What issues would a leaky riser cause?

[jbutler67]

What have others found as the reason for low compression following a leaky riser? What issues would a leaky riser cause? Valve problem? Ring problem? It depends?

Thanks in advance for any additional info,

1998 5.7, with 2 bbl carb, FWC
M048938

 

[sean.deangelis]

What have others found as the reason for low compression following a leaky riser? What issues would a leaky riser cause? Valve problem? Ring problem? It depends?

Thanks in advance for any additional info,

1998 5.7, with 2 bbl carb, FWC
M048938

If it's leaking raw water into the exhaust the water can make its way into the combustion chamber (through exhaust valve) and because the water doesn't compress you can make the head gasket leak. If that happens you will likely see water-milk in the engine oil or run it on muffs and you might notice abnormal amount of steam in the exhaust. If you have one of those IR temp light guns (laser pointer looking gun thing that reads temperature) run the boat and hit each port on the exhaust manifold/header near the cylinder head and see which one is significantly is cooler. What does that spark plug look like compared to a plug out of one of the normal temp cylinders?

 

[jbutler67]

If it's leaking raw water into the exhaust the water can make its way into the combustion chamber (through exhaust valve) and because the water doesn't compress you can make the head gasket leak. If that happens you will likely see water-milk in the engine oil or run it on muffs and you might notice abnormal amount of steam in the exhaust. If you have one of those IR temp light guns (laser pointer looking gun thing that reads temperature) run the boat and hit each port on the exhaust manifold/header near the cylinder head and see which one is significantly is cooler. What does that spark plug look like compared to a plug out of one of the normal temp cylinders?

I changed the exhaust manifolds and risers in May 2019.They may have been leaking before I changed them. I can’t remember if I found any water in the exhaust chamber. There was a lot of rust on the outside at the seam between the riser and manifold.
Right now, oil is good. No steam in exhaust either. But compression on #5 cylinder is now low... only 30. That cylinder measured 175 in June 2019.

I haven’t yet started tearing down engine, to pull the heads. If issue with #5 is head gasket or valve(s), I’m all set to fix it. But, If issue is piston ring, then I have a bigger project. If problem is piston ring, I may instead decide to replace the block. Wish I knew more before I start pulling the heads.

The first leak down test I did confirmed the cylinder is leaking air, but I couldn’t pinpoint where. Listened at throttle body/carb. But I didn’t pull exhaust pipe off riser, so couldn’t listen there. Also couldn’t confirm I heard air in crankcase, listening at crankcase breather elbow.

Next leak down test will be setup so I can listen at all 3 places... throttle body/carb, exhaust pipe off riser, crankcase breather elbow.

I asked about leaky riser to see if I could learn more before I start pulling the heads. Learn more about the type of damage others experienced, like rusted exhaust valve.

thanks

 

[tpenfield]

I think you are wise to perform a second leak test to (hopefully) isolate the source. Going from 175 psi down to 30 psi indicates a significant issue . . . perhaps more than just a head gasket.

You mention Closed Cooling (FWC) in your first post? You can confirm a head gasket leak by also listening for air in adjacent cylinders (#3, #7) while you run the leak test or look for bubbles coming out of the 'closed' side of the heat exchanger (cap off).

Other possibilities might include a tulip'ed valve or similar valve damage.

Is this a 'half' or 'full' closed cooling system? Salted or unsalted water that you boat in?

Usually, a leaky riser gasket alone would cause some pitting of the exhaust valve seats. This would give you a fairly high leak (25+%), but your compression would still be in the ballpark. Maybe 140 ish vs 175 psi.

If lots and lots of water got into the cylinder(s) for a leaky riser gasket then you could hydro-lock the engine and bent a connecting rod or 2. that would seem less likely if the engine still starts and runs OK.

The fact that you got 175 psi AFTER replacing the riser gasket should be encouraging, unless something happened afterward. Are the riser/elbow bolts still tightened?

See what the second leak test tells you . . . once you pull apart the engine, it will be harder to trace the issue, unless it is glaring.

 

[jbutler67]

If lots and lots of water got into the cylinder(s) for a leaky riser gasket then you could hydro-lock the engine and bent a connecting rod or 2. that would seem less likely if the engine still starts and runs OK.

The fact that you got 175 psi AFTER replacing the riser gasket should be encouraging, unless something happened afterward. Are the riser/elbow bolts still tightened?

Thanks for your response. I appreciate additional inputs and thoughts as I try to figure this out. Like you noted, it’s going to be harder to find once I tear it down, unless issue is obvious.

I run in saltwater. Saltwater runs through the heat exchanger and out the manifolds. Coolant in the block.

Elbow bolts are still tight. No leaks at joint between elbow and manifold seen on outside.

One other recent event that MIGHT be related?Replaced the ignition wires (spark plug wires) this summer. I accidentally swapped 4 and 6. It ran VERY rough when I started it up. And I rev’ed it a bit thinking that might “clear it out”. But I didn’t run it long like that. Once I figured out the issue, I swapped 4 and 6 back. It started right up and ran fine. Has been running fine all summer. No loss of power. Good acceleration to WOT.

I first started noticing a “different sound”in engine in September. It was making a “chugging” sound in the exhaust. As engine runs, I hear a “chug, chug, chug” but engine isn't vibrating and appears to be rotating smoothly, watching the belt and harmonic balancer. I put the timing light on it and it’s running smoothly and right on the mark for 10 degree BTDC.
That chugging sound in exhaust can definitely be heard when it’s running on hose out of water. And “chugging” is coming from port side, same side as low compression cylinder - #5.

Compression test sowing low compression on #5 was first clue into reason for the “chugging”.

Thanks again for sharing any thoughts.

 

[tpenfield]

The firing sequence is . . .

So when #4 plug wire is firing, #6 cylinder is at bottom of the intake stroke. When #6 plug wire is firing #4 cylinder is at the bottom of the power stroke.

Considering spark advance and a slight valve overlap, I would think #6 would be back-firing (or knocking) and #4 would be miss-firing. #6 would be the most susceptible to damage (basically 'detonation' aka knocking) with this plug wire mishap. The fact that #5 is showing the compression issue, it is not related. You will get a look at #6 piston for signs of damage if you pull both heads.

 

[Simoniz]

Have you given the cylinder a squirt of oil to see if that brings the 30psi back up to anywhere near the previous 175? I think the piston rings / bore would have to be in a very bad state to give just 30, and would expect its more of a valve problem, maybe sticking, which water ingress into the exhaust port from the riser could cause.

 

[Scott Danforth]

why did you start a second thread on the same topic?

 

[jbutler67]

why did you start a second thread on the same topic?

I’m sorry. Is there a way to delete the other thread?

 

[Scott Danforth]

I’m sorry. Is there a way to delete the other thread?

no problem, we can just have the mods merge the threads

 

[jbutler67]

Have you given the cylinder a squirt of oil to see if that brings the 30psi back up to anywhere near the previous 175? I think the piston rings / bore would have to be in a very bad state to give just 30, and would expect its more of a valve problem, maybe sticking, which water ingress into the exhaust port from the riser could cause.

I did compression test again today after squirting 2 tbl spoons of engine oil into cylinder. No improvement.

Did another type of leak down test.
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.

 

[tpenfield]

Looks like you found the problem. I like the plastic bag method, seems like it helped pinpoint the issue.