In my experience with prochargers, your belt being worn on one side would make me think you were either losing a idler bearing in a idler on the tenssioner is getting worn and ;'cocked", which either one can rub its respective pulley on a angle on belt and wear outside unevenly. If you grab the tensioner with belt off and try to pull it in and out, if it has slop, its worn. Another thing I learned is anytime a seprentine blower belt slips under boost it leaves belt dust AND pulley shoots over 200 degrees, hot enough where you cant touch it without getting burned!

Arctic,

I checked the tensioner and pulley. There was no more end play or axial/radial runout to the pulley than the new one I had on hand. It spun smooth and free with no wobble to speak of. I was not up for the swap, given the extremely limited space there is to work with, so I skipped it. I'm betting there was something rubbing on it at one point in time prior to our owning the boat. There are a number of hoses and wire bundles in the general area that could have been the culprit.

Thanks. Brad.
(937)545-8991

Guys,

Having the new belt on, it's time to move on to the rest of the effort. My thinking is to go ahead and replace the plugs as I do the compression test. I'm thinking the plugs are going to be fouled beyond burning them off. This would enable a clean slate to check burn pattern on the plugs. Sound logic?

Also.... I'm assuming the engine should be warmed up before doing the compression test.....?

Thanks. Brad.
(937)545-8991

You can do a comp test on a cold engine. Numbers might be marginally less than when warm, but you're looking for cyl-to-cyl variation rather than being concerned with absolute numbers. So it's fine either way.

Leakdown is another story.

Griff,

Dang. That made THAT easy. :ernaehrung004:

Thanks. Brad.
(937)545-8991

Guys,

Compression test complete.

L R
1 - 137 2 - 140
3 - 140 4 - 149
5 - 138 6 - 148
7 - 140 8 - 150

These are all guestimates, as the gage was an analog unit, with 5lb graduations on the dial, and I was trying to split hairs, reading between the lines. From memory, these seem like similar numbers to what I've seen others post here on OSO, but I really don't know if these are good numbers or not. I tried cranking the engine as consistently as I could. I had no way of watching the gage to see if peak was established or not, so I ran each cylinder twice, releasing pressure each time, before moving on to the next. I did notice that the right bank seemed consistently and, on average, higher than the left, so I went back and re checked two of them, just to rule out whether I was a factor, as I started on the left bank, and I might have gotten more confident in tightening the gage in the plug hole and cranking the engine longer. I was no real difference the second time around. This is my first time actually doing this sort of thing myself.

Thoughts?

Thanks. Brad.
(937)545-8991

Assuming those are accurate, you have less than 10% variation across the board. I'd say those are perfectly acceptable numbers, as far as consistency among the 8.

Did you have the throttle open while doing the test?
I’m not that far away from you. Just outside Camden Oh. is where we live. You probably go through here if you go to Brookville any.

Rosie,

Nope. Didn’t realize I should have. If somebody mentioned it, I’d forgotten. 😕

Thanks. Brad.
(937)545-8991

That’s why I asked. I didn’t see it mentioned and you stated that was your first time doing that.
The throttle blade will restrict airflow into the engine. You’ll likely see higher numbers with the throttle open.

How’d the plugs look? If the ignition side of things is getting weak you could be blowing the spark out under boost. I’d think you’re down on boost because you say you aren’t getting the rpm you did before.
Stay on the tighter side of the plug gap when you gap the new plugs.

Rosie,

The plugs looked great. Some minor carbon deposits around the base, just above the threads, but none on the insulator it electrodes. There was also no erosion to the electrode. They looked essentially new, which they are; they were replaced about this time last year. I have not had a chance to do a plug chop, to get a proper plug reading, though

It’s looking like we’re going to be going out on Saturday. I’ll report initial findings. Don’t know if I’ll get a chance to do the plug chop, though. The water stays pretty rough at Caesar’s Creek.

Thanks. Brad.
(937)545-8991

Rosie,

We went to Brookville Lake exactly once. We didn’t care for it. Too rough. Too much of a free-for-all. The “party cove” was OK, but standing on bottom right behind the boat was a bit unnerving. I guess I’m just spoiled on Cumberland.

