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hot starting issue any advice as falling out of love :-(

backitoff

Well-known member
Joined
30 Apr 2006
Messages
118
Hi guys
My 964 C2 has decided not to start when hot. I recently went on an 80 mile journey and stopped for fuel and it refused to start. Cranking was fine with loads of battery power.
After about 30 mins I tried again and after considerable cranking she lumpily fired up then cleared her throat and ran fine.

I had already replaced the crank sensor, dme, and ecu due to a previous refusal to start from cold or hot.

So, after further investigation I have just replaced the fuel pressure regulator. Started up from cold no problem. Took it for a 10 minute drive, switched off and hey presto it wouldn't start.
Fed up to the back teeth with it at the moment. I can live with the little ongoing repetitive issues but i cant suffer unreliability.

Anything i haven't thought of, or pointers in the right direction would be greatly appreciated.
Thanks
 
Might be worth checking earths. I had this on my speedster turned out to be lower earth cable. Goes from engine to body over rear suspension. My contacts clean either end but when lifted strap up from suspension was green with corrosion , replaced and never failed again. Not sure if 964 is sane layout but worth checking
Good luck, great car
 
I had a similar problem with my 993, but only after a long run and a lot of engine heat. I have had my injectors ultrasonic cleaned and with over 1000 hot miles through the Alps the issue has not returned. I hope this may help.

Anthony
 
My car suffered similar issues for some time while I worked through various fixes on the forums to try and remedy my hot starting issues.
Scenario: Stop at petrol station or drop Daughter off at school, get back in car, whirl whirl of starter motor requiring pumping of accelerator to kick car into life, swear at car and try not to make eye contact with amused bystanders.
Most forums will point in the direction of the Starter motor needing replacement or associated wiring degrading due to heat. I did replace the DME relay first but didn't get any improved results so jumped straight in to replacing the starter motor. This is a PITA of a job and requires a lot of patience, arms like an orangutan and the dexterity of a circus performer.
I ended up changing the starter motor twice as the first refurbished unit started to exhibit the same symptoms about a week after installation. The supplier kindly sent me another one which performed for about a month before the hot start issue came back.
At this point I decided that the Starter motor wasn't the only factor causing my starting issues so I looked for alternative solutions. The car now starts fine but I went through a number of fixes before getting there. I have listed items to check below in simplicity, time & cost to check, all going well one of the easy fixes might get you back up and running smoothly. Pay attention to (item 4) which yielded unexpected improvements.
1) DME relay.
2) Earth straps, replace the one under car connected to starter motor and one at the front connected to battery.
3 Replace ignition switch, I first removed the back and cleaned the contact points which had some noticeable improvement but eventually replaced the switch for good measure. There's a small yellow wire directly connected between the starter motor and ignition switch.
4) Pump accelerator a couple of times before turning ignition key. As stated earlier when the car would whirl upon starting, pumping the accelerator helped it cough into life. I decided there might be some sort of fuel starvation issue so replaced the fuel filter. For a small fix this had immediate results.
5) If all else fails replace the starter motor.
Best of luck.
 
Thanks for the reply's so far.

Regarding the starter motor, if it is cranking fine and shows no issues, how can it stop the car starting? Is it to do with current draw etc?
 
I'm by no means an expert on starter motors so I'm sure the other will correct me if the following isn't correct.

If you are still on you original starter motor chances are the components inside the motor (big cylinder) or (solenoid), small cylinder on top start to bind up when they get hot. Dust & general degraded materials start to build up inside which means the starter doesn't turn fast enough to kick the engine into life. If you open the car door and turn the key you should hear an audible click from the solenoid.

How old is your battery? It may appear to hold enough charge with a multimeter with the ignition off but if its getting old you need to test voltage drop when its being cranked to make sure there's still sufficient voltage to spin the Starter motor fast enough.

I believe this is why good earth strap contacts are also important as these also impact on the strength of the electrical circuit to the starter motor. Its all about maximum juice getting to wear it needs to. The wires around the starter motor sit next to the engine and get brittle. Same deal if they can't carry enough current they slow the crank down of the starter motor.

As I said some of the more knowledgeable amongst us might give you a more accurate description.

I'd be interested to hear if (Item 4) above, pumping the accelerator before turning key makes any difference. If so this might point in the direction of fuel supply rather than the starter motor.

Keep us updated if any improvements occur.
 
backitoff said:
Thanks for the reply's so far.

Regarding the starter motor, if it is cranking fine and shows no issues, how can it stop the car starting? Is it to do with current draw etc?

If the engine is clearly turning over then the starter motor is fine but you're getting no spark or no fuel or way too much fuel (flooded). Hence the engine does not fire.

The classic 964 symptom of the engine turning over but not firing is caused by a dud DME relay that is not allowing the fuel pump to supply fuel. However as you've changed that (presumably for a Porsche original and not an aftermarket one) I'm hoping the information you got from Rennlist about the carbon canister will help fix your hot starting problem.
 
Hi Steve
Thanks for the input. I mistakenly bought an aftermarket DME when I first got the car. I then replaced it with a good branded one, then I replaced that with one from my local OPC.

When I had a previous refusal to start issue eventually I had no choice but to send it to my local Indie with the new OPC DME fitted.

I have just discovered that it is now NOT the DME currently in my car. (indie has swapped it out for an old one Grr)
I have replaced it with one of my others, and the car now starts fine!
I figured to prove it is the DME I'll put the one fitted by the indie back in and try it. To my dismay, the car starts absolutely fine with any of the DME's fitted
In one way this is great news, however it does not get to the bottom of the problem and now I am really confused lol!!!!!!
 
Just to put this to bed, well for now anyway.

I managed to finally trace the problem by starting the car then going through all of the electrical connections in the engine bay. I found the crankshaft connector plug was poorly wired. When I moved it, the car cut out.

I had replaced the sensor (from my local OPC) which is now a 993 part so the plug on the original loom has to be changed. I knew I had not done it that well, however, it went to an indie and I asked them to do it properly, whilst checking the sensor gap. Sadly, their interpretation of properly, and my interpretation are completely different.

I cut the plug off the main wiring harness and started again soldering the wires to the pins as well as crimping.

Problem now solved, the car is 'on the button' Hooray! Back in love with it again!

It' amazing how these cars mess with your senses. Complete elation when running well, to complete frustration when there is a hard to sort problem.
The challenge of 964 ownership!

Just a note regarding the crank sensor. I have been told that if you order one for a 928, it has the same plug connector as the existing 964 loom, so you do not need to cut the wiring.

Thanks for your help chaps.
 

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