Okay. Different definitions of cold weather but when the temperature drops into the 40s here in Florida, my son's 1987 924S will not start. I am assuming it has to be DME related.

Here's the observable conditions when no start:
- Crank with no Tach Bounce
- As it warms up in temperature outside (over 50F), the car will immediately crank

What have we done to alleviate:
- Car has lots of new parts (from restoration) inclusive of all sensors, including Speed/Ref sensors
- No issues with Fuel delivery (pressure within spec), Fuel Injectors new (not rebuilds)
- DME relay was replaced with the Solid State relay with the Fuel Pump trigger at ignition

Car is at my house on weekend's typically. He left car with me and one morning when it would not start I swapped in the solid state DME relay. Car cranked up. Thought we had it solved.

Today, Solid State relay with Fuel Pump trigger stayed triggered even with no ignition or key. Son couldn't get car to start and hitched ride with a buddy at 7AM, Fuel Pump ran from 7AM to 9AM (when I got to his Mom's house to disconnect battery) and car fired right up (temp 52F). I shut car off and Fuel Pump continued to run.

I don't like the Solid State relay but am wondering if one of two things is happening.

1. Sensor Gap might change in cold weather (thought about pouring hot water on sensors if car does another no start)
2. DME Computer is on it's way out and doesn't like Cold Weather plus is triggering DME relay to run fuelpump

I had the issue with the DME relay running fuel pump previously and believed it to be an ignition switch. Replaced ignition switch last month.


Any thoughts on whether sensor gap changes in colder weather? Other thoughts?

 

You could try the following:
Disconnect the DME Computer, and put it inside of the house, and hook it back up once the temperature is below 50F. This should rule the DME itself out as a cause. I ain't no electronics expert, but god knows what might change in that thing at colder temperatures.
I don't think it's a change in the sensor gap. My car starts without problems at 14F

 

Had the car at my house this morning. 45F outside. Car would not start. I swapped for another DME (which I believe is good) and it did not start.

The sensor gap seems far fetched to me because 40F isn't that cold to cause the gap to widen. Also, the fact that the Fuel Pump Relay (which is Solid State with Pump Prime) runs constantly when battery attached makes me think something else is also going on.

Link to relay ---> http://www.ftech9.com/new-products/993-ssr-pp

I'm going to write to the relay maker (FTech9) and see if they have any thoughts. They seem to also sell modernized DME computers so that may be one way to move on from all the 1987 tech in the car.

Any thoughts?

 

Good Lord don't do this! You'll get water all over the clutch area

Did you try to hard-wire the relay slot? See here -> https://www.pelicanparts.com/techart...leshooting.htm

 

I'd say you have more than one problem going on right now.

1) The fuel pump should only be running when the engine is rotating above an estimated 300 rpm. If it runs when the motor is off and not cranking there's a problem. That's either a bad DME relay or a DME problem. Does it do this all the time or intermittently? If it does it with the key OFF, as you said, it would drain the battery pretty quickly.

2) The hard to start when cold is a separate problem. Have you checked the resistance of the DME Temp Sensor located under the J-Boot? When that sensor starts to fail it causes a lean fuel condition on a cold start. At 59 °F approx. resistance should be 3.3 k-ohms. At 86 °F approx. resistance should be 1.46 k-ohms. Mine recently failed. At 70°F mine read 17.8 Ohms (That's 17 Ohms, not 17.8K Ohms).

Regarding No Tach Bounce. A working reference sensor only gives a very slight bounce — it's not a huge bounce. Here is a video of what that looks like:


A good place to look for troublehsooting cold start issues: http://clarks-garage.com/ Scroll to the bottom of page, Click TS-01, Engine, under Troubleshooting. Then click Engine Is Difficult To Start When Cold

 

You sound like my ex-wife I appreciate you taking the time to explain. You're probably not familiar with the Pump-Prime DME relays by FTech9 nor did I probably do a good job of saying I know what the bounce from a reference sensor looks like on this car.

Every reference to engines not starting when they are cold is not exactly applicable to my situation of not starting at all when the temperature outside is cold.

