cylinder leak down test help

tfret · Oct 20, 2012

1984 Mercruiser 5.7L. I keep finding small amounts of water on the #7 spark plug. Have replaced risers and leak tested exhaust manifolds so I'm fairly confident it's not coming in from the exhaust circuit. I'm doing a leak down test on that cylinder today. I'm not sure when its at top dead center. With the piston all the way up at one point I get 80% leak. I spin the motor over and bring the cylinder back up again and I get 70% leak, so I assume that is the compression stroke. (is that correct?) Anyway, I couldn't really hear the leak source at the 10psi test pressure, so I disconnected the test gauge and just piped 50psi striaght into the cylinder. I can clearly hear gurgling in the water circuit with the T-stat housing top off and putting my ear to the T-stat. I think this tells me the head gasket is leaking into the cooling circuit. Please tell me if my logic so far makes sense and any other tips that could help me. Thanks!

Don S · Oct 20, 2012

Re: cylinder leak down test help

Yes, your logic is sound. It could be your head gasket, or a cracked head or block.

tfret · Oct 20, 2012

Re: cylinder leak down test help

Unfortunately I don't have indoor storage, so I have to do all my work outside. I am planning to winterize the engine today which includes an oil change. Should I skip the oil change since I'm going to be pulling the head, or go ahead and change it so that the oil is clean during layup? I don't know how long it will take me to get the repair done since I can only work at it when time/money/weather allow. What do you think?

Don S · Oct 20, 2012

Re: cylinder leak down test help

I would go ahead and change the oil and fog the engine. When done and the engine drained, remove the impeller, start it again for about 10 or 15 seconds and fog some more. This will make sure that #7 cylinder is dry of any water so it doesn't rust.

tfret · Oct 20, 2012

Re: cylinder leak down test help

thank you Don

tpenfield · Oct 20, 2012

Re: cylinder leak down test help

tfret said:
1984 Mercruiser 5.7L. I keep finding small amounts of water on the #7 spark plug. . . . I'm doing a leak down test on that cylinder today. I'm not sure when its at top dead center. With the piston all the way up at one point I get 80% leak. I spin the motor over and bring the cylinder back up again and I get 70% leak, so I assume that is the compression stroke. (is that correct?) . . . Please tell me if my logic so far makes sense and any other tips that could help me. Thanks!

Been there done that on a 7.4L . . .

When doing a leak test on a V8, I always start by ID'ing the #1 Cylinder TDC mark on the Harmonic Balancer at the front of the engine. Then I make "Criss-Cross" marks on the balancer with tape or chalk, so that the marks are at 90 degrees of rotation. This gives me a TDC mark for the other cylinders. Then, to test any particular cylinder, it is matter of getting to the #1 Cylinder TDC (plug the #1 spark plug hole with you finger and you will feel the pressure as it is on the compression stroke). Once you are at TDC for #1, it is a matter of rotating the engine 90 degrees for each cylinder down the firing order until you get to #7 (in this case). The marks on the harmonic balancer will keep you in check throughout the rotation sequence.

80% and 70% leak sound high, even for a blown head gasket, so I'd make sure that you are getting TDC on #7 and not BDC. However, if you are getting air bubbles up the cooling system to the thermostat housing, then there is high probability of it being a head gasket.

Other possibilities to keep in mind with a 1984 engine are rust through of the cylinder head or piston sidewall. . . these things are more probable if it has been used in salt water.

I'd come help you, but I am installing a closed cooling system on my 7.4's today . . . and maybe into tomorrow.

Good luck with it. Keep us posted.

tfret · Oct 20, 2012

Re: cylinder leak down test help

I went ahead and did the leak down test on all 8 cylinders. A youtube video showed using a wad of tissue paper stuffed into the spark plug hole. On the compression stroke it pops out. So thats how I determined compression TDC for each cylinder. I'm getting 65% to 75% leak on all of them. Here is where I hear the air going on each cylinder: 1-carb, 3-carb, 5-exhaust, 7-water, 2-water, carb, 4-carb, exhaust, 5-exhaust, 6-carb, 8-carb. I'm thinking all the valves are weak?? And both gaskets bad. The engine actually runs good if not great. I slalom skied behind it all summer. Fresh water only. I guess these 5.7L just have power to spare so performance loss isnt as noticable as with a 6 or 4 cylinder?? Anyway, I'm wide open for suggestions / comments on how best to proceed. Thanks.

tpenfield · Oct 20, 2012

Re: cylinder leak down test help

Wow, I'm thinking that you are looking at a full valve job. Seems like water got into the cylinders and trashed the valves, etc.

When you have the heads off, flip the block over and remove the oil pan to get a look at the crankshaft and connecting rods etc. With water going through the cylinders, there is always the chance that the engine was hydrolocking a bit and may have bent some rods, etc.

Bondo · Oct 20, 2012

Re: cylinder leak down test help

tfret said:
I went ahead and did the leak down test on all 8 cylinders. A youtube video showed using a wad of tissue paper stuffed into the spark plug hole. On the compression stroke it pops out. So thats how I determined compression TDC for each cylinder. I'm getting 65% to 75% leak on all of them. Here is where I hear the air going on each cylinder: 1-carb, 3-carb, 5-exhaust, 7-water, 2-water, carb, 4-carb, exhaust, 5-exhaust, 6-carb, 8-carb. I'm thinking all the valves are weak?? And both gaskets bad. The engine actually runs good if not great. I slalom skied behind it all summer. Fresh water only. I guess these 5.7L just have power to spare so performance loss isnt as noticable as with a 6 or 4 cylinder?? Anyway, I'm wide open for suggestions / comments on how best to proceed. Thanks.

