Signs of a bad manifold?

[boater2065]

Hi all,

Curious if there are any ways to tell when manifolds needs replacing? I know that if they are clogged enough, water won't pass through and the engine will over heat, or even worse if there are leaks. But, are there signs to look for before it becomes catastrophic? Will the engine temp slowly become higher? (i have noticed my temp gauge above 170 at times, but i've been fooled by bad gauges in the past). Will the cast iron get so hot you can smell burning?

The last records i have of the manifolds being replaced was around 2016, so worst case they are 4 or 5 years old.

I may have a mechanic scope one of them this fall while doing some other needed maintenance.

Any ideas help, thank you.

 

[Tacoma290]

How long you get out of them in PNW is based on how long the boat is idle (especially over the winter if it stays in the water). Idle manifolds and risers rust. At 3-4 years you should be checking them annually. You should also have an IR temp gun and shoot the manifolds in a few locations and record it for benchmarking a few times per year starting when they are new (or when the boat is new to you). When you see a change in the readings, its time to get ready to redo them.
Regarding temp gauges getting higher, it could also be an impeller that needs to be changed. I was happiest changing the impeller annually, but many get away with 2 years. But you have to change the OD oil annually, so it just makes sense to do both.
Hope this helps!

 

[boater2065]

@Tacoma290 Thanks for the info! I have had the boat since spring of 2019 and it's been the water the whole time. I will look into an IR temp gun, that sounds like a great idea and easy plan to keep track of manifold temp. If i get one, what is the ideal range of temperature, roughly speaking? I guess, is it worth it spend 50 bucks on the IR temp gun to see the current state of the manifolds, or rather should i have the mechanic check the manifolds for me while they service the outdrive?

I also thought about the impeller for the rising temp and plan to have that replaced this fall when i have the outdrive serviced.

 

[370Dancer]

My rule of thumb, if I see water stains at the gaskets on the outside, you can bet it's doing the same thing on the inside. I'll be scoping my exhaust this week with a bore-scope to have a better look at the exhaust side of the engine. I don't have any outside leakage indication, but it has been 3 years, and that is my limit. Ordered all new stuff tonight. Weight lifting class resumes in about a week.

Which leads me to this question...... why hasn't some aftermarket manufacturer built a dry manifold for us poor 7.4 liter guys? Are the dimensions THAT far off? or, are we looking at an incentive program to move us to 8.1 repowers?

 

[boater2065]

@370Dancer thanks for your input. My port side manifold has a little rust between it and the riser, so ya I assume it needs to be done. It's so expensive, I'm wondering if it's a job I want to tackle on my own. Question, can a manifold replacement be done while the boat is in the water? I have an IO without a seacock, so I'd doubt I can do that...?

A dry manifold system would be ideal, it's a shame!

 

[scoflaw]

That replacement can be done in the water, no problem. There are several aftermarket companies that make a dry joint for the 454/502 crowd, there're aluminum manifolds though. With regular flushing most say 10 year life span.

 

[370Dancer]

That replacement can be done in the water, no problem. There are several aftermarket companies that make a dry joint for the 454/502 crowd, there're aluminum manifolds though. With regular flushing most say 10 year life span.

I knew I should have asked you first. You can get dry joint conversion for 5.7 liter engines easily. Just break out more wallet.