911SC won't start

[AUSDave]

911SC won't start
Hi all. New to the forum and 911 ownership. Just wanted to ask for some help diagnosing a problem with our car. It's a 1980 911SC LHD from the US purchased earlier this year. Had some issues resolved when we bought it such as overhauled WUR and cold start valve, replaced cracked airbox, new fuel accumulator and some other minor things. I was a mechanic in a previous life and do some basic work on the car. Have changed the plugs, leads and removed the A/C compressor ( so much easier to get to plugs etc)

Anyway the problem is we went on a drive on a fairly hot day recently and the car got very warm and then just died. We let it cool a little and it started but we didn't get far. We let it cool a little longer, it started again but it didn't go far before stopping again. This time it wouldn't start after a very long cool down. We got it towed home and hasn't started since. It turns over easily but has a weak spark. I can hear fuel getting to injectors and smell fuel so i don't suspect its a fuel issue. Coil is original black Bosch - reads 0.5ohm on primary and 720ohms on secondary. I opened the CDI box and visually looks ok but i am not sure what else to check for there. Rotor and dizzy cap look fine.

Any ideas?

I suspect the coil and am thinking of getting an MSD P/N82222 High Vibration Coil. Do these require the non resistor rotor? We currently have the original rotor which looks fine.

I was wondering if anyone with an SC in Adelaide has a known good coil and CDI unit i can borrow to test if one of them is the issue.

Thanks

David

[blucille]

Hi David. Cdi is the first thing I’d check. It usually hums or buzzes with the key on. If yours is silent, it probably is the reason your car isn’t starting.

[kltarga72]

Follow blucille advice, those Bosch cdi will leave you sitting along side of the road until they cool down. I would personnaly stay with a Bosch cdi box verses the MSD. Contact Bob Ashlock, very helpful. Check out this link, https://ashlocktech.com/bosch-cdi
Also a corroded O2 sensor relay under the passenger seat can cause rough running and stalling when hot. Sometimes just unplugging and reconnecting can fix a bad connection. The relay sits next to the ECU (brain) Lamda system.
You have also replaced or fixed the fuel accumulator and cracked air box so your going in the right direction

[mysocal911]

Originally Posted by kltarga72

Follow blucille advice, those Bosch cdi will leave you sitting along side of the road until they cool down. I would personnaly stay with a Bosch cdi box verses the MSD. Contact Bob Ashlock, very helpful. Check out this link, https://ashlocktech.com/bosch-cdi
Also a corroded O2 sensor relay under the passenger seat can cause rough running and stalling when hot. Sometimes just unplugging and reconnecting can fix a bad connection. The relay sits next to the ECU (brain) Lamda system.
You have also replaced or fixed the fuel accumulator and cracked air box so your going in the right direction

Not really! You are aware that the engine should start & run without the Lambda system, right?
Check this link for CDI repair, https://www.systemsc.com/products.htm

[RDM]

This sounds much like a problem I chased for years. It was intermittent, only when hot out, and where I lived wasn't hot very often. CDI box is a good place to look for trouble.

My thread is here. See if this resembles your problem.
https://forums.pelicanparts.com/porsche-911-technical-forum/958213-memorial-day-diagnosis.html

Ingo Schmidt also rebuilds CDI boxes, and new modern ones are available from Jonny at Classic Retrofit.

[kltarga72]

Originally Posted by mysocal911

Not really! You are aware that the engine should start & run without the Lambda system, right?
Check this link for CDI repair, https://www.systemsc.com/products.htm

Never said the Lamda system would keep an engine from running but it can cause stalling and rough running. There are always variations in helping to diagnose and solve problems posted on this 911 forum. Having the same year 911 and same issue in the past I was letting the OP know what I did to fix the problem.

Lash

[ant7]

Your point was valid Lash, don't be put off, however; in this instance I think the suggestion of the CDI box going bad is probably more likely.
Ant.

Originally Posted by kltarga72

Never said the Lamda system would keep an engine from running but it can cause stalling and rough running. There are always variations in helping to diagnose and solve problems posted on this 911 forum. Having the same year 911 and same issue in the past I was letting the OP know what I did to fix the problem.

Lash

[PeteKz]

Agree with ant and several others. I also experienced a Bosch CDI box intermittently stopping, before it died completely. Also check the coil, as the symptoms you describe can be caused by a coil going bad. .

[mysocal911]

Originally Posted by AUSDave

911SC won't start
Hi all. New to the forum and 911 ownership. Just wanted to ask for some help diagnosing a problem with our car. It's a 1980 911SC LHD from the US purchased earlier this year. Had some issues resolved when we bought it such as overhauled WUR and cold start valve, replaced cracked airbox, new fuel accumulator and some other minor things. I was a mechanic in a previous life and do some basic work on the car. Have changed the plugs, leads and removed the A/C compressor ( so much easier to get to plugs etc)

Anyway the problem is we went on a drive on a fairly hot day recently and the car got very warm and then just died. We let it cool a little and it started but we didn't get far. We let it cool a little longer, it started again but it didn't go far before stopping again. This time it wouldn't start after a very long cool down. We got it towed home and hasn't started since. It turns over easily but has a weak spark. I can hear fuel getting to injectors and smell fuel so i don't suspect its a fuel issue. Coil is original black Bosch - reads 0.5ohm on primary and 720ohms on secondary. I opened the CDI box and visually looks ok but i am not sure what else to check for there. Rotor and dizzy cap look fine.

Any ideas?

I suspect the coil and am thinking of getting an MSD P/N82222 High Vibration Coil. Do these require the non resistor rotor? We currently have the original rotor which looks fine.

I was wondering if anyone with an SC in Adelaide has a known good coil and CDI unit i can borrow to test if one of them is the issue.

Thanks

David

This problem can occur when the small SCR capacitor, which limits the SCR's anode from a too short of a dv/dt rise, fails and prevents the SCR from turning-off when hot.
A cool-down of the CDI allows the SCR to function properly until heat takes effect again.

[mikedsilva]

Most likely CDI.... where in Aus?

[Guardsred911]

if you haven't fixed it yet, I would check these two things:
1) check the multi prong plug in the engine compartment thats on the left side on the fire wall... (by cylinder 3) - Pull it apart and make sure it's firmly together.
2) check under the front seats, there are a couple relays that go bad and cause this exact symptom. Mine are under the passenger (right) seat, I am unsure where a car that has the driver on the right would locate those... but easy to check. I just replaced mine and ran fine