1990 928 GT - Failure to start following rain...

This is a new issue. The car runs great until driven in the rain, & then it won't start for roughly 5 days afterwards.

The exact symptom is: first time turn over (day after rain) tries to fire one time only, & quickly chokes out on the first puff. From that point on it only turns over and never attempts to fire or puff again (until left alone and retried again roughly 4-6 hours later, whereas it will puff once again, & then only turn over). We checked spark on both coil outputs, & both fire. However, the right one seemed just slightly less than left, & both coils are just two months old! Checked spark at all eight plugs, & all are perfect. Can smell fuel strong, and fuel pressure is 50# at rail. Checked injectors with NOID light, & have proper pulse. Pulled Mass Airflow connection and no change. Left airbox off to ensure proper air inlet, & same.

We considered reference sensors or timing, but how could rain and 5 days to dry out possibly affect that? What symptoms would a failing Reference Sensor give?

Once it dries out, it runs like a beast and never stumbles once. Have driven it for days perfectly, so long as no rain. The really strange thing is it will fire fine on the day of the rain. It just won't start the following day.

I pulled the fuse compartment cover and checked for possible water intrusion, and see none. I pulled ever single fuse and relay, & cleaned the connections with fine sand paper, but same. The ground connections all just got checked recently and cleaned, by a mechanic. I can't imagine those causing issues this quickly.

I replaced the three known problematic relays (LH, EZK, Fuel Pump), with spares I keep in stock. No change.

Anyone ever see anything like this? What am I missing?

Where is Wally P??? He might have an idea on this! That is VERY STRANGE!

Hmmm I am going to shoot from the hip.

If you have spark and fuel it will try to fire.

But it has to be a dark blue spark or it will not fire unless lean.
You need a strong hot spark to light a rich mixture or it will flood the plug.

Moisture impedes spark at the plug and coil wires.
try this next time it happens.
remove both coil wires and spray the ends (not to much on the wire) of the wire (both ends)
Then try to start it.

Also, any of that white powder in the dist cap or rotor will collect the moisture and short the system out causing weak spark and it wont start.

Moisture in the air causing a starting problem tells me that its time for a tune-up IMHO

Several odd things about the situation.

Requirements are fuel, fire, air, compression and timing. You are losing one of these after, but not during, rain. The problem goes away with no action other than time. Tests apparently show all requirements present, despite the fact that the engine will not run...

My best guess is that you are losing spark in all eight cylinders. Unless the plugs are new, my first action would be to replace the plugs with standard copper, single electrode Bosch plugs, properly gapped.

My reasoning: The ignition system develops only enough voltage for the spark to get to ground. You want the spark to go to ground across the plug gaps, which means that the secondary insulation in the coils, coil wires, distributor caps, rotors, plug wires, plug caps, and plug insulators must be good enough to keep the voltage required to jump the gap away from ground.

Somewhere in your secondary ignition system, the voltage is going to ground instead of firing the plugs. Since your test seems to indicate that you have ample voltage available, one possibility is that the plug insulators are dirty enough that dampness allows the voltage to travel down the tip insulator instead of jumping the gap. If the plugs are worn, the tips will be rounded, which increases the required voltage. Since plugs need replacing eventually anyway, you won't really be wasting money if this doesn't fix the problem.

You basically have two independent ignition systems. One bad coil, distributor cap, or rotor button shouldn't keep the engine from starting. But - the next time that the problem occurs, remove each distributor cap and rotor. Using a good light, very carefully check for any traces of moisture on the inside or outside of each. Also look for any tiny little punctures or burn marks, cracks, dirt buildup that could hold moisture, or corrosion. Some minor corrosion on the tips of the rotor and the inner connectors is normal.

If the crank position sensor is bad, you will normally get no spark and no injection pulses.

I can think of no mechanical problem (cam timing, low compression, etc.) that is moisture-sensitive, or is self-healing.

Wally

The car just came back from getting tuned by Howard Wright at European Classics of Atlanta, who is pretty much "The Man" when it comes to 928's. He did replace the spark plugs with gapped copper Bosch plugs, as well as a slew of other new parts, and pulling and cleaning every ground connection on the car. It ran perfectly for about 4 weeks following his work, including through several rainy days. Then all of this started, out of the blue.

Just to give you an idea on recent new parts replaced by Howard, to get it running properly (it was stumbling before):

- Replaced Spark Plugs (Bosch Copper Cores)
- Replaced all vacuum line elbows (that were visible above Intake)
- Replaced Ignition Wire Set (Beru OEM set)
- Replaced Fuel Filter (Bosch)
- Replaced Air Filter (Mahle)
- Replaced LH Relay
- Replaced Fuel Relay
- Replaced Ignition Relay
- Replaced Coolant Temp Sender
- Replaced Temp II Sender
- Replaced Fuel Pressure Regulator
- Replaced both Coils (Bosch)