help - engine dies and won't restart

[Keebler]

I have an 88 911 with a twin plugged coil-on-plug engine controlled by a Motec M84 ECU. It’s configured to run in fully sequential mode with a Clewett 60-2 crank trigger wheel with a hall sensor and a Clewett cam position sensor. All’s been fine for a few years but recently the car died while on the move and had to be flat bedded home. I have not been able to get the car running again and am clutching at straws as to what’s wrong. Here’s what I’ve done so far:

I’ve checked for errors within the ECU using the Motec software. There are no errors other than sync errors (more on those in a moment). When cranking the ECU displays a sensible cranking speed of approximately 300 rpm. The throttle position sensor values correspond to the throttle pedal position. There is a MAP sensor connected but in the ECU setup it only uses throttle position to determine fuel and timing.

The ECU reports sync errors as per the attached image. The sync errors only show after cranking stops; they don’t display while cranking. So not 100% sure if they’re relevant.



I was given advice to replace the cam position sensor. I did that but no joy.

I was given advice that I might need to change the CRIP value after changing the cam sensor. I connected a timing light and the CRIP value is spot on.

The ref/sync graph looks pretty good to my layman’s eyes.



When using the timing light I can see that the engine turns over a couple of revolutions without any flashes, presumably as the ECU establishes where the cam and crank are. It then flashes regular as clockwork at TDC. But a noid light on an injector only flashes once and then never again. It’s as though the ECU is happy to keep firing the plugs but not the injectors? Why?

What condition would result in the ECU being happy to fire the plugs but not the injectors? I wondered if the injector drivers had gone south so I took the cover off the ECU and looked inside - no smell of burnt components or signs of heat damage.

[Stephen Dean]

Hi,

If you have the View screen open to the Diagnostics tab when cranking do any of the errors return when the Reset Errors key is pressed? You will have errors showing when the engine is not turning as it isn't seeing the teeth on the Crank or Cam, pressing the reset errors key clears these issues and shows the current events.

If you do a Output test on the injector (without the fuel pump enabled) outputs, do they all fire? Same as with the Ignition coils.

From the screen capture of the RefSync Capture, it looks okay (one of your cylinders may be a bit down on compression) though a Hall Effect sensor should not have a voltage dip on the output unless the supply voltage also drops below the minimum sensor voltage. I would need to see the actual capture file to verify that the Sync is in the correct position.

[Keebler]

Hi Stephen, thanks for replying and for your help.

The SynErr and and No Sync errors go away when the reset button is pressed but the "Synced NotSYNCED" error stays.

They all clear when cranking but then come back when cranking stops.

I have tested the injectors using the testing functions and I cna hear each clicking away in turn. The same with the coils; I can faintly hear the plugs sparking away.

I've attached the latest sync image and data capture files.

eighth.png (86.64 KiB) Viewed 9894 times

[Keebler]

A couple more pieces of information.

I have confirmed that the injectors continue to receive power while the engine is cranking. But they do only fire once and then that's it. Voltage drops to 10.4V.

I have tried loading the standard "Start" map into the ECU and then reloading my map. No change.

[Stephen Dean]

Hi,

So when the engine is cranking and you hit the Reset Errors button, all of the errors clear and do not return?

If this is the case, then the ECU should be synced and trying to drive the injectors and coils. I would start by confirming that the injector power remains live during cranking, and also, if you have one available, connect a oscilloscope to the injector output and make sure that a signal is being sent.

[Keebler]

The errors clear by themselves when cranking starts; they reappear when cranking stops.

I’ve confirmed with a voltmeter that the injectors continue to receive power while cranking. I haven’t used a scope but a noid light to determine that they fire one time only.

[Stephen Dean]

That's normal behaviour for those errors, you will get a NotSynched error when the engine isn't turning as the ECU is not seeing the tooth edges on the crank trigger.

Can you post your calibration file thanks, I want to load it into a M800 here and see if I can replicate the issue.

[Keebler]

Thanks Stephen. Config file attached.

The chap who mapped the ECU for the engine builder is going to try to connect up via TeamViewer or the like over the next couple of days to see if he can spot something. He may also be able to loan me another M84 ECU to rule that out.

[Keebler]

Turns out it was the MAP sensor. I was wrong when I said it's not used; it's very much used. The sensor puts out 0v on its signal line making the ECU think there's a very high vacuum. Fueling was reduced by 90%. When I changed the table to something more suitable for the idle vacuum level the car fired right up. The default value in the config is 0kPa so the engine wouldn't run. The high/low input voltage thresholds are also blank so the ECU never flagged the 0v input as a problem. The sensor was mounted to the engine which probably didn't help its longevity; I'll be relocating its replacement to the firewall.

New sensor on order and I'm confident it will solve the problem. Thank you for the help and suggestions.