1987 924s hard start and will not or slow rough idle when cold

[DrewPlaster]

1987 924s has decided to be difficult to start when cold and when it does start it will not idle or will idle very slow rough until it is warmed up. You have to depress the throttle to encourage it to start and stay running. Once it has been warmed up thoroughly it will start and idle normally.
I have checked the resistance values of the DME temperature sensor with the engine cold and warm and it is within spec.
I have checked the resistance values of the speed and reference sensors, they are within spec on the 600-1600 ohm terminals and infinite on the > 1M ohm terminals, infinite is greater than 1M ohm but is that within spec? I do not have access to an oscilloscope so I could not accurately perform the voltage operation test.
I checked speed and reference sensor wiring harness:
on the speed sensor harnesss
pin 8 goes to pin 23 on the dme connector
pin 27 goes to pin 27 on the dme connector
pin 23 goes to pin 8 on the dme connector
on the reference sensor harnesss
pin 25 goes to ground and shows up at different resistance levels on multiple pins on the dme connector
pin 26 goes to pin 26 on the dme connector
pin 78 goes to pin 25 on the dme connector
If these are all good, what else might cause the hard start and slow rough idle when cold?

Thank you,
Drew

[John_AZ]

Originally Posted by DrewPlaster

1987 924s has decided to be difficult to start when cold and when it does start it will not idle or will idle very slow rough until it is warmed up. You have to depress the throttle to encourage it to start and stay running. Once it has been warmed up thoroughly it will start and idle normally.
I have checked the resistance values of the DME temperature sensor with the engine cold and warm and it is within spec.
I have checked the resistance values of the speed and reference sensors, they are within spec on the 600-1600 ohm terminals and infinite on the > 1M ohm terminals, infinite is greater than 1M ohm but is that within spec? I do not have access to an oscilloscope so I could not accurately perform the voltage operation test.
I checked speed and reference sensor wiring harness:
on the speed sensor harnesss
pin 8 goes to pin 23 on the dme connector
pin 27 goes to pin 27 on the dme connector
pin 23 goes to pin 8 on the dme connector
on the reference sensor harnesss
pin 25 goes to ground and shows up at different resistance levels on multiple pins on the dme connector
pin 26 goes to pin 26 on the dme connector
pin 78 goes to pin 25 on the dme connector
If these are all good, what else might cause the hard start and slow rough idle when cold?

Thank you,
Drew

Need more information but here is the DME pin connector reference.






Source:
No Injector Pulse

You have checked the DME engine temp sensor.

It could be a vacuum leak in the rear hose from the ICV-idle control valve.
It is under the intake-impossible to access unless intake removed.

Have you ever removed the intake and replaced the hoses and clamps?

You might want to check for a vacuum leak.

vacuum system test.....soooo easy

GL & welcome,
J_AZ

[John_AZ]

Just another followup:

Check the reference sensors for correct gap and wire/connector damage.
Check the FW flywheel pins for dirt and damage.

J_AZ

[DrewPlaster]

I performed the sooo easy vacum system test and found two leaks, one on the throttle butterfly pivot (There's a repair kit available from Paragon for $20.50 that will fix this.??? Does anybody know what this kit is? Is it available via Pelican Parts?) and another under the intake manifold; unfortunately I won't be able to tear the intake manifold off until next weekend to determine exactly what is causing the leak. I'll post what I find after I get the manifold off next weekend.

Thanks,
Drew

[John_AZ]

I got mine from Bruce Arnn

Throttle Body Reseal kits for Porsche

Excellent kits and other products.

J_AZ

[DrewPlaster]

OK, ordered and received throttle body repair kit and fuel pressure gauge from arnworx.
Performed fuel pressure test and fuel pressure leak down tests all withing spec.
Installed repaired/resealed throttle body and fixed leak under intake manifold, replacing all hoses while it was off to future proof other leaks.
Now when car starts it runs smooth and steady no more speed hunting/stumbling rought idle cold or warm but it still intermittently refuses to start when cold so still looking for culprit.

[Gawernator]

Is your DME temp sensor original? It's inexpensive so couldn't hurt to replace it if so.

[John_AZ]

A couple of ideas.
I know you tested for fuel pressure.

1. Just remove one connector to an injector before a cold start. If the car starts it probably is a leaky injector (maybe the FPR-but you just tested it).

2. DME coolant temp sensor-replace

3. New cap and rotor?

More info from RL member "Danno"
Rennlist Discussion Forums - View Single Post - Could it be a bad AFM?

Drew,

This is a good read since you've tested a few connectors. Usually available on eBay.



GL
J_AZ

[onZedge]

I have the same car and had the same problem.

Intake Manifold Gasket leak.