Stumble and Missing when damp and cool
 

Since I got my first 911 a few months ago I've been triaging one issue, and could use some help from the tech gurus here. The car is a 1974 911. Engine is a 3.2 short stroke with DME controlling the FI and ignition. This has only occurred when the 911 has been sitting all night outside and there is a considerable amount of dew or light rain about. Starting is no problem, the engine goes to a high idle for a minute or so, then it drops rpm and stumbles much more than normal. This will continue for 5 minutes or so and I will have to nurse the throttle to keep the engine from dying. After it clears it will run exactly as expected with no issues. The challenge for me on this is this small window of poor performance, which limits troubleshooting.

Here is what has been done so far in an effort to fix this:

• replaced the head temp sensor with no change in behavior
• installed new Bosch cap and rotor with no change
• installed new NGK BPR6ES plugs
• checked resistance on existing Beru plugs and ends (ends all at approx 3k and near 0 on actual wires)

My initial thought was that perhaps I had bad plug wires, but if this were the case I would expect spraying water or driving in the rain would exhibit the same problem. The car only exhibits this behavior when starting from a dead cold, the common aspect is that it's damp outside.

Thoughts?

Have you actually sprayed water onto the plug wires while running? Might want to hit it with a spray bottle (mist) in the dark. You'll find out really quick if the wires are arcing.

Yes. I used a water bottle set on mist, and just sprayed around the entire engine bay, in the garage with the lights off. I didn't see anything at all to suggest arcing. I'll admit I was a little disappointed not seeing the arcing as it would have been a simple solution. I just hate to throw money to replace things that are functioning fine. Perhaps I should try it again, but pretty sure I did it right.

Hmm. Very odd. Certainly sounds like a condensation issue.

Do you have the O2 relay under the passenger seat? at $12 being the solution after I chased a TON of other rabbit holes, I always like to mention that... you could check by unplugging it when its running rough... if no change, then there is a good chance that it's your dealio...

Interesting. The transplant into my 911 was done by a shop back in the mid 2000's, so there are certainly some differences with a regular DME install. Back to your suggestion. There are no relays under the passenger seat, and only the DME relay under the drivers seat. It is an interesting thought however. This engine does not have an O2 sensor installed, but I wonder if there is something relay related. Thanks for the suggestion.

Thinking electrical now. Is there any sensor or module on the DME FI system that would only be operable when the engine is cold or hot? I might be able to troubleshoot this if that is the case.

mca, I went out last night again and completely saturated the spark plug wires and watched. This time I did see a very few sparks in two placed. I'm going to move along and just assume the wires need replacing. hopefully this will sort out the issue.

Can we assume the wires on your engine are the original Beru cables?

Likely not the original base on the 3.2 transplant and they do not appear to be that old. That being said, they could easily be >10 years old when the transplant happened.

My father's has arcing around the plug at the center of the dizzy (brand new braided Beru). I applied dielectric grease to the area where the wire enters the plug - cured the arcing issue.

Might want to give this a try and see if there is any improvement.

Regardless, old Berus or even new ones are still notorious for shorting out at the slightest hint of moisture.

Not a bad idea. I'll give it a shot.

I ended up going with a new set of Clewett wires, and used di-electric grease on both plug end and distributor end of the wires. Unfortunately it didn't solve the issue. It was damp this morning with the same behavior. Starts up, and high idle rolls down to normal. If I try and hold it at about 3000 rpm with no load, it will stumble, and need to feather the throttle just slightly. After a few minutes, it will magically shoot up in rpm and be fine from that point on. I have not replaced the key ignition parts (cap, rotor, plugs, wires) with the exception of the coil. I'm thinking I'll hook up a fuel pressure gauge to verify fuel, but I'm at a loss. I don't feel too bad replacing the ignition parts since they were all unknown, but I'm running out of ideas here. Anyone have any additional suggestions.

I had a 75 2.7L with similar issue and it was condensation inside the cap/rotor. Was living in S.Florida at the time and anytime it rained heavily the car would not start and sometimes even stalled. I sealed all openings to the distributor with silicone, where wires go in and out of it. Also applied small bead at the cap to seat it to the distributor and problem was solved.

Next time this happens take a heat gun or blow dryer to the inside of the cap and distributor to see if this fixes the hard start issue.

The strange thing is that it will start with no issues, but put it into gear and try and take off it stumbles. Give it a few minutes at 3k and it clears. Once clear it's perfect. Even in rain. The only time it happens is from a dead cold start when it's moist outside.

I'd still try drying the inside of the distributor on these very damp days prior to startup and see if it makes a difference.

I sure hate to bring this back up, bit it haunts me still. As a last ditch effort on the ignition I replaced the bosch coil with a flame thrower2 I had laying around (low impedance). Ran just fine, until it rained. So to recap:
What the engine does:

• When wet (or high dew) it will stumble until I run it 5 or so minutes
• It will not keep an off idle rpm, but will hold 3k with some foot adjustment
• Once it clears it doesn't show back up (unless sitting in traffic in heavy rain)
• frustrates the crap out of me

What I've done:

• New Clewett spark plug wires
• Dieelectric grease at all plug fittings
• New Bosch cap / rotor
• Known good coil
• Checked cap for moisture, finding none.

I had put lower on the list the need to make an adapter for the fuel rail and it seems I will need to make that happen now to correctly diagnose the FI system. What puzzles me is the relation to damp/wet weather after I should have resolved the ignition system with new good parts. Any thoughts on what may cause this behavior till I can put a pressure gauge on the engine?

I know you checked the coil but sometimes it is the culprit in these cases. A micro crack in the case near where the plug connection is. Water gets into the crack and creates a grounding pathway resulting in missing and general poor running. Nothing in dry checking the coil shows up as a fault since it is doing it's job. My thought is coil or or leads. Cheers

wait for wet weather and then take a heat gun to one area; test; if not improved, try another area; repeat

you transplanted the ECU? if so, don't forget that