A hole in the piston would show up as no compression during a compression test and most likely an very oily plug that is physically damaged.

Carnuts,

Pretty sure we can rule that one out, then. Compression test showed a pretty even reading across all eight cylinders, and none of them much below 140. And no plug showing any signs of oil.

Thanks. Brad.
(937)545-8991

Guys,

How would one go about checking these coil packs with a multimeter. I'm told there is a way, and I'm a pretty handy guy, but I can't seem to scry the details off the interwebs.

Thanks. Brad.
(937)545-8991

Guys,

Here are the plugs, BTW. No throttle dump. Just standard idle onto the trailer. But, I think they still tell a story. These plugs are about a year old, with about 50hrs on them.

https://cimg2.ibsrv.net/gimg/www.off...c5c948f65e.jpg
https://cimg3.ibsrv.net/gimg/www.off...e191c28d88.jpg
https://cimg4.ibsrv.net/gimg/www.off...dcda2ff1f5.jpg
https://cimg5.ibsrv.net/gimg/www.off...8a73170f2f.jpg


Thanks. Brad.
(937)545-8991

4, and to lesser extent 7, look to be dark and shiny, maybe leaky injectors?
Have you done compression/leakdown on this motor?

DrFeelgood,

We are looking at both the coil packs and injectors, but advice comes that the coil packs are more likely the culprit, so I’m planning on checking both.

I’m prepared to swap the coil packs for those two cylinders with adjacent ones, but I thought I’d look into verifying them with a multimeter first, if I can get some guidance on how to check them. (Cough, cough…)

Thanks. Brad.
(937)545-8991

Pretty sure, but double check, your coils are

ACDelco D581 GM Original Equipment Ignition Coil.

They are not crazy expensive.

i probably asked before, but what plugs and gap do you run ? The factory .060” gap is hard on ignition parts.

I am running AC41-983 plugs, gapped at .060”. I’ve read a couple people reducing the gap to as little .035”, but two people with strong reputations have indicated the .060” is ideal. This is WAY out of my wheelhouse, so I’m just going with the consensus as of yet.



Thanks. Brad.
(937)545-8991

Ideal, maybe for N/A engines, but when you put some boost to that much gap, it can blow out the spark. Personally, I'd reduce it to .050.

Guys,

Today's lesson in metallurgy....?

https://cimg2.ibsrv.net/gimg/www.off...75719191d5.jpg

Don't use stainless bolts in aluminum. This is what they look like after twenty years. The shifter bellcrank assembly is mounted on a bracket that is attached to two points on the lift ring plate at the right rear of the engine and one point on the exhaust riser base. This is the bolt that mounts the shifter bellcrank bracket to the exhaust riser base, which is obviously aluminum. Bad, Mercruiser.... BAD! I had a HELL of a time getting it out. I'm just hoping I can run a form tap back into the hole and salvage it without having to helicoil it. 😣

That said...

I am in the process of swapping the #4 and #7 coil packs with #2 and #5, respectively. I currently have #4, #7 and #5 out, and I'm headed to AutoZone to see if they can test them. I installed #2 in place of #4, but kept #5 out for a "good" reference for testing purposes. Assuming we find them faulty....

They are Denso coil packs, and I can't seem to find replacements. In the event I have to, are the ACDelco units interchangable? If so, can I just replace the bad ones, or should I replace them all, keeping them uniform?

So....
Many....
Questions.....

Thanks. Brad.
(937)545-89914