Gen VI 7.4L Valve tap

[mr300z87]

Over the weekend my GenVI 7.4L developed a valve tap that can be clearly heard in the attached video. Using the method described in the Merc Manual i think I have isolated it to the Exhaust valve on number 1 cylinder. From what i read this engine should have Hydraulic roller lifters. When the engine is above 1500 RPM the tap almost goes away and she ran fine yesterday, although I have no plans to run her again until it is fixed. A couple of my old school friends suggested I run a quart of Marvel Mystery oil in the engine to see it is mysteriously fixes the issue a quickly as it started. BTW this is in a Sea Ray Sundancer 250, we usually cruise at 3200-3600 RPM and the BB pushes her effortlessly up on plane and will cruise at 25-27 MPH. Also of note is we have run her regularly all summer long with no more than a week down time. Here is the link to the video.

https://youtu.be/pbsG-v_DZJM

As away any advise is appreciated. Thankfully our season in NJ is almost over and I have all winter to sort this out. The link in my signature show some of the history of this boat.

 

[alldodge]

Agree, sounds like lifter

Could try Marvels and it may help. How many hours is on the motor? Is oil pressure good?

 

[Scott Danforth]

take the belt off and run it for about 10 seconds. the ticking corresponds with the white mark on the belt coming around.

eliminate the accessory drive

 

[mr300z87]

Thanks for the responses guys you are the 2 people I was hoping would chime in. Oil pressure is good, I do not know how many hours are on the engine. Unfortunately work has been in my way of doing more trouble shooting. I will update this thread once I get down to the boat but do not think I will tear to deep into into it until she is on the trailer at home. We still might have a couple more weekends of hanging at the marine with our friends, as she is still floating and there is beer in the frige. LOL

 

[alldodge]

After the belt check if it sound is still there, my guess is you can still run it and be good for couple more weekends, then tear into it this winter

 

[mr300z87]

Well 2 weeks ago I put a quart of Marvel Mystery oil in and only ran it at the dock, thought maybe is sounded better, Today was haul out day and early this morning my friend and I went to get her, This requires a no wake run across the Tom's River but who can resist a flat calm only boat on the water blast. So I ran her up to normal cruise speed 3400RMP 26MPH on GPS. At that RPM I now could hear the tap, I ran her for maybe 2 minuets then back to no wake to approach the ramp. Now the tap was very loud and would not fully go away at any RPM.

On Tuesday this week I am going to first remove the belts as suggested and run for a couple seconds. I can say I am pretty certain that it is the exhaust valve on number 1, I used a piece to aluminum tube as a stethoscope and and the tap is most prominent right the location of that valve. Then my plan is to run up to temp and do a compression test on all 8 to see what I am working with. Then depending on what I find I will fog, change oil and drain block for winter storage. Next I will set to number 1 TDC loosen the exhaust enough to try and remove the valve cover to check for issues up top, After that is real decision time do I pull intake to check lifter. Hopefully find a bad one replace it re assemble and go boating in the spring. There is mention of noisy lifter mentioned in manual 16 how to find it and replacing lifter/lifters. I do not know the SN of my engine but is has the plastic timing cover, so all my sources say it is a roller/hyd lifter are these a known issue. Anybody have a GM part number instead of Mercs? I would assume the lifter from a 1996/1997 7.4 2500 truck is the same. Can you guys let me know if I am headed in the right direction. I will post compression and other findings during the week.

Mike

PS what do you know about 8.1L/496mag my friend is in contract on a formula 31PC with twins.

 

[QBhoy]

If I was feeling optimistic...I’d maybe tell you that a sometimes alarming tapping noise can often be something as little as exhaust flappers. Especially if it goes away after 1500 rpm. Even more noticeable if you have a corsa marine captains call diverted system.
might be lucky.

 

[alldodge]

Gen V blocks used flat tappet cam and steel timing cover
Gen VI blocks used roller cam and composite timing cover, casting number 12561357

Look for casting number below distributor and on the port side

The 496 motor has a love/hate relationship. Some like them and some don't. I'm in the don't like them side. There great motors so long as there are no problems

 

[Scott Danforth]

the 8.1 was around from 2000 to 2008 (a bit later if its VP). there is limited parts support for it from an OEM level. you would be better off with a 7.4 or 8.2 for parts service.

 

[mr300z87]

QB I wish it was a flapper my old boat had a noisy flapper that did not sound like this, and also that would be in the back by the exhaust where this is definitely up front.

I plan on sticking with a gen VI 7.4 and will either fix the tap or put a reman in. My engine definitely has the composite timing cover but I will get the casting number on Tuesday when I am working on the boat to confirm.

If reman becomes reality, how difficult is is to change to a stroked small block 377/6.2L and stick with carburetor? I like BB power but SB are cheaper.

Thanks for the info on the 496mag. My friend will be the owner of the pair of them not me!!! Make my next boat to be one of those new 32 cruisers with outboards. Now if I could just pick some winning numbers so I could afford a boat that is more than my house.

Have a great night

 

[alldodge]

BBC to SBC just requires the movement of the motor mounts.

Note: Smaller motor doesn't mean less fuel:
It takes the same amount of fuel to move weight (boat) at a certain speed. Getting a boat out of the water is the hardest part and BBC produce more torque at lower rpm then SBC. The BBC will rotate at lower rpm then the SBC

 

[Lou C]

If this engine has an adjustable valve train try doing that first and also check for a rocker arm stud pulling out...

 

[mr300z87]

AD the only way to go small block would be a stoker as I mentioned, Merc sell the 6.2L/377 CID as a replacement for a 7.4L in a repower situation. I am old school MOPAR so the only substitution for cubic inches is more cubic inches. I am guessing that any cost saving buy a SB would be spent on item that need to change. Since I am a do it my self kinda person if repower is needed a long block reman 7.4L is the plan. I will know more tomorrow when I start digging in.

 

[Scott Danforth]

if you watch how you source parts, a BBC is as cheap to build as a SBC.

Here is my BBC build.... https://forums.iboats.com/forum/eng...rds/10101672-bbc-build-don-t-tell-the-admiral

lots of good links and articles.

 

[mr300z87]

My casting # is 10237297 when I searched it, it comes up as 454, 4 bolt, Vortec 7400, L-29 Generation VI. Going out to warm it up for a compression test and oil change/winterize. Will post results later this afternoon

 

[Scott Danforth]

pull the valve covers to make sure you didnt yank out a pressed-in valve rocker stud

if you spun a bearing, it will most likely be cylinders 3/4 (dont ask me why, however per my machinist that appears to be the two cylinders that loose their bearings the most frequent)

 

[scoflaw]

There are no pressed in studs on a gen 6

 

[Bodean1]

I just fixed a exhaust leak that sounded EXACTLY like this.........

 

[Scott Danforth]

There are no pressed in studs on a gen 6

you are correct...... its 3/8 thread vs the normal 7/16"..... for some reason I was thinking pressed in like the SBC

 

[Scott06]

I just fixed a exhaust leak that sounded EXACTLY like this.........

Was thinking the same leaking exhaust manifold gasket