plumbum

Twice my car has taken a few long attempts to start, both times after the car was sitting for a bit but started in less than a minute of me cycling the key and cranking.

Today I finished installing a radio and it would crank and not start..I checked and had good spark however there was no power to the fuel pump fuse.

This lasted for around 30min of me trying to start and then starting to diagnose the problem and then the car started fine and continues to do so.

Any ideas? I know there are lots of possible faults however I would like to try and pinpoint the cause rather than try a few parts and have it not start on me again. So far my only clue is no power to the fuel pump fuse when this occurred (eliminating the ignition side as I did test and had good spark)

joejoe

Tach bouncing when trying to start? If no then may be crank sensor. Relay is good? Just guesses here.

plumbum

No tac bouncing that I noticed.

Does the crank sensor power the fuel pump fuse? Which relay does the fues use..did not see a fuel pump relay.

My guess would be the relay would be the next in line ahead of the fues..I just didn't locate it before she fired up

16valver

I'm chasing the same issue right now. What I've learned is that the fuel pump doesn't prime in the "on" position on our cars. Apparently the fuel pump kicks on and pressurizes when the reference sensors read 200 RPMs and up (what the starter motor spins the engine at). If your reference sensors can no longer *always* read that low, then you pretty much just end up cranking until you eventually build enough pressure to fire.

Try and find a time when it doesn't want to start, pop the air box off and have someone spray a couple light shots of starting fluid in there. If it fires you know you're not getting fuel for whatever reason. No tach bounce usually means ref sensors.

Der Treiber

I once had a similar problem, where the S2 started cold every time, but after 30 minutes of running, no way it would start. Had to wait for 1-2 hour cool down, and then it would start fine again. Turned out to be the DME/Fuel Pump Relay. Swapped it out. Haven't had a problem since.

plumbum

I'm defiantly not getting fuel..no fuel smell with all that cranking and no back fire/smoke when started.

How are people testing the reference sensor? I will pack some tools so if it does not start again I have something to do when waiting for a tow

harveyf

There was a thread just last week on testing of the sensors. It is not easy unless you have the right equipment. The thread might have been on the turbo forum, I follow both.

Regardless, intermittent will probably be wiring, not the actual sensor. Next time your down and wiggling wires, be sure to exercise the connectors and harness at the rear of the engine.

No tach bounce is a sign of sensor problems. Look at tach bounce when the car does start to make sure you appreciate the difference. Start the car. Stop it. Remove the coil lead. Try and start. You should be able to clearly see the tach bounce but it is subtle.

The speed and reference sensors push a pretty low voltage. It doesn't take much in wiring issues to loose the signal.

turbodave

While I think that your installation of the new stereo does not have a direct effect on your no start condition, I do think it may have contributed. During the install and the testing with the ignition key in the #1 position your stereo is only one of many items that are being powered by the battery alone. It doesn't take long to drain a battery doing this. Also, Porsche stipulates (owners manual) that a 15 second starter engagement is max. duration followed by a 10 second cooldown between start attempts to cool the starter as well as well as give the battery a chance to recover. So before suspecting anything else I would check my battery under load. Under static conditions (ignition switch off) it should read about 13.4 Volts or so measured with a multimeter post to post on a fully charged, installed battery. Now, have a helper try to start the car and watch the readings fall off. A weak battery will quickly fall below 12V. The DME computer must have at least 12V to function properly. "Everybody!, Repeat after me!, The DME must see 12 Vee"! and the lack of that is a root cause of many no start/intermittent no start problems. The DME computer doesn't care how new the battery is or how much it cost or how many CCA it has. It is digital and needs a constant 12V to function properly. But it will try with less as long as the voltage is enough to complete a circuit, if only partially (spark). However as the voltage drops it will not close the DME relay (it's last line of defense) to send fuel to prevent cylinder washing. Porsche and Bosh are looking out for you! The FUEL PUMP FUSE HOLDER WILL NOT SHOW POWER UNLESS THE DME RELAY IS CLOSED BY THE DME COMPUTER... THAT NEEDS 12 VOLTS. Sorry for the caps, but I can't emphasize this enough. So, the bottom line is make sure your battery is up to the task before you try to diagnose any further. While the DME computer in these cars are old and outdated by todays standards (but much more robust IMO) their architecture still requires the baseline fundamentals, a constant and consistent power source. And while I'm on my soapbox, I'll say this for all components of these ageing cars (kills me to say that because I was considered old when these cars were introduced) that good grounds and good clean positive connections can mean the difference between a wonderful experience that will keep the car in the family for many years to come or a car that is parted out or sent to the junkyard. I hope this helps you and others in these no-start matters. Keep us posted! Dave

harveyf

Dave,

A fully charged battery with the engine running might show 13+ volts but with the engine off, you're only going to see about 12.6 volts. My experience and based on a quick survey of several internet sources.

plumbum

Much appreciate all the advice!

The issue has occurred three times, once after being driven for an hour and parked for 20min, the second after being driven for 10 min and parked for 1h and the third was when I installed the radio, the car has been parked for a few days. I am confident it is not a hot or cold issue.

Great battery advise, I did not have the key on for much of the install except to confirm which wires were power. The battery was near 12.4v when I first started cranking..and dropped to just above 12 for the first crank (Fluke was installed on the leads) it did go lower...around 10V maybe a bit less when if finally started. I did not realize the DME was that sensitive.

The engine cranked normally and i was using an adjustable spark tester which I open up to well over three times the normal spark gap just to confirm I had a strong spark, which I did.

Interesting information that the Fuel pump relay (which is the DME relay) requires a minimum RPM to close and provide power to the fuse...like many of you have said it could be the reference sensor or wiring.

I never had a chance to chase any wires as it started but I will try and find them and see if I can see anything obvious...It is to bad it started...as the car was in the garage with tools at the ready...next time I am sure it will be no where convenient.

This weekend I am going to repair the clock.. maybe that will make it happen again

plumbum

*Update*

Repaired the broken clock last night and took her to work today...8h later I go out and crank no start!

I did notice the Tac was bouncing with the crank but only a few mm...not sure if that is the tac bounce people are talking about.

I checked and wiggled the reference sensor wiring...no change ...Having no tools on hand and the sun going down the car stayed parked for the night.

At the moment I am going to try not to complicate things and chase back from known problem areas...Starting with the fact the fuel pump fuse has no power. Next step back would be the relay which I will remove and test to see if it is getting power tomorrow..I located a wiring diagram for the relay...having read a few posts I think (hope) my relay is bad..I just hope it does not start tomorrow...allowing me to jump the relay and get the car home and after take the relay apart to find a fault.


Another thing I noticed is my car only has one reference sensor (that I can see) my 951 had two (reference and speed?) The car has never quit while running...if this signal was lost or intermittent would it affect drivability of the car or is it simply used for start up?

Chunkerz

The S/S2 use one sensor for the reference and speed. Supposedly it uses the teeth on the flywheel to sense the speed and there is a missing tooth that it uses for the reference mark.

plumbum

Thanks for the info Chunkerz!

william_b_noble

I wasn't paying attention to what car you have - the 16V cars have one sensor on the flywheel and a hall effect sensor on the cam shaft which operates an ignition control module located behind the driver's side headlight

Grandpa#3

The DME Relay has got to receive the 200 RPM signal in order to switch on power to the Fuel Pump. Either buy a spare DME Relay OR make a Jumper Wire Set as per Clark's Garage. com.

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

Cheers,
Larry