amr89c4

Greetings,

I am new to this forum as well as a new 964 C4 owner. The car is a mint, '89 model with 48,600 miles on the clock. I'll post pictures later. I finally picked the car up Aug 31 and it has been running flawlessly for 9 days and about 600 miles.

However, yesterday morning (6:30 AM) the car failed to start. What I mean is that it cranks perfectly but it has not fired. Not one pop. The tach needle is not moving during the crank, so I'm inclined to believe I have no ignition for whatever mysterious reason.

I picked the car up in Indy and was visiting a friend in Ohio on my way home to TX, I'm relying on her shop to get me going. An unexpected business trip "detoured" me and I was headed to the airport this morning and had no time to troubleshoot before leaving.

I am very well versed in EFI and digital ignitions and am pretty familiar with the C4 systems as I studied the system as much as I could considering there are no readily (that I have found) available service manuals specific to the C4, prior to purchase. I am not, however a Porsche technician so am seeking some opinions.

My friends Bentley "Porsche 911 Carrera Service Manual 1984-1989 - Coupe, Targa, and Cabriolet", did not include the C4, so I would appreciate a recommendation for a good technical service manual. I have ordered the "Porsche Carrera 964: 1989-1994 Technical Data - Without Guesswork: Carrera 2, Carrera 4, RS America, Turbo 3.3, Turbo 3.6", but I realize its more a technical data manual than a service manual.

In the outboard systems I am familiar with, a failed CPS (crank pos sensor), which I assume failsafes on this engine the same as our outboard and could cause this issue, as well as a main power relay or DME/fuel pump relay in the case of the Porsche. Worst case is an ECM failure, I suppose. In the back of my mind lurks the alarm system, though its my understanding it "kills" the fuel pump, blows horn and flashes lights for 90 seconds. I have no alarms going off.

Someone locally suggested a failed fuel pump, but I find it hard to believe the car would not at least fire once or start and run ragged for a second or two. The car was running fine when I turned it off Saturday evening. Since I was trying to get to the airport this morning, I was unable to verify spark or fuel pressure.

Of course, not being where the car is makes it tough to diagnose. Do you folks have any thoughts? Have you ever heard of a car running fine and then a day and a half later, cranks fine, but won't start? The shop is supposed to check it out later this week and I return to the car on Saturday AM.

Any ideas or experiences would be appreciated.

thx,

Lynn
89 Guards Red C4 coupe

DAVISRILEY

dme relay perhaps?

amr89c4

Thanks, that was a prime suspect for me. I was not sure if a failed CPS would failsafe to a "no start condition".

DAVISRILEY

Dme relay can make it a crank, no start. A lot of people carry a spare, me included. If there is a dealer near where you are, most will have one in stock. Good luck.

Andy Roe

I'll 2nd the DME Relay. Cheap part and quite a common failure point. I'd try replacing that first. A lot of people carry a spare in case this happens!

dfinnegan

DME Relay part no. 993.615.227.00


You can also jumper it for a test:

jimq

if you can't hear the fuel pump running when you turn the key then I also agree DME. I had the same thing happen to me a couple years ago.

sshenton1975

Hi I had a similar thing happen to me a couple of years ago on my 89 C4

Basically, the DME was giving up the ghost.
This was then compounded by me pressing the gas pedal when trying to start the car.
I since learnt that these cars are extremely prone to flooding and that you should never touch the gas pedal when trying to crank them over,.

This could be your problem?

Cheers

Simon

911URGE

One last thing to try...take the DME relay out and smack it hard in your hand several times and put it back in and see if it fires up. My relay appeared to be sticking at the time. That technique at least got me home from the store.

Replaced with a new one and put a spare in the trunk.

amr89c4

Looks like the DME relay is the hands down favorite for being the culprit. I have one arriving tomorrow.

The shop called today and had a good news, bad news message. When they rolled the car off the tow truck platform, they hit the key and of course the car started instantly. After a half dozen cycles they decided to see if it would start in the morning (this morning). It started fine. Nothing like an intermittent to chase down.

I also thought that the key switch could be at fault, but based on the input from you guys, I think the DME relay is the prime suspect and a heck of a lot easier to swap out.

Thanks for all the responses and the relay "bypass" test/emergency suggestion.

Lynn

drisin80

best of luck swapping out the dme relay, and i hope it solves all of your problems. i had similar problems which started of very intermittently, and then became more and more frequent. i have a 90 c4 with 60,000 mi. in my case it turned out that the dme relay was ok, but i finally tracked it down to a faulty circuit board in the factory alarm system. there turned out to be a fault (crack) in the circuitry of the board which would disable the fuel pump, but give no other simptoms. in 89 and 90 models the alarm is behind the main fusebox attached to a clip in the fenderwell. i found it not to difficult to remove and check the board. bad circuit boards (others as well--ie convertible top unit etc.) are a fairly common problem on these cars. hopefully the relay will fix things, but this is a good place to check if the problem does not go away completely. lol---tom

amr89c4

Thanks for the tip on the pc board in the alarm system, which raises a few questions. When you say a crack in the board, did you find a trace broken? Cracked or cold solder joint? Were you able to duplicate the problem through vibration or temperature variations? What was your test criteria?

thx,

Lynn

drisin80

i actually found a crack in one of the traces. how i figured out that this might be the problem was a long story, but the jist of it is that i eliminated the possibility of it being a dme relay by jumping the relay as in streather's manual (there are posts with pictures on how t his is done) when the car wouldn't start. i had previously bought 2 new relays, and was fairly certain that this was not the problem. after looking at the wiring diagrams and some help from others in looking them over (sorry if i am not giving proper credit) the alarm module simply looked like the next place to check. after finding and disconnecting the module--i gently removed the circuit board from the cover and the fault was obvious. i bought a new one from sunset porsche--around a 100 bucks---and everything has been perfect. so i guess the test if your car will not fire would be: if you know that the dme relay is ok, and yet the pump runs and the car fires if you jump the relay---- then this means the pump is obviously ok-- take a look at the alarm. tom

amr89c4

Perfect. Thanks Tom for the great explanation.
lynn