What did I break? Or did I break it at all? (Chevy 454 question)

Long story short, I think I have some kind of valvetrain issue with one of the 454s on the boat.  But I'm not quite sure.  For background, these are Mark IV 454s, so flat tappet cams. 

The first occurrence of the issue was about 2 years ago on the first test outing of the season.  I ran them up to WOT for a brief check.  Everything seemed to be running fine, but once I slowed down enough it became apparent that one engine had developed a misfire at low RPM.  At idle it was pretty much running on 7 cylinders.  I didn't dig into it much that day, but I did the following weekend.  I fired that engine up and it was very clearly running on 7 cylinders with low exhaust temp on #4 according to the IR gun.  But while I was crawling around the engine thinking of things to check, it started to sound better.  By the time it had been running for a minute or so, it was running perfectly fine, sounded normal, all 8 cylinders firing, exhaust temp on #4 was in line with the other cylinders.  There was no change in valvetrain noise between when it was missing and when it was firing. 

After that day, the issue never came back and the engine has run perfectly fine for 2 years and about 130 hours (engine has about 2000 hours on it total).  The only thing that seems any bit off is a valvetrain speed tapping noise, but I can't say if that existed before the issue.  And it doesn't sound like it's in the #4 area, it sounds like it's the fuel pump pushrod (the noise is right up front and louder from the front of the block than the valve covers).  Plus that noise was present at the same volume both when the misfire issue showed up and when it was running fine. 

Fast forward to yesterday.  Engines had been sitting for about 2 weeks and I had just changed the fuel filters, so it took a bit of cranking to get them started.  While cranking the problem engine I noticed an upward hiccup in cranking speed on every revolution as if one cylinder had low compression.  And when it fired, the exhaust sound and behavior was an unmistakable misfire.  So I grabbed the IR gun and headed for the engine room.  Sure enough, low exhaust temp on #4.  Not dead cold, but much colder than the others.  And sure enough, after a minute or so the engine started to smooth out and then ran perfectly fine with no signs of a misfire, good exhaust sound, etc.  I shut it down, let it sit for a bit, and re-started it and it started right up normally on all 8. 

I'm thinking this has to be a valvetrain issue, but not a flat cam lobe, as that wouldn't be self fixing.  There's no popping through the intake, so I'm thinking either I have a lifter collapsing and causing the #4 intake valve to not open much, or something is causing the #4 exhaust valve to hang open slightly.  My thought after the first incident was that the valve springs might be getting weak and I floated a valve and pumped up a lifter, but that doesn't explain yesterday's occurrence as the engine hasn't been above 2000 RPM yet this year (and even that was just a quick blip to confirm good throttle response after things warmed up on launch day).  

Today will hopefully allow a test run, so I'll get to see how it's running again (and under load).  I'm betting it'll run perfectly fine today (not planning to run it all the way up to WOT), but we'll see. 

What does the hive think? 

Lifter is stuck up, hanging a valve open and preventing compression.  When it warms up and the engine block and heads expand, things get better.

That's my guess.  Lifter stuck down makes noise but runs okay, lifter stuck up is quiet but has no compression.

I had a 460 Ford do something like that.

Started it up on the stale gas in the tank and it stuck a valve. Not stuck solid but gummed up enough that is was closing slow.

Both of those theories make sense to me.  I'm not convinced it's temperature related though, as the first occurrence happened while it was already running and still existed on the next start but then cleared up and didn't happen again for 2 years.  This second occurrence was out of the blue.  2 weeks ago it ran fine after sitting all winter.  

Fuel wise, I changed the filters yesterday before starting it.  The fuel that came out of the old filters looked and smelled fine.  

Either way, what are the thoughts on next steps?  I think I'm going to dump a few ounces of MMO into the oil before I run it today as it won't hurt and there's a chance it'll help if something is gummy/sticky.  

That's what I would do. MMO, Seafoam, ATF or something like that in the oil and give it a good run to get everything up to temperature.

