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I'm trying to diagnose an ongoing ignition problem with a 1996 Chevy Suburban. Sometimes it won't start, and the previous owner said this happens regularly, and every time the problem occurred, replacing the distributor cap made it work again. Obviously replacing the distributor cap again and again is not a reasonable solution in terms of cost or inconvenience, and it's obvious that there's some other underlying problem that's either ruining distributor caps in a hurry, or (more likely, in my opinion), which is going away for a while just as a result of doing work in that area, most likely moisture- or grounding-related.

Unfortunately I have not seen any of the caps that were replaced, so I can't comment on their condition, however I did find this:

http://www.handymanlyness.com/archives/auto/repair/ignition/distributor_cap/GM_truck_1/replace_cap_96_Yukon.html

which mentions:

With another truck, I've had problems with a coolant leak letting traces of anti-freeze into the distributor cap and causing the exact same problem this truck had... difficulty starting in damp weather with a cold engine. The anti-freeze would absorb moisture from the air and turn into tiny droplets of conductive liquid, which would ground out the spark and prevent the vehicle from starting. I could solve the problem by heating the cap with a heat gun or hair dryer, and the truck would start just fine.

Unfortunately, nothing is said about where the coolant leak was coming from, and I'm a bit surprised that coolant would be leaking into the distributor, which is situated on top of the rear part of the (V8) engine. Does this explanation make sense at all? If so, where could the leak be coming from?

I didn't check (and I'm not near the vehicle now) whether it's using Dexcool, but based on the year, it might be. I've also heard reports that Dexcool has, or deteriorates to a form that has, higher conductivity than proper antifreeze, so if there is a very slight leak causing the ignition problem, is there a chance replacing the coolant might fix it, and can this be done safely in a vehicle made for Dexcool?

Update: Here are some other threads with a similar issue:

http://townhall-talk.edmunds.com/direct/view/.f14f5d1/141

Also, I should mention that if the issue is moisture related, it's almost certainly not ambient moisture, since the vehicle started yesterday after a long time of sitting (it hadn't been working before) with overcast weather (previously heavy rain) and near 100% humidity.

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Something which might help you diagnose the issue better is to do an old school trick on your just replaced distributor cap. This would be to generously spray the inside of the cap and the rotor with WD-40. Many people use this as a lubricant to help with stuck nuts and such. The WD in the name actually means Water Displacement. By spraying this inside, it will keep water out of the cap and off of the parts. WD-40 is also non-conductive, so will help to eliminate carbon tracking, which can cause the engine to miss on one or more cylinders.

I am with you on this, though, thinking about you saying the engine will not start. Unlike the older HEI distributor, your distributor does not have the coil located in the cap. If I were a betting man, I would think you have some sort of grounding problem to the coil or the electrical connection to it is lose, corroded, or otherwise not allowing things to work right.

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