Ongoing 8.1 Issue Still Unresolved

copb8tx

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Jan 26, 2008
2,829
Highland Village, TX/Port Richey, FL
Boat Info
SOLD 2005 420 DA w/T-8.1S Horizons
Engines
2018 Sea Hunt 255se w/Yamaha 300hp
I'm still fighting an issue with my port 8.1L engine where it 'pops' under heavy load or at 3500+ RPM's. You could almost describe it as a very very light backfire maybe. But's a rapid pop-pop-pop sound.

It's been discussed here: http://clubsearay.com/index.php?threads/8-1-power-issue.84318/page-5 however it was somewhat buried in a long thread and I didn't want to keep adding to the OP thread so I'm starting my own.

In a nutshell, I've replaced plugs, wires, fuel filters, oil (not related) and now the fuel injectors. The engine idles and runs flawlessly up to the point of the problem. No smoke of any kind. They engine has 400 very well maintained hours on it.

So now what could be failing? I was thinking possible the one of the individual coil packs. Does that make sense? If so, how can I find which one it is? They are very expensive at almost $800 for a set of 8.

What other components should I be considering? A friend mentioned the Throttle Position Sensor but I'm not sure how that would cause the is issue. Or could it?

If I don't get a clear path then I'm going to have to have my marina look at it and quite frankly I have very little faith in their capabilities.
 
Start with removing the valve covers and checking for a broken valve spring.
If the springs all look ok , try pushing down on each rocker, they should NOT move by hand pressure.
Next a leakdown test and compression test
 
Start with removing the valve covers and checking for a broken valve spring.
If the springs all look ok , try pushing down on each rocker, they should NOT move by hand pressure.
Next a leakdown test and compression test

Would a good leakdown and compression check eliminate the valve spring issue? If so, would these tests be easier to do than removing the valve covers?
 
couple of simple tests, pull the sparkplugs and examine them pure white is a water ingestion problem, taste the tips, salty is water ingestion/reversion.
 
sometimes the popping is due to a valve not seating or "floating " at high rpm`s because of a broken spring. GM engines use a inner and outer spring setup. Leakdown and compression test will indicate any problem with a valve suck as a burned seat or damaged intake valve face or seat.
Another quick test is to see if all the valve stems / rocker arms are all the same height. Pressed in studs have a habit of pulling out and loosening the hyd lifter preload, "a soft valve"
Do you need the manual?
 
couple of simple tests, pull the sparkplugs and examine them pure white is a water ingestion problem, taste the tips, salty is water ingestion/reversion.

I pulled the old plugs and examined them to see if they were chalky, oily or in any way different. They all looked as you'd expect them to and they all looked pretty close to the same as each other. No since of any water related issues. They were then replaced with the new ones.
 
sometimes the popping is due to a valve not seating or "floating " at high rpm`s because of a broken spring. GM engines use a inner and outer spring setup. Leakdown and compression test will indicate any problem with a valve suck as a burned seat or damaged intake valve face or seat.
Another quick test is to see if all the valve stems / rocker arms are all the same height. Pressed in studs have a habit of pulling out and loosening the hyd lifter preload, "a soft valve"
Do you need the manual?

Yeah, I'll need a manual if I decide to get this deep into the motor. I may get past my pay-grade here pretty soon though. How complicated is it to pull the valve covers off with all the other stuff mounted above the motor? Is this a 1,2...8 hour job?
 
Popping sounds like a timing issue.

Lets hope it's nothing internal.

Cam position sensor could be the issue.

For cars they are not pricey to replace but have no clue for a boat motor.
 
Popping sounds like a timing issue.

Lets hope it's nothing internal.

Cam position sensor could be the issue.

For cars they are not pricey to replace but have no clue for a boat motor.

By my untrained ear it sounds like some sort of ignition issue. The engine runs up on plane at 3400 rpms completely smoothly without noise.

Can coil packs have a current draw issue that would manifest itself this way?
 
By my untrained ear it sounds like some sort of ignition issue. The engine runs up on plane at 3400 rpms completely smoothly without noise.

Can coil packs have a current draw issue that would manifest itself this way?

Like a car ..... newer boats have a computer that can be read/downloaded. Has anything been logged? I bet if there is a miss fire it would show up and indicate which cylinder(s) or faulty sensor.
 
a weak ignition coil can fire a spark plug fine at lower engine rpm's but not supply enough spark at higher engine rpm's to properly fire the plug thus causing a mis-fire.....

my bet is on a weak ignition coil.....

good luck....

cliff
 
This sounds very similar to what happened on my last boat. Engine would run flawlessly, then after getting it up to cruise it would start 'coughing'. It was intermittent at first (every 5-10 minutes or so) and then got more and more frequent. I also noticed a power loss as it would be very difficult to get up on plane with more than 4 people aboard (this was a Bayliner 245 with a 5.0). I did everything you did to no avail, and then finally had a compression test done. No compression in #7 cylinder and #8 was low. Turns out it was a bad valve intake. I ended up having to have both heads replaced. It ran flawlessly since then up until I sold it. I highly recommend having a compression test done. It will let you know if the problem really is internal, or something more minor.
 
Is popping noise rapid,loud and coming from intake ?If you do decide to pull valve covers to ck valve springs and valve stem heights,turn engine over and visually see that all valves fully open.If you have flat cam at an exhaust valve ,pressure that would normally exit out exhaust will exit thru intake and create rapid popping noise thru intake
 
Lots of good info there, but Before you get too deep into this, I would swap all 8 coils from one motor to the other. Label them with a sharpie or paint pen to keep things organized.

See if the popping moves to the other motor.

Also closely inspect the wire harness connectors that go to the coils, look for damaged, corroded, or loose crimps.
 

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