Ayuh,.... Pull the heads, 'n see what you've got to work with....

tfret · Oct 21, 2012

Re: cylinder leak down test help

I'll start out by pulling the heads and we'll see what I find. Then we'll go from there. It will probably be a while before I can get into it. I'll post again then. Thanks.

tfret · Nov 10, 2012

Re: cylinder leak down test help

Well I got the intake and heads off last weekend. Upon inspection I didn't see anything obvious wrong anywhere. The heads aren't warpped. I can't really tell how the water was getting into the #7 cyliner. No obvious cause on either gasket of a leak but it had to be there somewhere. Everything appeared to be in decent condion, just very black with carbon on everything. Today I pulled the valves from both heads. They have a "glaze" baked onto the seat surface, and so does the head side. I think I can clean the valves up ok. I'm puzzled that the seat surface on the heads seems to varry from valve to valve. Some seat surfaces are close to 3/16", and others only 1/16". I'm not sure why that is(?). Now for the bad news, both heads have a cracked exhaust seat in one cylinder. It doesn't look like the seat is a separate pressed in peice, rather it appears to be machined into the head casting. Is this correct? I assume if that is the case I will be needing new heads(?). I'll try to attach some photos. Please feel free to give me some advice.

tfret · Nov 10, 2012

Re: cylinder leak down test help

I have another question. I descovered that the 3,4,5,6 cyliners exhaust ports are not only ported to the exhaust manifold, but also to the intake manifold. What the heck is this for? Because of this, it appears the "inner" cylinders get less water cooling than the "outer" ones. This could explain why the cracked exhaust seats are both in these inner cyliners.

tpenfield · Nov 10, 2012

Re: cylinder leak down test help

Valve seats are pressed in and usually a full valve job gets new seats and a fresh grinding with the 3 angles.

The cracked valve seat looks like the port is also cracked, so that should warrant some investigation. If that crack is on #7, then there is the source of the water . . . .

The middle cylinders (usually 4 & 5) send some of the exhaust up the intake manifold to provide some heating of the intake manifold. That is what you are seeing . . . they are "cross-over" ports.

Bondo · Nov 10, 2012

Re: cylinder leak down test help

tfret said:
I have another question. I descovered that the 3,4,5,6 cyliners exhaust ports are not only ported to the exhaust manifold, but also to the intake manifold. What the heck is this for? Because of this, it appears the "inner" cylinders get less water cooling than the "outer" ones. This could explain why the cracked exhaust seats are both in these inner cyliners.

Ayuh,... That's the heat cross-over, used on SBC's up to '96, 'n the Vortec heads, which Don't have the exhaust/ heat cross-over...
It heats the plenum to operate cleaner/ smoother...

Speakin' of Vortec heads,.... You oughta be lookin' into a set, 'n matchin' intake manifold....

My guess is, yer needin' a set of heads anyways...

tfret · Nov 10, 2012

Re: cylinder leak down test help

I the two cylinders with cracked exhaust seats are #4 and #5. Do you think I can have new seats installed? Why would they have cracked? The engine has about 1000 hours on it.

tpenfield · Nov 10, 2012

Re: cylinder leak down test help

I had a couple of seat installed on my 454 Mercruiser . . . done at a local machine shop . . . it cost about $100.

You might find some re-built heads on CL or eBay.

Tail_Gunner · Nov 10, 2012

Re: cylinder leak down test help

tfret said:
I the two cylinders with cracked exhaust seats are #4 and #5. Do you think I can have new seats installed? Why would they have cracked? The engine has about 1000 hours on it.

1000 hours It might be more cost effective to find a low mile truck engine and marinize it..There a dime a dozen

tfret · Nov 13, 2012

Re: cylinder leak down test help

Well I did some research. The heads I have cast number ends with 624. These heads were produced from 76 to 87, or there abouts, and are well known for cracked exhaust ports. Presumably due to overheating caused by the exhaust crossover circuit. I did some reading in the Merc maintenance manual and it says if a seat is cracked on the head, throw it away. I also discussed it with a local automotive machine shop and they say it "can" be fixed by boring it out and installing a sleeve, but it isn't feasible or recommended. Therefore, I'm now on a quest to find some heads. sbc 350's don't seem too hard to come by so I don't think I'll have too much trouble there. I did find a 92 GMC truck engine that is fuel injected. I'm guessing it would be fairly difficult to upgrade my engine from carb to fuel injection(?). I'm sure there are a lot of electrical connections that my existing engine doesn't have. I'll probably opt to keep it simple and just get new heads. The machine shop I spoke with has some rebuilt heads that are ready to go, but have automotive valves, not marine valves, and they tell me there is a difference. Is this correct, or just a line to charge me more?

tpenfield · Nov 13, 2012

Re: cylinder leak down test help

Me thinks the valves are the same.. .

tfret · Nov 14, 2012

Re: cylinder leak down test help

A couple of questions:
I was loosing around 70% on every cylinder during the leak down test. The two cylinders with cracked exhaust ports obviously will leak down. But why would the other cylinders be about the same loss?
Secondly, if these 462624 heads are know to crack I'm reluctant to go back with them. What other head options do I have?

cylinder leak down test help

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