Thanks. Brad.
(947)545-8991

I know what spot you’re talking about at Brookville. We avoid that spot because it’s so shallow and nothing but rocks.
it is a ROUGH lake on the weekends but we like it because you’re allowed alcohol in the boat in Indiana and my wife likes her beer when she’s in the boat. lol.

The compression numbers are very good. Go run it with new belt. I’m 90% sure it will be back to normal

Griff,

I'm certainly hoping so. I'm optimistic. The pessimist in me is still thinking about the injectors, though. We'll see.

Thanks. Brad.
(937)545-8991

Rosie,

FWIW.... I don't believe alcohol is allowed on Caesar's or East Fork Lake (the two overgrown ponds in Ohio that we frequent when we can make it to Cumberland), but we do it anyway. And I KNOW alcohol isn't allowed on Cumberland (at least, it USED to be prohibited), and the lake patrol would only harass you if you were being a problem.

Thanks. Brad.
(937)545-8991

Boost is down? Belt will be the culprit if you don't see any obvious perforation on the plumbing between the procharger and intake (I suppose you could have a rotted out intercooler, which wouldn't be visibly obvious).

Fuel pressure is down? No, it looks like fuel pressure is where it should be for the amount of boost you report.

Check the idler/tensioner bearings, and take a straight edge and verify that the tensioner is aligned properly and not at some angle.

Plugs? Sure, if they have been running pig-rich then swap em. I see you did a compression test. But male a note to open the throttle next tine you do it so your results are correct.

Speed? If you got 68 out of the boat with hea y fuel load, cooler, gear, people, then if you run it with 1/4 tank of fuel, one person, no gear, no water in the freshwater tank (if you have one) and it's a cool 60 degree day then you'll see that 71 on the clock (assuming you run a prop that doesn't bump the Rev limiter).

Weight matters if you're looking for that bug number.

Throttle chops aren't spooky in a vee hull at the speeds you're running. But reading plugs is a lost art. If you aren't sure what you're looking at, then it wont tell you much
Insulator color, heat line on the electrode, evidence of bright specks, these are all part of the insight of what your plug is seeing at soeed, but the differences are very slight.

Before you look ANYWHERE else, get your lost boost back.

Has anyone actually ever compared compression test results with throttle closed vs open?

Closed throttle blades still permit air passage, along with IAC passages. How else could an engine idle? It might take an extra revolution or two, but since a compression tester "stores" the pressure until released, the end result should be the same assuming you crank the engine long enough to get a stable reading.

McCollinsTN,

Belt is changed. Testing on Saturday. I'll review fuel pressure then. Tensioner and pulley are good. Pretty much identical to the new one I didn't replace it with. Maybe a bit more free spin on the pulley, but that's to be expected with some run time on the bearings. Plugs are changed. I cannot, for the life of me, find anything that might be a leak in the intake tract between the blower and the throttle body. I've looked every time I've been under the sun deck since the power has dropped off. The intercooler is sound. Again... I've looked. The boat only has 190-something total hours on it and was absolutely babied for the first 143 of that (not that we are abusing the boat in any way). I think, as I gather, what is most telling about any compression test is the consistency across the cylinders, and I'm told mine are within an acceptable range. I was very happy there wasn't that ONE cylinder at, like, 50, or whatever.

I'll take pics of the plugs after a throttle chop, because... NOPE... I don't REALLY know what I'm looking at. I'll post those pics here for more knowing eyes to review. Rev limiter is set at 5250, just to ensure we don't hit it under normal conditions, and we don't air the boat out, literally AT ALL, so....

I'd like to build a monster engine, replace the drive, and reach for 85, but I don't know as I'll ever convince the wife that's "necessary" (SIGH). For now, I'm happy with 65MPH or so, given our current set-up.

Thanks for the input. It's all being absorbed as best I can.

Thanks. Brad.
(937)545-8991

Guys,

Took the boat out yesterday with the new blower belt and new plugs. Nothing has changed. Still hitting a rev wall at ~2500RPM at takeoff until it starts to plane off. Still takes what seems like forever to plane off. Still hits a rev wall of ~4100RPM at something just short of 2/3 throttle on the stick. Still topping out a good 10MPH short of what I’ve had the boat at. Still only showing ~1.5 lbs or boost. Still only showing ~50lbs of fuel pressure above zero manifold pressure. Still crossing zero into positive manifold pressure at ~3600RPM.

