Snakeoiler

Hi,

I am new to this forum. I have been a Porsche owner for 28 years. I am on my third car. I have a 1988 911 that has starting issues that seem to relate to temperature changes. I have had this car to a couple different shops with no luck. I figured I would go online and see if other have had simaliar issues.

It cranks but will not start. It is getting gas. No spark verified by pulling a plug and grounding it per the shop manual. I have voltage at the coil with the key one. I unplugged the speed and crank sensor and check ohms. Pin 1 and 2 read correctly. I get no reading on pins 1 and 3 or on pins 2 and 3. I heated the garage and it started right up.....I let the garage cool down and it would not start. I heated the garage and fired it up. I took it for a drive and heated the engine up. When I got back the sensors gave me the same reading as it did when it was cold and not starting. I hope that is enough history. The car has done this 3 times over the last 8 years. once it snaps out of the funk.....it will run flawlessly for a year or more before it does the same thing again.

Any idea. I am tired of paying the mechanic to not fix it.

dshepp806

No way those sensors should give you the same ohmage readings at (both) hot and cold situations..

Snakeoiler

The shop manual stated I should read 100,000 ohms accross pin 2 to 3 and 1 to 3. I got no reading on those pins.....it appreared it was an open circuit. I did get the correct reading accross pin 1 to 2......according to the maunal. When the garage is warm.....the car will start just fine.....and I still get no reading accross pins 2 to 3 and 1 to 3. The last couple of times it did this the mechanic never found the problem as it started for him while in his garage. I usually never drive it in the winter....so not sure what is up. Maybe I'll buy a better ohm meter to see if it is not displaying properly at the 100K ohms.

I hope someone on here will be able to out smart my car.....it has me scratching my head.

theiceman

what about the cylinder head temp sensor , tHESE CANBE WEIRD and may be worth changing even as a PM measure.
keep in mind it may have nothing to do with an actual sensor but coud even be due to flakey connections or other components like a DME relay or such thing

Snakeoiler

The cold weather triggers it not starting. I was hoping someone else has run accross this problem before. I am not sure where those temp sensors are you are writing about. Could that be the problem? I have a call into Rick at weekend rides to see if he has any ideas.

rusnak

snakeoiler, the Bentley manual gives the correct resistance values for the crank, speed, and cylinder temp sensors. If you have a thermometer, you can use ambient as the reference temp. Also, the air flow meter (between the throttle body and the airbox) has a temperature sensor. All you need is a dependable digital multimeter, and you can check all of these things. I wouldn't try it with an analog meter. We just had a thread in which a faulty connection to the CHT caused flooding and a no spark situation. Make sure your battery connections are in good shape as well as all of the grounds.

cal44

I always like to throw my useless two cents in. Do you have a theft control device on the car?

theiceman

yeah it must be the anti jack frost device ... it only prevents you from stealing it when it is cold

j/k

Snakeoiler

I have the factory alarm. I ruled that out when I was able to start the car when it is warm in the garage, and it won't start when I let it get cold in teh garage.

My friend is bringing over a digital multimeter today. I will check those readings.

Thanks for the advice. Hopefully one of those sensors will read faulty.

How hard are the crank and speed sensors to replace. I do all the maintenance on my H1, so I am decent with wrenches. This car's problem has owned the local mechanics. There appears to be much more knowledge on this forum than in those shops.

rusnak

be sure to check the cylinder head temp sensor

Snakeoiler

Is the cylinder head temp sensor the one that in above the referrence and speed sensor? That sensor checked okay with the ohm meter. I borrowed my buddies good ohm meter and still no resistance across pin 1 to 3 or 2 to 3 on either sensor. Will the car run okay with the speed sensor not working? It seems odd that both sensors would stop working at the same time.

Will the cylinder head temp sensor keep it from starting? I will look for it in the manual and test it.

Thanks for everyone's help.

rusnak

the car will not run with any of the 3 sensors missing input signal or faulty.

The cylinder head temp sensor is the only one that changes resistance according to temperature. If the engine has been sitting and not cranking, then the CHT will be at ambient air temp, so you can check the resitance against the table in the manual.

You should also check the afm as outlined in the bentley manual. If you have the manual, then all the info is in there. If you don't have one, then go to Amazon.com and order one. That, and get a digital multimeter. An analog one is not safe to use on some tests.

theiceman

to answer your question .. the car will not start with a bad CHT sensor. also keep in mind you may have an intermitently faulty sensor. it may test fine but still not work correctly .. you did say you have swapped your DME relay correct ?

Snakeoiler

I have not replaced any parts yet. I am still trying to figure out which one/ones to buy.

What is the AFM?

I have a manual. I will look up the CHT sensor reading and check them in the morning. I also have a good digital ohm meter to use.

The manual tells me on the crank and speed sensors....I should have 100,000 ohms across pins 1 to 3 and 2 to 3. I have not reading on these pins. I also checked them once I heated the garage and still no reading across those pins. I did get the correct reading across pin 1 and 2 as outlined in the manual.

I need to pull the seat to get to the DME relay, correct?

theiceman

nooo i think you can just reachuner there ... and it is by far the biggest cause of none issues on these cars .. even if it turns out not to be this you definitely need to keep one on your spare parts in your car ... so it is NOT wasted money.