Damp, wet weather = engine misfiring. Help please.

[911obgyn]

Clean the grounds while you are checking everything else, they may look good, but remove wires and clean connections.

[JCFL]

Hello guys, and thank you for all the suggestions on possible solutions.

I have followed many pieces of your advice and was able to discount these as possible problems:
- coil
- two sensors at crank
- some grounds

With the help of my son, we were also able to establish bad fire in the distribution cap (several black marks) and on the rotor. We were also able to confirm black residue on spark plug (first on the left of engine). This would tend to confirm bad firing...

We were able to establish that DME relay worked fine, on smelling the gaz fume at the back of the engine.

So, I have decided to treat myself to a rebuilt DME unit from Steve Wong c/w performance chip. At the same time, I will also change the rotor and the distributor, and the spark plugs.

Will let you know guys how it turns out...

[snbush67]

I think that it’s possible that your plug wires are arcing.Maybe your rotor has been going bad as a result of plug wires arcing.

[JCFL]

SNbush67,

I changed the plug wires last July. I Was hoping that they would be ok. If the new/rebuilt DME does not fix the problem, I will check all the plug wires again. Thanks for your input.

[PeteKz]

How did you discount the coil as a possible problem? Simply measuring the resistance is not sufficient. Since you have a spare coil, swap it out (5 minutes if you are slow) and see if the problem persists. If it doesn't fix your problem, rule it out and then chase more expensive stuff.

But DON'T make lots of changes at the same time. One thing at a time.

[snbush67]

Originally Posted by JCFL

SNbush67,

I changed the plug wires last July. I Was hoping that they would be ok. If the new/rebuilt DME does not fix the problem, I will check all the plug wires again. Thanks for your input.

It doesn’t matter if you changed the plug wires, brand new wires don’t care, they can arc. Even if the new DME works fine, you still have to check to see if the new wires are arcing.

The rotor could have been damaged by arcing wires, so if you didn’t change the rotor when you changed the wires then the problem would remain. Or as Pete recommends, if your coil is on the fritz then only a different coil will work. You really have to do some serious troubleshooting and determine what exactly is wrong. Without an accurate diagnosis then you’re only risking damaging the new shiny parts you’re throwing at it, it becomes an expensive circle of destructive mechanical and financial frustrations.


Cheers, Shane

[brighton911]

I suggest checking the distributor bushing by seeing if you can move the rotor side to side. My nephew's 87 911 distributor upper bushing was worn allowing the rotor to contact the cap. Mine at the same mileage was just fine.

[JCFL]

I did verify the distributor bushing - they seem fine.

I also verified the coil, by swapping a new one - but as it was said, perhaps arching was the culprit and damaging the setup.

This Forum is unbelievable - thank you again all, for these helpful suggestions.

[David Inc.]

Run the engine in a very dark room and give the wires a spritz with a spray bottle of water. If they're arcing you'll get a great light show. Many cases of bad, new wires out there.

[JCFL]

Hello Shane, and thanks for the comments.

In June 2022, I changed spark plug, spark plug wires, cap, rotor and coil. This June 2023, I changed the plugs, the cap, the rotor, and I checked the rotor.

Unfortunately, my knowledge of mechanics is such that I having a very hard time doing diagnostic. I HAVE BEEN doing troubleshooting, but I am unable to get the car to go.

Originally Posted by snbush67

It doesn’t matter if you changed the plug wires, brand new wires don’t care, they can arc. Even if the new DME works fine, you still have to check to see if the new wires are arcing.

The rotor could have been damaged by arcing wires, so if you didn’t change the rotor when you changed the wires then the problem would remain. Or as Pete recommends, if your coil is on the fritz then only a different coil will work. You really have to do some serious troubleshooting and determine what exactly is wrong. Without an accurate diagnosis then you’re only risking damaging the new shiny parts you’re throwing at it, it becomes an expensive circle of destructive mechanical and financial frustrations.


Cheers, Shane

[911 Rod]

I had the same problem and it was an almost new cracked distributer cap

[JCFL]

Hello guys,

Today, I have done the water " mist " test in the dark, and no, the plug wires do not arc. I am glad that this issue has now been discounted as a possible source of the issue with my car. There was no arching anywhere in the engine bay.


I am now going to start the clean up of the grounds.

