Help! 911 runs terrible in rain

[micky_ray]

I posted a question about this last week, and got several good suggestions, but the behavior continues and I now have more information.

I have a '85 911 cab, euro spec, 34k miles. Whenever I drive in the rain it starts out running fine but after a while the power falls off severely (takes a long time to make it across an intersection from a stop). The idle drops from a smooth 900 rpm to a lopeing (sp?) 750rpm and I occasionally get a backfire.

I spoke to a mechanic, and he suggested that it was the ignition system getting wet or the plug wires being worn out, so I put new plugs, Magnecor wires, rotor, and cap on, with dielectric grease on all boots and where the cap meets the distributor. I also inspected the coil for cracks as was suggested on another post but it looks fine.

This morning I drove to work in the rain and the problem started again. The upper engine compartment is dry, there is no moisture in it. I also checked out the DME unit, and there is no moisture there either. I am fairly certain now that the problem is not with the ignition system, but I really don't know where to look.

The only clue I do have is that the last three times this has happened, it was immediately after I opened the heater vents.

Thanks for any suggestions you all can offer!

Micky

[TRE Cup]

"immediately after opening heater vents".. hmmm. clue there for something unusual: look at the heater control cables up by the orange colored flap valves - most likely the one closest to the starter. you could have some fouling of cables and electrical there. have someone get in the car pulling the lever up and down, while you are checking underneath
secondly: find all the ground contact points on the engine wiring harness, remove the nuts, clean the contact terminals, and reassemble- don't forget the ground strap from the trans to the body- located near trans mount on passenger side of car

[micky_ray]

Thanks for the tip, I'll check out the harness and ground straps. I think I read somewhere that there is wiring in the heater ducts, do you know if this is true? Is it possible that water is getting in there and shorting something out?

[Wil Ferch]

The heater dump valve in the drivers's side wheel well has a temp sensor in it if your car has the 9 position dial "auto" heat feature in the console between the seats...that is...if it doesn't have the red levers between the seats.
---Wil Ferch

[micky_ray]

Nope, mine has the manual levers. Thanks for the info though, I always wondered what they meant by "auto" heat and if I had it.

[Steve 82SC]

I responded last week with a suggestion about the ignition coil. Sometimes the crack can be so hairline that it can be difficult to diagnose. It's always easiest to start with the simplest things. If you have a spare coil, change it out. Maybe you'll get lucky. Good luck.
Steve

[Eric Coffey]

I think I mentioned to double-check the HTS. When it was replaced, did they install the updated two-wire (grounded) version?

[micky_ray]

Eric: Yes, the HTS I installed was the upgraded 2 wire version.

Steve: Unfortunately I do not have an extra coil, or I would try your suggestion. Do you know of a way that I could seal it that would not permanently damage the coil if it is not the problem?

Thanks,
Micky

[Steve 82SC]

You won't damage the coil if you use a clear spray LAQUER. The laquer, as opposed to an enamel, can be easily removed with paint thinner. Just use a quick-drying laquer...it's worth a shot and is hopefully the simplest solution. If it doesn't work, no harm done.
Steve

[dickster]

micky

i have a similar, though not so severe (yet), problem in my '86.

i have just bought a new dme relay (not replaced since i had the car, so no idea how old it is) to try. just a thought that if you havent changed it already, its good, cheap, preventative maintenance to do anyway.

i'll let you know how i go with the new relay.

good luck.