Anything I could do to prevent rusting of internal surfaces?

[jbutler67]

1998 Mercrusier 5.7L with 2 bbl carb. FWC.
M048938
720 hrs

Looking for advice. I am considering pulling the heads to have them refurbished during the offseason. (Reason why added below.) Boat will be outside under shrink wrap while heads are off.
I read that it is NOT recommended to drain engine coolant for storage over winter, as this will promote rusting of internal surfaces. But, coolant will need to be drained in order to pull the heads.

Anything I could do to prevent rusting of internal surfaces? Add coolant back into block after heads are pulled? Other?

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Reason for the work:
Compression test at the end of this season came back with very low compression on #5. These are the results from Oct 2020, which was a "dry test" on a cold engine:
1 - 180
3 - 180
5 - 30
7 - 180
2 - 180
4 - 185
6 - 185
8 - 185
I sprayed about 2 tablespoons of WD40 into #5 and ran compression test again. No improvement. Still only 30 psi.

Prior compression test, June 2019 (also a "dry test" on a cold engine) but using a different gauge:
1- 175
3 - 175
5 - 175
7 - 175
2 - 180
4 - 175
6 - 165
8 - 165

The exhaust manifolds and risers were replaced in May 2019. However, i don't know the age of the previous ones. So, I won't be surprised IF I find some rusting in the valves and valve seats on cylinders below the risers (on 3,5 and 4,6) once the heads are pulled.

This weekend, I'll be doing the following leak down test to try to further pinpoint the problem before i pull the heads:

• Bring #5 to JUST BEFORE TDC
• Disconnect exhaust pipe extension
• Remove flame arrestor
• Put plastic bag over top of carburetor
• Put plastic bag over exhaust pipe
• Plastic bag on xxxxxx, to check for air blowing into the crank case
• Pump compressed air into #5 and watch plastic bags

 

[Scott Danforth]

first, you are over thinking it.

no value in putting a plastic bag over the carb, the whole intake will be off to get to the heads.....

drain the gas from the carb and forgetaboutit until you bolt it back on. if you wait more than 6 months, you may need to address the carb with a gasket set

no need to put a plastic bag over the exhaust pipe unless you want to. serves no purpose other than to give the mice something to make a nest with

• drain the anti-freeze
• pull the heads
• hose the bores down with 30 wt oil or a smear of grease
• and if you are worried about it, spray WD40 in the water jacket.

also, any compression reading over 150 is a crap gauge or you have water or oil in the cylinder. you cant make more compression than the published cranking numbers of 150 (think about it, atmospheric pressure is 14.7 psi, at best, a 10:1 motor will be about 150 psi).

 

[jbutler67]

Thanks. I'll do the following...

• hose the bores down with 30 wt oil or a smear of grease
• spray WD40 in the water jacket

The plastic bags over the carb and exhaust pipe are for leak down test... to help identify where air is leaking.
During the first leak down test, I could hear the air in carburetor/throttle body but couldn't tell if it was coming out the intake or if I was hearing the air pass through the cylinder.

"any compression reading over 150 is a crap gauge or you have water or oil in the cylinder... atmospheric pressure is 14.7 psi, at best, a 10:1 motor will be about 150 psi"
I didn't realize that. I learned something new tonight.
Those were readings after letting starter crank motor over for a 5 count. Gauge jumped 3 times. I didn't see the gauge's first reading on first pulse of air, but second jump readings were around 160/175, then third jump readings are the ones I noted.
I'll have to do the tests again. But, I now think there may be oil in the cylinders.

 

[Lou C]

When I did a similar repair (removed cyl heads due to blown head gaskets, found heads to be cracked and replaced them) the cyl heads were off for about a month. I usually leave the engine filled with marine antifreeze (raw water cooled) during the winter but did not that year because I knew I had leaky head gaskets and had to leave it empty. That was fine really, I don't think it caused any additional corrosion compared to what was already there due to raw water cooling. I did notice that sealing surfaces will start to rust if you leave them exposed to air, so I took a rag sprayed it with fogging oil and wiped the engine down and kept it covered to keep dirt out. Ordered a set of reman heads and put it back together. Overall not a hard job, just keep the parts organized, really clean up the block deck surfaces and get a set of thread chasers and clean out those cyl head bolt threads. On mine they were pretty dirty due to raw water cooling. I also replaced the head bolts with APR bolts because the originals were rusted.

 

[Simoniz]

Where you say you sprayed WD40 into the No 5 cylinder, I reckon this is too thin to really tell what is going on. Use engine oil so that if it is a ring / bore problem, it has a chance of imporving the seal. With WD40, by the time you have got the compression tester screwed in, most of it will have drained away, or evaporated, and bear in mind, WD40 leaves no oily residue, so you are actually washing away any oil that might be on the piston rings and bore.

But as I mentioned in my reply to your other post, with only 30 psi I dont think its a piston problem, more like a sticking valve.

 

[jbutler67]

Where you say you sprayed WD40 into the No 5 cylinder, I reckon this is too thin to really tell what is going on. Use engine oil so that if it is a ring / bore problem, it has a chance of imporving the seal. With WD40, by the time you have got the compression tester screwed in, most of it will have drained away, or evaporated, and bear in mind, WD40 leaves no oily residue, so you are actually washing away any oil that might be on the piston rings and bore.

But as I mentioned in my reply to your other post, with only 30 psi I dont think its a piston problem, more like a sticking valve.

Did another compression test today. Squirted 2 tbls spoons of engine oil into cylinder with low compression (only 30 psi). No improvement. Still only 30 psi.

Did a type of leak down test using plastic bags. (My hearing isn't what it used to be, so needed a visual to help identify where air may be leaking.)
Brought bad cylinder (#5) to TDC. Loosened rockers to ensure valves weren't being compressed by rockers.
Put plastic bags over carburetor, exhaust pipe off the riser, and breather elbow on valve cover.
Pump compressed air into #5.
The plastic bag off the exhaust pipe filled right up.

Bad exhaust valve on #5 is what is most likely causing the low compression.

Did same test on with the plastic bags on other cylinders. None of the bags inflated.