Unfortunately, due to our being significantly out of routine getting to the lake, I did not have the tools to read the plugs, so I still have not done a proper throttle chop. I know this is still a necessary step, and I fully intend to do so. In the meantime….

Where else might I might look? I’m still convinced there is not a compromise in the ducting between the blower and the throttle body. I cannot speak for the internals of the intercooler, but I can vouch for water flowing through, and I see no evidence that water is going through the engine. Not sure if these two facts indicate a lack of compromise to the intercooler, but I can’t imagine one or both wouldn’t change if there was an internal compromise to the intercooler. Surely, if there were a breach. In the water passages in the intercooler, water would be going through the engine, right? Any way to check without pulling and inspecting? Not that I’m opposed to doing so…

Anybody want a free weekend on Cumberland? Just bring your intuition, experience and diagnostic tools. 😃

Thanks. Brad.
(937)545-8991

Just a random thought, is the intake loose? This was a problem with the 8.1 in the trucks years ago .

Carnuts,

Intake manifold? I wouldn't think so, but I can certainly have a look. I've never put a wrench to it, but that's not a reason it wouldn't have loosened up over time.

I also keep thinking about the ProCharger impeller and that drain plug incident. I don't even want to THINK about what it's gonna take to get that ProCharger out, short of pulling the engine, to check the impeller. I know it turns smooth and free, aside from the obvious over-gearing drag, when turning it by hand.

I also keep thinking about the injectors, and seeing those pictures in my head of clogged fuel injectors squirting out streams of gas, instead of atomizing it properly. That could result in reduced engine power AND raw gas in the exhaust, right?

Would this also be a potential behavior if it were running lean? Could the knock sensor be triggering a rev limiter through ignition retarding? That's what it's there for, isn't it? I don't know. I understand the rev limiter works by restricting fuel, which seems kinda dumb. But how does it deal with a lean condition? Or does it?

I'm open to any and all suggestions.

I know. I know. Read the plugs. I'm working on it.....

Thanks. Brad.
(937)545-8991

Either its not making boost or its leaking boost, in a car I would smoke test for any leaks to put that question to rest. So new plugs , new wires , clean filters, good fuel... have you used an temp gun to make sure all cylinders are hot ( you probably would have seen a bad plug)
can you read the data stream in real time somehow?

I’ve been doing a bit of reading and searching. I keep thinking about the injectors. Where would a guy send them for cleaning and inspection?

Thanks. Brad.
(937)545-8991

Send them to Articfriends on here ( Smitty ) he has done several sets for me and others over the years, top quality job and a good guy

Another vote for Smitty / articfriends. He did 2 sets of 502 injectors for me. Excellent!

Imo you have to see where the boost went. You can't go from 5psi to 1.5/psi and something not be right.

8.1 have weak pistops and are known to break.

If you don't have a boroscope, I would invest in one (cheap on Amazon) and take a look thru each spark plug hole. I wonder if you will see damaged pistons. I hope I'm wrong.

Not sure of the years, but some 496s had an issue with the cool fuel system. There was overspray black paint inside the cool fuel that would flake off and clog the injectors

Underpsi,

I misstated in my OP. The ProCharger kit we have is a 3lb kit, not 5lb. So, it's only "down" about 1.5lb. Since these centrifugal "superchargers" are not linear, there is a possibility that our remaining boost is in that last 500-600 RPM that we are not reaching. After reading several threads here on OSO and a couple other forums, there are a myriad of reasons these 496s drop off this top end, and most of them are either electrical (IAC, etc) or fuel injector related.

I'm aware of the piston situation for this engine. If there were a hole in the top of a piston or three, surely there would be other symptoms, right? I have noticed we are going through a bit of oil (consuming, not leaking), but it's pretty minor. I'm definitely hoping you're wrong, too. 😐

I may just acquire a bore scope, depending on what they cost, just for the academic opportunity. Might have a look when I do the throttle chop and inspect the plugs.

Thanks. Brad.
(937)545-8991

Griff,

I will definitely look into that, too.

Thanks. Brad.
(937)545-8991

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