point.

[Porsche 2]

Thanks for all your prompt responses. I have already replaced the fuel pump relay with no improvement. Next, I will check spark and CHS resistance with the engine hot. Checking the maintenance records shows the CHS was replaced some 9,000 miles ago (8 years), and it appears the two flywheel reference sensors are the original units, but now with cracked outside sleeve protection. I now suspect these reference sensors need to be replaced subject to the hot tests above. One more observation, there is no gas smell while cranking the engine. With a new fuel pump installed, and the engine running perfectly until shut down hot when it won't restart, I suspect this is not a fuel pressure issue.
Will keep you posted.

[Rodsrsr]

You should try the relay before going any further. I spent over a month troubleshooting every piece in the system and even sent out the DME for testing. I overlooked the obvious because I assumed that if the car ran the relay must be working. Its fairly inexpensive and most of us carry a spare in the glovebox anyway, since they are known to stop working without warning.

[rick-l]

Not advocating this as a fix but, just for info, the car will run fine if you disconnect the reference sensor after it is started. It won't start without a reference sensor.

[jlex]

Is that true for the speed sensor as well?

[T77911S]

you should check the FP before throwing more money at it. test before spending $$$
you have FP regulator.
also check injectors. they could be leaking and flooding the engine.

when hot, if you turn it off will it start right back up or is it a matter of sitting that makes it not start.

if you have spark when it will not start then the CPS is working

[pmax]

The position sensors are sensitive to heat when they are about to fail.

[rick-l]

Originally Posted by jlex

Is that true for the speed sensor as well?

No based on what I have read, mostly the FRwilke site (motronic math), the reference sensor establishes top dead center and from there on the DME just counts the flywheel teeth (using the speed sensor) to see where it is.

[Porsche 2]

Just started cold engine, no problem, went for a short drive, engine heated up to normal operating temp (approx 190), shut engine off, will not immediately start again. Immediately, while engine is still hot, checked for spark, --- no spark! This compares with nice strong blue spark with engine cool. So, appears DME is not allowing spark due to some signal. I conclude I need to change out the two old (probably original) flywheel sensors, even though while cold they have the correct resistances (per Bentley). Comments please.

[wrxnofx]

Two comments:
1. ) If memory serves, you can simulate a "normal warm" cylinder head temp sensor reading by jumping the leads with a resistor. You'd have to search. Edit - here is some info I found:

The CHT should read approximately 3500 ohms cold, and approx 300 ohms hot. You can "fake out" the computer by substituting a resistor for the sensor.

Go to Radio Shack and get a 330 ohm 1/2 watt resistor (or a selection).

Once the motor is warmed up:
1. remove the CHT connector,
2. bend the ends of the resistor leads over on themselves
3. shove the ends of the resistor leads into the female connector for the CHT and use some masking tape to hold it in place.

2.) If you do decide to replace the sensors, you can buy the BMW ones for half the price. The only difference is that they are a bit longer. Use BMW P/N 12-14-1-708-619-M14. This is Bosch 0261210002

[Porsche 2]

Update -- old flywheel sensors removed -- after removing lower shock bolt and hot air hose to help access -- still awkward but doable. Expect delivery and install of new sensors today -- will report after. Thanks for your suggestions wrxnpfx, BMW part on back order

[Porsche 2]

New flywheel sensors installed. After engine warm up run to operating temp, engine starts and stops after repeated attempts as it should. So problem of hot start is eliminated --- so far so good. Sensors supplied by our host appear to have done the trick!

[DRACO A5OG]

Congrats!

Kudos to PMax

[Porsche 2]

Here is a pic of the old sensors along with a few remaining "chips" of the old insulation!

[pmax]

Cool ... Thx

Haven't replaced mine yet but the one in my other German car failed just like this.

How bad was the installation ?