Car is a 88 951

All of a sudden I started having hot start issues. Which mimic both DME and sensor trouble, so I am going to lay it out in chronological order and get some suggestions on what to change first. Right now the car is broken down at my uni. parking lot so it's not easy for me to be swapping out a lot of parts without needing to be done.

Day 1

• Cold start
• Drive 5mins down road, park.
• Come back out 15mins later, car will crank and start to run but won't stay running, giving gas had no effect.
• Left it parked there for 5 hours
• 5 hours later it fired up and ran perfect, nothing had been touched.

Day 2
Moved it around my drive way while mowing lawn, Total run time < 5mins but had several (5+) start/stop sessions, no issues what so ever.

Day 3

• Cold start this morning, cranks hard fires quickly
• Drive 10mins down the road to uni. Parked.
• 1hour later car has same symptoms as day one, starts to run but won't stay running.
• Tap on the DME relay thinking it might have something to do with it.
• Still won't stay running.
• Leave the car for 3 hours.
• Wiggled the ref/speed sensor connectors
• Now the car is acting like a DME issue, tach bounce but no fire at all. (Same symptoms I had when I replaced the DME relay last year, which fixed the problem then)
• Tried pulling the DME relay a couple of times tapping again, and re-seating it and still the no start tach bounce remains.

I have never had any start problems like this other then when my DME relay went out last year and that gave me several weeks notice when it would randomly not start for a couple of mins.

The best I can come up with is that I have a start issue when the engine gets a good heat soak, and some how I busted my DME relay tapping on it? Or could the problems because of one common cause?

Also just realized that the first time this happened (Day 1) happened after leaving it outside in the rain. It is normally garaged. Maybe related. But it has had a couple of days to dry out since then...

Thanks in advance!

Updates

Day 4

After testing the relay from the above guide and plugging it back in the car fires first try and starts right up. (Relay checked out fine and its only 1-1.5 years old so don't expect problems)

So I still haven't learned anything. I did notice some new things though, when the car is idling the A/F goes from 14.7 up to 16-17 for a little then back down all why the engine has a 50rpm surge up and down. Then on top of that while it is idling it will have a random hiccup, the engine stumbles drops rpm and goes to 10:1 afr before returning back up to 14.7.

Day 5
Today the car wouldn't even start after sitting for a couple of days, it did its usual fire, run for a second, then shut down, and just crank there after like a DME relay problem

I then pushed the car to a new location 15ft away, and tried it again and it fired right up and ran fine...

 

Follow the engine trouble shooting procedure found on
http://www.clarks-garage.com/

Test Procedure Fuel-05 uses test jumpers to replace the DME relay. If the engine starts then the DME relay is bad.

Hope this gets you going
Keith

 

Thank you, some how I missed that section when I checked there the other day.

Well I have an update.

Today Day 4

After testing the relay from the above guide and plugging it back in the car fires first try and starts right up. (Relay checked out fine and its only 1-1.5 years old so don't expect problems)

So I still haven't learned anything. I did notice some new things though, when the car is idling the A/F goes from 14.7 up to 16-17 for a little then back down all why the engine has a 50rpm surge up and down. Then on top of that while it is idling it will have a random hiccup, the engine stumbles drops rpm and goes to 10:1 afr before returning back up to 14.7.

 

Added updates to first post.

Still looking for suggestions on what to replace first...

 

Can you access a known good DME and try swapping yours out for a test?

 

Replace the relay. It will give you odd failure modes. After 15 years they do give out. I know of numerous 944 owners that keep known good spare in the glove box for just such occasions.

 

I can't think of any unless someone near Chattanooga, TN is willing to let me test with one of theirs.

As per post, relay is 1 year old, already tested and it checks out fine. I also tried a brand new one that I have for such occasions and it made no difference.

 

Are you still working on the hot start issue?

 

Yes sir I am. Well kind of it has now started to mess up not only on hot start but now sometimes on a cold start as well.

I guess I am going to look in to testing the ref/speed sensors next and see what I can find there, seeing as I don't have access to a known working dme at this time.

 

Does the Tack bounce a little while tring to start. If not the The sensors need to be tested. http://www.clarks-garage.com/
Are you able to check your fuel pressure and fuel pressure bleed down ? There is a procedure @ http://www.clarks-garage.com/