Help! 1988 3.2 rhd. Replaced CHT & speed & reference sensors now engine won't start.
 

Hi,

I was having cold start problem and might have flooded the engine when I kept cranking it. So when the engine started it backfired. My mechanic suspected the 3 sensors may be the cause as the wire insulation was cracking due to age. Engine was running fine after the bacfired but occasionally would run rough (happened recently after almost 10 years ownership).

So he replaced all 3 sensors but now engine won't start. Old CHT sensor is single wire, new CHT is 2 wire. Speed & reference sensors are BMW parts bought from our host. He went back to old sensors, but engine wont start either. He did not start the engine before replacing the sensors.

There is fuel by the fuel rail and there is spark on the cable that connects to the cap distributor. He also squirted some fuel into the air intake pipe that is attached to the air flow meter but the engine still would not start.

Can the backfire damage the AFM? It was strong enough to unlatch the air filter housing.

I have the Bentley manual and I will ask him to go thru each fuel injection test, but what could be the cause here?

How to check if DME is not getting signals from the speed & reference sensors?

Could it be corrosion on CHT ground connector of the engine harness?

DME was recently serviced by Steve Wong 911chips. Injectors were also service thru him.

Thanks in advanced,
Oemar

http://forums.pelicanparts.com/uploa...1518440340.jpg
http://forums.pelicanparts.com/uploa...1518440340.jpg
http://forums.pelicanparts.com/uploa...1518440340.jpg
http://forums.pelicanparts.com/uploa...1518440340.jpg
http://forums.pelicanparts.com/uploa...1518440340.jpg

Try switching sensor positions in engine bay. Check gap at flywheel.

Hi..did you adjust the sensor bracket to 0,8mm?
https://www.porscherepair.us/porsche...rsche-924s.jpg
here is some reading.
http://forums.pelicanparts.com/porsche-911-technical-forum/311570-3-2-sensor-question.html

Are you sure you didn't mix up the speed and reference sensor wires when you reinstalled them? Have you pulled a plug to check for spark?

Hi Oemar - beautiful car!

I don't know why your CHT harness isn't set to accommodate a 2-wire CHT - One wire is 1984 vintage I believe (I don't know about ROW), but the CHT shouldn't be the cause of a No-start. It would cause rich start and rough running when warm.

You can do a resistance test of the Speed & Ref sensors per bentley, but to do them in circuit you would need a break-out box and scope.

If the Ref sensor path is bad or swapped, the DME will prevent the Fuel pump from staying on after a start attempt. Swap these at the engine connector as mentioned to test - it won't hurt anything.

Glad you have spark at the dizzy, but check at a plug or inspect the rotor.
Test the AFM per the Bentley or search here for a test using a 9V battery.

Let us know and good luck

Thanks everyone for the reply.

My mechanic swapped both speed & reference sensors to their corresponding connectors, he also reinstalled old sensors.

How to check to sensors gap or adjust sensor bracket? I did remind him repeatedly about sensors gap but he said he couldnt see the gap / no access to the flywheel.

What is a break-out box and scope? Where can I get them?

Thx a lot Dan

@Proporsche

"Hi..did you adjust the sensor bracket to 0,8mm?"

How do I do this? This maybe the problem, incorrect speed sensor gap. But my mechanic re installed old sensors and the engine still wouldnt start.

Just measure speed and ref sensor voltage output on pins 1 and 2 (same pins that have resistance between 500 and 1000 ohms), it should read AC voltage during cranking, approximately 0.5 volts or so. Speed sensor reads voltage quite constantly but ref sensor has one pulse per rotation.

If he was able to do the swap without actually removing the bracket, no need to check the clearance as long as it was okay before the swap. If he had to take off the whole bracket because the sensors were corroded and couldn't be removed otherwise, then yeah, the clearance has to be re-established. Clearance is critical. There are posts showing how others have done it.
If your mechanic says he couldn't see the gap & had no access to the flywheel, I think you've found your problem... :rolleyes:

Lightly Glue a washer to the reference sensor for the flywheel.
I think the used 8 mm spring washers are .8 thick. There is a good +/- variable on the spec
Bruce

Get a new mechanic.. asap.

Did they happen to be URO parts?

remember you have used BMW part i`m not sure it is the same length as the original sensors.As Bruce said ..follow his suggestion.
Nobody can see the gap ..it has to be set before you remove the bracket or replace it without removing the bracket.If your mechanic has removed your bracket and you are using the BMW sensors.
Again glue the washer on the end of it and insert.secure your bracket and remove the washer and reinstall
Ivan

If you have the spare sensor just do as Bruce suggested on the gap setting. You'll remove the speed sensor, loosen the bolts that hold the sensor, rotate the bracket until the washer bottoms out on the flywheel,tighten the bracket bolts down, remove the sensor and install the new sensor or just remove the washer you glued onto the sensor if the sensor is good.

If voltage verifies correct, wouldn't this indicate that the speed/ref sensor gaps are proper (and eliminate need to reinstall/use washer/etc. suggested by subsequent threads)?

The BMW parts are interchangeable and about half the price of the Porsche pieces on the aftermarket but be sure to order Bosch.
Bruce

He was able to remove both sensors without removing the bracket. We both compared the old sensors with the BMW sensors, identical parts but with different BOSCH part#, he was initially doubtful about the BMW parts.

He swapped sensors to connectors and swapped back to old sensors but to no avail.

We will measure the gap with feeler gauge. Thanks.

Not easy getting a Porsche mechanic here in Jakarta, Indonesia. There is only one official dealer here and opened in 2005 so they have no knowledge on aircooled cars. They also charge approx US$300 per hour.

My mechanic has been working on my cars (3.2 & 964) for 9 years and has never failed in repairing things (going by advice from Pelican members & Bentley manual). I'm sure he'll eventually figure this out.