Gen VI 7.4L Valve tap

[mr300z87]

Tuesday seems like forever ago. sorry for no updates until now work as always prevent fun stuff. After running up to temp on the hose I pulled the plugs which all looked good, maybe a little rich. Then a compression test on all 8 which were a little lower than I was hoping for but consistant. So I changed the oil fogged the engine then drained block and manifolds which is good since there is frost warnings tonight. After that it was some basic clean up of the boat in prep for winter. By this time I ran out of steam and decide to grab a beer. Needless to say I did not go any further.

I wish this was as simple as an exhaust leak but since the noise started it has gotten progressively worse every time it has run and it is very metallic sounding. It is definitely in the top of the engine when I put my stethoscope (aluminum tube) on the front corner valve cover flange which is in very close proximity to the number 1 exhaust valve the tap is transmitted right into my ear and hand. I'll know more in the morning. I have a another video but need to up load to youtube.

Can anyone explain the L29 designation? Btw I know lots about Moparsand Nissan 300ZX but these Chevy casting and engine designation are foriegn to me but I am learning.

Have a great evening

Mike

Have a great night

 

[alldodge]

The L29 (vortec 7400) was the first MPI 454 and designed for light duty trucks and had a bit more power and better fuel economy. Was later replaced with L18 (vortec 8100) later.

https://en.wikipedia.org/wiki/Chevrolet_big-block_engine

 

[mr300z87]

So this am I pulled the rocker cover which revealed nothing. At number 1 TDC I found no slop in the valve train nothing looked warn. With that said I put the rocker cover back on and we split some firewood. LOL My plan is to order a lifter and intake gasket set so I can change out the noisy lifter. I am hoping that there are a lot of forces being generated on the valve train when the engine is running causing slop that I can not feel at rest. For less than $100 I can potentially be back in business. Scott or AD do you know the GM part number for the lifter or should I just call a Chevy parts dealer and ask for lifter for a 1997 2500 series truck with a L29 engine? If that doesn't fix the problem we are thinking about either a rebuild or reman long block.

 

[scoflaw]

I'd start it up with the cover off, and put some pressure on the rocker arm. Doing all that labor for 1 lifter doesn't make any sense to me

 

[mr300z87]

Another question, since my casting number is for and L29 which was different from the casting number AD posted can I assume this engine was originally from a truck and was a reman block (no Merc serial number). I bought the engine dresses up with all the Merc stuff on it which included the brackets alt, hoses/coolers and such. I add exhaust, carb PS pump and fuel.

 

[Lou C]

I'd start it up with the cover off, and put some pressure on the rocker arm. Doing all that labor for 1 lifter doesn't make any sense to me

Agreed its worth doing some more diagnostics. Get a junkyard rocker cover and cut off the top half to keep oil from going all over the exhaust.
If you pull out that one lifter you can see if the cam is wiped out there. In fact in a boat, changing a cam/lifters is not a big deal as long as you have access in front of the timing cover.

 

[alldodge]

Scott or AD do you know the GM part number for the lifter or should I just call a Chevy parts dealer and ask for lifter for a 1997 2500 series truck with a L29 engine?

The Merc part number is 850466T
but going to the auto store will get you the same thing. Only issue might be if they have them in stock

Another question, since my casting number is for and L29 which was different from the casting number AD posted can I assume this engine was originally from a truck and was a reman block (no Merc serial number).

No that isn't what it means. The L29 was only made for a few years and were brand new blocks. Then GM redesigned it and this would be the Gen VI (cast number I posted) which turned into the 454 LX 330hp and the 454 Mag 385hp using 502 heads

 

[Scott Danforth]

the L29 is the Gen VI motor. 1996-2000

if a lifter went, the cam also may have gone. these are roller lifters

 

[mr300z87]

Going to dive in tomorrow morning and see what I find. Work has been crazy so there is no having fun until the weekends.

 

[Scott Danforth]

if you pull the heads, dont drop it on your knee..... that is 85# of cast iron and they hurt like a MF when dropped on your knee

 

[mr300z87]

Scott it sounds like you know that form experience. LOL

 

[Scott Danforth]

That and breaking my toe on the engine stand because i was wearing flip flops.....

 

[mr300z87]

Well, here I am Nov 2nd and I just finished winterizing the boat an putting her winter cover on, although easily remove if mother nature cooperate and I get some time to work on her. What I have done is replaced the lifter on the noisy cylinder to to no avail, sound exactly the same. Would have been a 2.5 hour job if I did not spend an hour cleaning the gasket surfaces and stop to take my wife to NJ motor vehicle to register our daughter's new/used car. All in all not a bad job. So today before closing up the boat I pulled number 1 plug wire while running and the tap was gone. So I figured it is something deeper in the engine. Had a friend of a friend who is motor head come by and have a listen. He said the motor sounded good and it could be a burnt exhaust valve and suggested pulling the head to have a look Could I possible see a burn valve with a boroscope?. Then just get the heads re-done. going to make some calls Monday and see if I can get cost to redo the heads. My concern is blowing out the bottom end with fresh heads. My focus at the moment will the to get my yard work done (I hate leaves) then concentrate and getting my friend 5.7L together and into his boat which will help fund my project. I update this as time and weather permit.