The elephant in the room seems to be that you have a boat with 2 big blocks! 😳😳

SV reX said:

The elephant in the room seems to be that you have a boat with 2 big blocks! 😳😳

Despite the 2 big blocks it's still slow.  27,000 lbs doesn't move that fast with 680 HP (total).  

In reply to rslifkin :

Just do what Ferruccio did:

https://www.lambocars.com/riva-aquarama-lamborghini/

27,000 lbs? 😳

SV reX said:

27,000 lbs? 😳

Agreed with a sticky lifter or valve.  It's about the only thing that would cause compression to come and go.

SV reX said:

27,000 lbs? 😳

Think of all the water drag on that thing!

In reply to Pete. (l33t FS) :

Exactly!

I've got a pretty good sized pontoon boat. It's basically the size of a one car garage. It weighs about 3200 lbs.

My truck weighs about 7500 lbs. 

So, if I parked my truck on my pontoon, then added ANOTHER truck and ANOTHER pontoon on top of it, I'd STILL have to add ANOTHER truck!

That's a helluva boat!

It's a mid 80s 38 foot Chris Craft.  Built a little heavier than the average pontoon boat.  And with full tanks we carry more than the weight of that pontoon boat in just fuel and water.  

Status so far is that we're getting ready to head out for a test run shortly.  I added a bit of MMO to the oil on the problem engine.  I'm not expecting there's a lot to clean though as I took a look through the oil fill and it's downright spotless in there, just clean oily metal, not even a hint of varnish.  Conveniently the fill cap is over #4.  Nothing looks obviously out of whack with the exhaust valve and spring for #4.  I can't see the intake side well though.  

So we got out for the rest run.  Total runtime of about an hour and 10 minutes.  It fired right up on all 8 and ran perfectly from the start.  Headed out and ran for a bit at slow cruise for a good warmup.  Gave it about 15 minutes at 3200 rpm under a good load to get everything good and hot then slowed down to head back in and let SWMBO do a little docking practice before we headed for our slip.  Engine sounded healthy and ran perfectly the whole time.  

I think I'm gonna have to put this on the "keep an eye on it and see if it reoccurs" list for now.  

Cain't diagnose what ain't acting up!

Here I was thinking you had one of those old concrete boats.

Generally if you have having a lifter issue with a BBC, you will know based on the tapping noise the rocker arm will make.  Misfires that come and go are a bit of a pain to diagnosis.

81cpcamaro said:

Generally if you have having a lifter issue with a BBC, you will know based on the tapping noise the rocker arm will make.  Misfires that come and go are a bit of a pain to diagnosis.

That was my thought, a collapsed lifter should make noise.  At least it has on every other engine I've seen it on.  Whatever is going on here doesn't sound any different when it's having the issue or not.  If the level of intermittent stays at "shows up once every couple of years and fixes itself in a couple minutes" then I'm not too concerned about it.  I'm just hoping it's not the beginning of a bigger problem. 

I can't confirm if the hiccup when cranking was gone yesterday though (I assume it was, as it wasn't there until the issue showed up on Saturday).  Engine started too quickly to really tell being that it had only sat for a day. 

I'm definitely going to write this issue off as a fluke and just ignore it.  Since the successful test run we've had the boat out a couple more times.  Longest the engines have sat is about 2 weeks, and they've had 5 starts and about 5.5 hours of runtime since the test run now with no signs of that issue coming back.  Although I haven't run the boat hard since the test run and our normal "slow cruise" is only about 1300 RPM with the throttles barely cracked.  Cranking has sounded normal without the hiccup every time, so whatever was going on that day is definitely not happening now. 

Good on you

Sitting unused is hard on any motor, but even worse on marine engines with all the humidity they're exposed to......sounds like it sorted itself out tho.

How many gallons per hour at WOT? at 1300 rpm cruise? Just curious.....

I think it's great that SWMBO takes the helm, mine wouldn't go near it.