You've been catfished. This is actually a boat engine topic, but since it's a Chevy 350 at heart I wanted to get you non-boat experts to read it too.
Here's the deal, I had a boat surveyed with a 2000 Mercruiser 5.7 EFI. The guy did a relative compression test using an ammeter on the starter cranking the engine with the ignition disabled and the plugs in. In short, the amperage peaks every time you hit a compression cycle and you can read the relative compression but not the actual compression. He says he uses the method all the time and has backed it up with traditional gauge readings many times and never found discrepancies. He found what reads as one low compression cylinder. Here's the graph:
According to him the high peak is an artifact of the low hole preceeding it, a common occurrence. The boat owner has the boat at a repair shop for a compression and leakdown test and we'll discuss afterward.
Now...here's where I need the GRM crew.... I'm bench racing the issue (and diagnosis) until I hear a result from the mechanic. Because of course I am.
Can you have a single piston with significantly lower compression and not have any symptoms on the running engine? We tested the boat and it seemed to run great. Hot and cold idle was sewing machine smooth and quiet. No stumbles, ran a dead-steady 175*F over an hour on the lake and at the dock.
Other general info:
Boat hasn't been used much lately. Only about 25 hours in the last 5 years if the records I've seen are accurate. Stuck rings? Something gunked up?
Had a stumble when hot issue recently that was fixed by a mechanic who replaced the coil.
Owner replaced the impeller recently, surveyor noted it was done after a failure. Might have run it hot?
Owner says the boat never ran the factory listed RPM at wide open throttle as long as he's owned it. Listed WOT is 4400-4800. I had it running a little over 4000 RPM and had just a little more throttle left. I didn't go full WOT. These numbers agree with the one example of speed vs RPM I've found posted on the same vessel. Could it have been down on one cylinder since he bought it in '13? Seems kind of unlikely. I've seen a survey he had done in '16 but compression wasn't tested.
I invite your idle (lolz) speculation...
Can you have a single piston with significantly lower compression and not have any symptoms on the running engine?
Yes, but without a proper compression test you don't know anything.
If it's only one cylinder, odds are it's an exhaust valve, but a wet compression test will point you in the right direction
This sounds promising. A bad valve would certainly be one of the better outcomes. Of course, whatever damaged the valve could have made other problems...
ultraclyde said:
This sounds promising. A bad valve would certainly be one of the better outcomes. Of course, whatever damaged the valve could have made other problems...
This is a pre-purchase survey, right? Whatever the issue is it should work out OK for you. Either insist on it getting repaired before purchase or get a quote from a reputable yard and have the purchase price reduced by that amount.
APEowner said:
ultraclyde said:
This sounds promising. A bad valve would certainly be one of the better outcomes. Of course, whatever damaged the valve could have made other problems...
This is a pre-purchase survey, right? Whatever the issue is it should work out OK for you. Either insist on it getting repaired before purchase or get a quote from a reputable yard and have the purchase price reduced by that amount.
yes, exactly my plan. In fact, once it's diagnosed and a quote is done on the repair, I'd rather buy it as is at a discount if the seller will work with it. The boat is 3 hours away so I'd rather have the work done local in case I have an issue with it.
Or do it myself, depending.
EDIT: my only concern about this is that the seller's wife said they wished they'd listed it for more after they looked around the market more. I think the seller is a stand-up guy, but if he's not then I could see using this as a reason to cancel our deal, do the repairs, and re-list it at a higher price.
11GTCS
Dork
7/28/21 12:19 p.m.
In reply to ultraclyde :
It's not unheard of in seldom used marine engines to have a stuck valve or valves. I'm concerned about the WOT RPM issue as it's really important to be able run in the recommended range so as not to lug the engine. There's no down hill in a boat after all. This is usually related to propeller (or propellers in the case of a Bravo 3) pitch, there should be information available on line from Pursuit to determine what they installed at the factory for this powertrain. It's possible, even likely that the propeller pitch is 100% correct and the drop in performance is related to the dead cylinder. Could be something has been stuck or otherwise broken as long as he owned it especially where the previous survey didn't test the engine compression.
My 22 cuddy is somewhat similar in size / weight to the Pursuit. 350 / 4V Mercruiser but with an Alpha drive / single propeller. With a 17" pitch 3 blade aluminum propeller I see an indicated 4,400 RPM and 44 MPH on GPS.