This is the closest thing to EXACTLY my problem on the Internet....

https://www.pelicanparts.com/techart...leshooting.htm

See the comments section.

 

I agree with the above comment, have you replaced the fuel injection temperature sensor? Is the intake boot tight?

 

I am not being difficult or ignoring advice but please try to understand the dilemma.

This is not a NEVER starts when the engine is cold condition. It is a "when the temperature of the air is cold" the car will not start condition.

The temperature sensor is less than 1 year old and the boot is tight. Neither should have anything to do with the no tach bounce until above 50F.

Anyone else who has had a Solid State DME relay that was pumping fuel regardless of the key being in the ignition or others who have experience DME relay or computer like failures in cold temperatures?

 

To clarify, the car will start and run at 50 °F+ but in the 40s, it won't start.
Is the fuel pump still running with the ignition off / key out, or was it just that one day (intermittent problem)?
You mentioned pouring hot water on the sensors, how about just a hair dryer or heat gun (on low heat) instead - pointed at the sensors?

 

Thank you, Chalt! You are correct about temperatures and when it will and will not start. Fuel pump still runs if battery is connected, regardless of key state. I am now on my second Pump-Prime Solid State relay from F9Tech.com and do not believe they make bad relays. I can put an original equipment relay in and the fuel pump doesn't run.

Not a bad idea bout the hair dryer/heat gun on low (just a tight space back there and I don't want to melt anything).

Monday Dec 2nd will be the next day we are below 50F. I will likely have two warm (inside house) DME computers to plug in and try when the temperature drops. If it starts, then the issue is cold weather and the DME computer. If it doesn't then I can only assume it is cold weather and the sensor gap.





If you have an F9Tech relay and have ever had an issue with the Fuel Pump running without the key, PLEASE comment.

 

Okay. Pretty sure this has been solved. The culprit is interference from the rebuilt starter and reference sensors likely in the original, not updated bracket. This combination produces an intolerable amount of interference (electro mechanical ?) in the cold weather. Thanks to Rennlist and lister @mahoney944 , he was able to confirm his cold start issue was ultimately solved this way....

"As it turns out the solution consists of two parts.

1. Make sure your speed and reference sensor bracket is the updated one. Which uses a sleeve to shield the signals. https://944online.com/index.php/refe...block-kit.html

2. And most importantly, the factory starters are known for interference issues. Most rebuilt " auto store replacements" suffer from this even more. Update the starter to this and you'll be good.
https://www.pelicanparts.com/More_In...RoCthkQAvD_BwE"

We only replaced item #1 as I have a complete disdain for gapping the sensors on the aftermarket performance flywheel with pickups set at the wrong height. My son (16 year old) reports that he was able to restart the car at 40F. I'm not quite sure if we have the original or new bracket but I think we have the original in there. I'm leaning towards either one of the above probably would solve the issue without having to replace both. I love the high-torque starter which is item #2.

Thank you very much Rennlist!

 

Is that updated speed and reference sensor bracket something that 944online specialty manufactures? In other words, did Porsche install this from the factory on certain 944 or 924s cars? Was this something designed and engineered by the aftermarket parts community to address a common problem?

The one from 944online...

https://944online.com/index.php/refe...ription_tabbed

...looks different than what Pelican sells...

https://www.pelicanparts.com/More_In...101-321-07-OEM

Which leads me to believe that the 944online one was modified/improved to address this problem.

 

No the updated bracket comes from Porsche oem. I believe pre 87 has the original non sleeved version and after 87 has the sleeve.. they also changed the bell housing design slightly.



What many people do is slot the bell housing if you have the closed version and use the updated bracket . This allows easy sensor gapping and clutch work. No need to remove the sensors to remove the bell housing . Here's an example.

 

Since you may have already found the problem, I offer this link for testing the flywheel sensors at the DME as confirmation:


Another good test is to use an oscilloscope in place of the DVM. You might be able to borrow one to test.

Good luck, it's sometimes frustrating, but imagine what it would cost to pay someone to fix it!

 

I have not heard of the starter causing the Ref/Speed sensors to malfunction. What kind of rebuilt starter did this?