Intermittent misfire

[Antz81]

The dielectric grease IS NOT for heat transfer!

I had always believed that it was used for heat transfer (in this application anyway), but that the thermal paste (or grease if you prefer) Is simply a better product to use for the job.

I've just been doing some research, and it seems that dielectric grease is very good and helping with heat transfer. It is better than a bare metal connection, and almost as good as thermal grease. Some people have even used dielectric grease in place of thermal grease with no adverse effects.

toobroke, this doesn't necessarily mean you are right though. Dielectric grease is normally used to keep moisture away from electrical connections (It does have other uses also). It would seem like the RTV is made from a similar compound, but with a big difference, the RTV will act as an adhesive. This is why RTV is not suitable. You will discover this when it comes time to replace your module.

 

[toobroketoretire]

If that was the case why not bolt it on directly without anything?
That would give you 100% heat transfer

Sent from my SM-N920P using Tapatalk

Yes, RTV also acts like an adhesive but so what? It just takes a little screwdriver to pry it loose then use the screwdriver to scrape the remainder off. Big whoopie.

The thermal grease is used to "heat sink" the heat away from the HOT module and into the cooler HEI housing. Its the same as using Wesson oil in a skillet to cook a weenie.

 

[Antz81]

Finally got some good weather and time to check things. I found two things, one which was likely to be the cause and another which wouldn't have helped.
When I had replaced the coil a while back I used the original screws to hold it down. Not sure if the new cap I did at the same time has a slightly thinner plastic below these screws or if the new coil uses a thinner core, but the screws had punched through and were exposed, there were some marks to indicate that the rotor was occasionally making contact with these screws. Probably not causing the misfire, but a weak spark. Anyway, I ground the screws shorter and put some RTV over the holes to fix this issue.

The second issue was most likely causing the misfire. I checked everything else but could find a problem so though I may as well try the new ignition module. started to remove a few thing to replace it and found that one of the grounds in the distributor wasn't tight. So I tightened it and took the car for a test drive. No more misfire.

 

[Tom Bryant]

Sometimes it's an easy fix that takes some time to find.

Tom

 

[toobroketoretire]

My new Proform HEI also caused misfiring until I found the coil/module ground wire had the ring terminal crimped onto the insulation rather than the copper wire. I gave that ground wire a little tug one day and the wire pulled right out of the ring terminal (???). I stripped the insulation back, re-crimped the ring terminal, then SOLDERED it on. End of misfiring (until I caused misfiring by not providing enough clearance to my air cleaner base recently).

 

[Antz81]

(until I caused misfiring by not providing enough clearance to my air cleaner base recently).

I still say the problem was the spliced wire and not the clearance.

 

[Hib Halverson]

(snip)

I have been running a vented HEI cap to help prevent module failures and so far it appears to be helping. No different than using fans in an electrical cabinet to cool transformers and such. As my blower is on HIGH almost all of the time when my A/C is on there is a significant amount of cool air that circulates thru my HEI cap.

Wait...what?!
You say you have a hose running from your hvac up to your distributor cap?


I won't say that's the most outlandish modification I've read about in all my years here at the CAC but, it's in the top five.

If you need information on how to service HEI distributors, it's in the Factory Service Manuals. I have them for 71, 80 and then from 82-12. What follows is based on content in 1991 book as that was the last year any Corvette Small-Block V8 had a distributor. Also, I have some info from various makers of RTV and thermal greases to which I referred in writing this post.

Why not just use proper silicone grease GM requires?

Some mistakenly ID it as "dielectric grease" but it is, more properly, "thermal grease". It's used between ignition modules and distributor "breaker plates" in HEI distributors. It was, also, used between LT5 ignition modules and the bottom of the LT5 intake plenum, but wherever it's used, thermal grease enables cooling of those modules by enhancing heat transfer away from them. It was NOT used to prevent heat from going from the engine into modules.

This grease is a temperature-conductive material which enhances heat transfer to a heat sink, ie: the the HEI's breaker plate. This heat transfer is done by direct contact between the two surfaces. Thermal grease improves the efficiency of heat transfer by filling the itty-bitty air gaps which occur between imperfect surfaces. The contact area between the module and the breaker plate has inherent surface imperfections–machining marks, casting irregularities and other, tiny, sub-microscopic spaces–which reduce direct contact and create air spaces. These very small air spaces greatly inhibit heat transfer due to air being a very bad conductor of heat compared to, say, steel or aluminum which are common heat sink materials.

I submit that:
if the CAC's in-house "professional engine builder and mechanical engineer" would not use RTV–which does transfer heat, but not nearly as well as will thermal grease–use the right stuff under his modules, he probably would not have had to "engineer" a system to pipe cold air :L out of his freakin' hvac into his distributor cap.