944S wont start (Sometimes?)
 

1987 944s has ALWAYS started flawlessly, 2 revolutions and idling at 1000, no sweat. Suddenly now though it takes a little more cranking ( 3-4 rev's) to get it going. Then, if i shut it down after 3-4 minutes it will not restart. Cranks a lot, but wont start. It has spark and the fuel rail is full with decent flow, but it just acts like it isnt getting any fuel. Kinda sputters a little once in a while. Then...after cranking on it till the battery is almost dead, i let it sit while i get my charger out, (couple minutes)and then it starts. Runs rough for a few seconds, and then seems ok. But it will start first thing in the morning or if i drive it long enough to get it good and warm. Any suggestions would be greatly apprecaited, fuel injection is not my area of expertise...Thanx

Might be worth checking the ground wires behind the engine that bolt to the firewall. These frequently cause problems that are very difficult to track down because they often only result in partial system failures. Even when they look fine clean they often benefit from the adding an additional lead under the connection and bolted to another area of the firewall. Battery leads are a pain too. Cheapest thing you can check.

http://www.connact.com/~kgross/FAQ/tb/tb.20.7601.html
http://www.connact.com/~kgross/FAQ/944faq08.html

Cebu beat me to it! Fuel pressure... ahhh just follow his first link!

Thx guys, i'll check the ground straps (i do have a few intermittent elec. problems that might explain). However , it acts like a fuel thing, so i'll also check the pressure, the rail is always full, but could be low press. I'll let you know....Thx again..............

What do the tips of your plugs look like? Condition? If you have decent spark and if your plugs are saturated from fuel after cranking till the battery is dead without ignition it's probably something electrical.

OK guys, makin progress... wont start at all now....lololol
Anyway, its fuel, the pump wont run. If i pull the DME relay and jump between 30-87b the pump will run, then reinstall the relay and i get a little fire. If i jump between the terms. and clamp the fuel rail return line to build pressure, then the engine will run for a few seconds until the presure drops. This must just be a result of the pump not continually running during cranking. The pump wont run during start or while the engine is running, but the pump is good. I thought relay obviously, but there has to be other criteria for the pump to run, i'm guessing bypass during start and then what, maybe oil pressue? But since it wont run during starter cranking, it must just be the DME relay.???? right???? ( or not ) Thanx

RPM. The speed sensor has to read at least 200 rpms or it will kill the pump. but it's probably the relay. Don't you have an extra one laying around?

But there has to be something that allows the pump to run under 200 rpm, like during the start. Does the DME relay provide this as some sort of bypass for the pump during start up to 200 rpm? Also, i have never carried a spare, and (knock on wood) never needed one. I take it this is cheap insurance as these cars age, eh? This relay must control numerous functions.,,,,,,,, Thanx

Yes the fuel pump is triggered through the DME by the same circuit that activates the solenoid on the starter. When you let off of the key there is a timer in the DME that keeps the pump running for about a half second. If the DME sees the required signal from the speed sensor then it keeps the fuel pump relay part closed.

The DME relay is a two stage relay. First stage/contacts close with the key on sending voltage to the DME and the injectors. The second stage of the DME relay, the fuel pump, can not close unless the first contacts are closed. Checking for 12 vdc at the injectors is one of the quick tests to determine that the first stage in the DME/fuel pump relay is closing.

I have 12 v. at the injectors, and the pump will run if I jump 30-87b, just wont run any other time. Sooooooooooooo, does this mean DME relay?.....................Thanx...and if so , suggest where to get one, I see pelican doesn't have them........

My first guess would be the DME/fuel pump relay. Good to have a working spare in the glove box.

Second would be the speed sensor.

Pelican parts does have them. I got one from them last year. If not try Zim's or Performance

But if its the speed sensor, i should still have fuel pump operation while the starter is cranking, right? But it wont run during cranking or after start, only when i jump 30-87b....but, if i understand you correclty, if i have 12 v. at the injectors, and thats the same circuit that powers the pump during start, then i should have power to the pump?????????????????????????sompin aint makin no sense...also, this is the relay, position 21 in the fuse box, correct...cause i've heard there is another fuel pump relay.......

Let me back track a little. You have an 'S'; twin over head cams right?

Then there is only one sensor. Used for both the speed and the reference signal. Uses "teeth" cast into the flywheel. So if you are getting spark you should also be getting the speed part of the single sensor signal.

There is only one DME/fuel pump relay. Don't know where or who told you otherwise. If you look close at the top of the proper relay it has the letters DME cast into it. G5 if my recall is as good as my memory.

No, the first set of contacts in the DME relay only supply voltage to the injectors and the DME. It's the second set of contacts that the DME closes (remember that theDME has to have current or it doesn't work) that provides the power to the fuel pump. The signal to the DME is the current to the starter solenoid.

The fuel pump contacts should be closing when you crank the car (use the starter) and when the engine has 200 rpms or better.

Here is a web page about the DME relay with a location picture for the later 944's.

http://www.clarks-garage.com/shop-manual/fuel-05.htm

Thanx Socal...yes it is an S, and it turns out the fuel pump relay is for earlier models....the link also helped alot, thanx...99% sure its the DME, run a couple checks and i'll know for sure.....i really appreciate all the help and i'll post my results....Thanx again. :-)

The DME relay is the same for all of the models. It's just refered to as the fuel pump relay in the early 944 parts catalogs. Later Porsche parts listings call it the DME relay or the DME/fuel pump relay.

Same thing and part number.

Just an update.....the culprit was the DME relay.....opened it up and sure enough, several solder connections were cracked,
resoldered them and now it works like new, starts right up and runs great...THANX to all for the help, an excellent site guys..
:)

Get a spare relay for your glove box. Unless you carry a soldering iron with you.

Send Clark a message thanking him for his web page.

Just had a similar problem with my 951. It would start right up some times and not start without a lot of effort other times. It finally quit all together Sunday (at the track, of course). Turned out to be the fuel pump itself. It had developed an intermittant problem that finally became terminal. Once replaced, it starts better than ever. As already mentioned, the relay for the fuel pump often is the culprit as well. I carry a jumper with me at all times just in case. Plus, it helps me rule out the relay as a source of the problem.