Mysterious (to me) non-starting issue

CatsEyes · 01-21-2011, 04:01 AM

Hi folks,

I have a 1986 944 NA European-spec car. It's been running fine, except today, when it simply wouldn't start at a carpark. And hasn't since. Except for one minute or so, when the road service mechanic (US term?) had it ticking over OK. However, I tried to drive it out, and it stalled almost right away.

Seems the battery is supplying sufficient power, and the fuel pump is working OK. The engine turns over fine at ignition, but nothing fires. Seems the fuel injectors aren't receiving any signal. And the plugs aren't firing.

All the relevant fuses and relays seem fine. Everything seems properly connected.

Background. A week ago I replaced the fuel injectors. Careful job: everything's seemed fine since. In fact, the car was happy as Larry. Until today.

I'm sure there's something obvious I've missed...

931GT · 931GT's Garage

Maybe the reference sensor is bad?

thekidd · 01-21-2011, 10:24 AM

A basic check for the reference sensors is to try and start the car (even if you know it wont) and see if the Tach bounces at all. It may bounce a little when you move the key from the ignition position, but that doesnt count, only when you are trying to start the car. If the tach does bounce when you are trying to start the car, they should be OK. If not, try shaking the speed and reference sensors wire connections. I have the BMW style reference sensors in my car and I have to do that every now and then. Good Luck

zedsn · 01-21-2011, 03:53 PM

There is a removable cap in the front of the fuel rail with a ballbearing. Remove both of them and put a hose in the hole securely and with the other end in a gas can and have someone hold it there while you crank the engine to see if your fuel pump is indeed pumping fuel. If not could be a DME/fuel pump relay or your fuel pump has gone bad. Do you have spark at the spark plugs? If not could be a reference sensor or your computer went bad possibly. My 2 cents.

CatsEyes · 01-21-2011, 04:23 PM

Thanks, folks.

Fuel pump is pumping. But no, there's no spark at the plugs.

Reference sensor seems the most likely culprit based on the comments above. Though if anyone has another suggestion I'd be delighted to hear it.

Pardon my stupidity, but is there a simple description of the reference sensor's purpose somewhere. Never even thought about it until now.

David

tmfast · 01-22-2011, 06:50 AM

Hi,
I had the same problem. It was the speed sensor. Try spraying starter fluid into the air intake. See if it will start.

TibetanT · TibetanT's Garage

CatsEyes:

The reference and speed sensors send electrical impulse information to the DME/ECU brain. These impulse signals are utilized by the computer (brain) to electronically control the fuel injectors and the flow of fuel to each respective cylinder, or rather intake manifold.

The reference sensor records the location of a pin at the top of the flywheel, TDC position, while the speed sensor records the actual movement (speed) of the flywheel. Thus, regulating the amount of fuel that is sent to the engine.

Okay, with this information, one thing to check is the whether your injectors are actually getting voltage signals and as it has been said earlier, whether your tachometer is bouncing. The tach gets its RPM information from the speed sensor.

If your car was running okay before this problem, the first thing to check is battery voltage. Voltage on battery should be 12.5 and while cranking if battery voltage falls below 12 volts, you probably have a bad battery which is causing your car not to start.

If battery is okay, then the next thing to check is the DME relay. These relays are the "go between" for signals from the sensors and the ECU/DME brain.

Check the website for Clarks Garage, they have a very good troubleshooting explanation for DME relay problems and how to "jump" the 87, 87a and 30 terminals to see if your fuel pump is running and getting fuel to the engine.

I would start there and then report back, so we can all help with diagnosis.

Good luck.

CatsEyes · 01-22-2011, 06:28 PM

Hi Ed,

Thanks for the many useful suggestions.

Some quick responses:

1. I tried jump-starting the car from another battery without sucess. So I doubt that's it. And the car turns over well, but there's no electrical pulse at the injectors.

2. No, there's no bounce whatever at the tacho.

3. I tried disconnecting and re-connecting the wiring at the sensors, with no effect.

4. I haven't tried the DME trick thing yet, since it seemed the pump was working OK. But I will, and will report back.

However, further to 1. above. I have a recollection of a strange conversation with the previous owner, where he described some problem with battery power to the engine at start, even when the battery was fully charged. But he said (of course) it had gone away long ago and not returned. Being a Euro-spec model, the battery is in the back hatch. Could the wiring from there be at fault?

One thing I want to get clear about. Previous posters had suggested the reference sensor. You seem to incline to the speed sensor. Is it one or the other, or possibly both?

Also, as I described above, it did start once, for a minute or two. But then died. My strong instinct was it was fuel-related.

TibetanT · TibetanT's Garage

CatsEyes:

Yes, my first thought, with regard to the sensors, is that the Speed Sensor is faulty since the Tachometer is not moving at all during the cranking of the starter.

I do agree with you that the wiring from the battery in the hatch area could also be faulty and worth looking into at some point. At times the ignition switch is the problem with voltage not getting to the starter as well, but in this case you did say that the car cranked, or rather turned over, so the ignition switch is working.

CatsEyes · 01-22-2011, 11:50 PM

OK, we tested the speed and reference sensors with a multimeter. The reference sensor is functioning: the speed sensor is not.

So that looks like the culprit. Hopefully.

Probably have to get the part from Pelican Parts, so it could take a while to this side of the world. I'm presuming Bosch doesn't still stock parts this old in these parts.

Thanks for the help everyone. I'll let you know how we go.

commdiver · commdiver's Garage

Reference sensors, unplug both and make sure they have good connection still, dont confuse them as they are identical parts but need to be plugged into the right plug.

Also DME relay has 2 jobs, fuel pump and sends power to computer so make sure you have a good one or jump it with a 3 wire arrangment!

Go to Clarcks Garage they have all the write ups on any 944 diagnosis you can think of?

CatsEyes · 01-23-2011, 03:06 PM

Hi commdiver,

1. Reference sensor is functioning fine; speed sensor seems to be dead, though.

2. Fuel pump is working fine. I jumped the DME relay with the paper-clip work-around, and fuel pump activated, but car still wouldn't start. Fits with 1. above.

Hope I haven't missed any other possibilities.

CatsEyes · 02-01-2011, 12:55 AM

A quick update.

The reference/speed sensor finally turned up today from Bosch's Australian division (probably would have been quicker to get it across the ocean from Pelican Parts!)

And success! Car now running normally. With the usual to-do list: hunting idle, motor mounts etc. But nothing awful.

Thanks so much to everyone for their sage advice.

David