Bad leakdown test results - 502 MPI
 

Some of you may recall my recent threads related to trying to find the missing speed in my Sunsation Dominator. Twin 502 Mag MPIs, barely cracks 70mph no matter the conditions or how I trim it out.

Hours unknown but estimated 425 since rebuild based on hour meters that were there when I bought the boat.

Today I decided to dig deeper.

Pulled plugs (pics later), Compression Test, Leakdown Test.

Compression Test:

Stbd

8-174 7-170
6-165 6-160
4-185 3-176
2-176 1-180

Port

8-190 7-165
6-185 5-177
4-185 3-185
2-187 1-180

Leakdown:

Stbd

8-1% 7-4%
6-8% 5-46% (audible through exhaust)
4-0% 3-6%
2-0% 1-0%

Port

8-1% 7-18% (audible through exhaust)
6-8% 5-5%
4-5% 3-3%
2-1% 1-7%

Looks like I've found some of my missing speed. I'll put the plug pics in the next post below.

Plug Pics, labeled S and P plus cyl number


https://cimg1.ibsrv.net/gimg/www.off...f1629e5a1c.jpg
https://cimg6.ibsrv.net/gimg/www.off...b925fb4b39.jpg
https://cimg2.ibsrv.net/gimg/www.off...a7b326c8fb.jpg
https://cimg9.ibsrv.net/gimg/www.off...99dc1530c6.jpg

Do you know what afr and how much time is on those plugs?

At a minimum looks like the heads need rebuilt, did you check the valve springs on those leakers

No clue on AFR. These plugs probably have 8-9 hours on em.

Nah I didn't pull valve covers because headers would have to come off first.

I figured I'd pull all 4 heads and have em redone. Big question is, can I run it like this the rest of the summer? Or do I need to stop using it and tear down ASAP to avoid damaging the bottom end?

If it's a burnt exhaust valve or seat its not going to get better or fix itself. It were me I would get it fixed sooner than later as the hot exhaust gas acts like a blow torch and slowly wrecks havoc until no compression or worst case it drops a valve. Look at the cost difference between buying new compleat heads vs buying all the parts and labor. With a set of heads you could do a swap in a day, have cores and not ruin your boating season with parts waiting in machine shop purgatory .

I think your readings on those two are flukes. Probably got a piece of carbon in there when it shut down. The compression is not all that far off on those cylinders compared to the others. If anything the plugs on P6 and S1 look the worst. I would run it some and then check it again. I cant imagine hurting anything in the bottom end by continuing to run it for the rest of the summer. Just use some common sense and if something changes drastically then call it a season.

To me the numbers make sense; the same cylinders low on compression have the highest leakage...but it is entirely possible you have some crap stuck on the exhaust valve(s) on the low cylinders. Back in the carbureted days we'd run some ATF thru the engine and then do the tests again to see if it knocked the crap loose. Much harder to do on a f/i engine though I suppose there's a way.
I would run it for a while and test again; after all now you have some good baseline data.

Thanks for all the input. I'll button her back up and start scoping out options for machine shops to do the heads this winter. I'll probably stay below 4K rpm in the meantime too in case there's some marginal valvesprings there, still gets me to 60mph so plenty fun for cruising around.