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1990 740 sputters then dies in rain

D

DavePolyakov

Member
Joined
Aug 3, 2020
Location
Shoreline, WA
Im on a camp trip and its the first time I?ve ever drove this car in the rain. After a few minutes the car started to miss and then it turned off and made a popping sound. It started after about 20 minutes, then the same thing happened again. It will not hold an idle and will keep shutting off in the rain. The dizzy is dry, I checked fuel pressure by loosening the connection on the fuel rail and testing if fuel would come out. What should I check? Please help me make it home
 
The most common problem with rain driving issues are bad spark plug wires. They may be cross firing and shorting to each other or shorting to the valve cover. You could try insulating them from each other and from the valve cover. Also check the coil wire for shorting to something nearby like an a/c line. If the top of the coil is very dirty that can also short out the spark.

Another location that may be a problem is the ignition amp. That is the module near the battery on the left inner fender and the connector can get corroded which may be worsened in the rain. Also make sure none of the air intake hoses are cracked or split.
 
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Checked everything you said, haven’t found the issue yet. The ignition amp is totally dry. Spark plug wites aren’t new, but in fine shape, not bad condition at all. Could I use blue tape on the spark plug wires to insute them? I feel like they’re fine but im not sure, maybe the blue tape would melt. I do not have the metal heat isolators on the plugs.
 
^ Might be worth a try. I'd use dielectric grease, as I don't know the flammability of anything else at high temperatures such as the plugs experience combined with the possibility of rogue sparks. Same goes with tapes, glues, etc. Be safe when working on the ignition system.
 
You can test the plug wires with a squirt bottle of water and see if that makes the car stall. Plug wires can look fine but still leak spark.
 
I did make it home. After stalling about every 15 minutes and waiting for 20 minutes before it would start again, it then drove for an hour in a half with no issues in very rainy weather. I almost made it home, then decided to do a 2nd gear pull at about 10 lbs of boost. It broke up and stalled again. I eventually limped it home.

So, because I replaced my dizzy a few weeks ago it's probably a good idea to replace the wires and plugs anyways. I also am not sure if the stalling is 100% correlated to the rain, but I think it is.
 
Probably half of the clips are gone, but it's still there. The dizzy didn't look wet from it leaking, it doesn't even really look like it is leaking.
 
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Is this an LH2.4 turbo? If so, check the connector for the RPM sensor, it may be getting wet/coming loose or the wiring may be shorting together, replace it if in doubt

Another thing to look at is the fuel pump relay
 
I don't know where to get them right now. I am going to look into upgrading to the later model version that I think rivets in.


Things I would check:

-Corrosion, fretting, oxidation, etc at the ignition coil connectors.

- Poor grounds at the intake manifold

- Poor connection/terminals at the powerstage

- Open insulation for the crank sensor (or just a dying sensor)

- King lead (coil to dist) shorting to ground

- Moisture in the cap. Due to a failed hood seal
 
- Moisture in the cap. Due to a failed hood seal

I've had this problem on several 740s

Its takes very little water in there for it to not want to run at all.
 
Ok, my crank sensor was ripped completely at the base. Maybe this was why my car occasionally stalls and breaks up in boost? I replaced it with a good condition used one I had laying around. I tried reinforcing the hood seal with silicone but I think I'll have to completely remove it and all the plastic clips and use only the silicone if I want to do it properly.
IMG-2300.jpg
 
My 15-minute test drive went smoothly. However, I don't think I'll know for sure until I can test it in the rain but I'm hoping this was the only cause of the issue.

Edit: Just went for a drive and I was parked for about two hours. When I went back to start the car it took three tries before it fired up. Maybe not related but probably it is. Guessing the problem isn't fixed.
 
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