Help needed: Engine dies while driving... details in thread.

[jmd88]

Hey friends. Some support needed as I'm a bit lost. 88'911 3.2

This weekend I was sitting at the coffee shop, idling in the drive through line and suddenly my car just stops. I was sitting for 7-ish mins and she just stopped. Kinda startled me. But I turned it off and it immediately started right back up, no issue. Thought it was the coil failing. but managed to drive without an issue for another 30 mins home.

then the issue continues.

Issue: Driving and the engine 'flatlines' meaning RPMS drop to zero mid-drive. Started as an intermittent issue - driving along and suddenly, mid/consistent-throttle the motor RPM drops to zero. When I say mid- throttle, I mean just cruising along at 60mph () the RPM randomly just drops to nothing. Electrical system doesn't seem impacted, only the motor. Just dies. I can clutch dump and bump re-start and it seemed to work the first few times (just to get to the side of the road safely) and then nothing. Just dead, roadside.

Relevant Details:
New John walker built motor ( totally rips!!! The entire thing was done top to bottom and all critical sensors etc.)
New Fuel Pump
Solid State DME
New Cap/rotor/wires/plugs (less than 10K on all)
New Coil (from dealership thinking that was the issue, Installed tonight - issued appeared within 4 blocks after the car was off/battery unplugged for 4 hours)

Pulled over while trying to get home. Let the car sit for 3 mins, started again and it ran enough to get another 4 blocks home... dead coasting into the driveway...

30 mins later car starts right up in the driveway. Let it sit and idle for 10 minutes, just runs perfect.

Checked all fuses both up front and the 3 on the engine side, and they all appear intact (stock plastic + bar style)


Bit lost. Where do I start poking?

[917_Langheck]

Confirm the DME relay is not the issue by installing the oem relay and replicate the conditions associated with the issue.

Check for the "in spec" values of the reference sensors tracking TDC and rpm.

Check the "in spec" value of the cylinder head temp sensor.

[jmd88]

Originally Posted by 917_Langheck

Confirm the DME relay is not the issue by installing the oem relay and replicate the conditions associated with the issue.

Check for the "in spec" values of the reference sensors tracking TDC and rpm.

Check the "in spec" value of the cylinder head temp sensor.

I replaced my old failed DME with new solid state and don’t have old one to try - any tips on testing the solid state DME that isn’t buying a new oem unit?

Do you have reference for expected values and method to test these sensors?

Thanks for the reply.

[wazzz]

A way to rule out the SS DME relay if you don't have a spare OEM is to remove it and jump terminals 30, 87, and 87b all together on the socket with proper banana plugs or similar plugs or pieces of wire that fit snugly into the female terminals. They need to make good contact so they don't themselves introduce an intermittent fault.
Fuel pump will be running permanently, so this is a temporary fix to test drive and see if the engine dies again.

[PeteKz]

Start with basics: When the engine dies, immediately squirt starting spray into the intake. If it fires, you have spark and are missing fuel. Then troubleshoot the fuel injection. If it doesn't fire, then you most likely do not have spark. Troubleshoot that part of the system.

[mysocal911]

Originally Posted by jmd88

I replaced my old failed DME with new solid state and don’t have old one to try - any tips on testing the solid state DME that isn’t buying a new oem unit?

Do you have reference for expected values and method to test these sensors?

Thanks for the reply.

You are aware that SSR DME relays fail too? You should always carry a spare relay!

[pmax]

Originally Posted by jmd88

I replaced my old failed DME with new solid state and don’t have old one to try - any tips on testing the solid state DME that isn’t buying a new oem unit?
...

DME relay.

[mysocal911]

Originally Posted by pmax

DME relay.

A simple 5 minute re-soldering of the original OEM DME relay results in a very reliable unit.
Remember about SSRs, they have about 5X the number of components compared to the OEM relay (3 components),
which increases the SSR's failure rate over the OEM's.

[jmd88]

Coming back around to this.

I've replaced the following:
1. Coil
2. Cylinder head temp sensor
3. DME (Solid State)

Summary:
It initially appeared to be the CHTS as the Solid State DME showed a change state after the sensor replacement.

Only for the DME solid state the subsequently fail right after the fix seemed good - even though the DME showed all the correct lighting. I swapped out for my old non-solid state DME and she stayed running for longer than 15 minutes.

Disappointed to get such low milage out of the Solid State DME and that all it's Diagnostic LEDS to work correctly even though the DME itself failed.

Thanks to everyone who provided insights and support!

[jmd88]

Originally Posted by mysocal911

A simple 5 minute re-soldering of the original OEM DME relay results in a very reliable unit.
Remember about SSRs, they have about 5X the number of components compared to the OEM relay (3 components),
which increases the SSR's failure rate over the OEM's.

Is there a DIY for this? Would like to understand what you mean by "5 minute re-soldering" as I'm pretty comfortable with an iron.

[pmax]

Originally Posted by jmd88

Coming back around to this.

I've replaced the following:
1. Coil
2. Cylinder head temp sensor
3. DME (Solid State)

Summary:
It initially appeared to be the CHTS as the Solid State DME showed a change state after the sensor replacement.

Only for the DME solid state the subsequently fail right after the fix seemed good - even though the DME showed all the correct lighting. I swapped out for my old non-solid state DME and she stayed running for longer than 15 minutes.

Disappointed to get such low milage out of the Solid State DME and that all it's Diagnostic LEDS to work correctly even though the DME itself failed.

Thanks to everyone who provided insights and support!

DME relay.

[mysocal911]

Originally Posted by jmd88

Is there a DIY for this? Would like to understand what you mean by "5 minute re-soldering" as I'm pretty comfortable with an iron.

Watch this; https://www.youtube.com/watch?v=rwhR2EBTPYg&t=30s

And this; https://www.youtube.com/watch?v=cfrErbd1RQ8

Doesn't get any simpler! And people waste money on a SSR??????

[jmd88]

Originally Posted by mysocal911

Watch this; https://www.youtube.com/watch?v=rwhR2EBTPYg&t=30s

And this; https://www.youtube.com/watch?v=cfrErbd1RQ8

Doesn't get any simpler! And people waste money on a SSR??????

This is a fantastic reference. Thank you for taking the time to post the links. Super helpful.

[kltarga72]

I had similar issues with engine as you described. If other suggestions do not help look at the fuel filter and fuel pump.
On my 1972 the fuel filter was so clogged that it would shut the engine down or it would drop to lower rpm or idle.
My 1980 SC had a new fuel pump, six months later It had the issues you described. I fought that battle for months and after removing steel plate that allows access to fuel pump, I was able to put my hand on the pump (I have a lift in my garage) and could feel that it was cutting in and out.

Lash