 

[alldodge]

Could I possible see a burn valve with a boroscope?

Not unless its burnt real bad, but in that case you would have next to no compression

Have you check to see if the valves on that cylinder move as much (up/down) as the one next to it?
As Scott mentioned about a cam lobe issue maybe.

My thought, is it may just be tie for a rebuild

 

[Scott Danforth]

only way to see a burnt valve with a borescope would be if there was a hole in the piston and you come up from underneath. however a compression and leakdown test will tell you that.

I say pull the motor and go thru it this winter.

 

[Bodean1]

I still think it's an exhaust leak. Especially since you pulled the plug wire and it went away. Mine did the exact same thing.

 

[mr300z87]

As always all comments are appreciated. Bodean1 I can say with 100% certainty that it is not an exhaust leak, although I wish it was as that would be any easy fix. As I stated above am now at the mercy of the NJ weather, but my plan is to first check valve movement as AD and Scott mentioned, I have already done a compression test that showed OK,, I will do a leak down as weather permits.. After that my plan is to pull the head and see what I see. Then I will make a decision to have the head redone or buy a reman head. If it turns out to be deeper in the engine I will just go with a reman engine as local engine builder I spoke to is not to keen on working on Marine engine used in salt water as there is to much corrosion. I will post update when I can. Mike

 

[Lou C]

I was able to take my cyl heads intake and cyl heads off using a De Walt electric impact gun, I thought that because of 15+ years of salt water use I would not be able to but actually I didn't break one bolt and the threads after being cleaned out were just fine. I was able to install re-man heads new gaskets and got it all back together, despite salt water use.
I listened to your vid again, it seems to be tapping about 5x each second, in a minute that would be 300 times, and if its idling at 600 rpm that puts it at cam shaft speed, not crank speed, which is easier for repair issues.
When I took mine apart the lifters and cam looked like new.
When you replaced the one lifter how did the cam lobe look? Are you sure none of the pushrods are bent, and is oil flowing thru all of them?

 

[mr300z87]

Lou I never even thought to time the tap to see what speed it is but I do think it is something in the valve train. The question in my mind though is if it was a wrist pin would it only make the tap on power stoke it would still only be at cam speed I think. The tap seems to be too high up in the engine and also metallic sounding be anything but valve train. When I put my stethoscope (aluminum tube) on the flange where the rocker cover bolts to near the No1 exhaust valve I and hear and feel the tap, when I move back towards the intake valve I hear the audio but not through the tube and I do NOT feel the tap. Right now as I stated i am at the mercy of the NJ weather which is not looking good for boat work for the next week or so and of course there is always th issue of work preventing me from having fun. I would gladly take back afternoon daylight instead of morning daylight. Let's abolish eastern standard time as it is not needed. LOL

 

[mr300z87]

Well here we are almost mid march and the third Monday in a row with temp in the upper 60s (saw 75 on the thermometer in my truck) so this day I took off to pull the head and figure out a game plan. To my surprise bodean1's post 36 was the winner.

"I still think it's an exhaust leak. Especially since you pulled the plug wire and it went away. Mine did the exact same thing".

After pulling the cover up and staring at the engine for a few minuets I dove, first order of business get the exhaust, what I found was the front most exhaust bolt was no more than finger tight and the gasket (Not a Merc part) literally disintegrated when the manifold came off. In the photo below you can see where the leak was in upper right corner, which is exactly where the noise was coming from.

This makes perfect sense because when I disconnect #1 plug the noise stopped. I just never would have believed that it would sound like it did. Also as the gasket continued to fail it got louder. My apologies to anyone else that said exhaust leak as I am total surprised but confident that this is the issue. Took a ride to my local Merc Dealer and bought the correct Merc gasket for the manifold and risers. I'll do both side since the other side used the same POS gasket which is bound to fail soon too. WORD to the WISE aftermarket may save a penny now but will cost in the long run.

This leads me to my next question here are photo of my Manifold, 3" riser and elbow, they look usable to me. Can I get a few more seasons out of them? I boat in both salt and brackish and the water at my slip almost fresh.
elbow riser side 1

riser side 2 manifold


Going to bolt it back together this weekend do a quick engine run then drain block in case we get any more below freezing weather, you never know March in NJ one day 75 the next snow!!!

As always any advice is appreciated. I'll post my results this weekend, fingers crossed I sold my 68 Plymouth Satellite for nothing.

Mike