Anyone familiar with Big Block Chevy 7.4 gas engines?

Trying to help my FIL to troubleshoot a Volvo Penta Volvo Penta 7.4gi marine engine. It's basically a Big Block Chevy that has been marinized, or whatever you call it.

It starts, runs till it gets warm and then locks up. Can't spin the driveshaft. It doesn't seem to overheat. After it gets cold it spins again and will start and run. Engine was rebuilt by the previous owner, who wasn't 100% confident in his skills.

Any thoughts on what it can be?

 

Sea water pump works?

 

When you say locks up...the whole motor locks up and wont turn by starter or by hand?

That sounds like lack of clearance in the bearings or pistons. As they expand from heat....oops not enough room to operate.

 

I presume it does, you check if there is water coming out of the exhaust? I assume that is how you check that impeller and cooling system is functioning.

 

Yep, wont turn by hand or with starter. Incorrect crank bearing clearances?

 

This.

If the guy wasn’t “100% confident in his skills”, it’s a fair assumption that he doesn’t have the skills to measure piston slop, ring end gap or bearing clearances.

 

Crank, Rod or piston clearance.

An old school rumor was F1 engines wouldnt turn over if they were cold. I dont think this was 100% true but they do pre heat them to near normal temp before starting so the clearances are correct. So its possible something has enough clearance cold and then after heating up and expanding it seizes.

Or your temp gauge is shit and the motor is getting really hot and seizing.

 

Yeah, sounds like the crank/piston assembly needs o come out and be re-checked.

 

If it is that tight and it has been started and run until it seizes it should be fairly evident what the problem is when you tear it down.

Did he do anything to the drive? I would eliminate that as a possible cause before pulling the engine. It would suck to pull the engine and tear it down to find nothing and start a more detailed look and find that a spacer or something was left out of the drive.

Does it make any strange noises when you restart it? If his skills were lacking for peace of mind I would be pulling and disassembling everything, much easier to do now in the shop than to get 25 miles off shore and you need to start rowing

 

If the clearances were that tight, I would imagine you would have some metal in the oil no? Pull the plug and check.

 

Need to check the oil. Will try that next.

Thanks for all the replies.

 

Nothing important to add, but I think when you change a motor to marine use, it has to be be marin-ated.

 

I’m no big block expert, but this seems like a cheap easy check might be a borescope to look inside the cylinders. I would think a lock up would show scars if it is piston related?

If nothing there, then drop the pan and plastigauge the bearings. If you are lucky it may just need a different size of bearings?

 

It's a bearing clearance issue.

I had a 400-blocked SBC one time that one main bore was a little tight. The machine shop that built it thought it would be ok. Running it just to break it in it shut off once like I hit the switch. Weird, but it started right back up and sounded fine. Locked up trying to get across the scale at the end of it's first night of racing.

I'd still unhook the drive and try it again. There needs to be some endplay in the drive. The drive could be causing the thrust bearing to seize when hot.

Unless it's a head/head gasket issue. With the drive unhooked if it still seizes I'd pull the plugs as fast as you can and try to roll it over. Look for water coming out a plug hole. The later 454s didn't have the heartiest of head castings.

 

The internals of the engine are the same regardless of the application, truck or boat.

The marine-specific items are external items like the starter, fuel pump, block drains, freeze plugs, etc.

I used to own a SeaRay with a Mercruiser 454. One year it hung an exhaust valve and everything I needed to order for the repairs came from an auto parts store, the valve, push rod, gaskets

Regarding the rebuild, what was the reasoning to rebuild in the first place?

It'd be interesting to know if the PO purchased new bearings and installed or perhaps didn't return the existing bearings to their original location but yeah, another vote for bearing clearance issue

 

Basically, just changing all the plugs to brass so shit doesn't rust out.

 

Hydrolocked??Pull the plugs and crank it over,easier thing to check w/o pulling the motor.

 

Is the oil pan serviceable sitting in the boat? Engine coming out?

 

Depends. Does the boat have to float afterwards?

If this was an electrician or a plumber with a strong case of "not my job" they'd just sawsall the hell out of the hull and get at it from there. Someone else can deal with the hull problem, I did my part .

 

Pulling a main cap will tell you immediately if it's been repeatedly seized

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