Weird '84 engine problem
 

Have gotten nowhere trying to figure out a very strange problem. Two weeks ago, after a heavy rain the night before (which may or may not have anything to do with this), it didn't want to start . It would crank strongly, barely catch, idle fine for 20 seconds, then the rpms would drop, the engine would stumble, then finally it would die. After a while I found that if I could hold the rpms above 2k, the engine would run perfectly; if I let off the pedal it would drop to idle and repeat the above symptoms. Finally, it seemed to go back to idling normally so I tried to drive it. The first few times I came to a stop, the car would stumble and try to die just as I let off the clutch, but that went away in a few minutes. Eventually it seemed totally fine and I drove it normally for about a week.

Then this week the same pattern (yes, another heavy rain) repeated but now the car won't start at all. New behavior though: again it cranks strongly but never tries to fire at all until just as I release the key it will cough gently a couple of times before stopping.

I've replaced the DME relay and fuel filter, checked the distributor cap & rotor, jumpered the fuel pump fuse... Any ideas?

Check your distributor cap....Wait, how did I not see your last line.

Any water in your ICV connection?

I will take it off, check and clean it. Thanks!

Spray some WD40 in the cap.
It will remove any moisture in it.

ICV was dry. A little sooty, so I cleaned it. No change. The distributor is dry and clean.

There may be water in the oil breather hoses. (This happened to my 84)
Remove all the hoses and clean them out.

I'd be checking for spark and fuel at this point. Possible dead fuel pump.

I have fuel to some extent -- not sure if the pressure is right but there is some making it to the injectors. And I know I have some spark, not sure if its intermittent, weak, or mistimed.

If the spark plug wires and connectors are old, they can "leak" electricity to ground when they are damp or it is very humid. When the car is running poorly, particularly if it has just rained, open the engine lid with the engine running in a dark or dimly lit garage. If the wires are leaking it should be visible. Also, I would pull the wires and examine each one for cracks in the insulation and check the resistance on each wire, connector to connector. The correct resistance will vary based on what type of wire you are using, but you will immediately see if one is much different, i.e. infinite resistance or 100k ohms when all the rest are 5-10k.

Checked resistance of several spark wires, all seemed good. I pulled a couple of wires and hooked up a spare spark plug, looked like a strong, bright spark.

Pulled the Motronic and cleaned the contacts. Decided to open the box and look for any issues. Saw one trace that looked like it might have corroded or burnt so I bypassed the damaged area. Nothing changed at all.

Looked for water in various hoses (oil, vacuum, etc) and found nothing.

http://forums.pelicanparts.com/uploa...1412456037.jpg

what is the resistance of the cylinder head temp sensor?

coil wire
 

Check the condition and resistance of the coil wire. There is enough voltage to jump a gap in the wire with a plug removed but maybe not in place. Another possibility is moisture or even a crack on/in the coil tower.

What about the AFM? The carbon tracks on these things go bad, and the cars run really poorly then. Whether this could be exacerbated by moisture, I don't know.

How old are you plug wires? sounds like they are going bad. At night spray them in the dark and you will likely see sparking where you should not. My wires are toasted and the car runs fine when its dry.

Went out this AM with the new ideas:
- CHT sensor 4.1k ohms @ 40F
- .3 ohms resistance for the coil wire
- Coil looks good, no cracks or defects to the eye
- AFM resistance (9v test) tracks smoothly as door is manually moved
- Plug wires are about a year old and don't spark in dark on a moist day. Will try spray test when dark

So, after checking these things I tried to start it up and it purred like a kitten. Since the last time I tried, I checked the things above and did nothing else. It is a very cold dry windy morning so if this does have some relationship to moisture, today would be the opposite situation.

Rained over the weekend, and I'm back right at the start (or in this case failed to start) with the same symptoms.

I would look real close at the wires for engine sensor. Cyl head temp, both flywheel sensors. (Think there are two)

Reason- I recall a few years ago someone posting same symptoms. Water was getting in the wiring harness for one of the sensors. Look at the wire sheath for all the connectors.

Now it's time to pop the distributor cap and dry inside the cap and distributor with a blow dryer on high.

Afterwards you'll be scratching your head and wondering why you never thought of that before.


Cheers,

Joe

I have owned many old VW bugs in the past and this was a common problem after heavy rain. Water just finds a way in .

FWIW - another possibility is the O2 sensor, especially if its not the 3-wire later version. The fact that the engine runs over 2k rpm could indicate an O2 issue - I had this in my '85 and it took 2 years to figure out. Ran fine dry but similar symptoms to yours when it was very wet. Good Luck