My son was just reminding me in a phone call that when we did check the coil, it was barely getting voltage upon cranking the engine over. It would light up a testlight, but very dimly, to sometime not at all.

And therefore, the coil itself could then not produce enough voltage for the distributor cap. We then tried to connect the positive terminal of the coil directly to a good power source with a jumper cable - getting the same sesults.

My son believes that this might indicate a bad ground linked to the DME unit. I am going to start checking those first, but I am not sure where I should be looking.

Does anyone out there has an image of ground points that would be linked to DME ?

Thanks.

[JCFL]

Today, I performed the DME relay test, by using jumpers in the relay socket.

I removed the relay from it's socket and connected a wire between pins 30 and 87 to turn on the DME and between pins 30 and 87b to turn on the fuel pump. Although the car was cranking, and trying to start, it did not.

As said in my previous post, it really does seem as if the coil is unable to produce enough power to all spark plugs. And yet it is a new coil, after another new coil last July.

I have now started to clean the grounds.

So, I have a new:
- rebuilt DME with Steve Wong performance chip
- 6 new spark plugs
- a new coil
- new rotor
- new cap
- 6 new plug wires from last July, tested for arching with water

Any suggestions guy ? I have a new DME relay on order, but based on my jumper test this morning, the relay is not at fault . . .

Thanks

Jacques

[911 Rod]

Try the old chip?

[JCFL]

Gone to supplier as part of the deal to buy rebuilt DME unit.

And the problem started with the old chip in (the original DME).

[r_towle]

It’s happening when wet.
You still have not tested nor replaced the crank/reference sensors.
You have not swapped in the old coil to test.

When mine died in a parking lot, mild weather, no rain. It was the crank and reference sensors.

Also I would consider testing the head temp sensor, another key to the ECU setting the proper mixture.

Lastly, or firstly…
Pull the gauges back out and verify and validate all wiring is in place still. Something may be loose.
What you did last…..the wiper project.
Some (temp) and pressure gauges route via the gauge to the ECU possibly, and a loose connection or one that fell off might do it

[JCFL]

Thank you very much Rich - your are very generous of ideas and possible solutions. I will follow up on your suggestions - the 3 sensors are 329 $, which will be about 500 $CAN with shipping. Herk !

[SkunkWorks]

Originally Posted by JCFL

Thank you very much Rich - your are very generous of ideas and possible solutions. I will follow up on your suggestions - the 3 sensors are 329 $, which will be about 500 $CAN with shipping. Herk !

You can use the BMW crankshaft sensors which are half the price of the Porsche one. Exact same sensor.

BMW part number 12141708619 / Bosch 0-261-210-002 $55.43 each on Pelican

For cylinder head temperature sensor make sure you get the updated one that's 2 wire, part number 93060691500

[JCFL]

Thank you Skunk,
Order is placed - yes to BMW crankshaft sensors.

[JCFL]

Hello guys,

I have been doing some more searching on the Forum, and also following up on the advice and suggestions received. Notably, Rich reminded me about the important 3 items for running the engine: 1) there must be FUEL, 2) there must be SPARK, and 3) there must be AIR. I have also recruited a neighborhood mechanic to give me a hand in identifying the problem.

ITEM 1)
We have confirmed that the fuel pump is working ok, and that there is fuel going to the engine. I did the bypass of the DME relay test (when we hear the pump operate), and I also have been cranking the engine. It tries hard to start, but it fails. And then we get this strong fuel odour as well - so yes there is fuel.

ITEM 2)
We have also confirmed that there is spark. My friend and I have tested the coil, the distributor cap, the rotor, and also the plug wires (for spark in the dark caused by water mist testing).

ITEM 3)
In testing the Air Flow Meter, we found resistance measures that were not up to par. I will be detaching the Air Flow Meter from the engine later today, along with air filter. We will then test the AFM more precisely.

NOTE
I have new DME relay, new (rebuilt) DME, new plugs, coil, rotor and distributor cap. So does it seem plausible that the AFM is causing the car not to start. Again, it does crank well, and we hear it trying hard to start, but just not able to.

We also tested 1) the speed sensor, 2) the position sensor and 3) the cylinder head temperature sensor. All three are showing the appropriate resistance levels, per the Bentley book, as well as info found on this Forum.

Any other suggestions. Thank you.

Jacques