Damn thing won't start!

jeabigwave · 05-25-2009, 09:33 PM

I only drive my 87 Carrera once every few weeks. Lately, it has been taking a long time to start up. In fact the last few times I cranked it over so many times the battery got drained and I had to jump it. However once it fires up it runs like a top and I have never had any issues starting it even after sitting for hours at a time on the same day that it is driven. However today, it will not start. I'm getting spark, I also jumped the #6 fuse (fuel pump) to the #7 fuse and the fuel pump makes noise. and while doing so even tried to starting the car. However, the damn thing doesn't want to start. I also took of the rotor cap to see if maybe the rotor was cracked.. Can anyone offer any suggestions besides using a stethescope in order to listen to the injectors. (I'm not an MD and don't own a stethescope). Thanks in advance..One frustrated 911 owner.

slodave · slodave's Garage

Have you swapped out the DME relay? If you don't have one at the moment, you can try to tap yours and see if it starts or take it out and apart and check the solder joints.

ischmitz · ischmitz's Garage

Here are some pointers:

How do you know that you are getting sparks - did you pull a plug wire and used a spark plug against the engine body. To test for fuel you need to check fuel pressure and get an LED test light. It will show injector pulses.

Usually, if your DME relay is bad your fuel pump doesn't come on during cranking. This can have to causes.

a) bad DME relay
b) DME relay doesn't get signal to turn on from the DME

If you jumper the fuel pump and it runs that tells you it's physically O.K. Now if the car still doesn't run this would suggest the DME is not turning the DME relay on during cranking or you have another issue. Check if the fuel pump runs while you crank. My guess it is running. If not you have an issue inside the DME.

My next guess is that you could have a bad flywheel sensor. Either the speed or the reference sensor could be marginal and on its way out. There was a recent thread here where a guy had one of the sensors (speed/reference) bad in his car. I forgot whether he wasn't getting spark or fuel. After replacing the sensor the car ran fine again.

Ingo

slodave · slodave's Garage

I'd listen to Ingo.

Phat Euro Cab · 05-26-2009, 06:54 PM

Quote:

Originally Posted by ischmitz

My next guess is that you could have a bad flywheel sensor. Either the speed or the reference sensor could be marginal and on its way out. There was a recent thread here where a guy had one of the sensors (speed/reference) bad in his car. I forgot whether he wasn't getting spark or fuel. After replacing the sensor the car ran fine again.

Ingo

Not sure if it was my thread that was being refered to but here's a link to it. http://forums.pelicanparts.com/porsche-911-technical-forum/452049-86-911-will-not-start.html#post4439060 Speed sensor fixed the problem and then I packed the car like a giant suitcase and drove 1600 miles from CT to FL afterward.

Por_sha911 · Por_sha911's Garage

To run your engine needs a)air b)spark & c)fuel.
1) Swapping out the DME relay is a cheap and easy test. You should have a spare DME relay anyway.
2) When you pull a plug wire, use any old plug to check for spark (but don't zap yourself).
3) There is a simple test to see if your flywheel sensors are bad. Any cheap multimeter will do. (I believe I've seen it either hear or in Bentleys).
4) Sometimes a cap or rotor will have a hairline crack you can't see but it shouldn't cause a no start only situtation. It would be more like a running rough problem all the time.
5) Harbor Freight sells stethoscopes for about $4-5. Cheap but gets the job done.

Take a deep breath, don't panic, take one item at a time and start eliminating variables until you find the one item that is out of whack.

ischmitz · ischmitz's Garage

Phat Euo Cab, your problem was pretty straight forward; a bad sensor and you lost both fuel and spark. But there was someone else who lost only one of the two. And against common sense it still turned out to be one of the sensors being just marginal.

The DME interprets both sensor signals by means of custom a trigger chip and generates timer interrupts at the MCU. That information plus other sensor input is used to calculate the fuel amount (injector pulse duration) and timing advance. So if one of the sensors is completely dead the DME won't produce spark or fuel. But apparently if one of the sensors is marginal the DME can still deliver fuel.

Ingo

jeabigwave · 05-30-2009, 07:44 PM

Had some time took look and test a few things today. Apparently, I'm not getting spark, but I am getting fuel, as I not only can smell it, but I also jumped the #6 fuse and could hear the pump running as well. Previously my son told me he saw a spark, but after further analysis he was mistaken. I pulled the Coil Wire off of the Rotor Cap and tested the voltage from the coil (11.98 vdc), so it doesn't appear to be a bad coil. So the DME relay looks easy to replace. However, how to I get to the "flywheel sensor" and how do I determine which one is defective? Also I searched the parts catalog for "flywheel sensors" with no luck. Is it maybe referenced as something else? Thanks for all the help..This forum is awesome!

kidrock · 05-31-2009, 07:08 AM

"flywheel sensor" is also known as the reference sensor....there is a metal stud screwed into the outer portion of your flywheel which acts as an electronic reference for Top Dead Center. In essence, it is a crucial timing sensor. You will find it screwed into a small harness near the front of cylinder #3, along with the "speed sensor". IIRC, the speed sensor counts the number of teeth on the flywheel and performs a different, although similar, function.

ischmitz · ischmitz's Garage

Jumpering the fuel pump doesn't mean your injectors are delivering fuel. Get a NOID light to test if you see injector pulses. Where do you smell the fuel?

Your coil test is inconclusive. The 3.2 ignition works like a Kettering ignition: The coil gets permanent +12V on one side and the other side goes to the DME. Inside the DME a "switch" provides a path to GND for a short time (dwell time) and then opens again to produce the spark. Testing the coil is more involved than finding +12 volts.

Start with a basic test: Switch on the ignition and feel the idle control valve on top of the engine. I should vibrate. If not your DME relay is bad. Next, test the injectors for fuel with an LED test light or NOID light.

jeabigwave · 05-31-2009, 08:21 PM

Looks like its probably the reference sensor and I have no idea on how to replace it, nor align it properly. I'll have it towed to Ken's in Campbell so that it doesn't spend another weekend in the garage.. Thanks everyone for all your input. Beleive it or not I learned alot about my car this weekend.