Volvo 5.0Gi compression issue - need to replace an intake valve

[allingeneral]

My port engine started having an issue with back-firing through the intake when under load near the end of last season with about 750 hours of runtime. I replaced a bunch of stuff before finally doing a compression test and determining that #2 cylinder was pushing zero compression.

So, I got the head off today, and found that the #2 intake valve has "tuliped" or "mushroomed". Presumably from too much heat in that cylinder for various reasons, but I can't identify which reason would be the most likely culprit. That discussion is probably for a separate thread.

My question has to do with replacing the valves. VP wants almost $100 each for the intake valves - this is ridiculous. $790 for 8 intake valves? Are these people insane? Where can I find the valves for a better price? VP part number seems to be 3857666. Where can I find proper valve dimensions?

I think I'm going to take the head to get the #2 intake valve seat ground. Should I get them all done? Should I pull the other head off and replace them as well? I'm leaning toward just replacing the intake valves on the problem side and leave the rest of it alone, slap it all back together and run it.

What do you think?

 

[bruceb58]

Bring the head to a machine shop and have them rebuild the head. The valves are not special...Chevy valves.

 

[G_Hipster]

Ditto. Just standard valves. And there are rebuilder valves available with a slightly oversized stem and head if the seat has really taken a beating or the guides are sloppy as opposed to doing guide liners and seats.

 

[Scott Danforth]

You can buy 2 heads from GM for $800

Take your heads (both of them) to the automotive machine shop. Estimate $300 for job including new valves

 

[allingeneral]

Take your heads (both of them) to the automotive machine shop. Estimate $300 for job including new valves

That seems like the right thing to do.

Thanks all for your input.

 

[Scott Danforth]

BTW, your exhaust valves tuliped because you were running really really lean.

if you dont fix the cause, your new valves will also tulip

 

[allingeneral]

BTW, your exhaust valves tuliped because you were running really really lean.

if you dont fix the cause, your new valves will also tulip

I replaced the fuel pump and sent the injectors off for sonic cleaning.

I believe it's possible, based on some brief research, that I am over-propped (Duo prop f5's) and that I'm working the engine too hard trying to get up on plane, especially when my running gear isn't clean (brackish marine growth). Basically, running WOT with low RPMs which causes overtemp in the combustion chamber.

Also looking at a possible blockage in the water jacket.

Plugs all showed indications of running rich with a lot of black carbon. None of them looked lean.

Here's an excerpt that a friend provided that makes a lot of sense:
I don't know what you are running but this was a good find on a thread: Heavy load and low RPM along with wide open throttle and no speed gain (lugging) will do it every time. Its real common on carburated engines. No vacuum at WOT and low rpm causes the carb to not have enough air flow through the venturies to draw out the fuel. Engine leans out, things get over heated in the combustion chamber. The intake valves are not designed for the heat spike and they go soft. Then the combo of cylinder pressure, heat, and valve spring pressures "suck" the overheated valve into the bowl.

Dragging a tube or wake board at half plane will get you as well.

 

[allingeneral]

Also considering stainless valves. Any opinion there?

 

[Scott Danforth]

stainless valves are standard. if you think you have a lugging issue, get Manley extreme duty valves (Inconel). If I recall, they are about $45 each. standard stainless are about $12 each

what RPM's are you running with the F5's?

 

[allingeneral]

what RPM's are you running with the F5's?

I've had this boat for 6 years and don't think I've ever gotten it over 4200 rpm trimmed out and running almost 45mph.

 

[Scott Danforth]

4200 RPM is a bit low

 

[allingeneral]

4200 RPM is a bit low

That's what I was thinking. Might be good to drop two sizes to F3's. But at the cost required to switch, I'll probably just run these F5's forever.

 

[Scott Danforth]

I would run F4's you can find someone that will take the F5's (ebay)

if your motors were carb'd, I would change the cams, get a bit more torque from 3000-5000 RPM. or pull the 5.0's and drop in 5.7's

 

[allingeneral]

Don't think for a second that I didn't briefly consider "dropping in 5.7's"!!