I would drop the thing in the water and hammer it for a while, then recheck. Stuff gets sticky when it sits.
Streetwiseguy said:
I would drop the thing in the water and hammer it for a while, then recheck. Stuff gets sticky when it sits.
If I was the owner, absolutely.
As the buyer, no. Let a mechanic diagnose it.
ultraclyde said:
Streetwiseguy said:
I would drop the thing in the water and hammer it for a while, then recheck. Stuff gets sticky when it sits.
If I was the owner, absolutely.
As the buyer, no. Let a mechanic diagnose it.
I meant the seller, because you are going to give him a few thousand fewer dollars because of it. Worth the effort, I'd say.
Why has no one suggested to just LS swap it yet?
11GTCS said:
In reply to ultraclyde :
It's not unheard of in seldom used marine engines to have a stuck valve or valves. I'm concerned about the WOT RPM issue as it's really important to be able run in the recommended range so as not to lug the engine. There's no down hill in a boat after all. This is usually related to propeller (or propellers in the case of a Bravo 3) pitch, there should be information available on line from Pursuit to determine what they installed at the factory for this powertrain. It's possible, even likely that the propeller pitch is 100% correct and the drop in performance is related to the dead cylinder. Could be something has been stuck or otherwise broken as long as he owned it especially where the previous survey didn't test the engine compression.
My 22 cuddy is somewhat similar in size / weight to the Pursuit. 350 / 4V Mercruiser but with an Alpha drive / single propeller. With a 17" pitch 3 blade aluminum propeller I see an indicated 4,400 RPM and 44 MPH on GPS.
Another owner with the same boat and drivetrain posted some time ago that his max was 42.67mph @ 4680 RPM. The 2016 survey states that the max achieved in this boat was 38mpH @ 4000 RPM. I got it to 40 and the RPM was a little over 4k but it was rough enough water I didn't push it to the stops. As I mentioned, that survey didn't include any actual hull testing or engine testing.
I was also told the risers and manifolds were done just before he bought it in 2013, They would have been my first stop.
So it sounds possible to me that the PO got a riser leak that caused a burned/leaking valve, did the risers, and sold it to the current owner who thought it was running ok and never had it checked. Sounds like this boat may have been running this way for nearly a decade but ran okay enough that no one ever thought to check the compression. From the current owner's mechanical knowledge I could see that.
Or...the current owner new it had a dead cylinder but it ran ok so he decided to sell it while the market was up and hope I didn't hire a surveyor. I don't think he's the kind of guy to do that, but that would be the point, wouldn't it?
Or it could be propped wrong but I'm pretty sure they're the OEM setup. They're not usually that far off.
Stampie said:
Why has no one suggested to just LS swap it yet?
I was drooling over the turn key Mercruiser 383... but they're pricey.
Stampie said:
Why has no one suggested to just LS swap it yet?
I've wanted to do that for 10 years, but marinization parts are unobtainium.
On the amperage/compression... I think you're getting an X-ray when you need an MRI. By that I mean - real compression test. I would imagine that something as simple as a different resistance plug wire or smaller plug gap could send those amperage spikes in any direction.
We are talking about 50kV in carbon-trace ignition wires with an intentional gap in the circuit that it has to cross.
The nice thing is, it's a 350 with no special internal parts. It's a bone-stock GM goodwrench longblock off the Chevy assembly line that Mercruiser finishes assembling. If there is something that is out of whack and isn't fixed by just running the cobwebs out, it shouldn't be hard to fix. Its rare that a stuck ring can't be fixed with a little kerosene or diesel or kroil. A valve will mean pulling a head. Partially collapsed lifters aren't out of the ordinary, but it's a roller cam motor so you could swap out one pair of lifters.
Anything bigger than that (hole in a piston, scored cylinder bores, etc) would likely show up as a misfire or a noise.
I spent some time talking with the seller. He agreed that it's probably been that way the whole time he's owned it but they never ran the boat hard so it didn't occur to him that anything was wrong. Also discussed the possibility that the method is at fault and verifying it with real testing is the right choice. Boat's at the shop and might get results before the weekend.
He's also very open to selling it a reduced price if it needs work because he's rather not deal with it. Which suits me.
In reply to ultraclyde :
Guy on a lake / boating forum I read had very similar symptoms to yours. Ended up being a stuck exhaust valve if I’m remembering correctly but definitely a valve. He swapped the heads and was back in business.
I wouldn’t put a lot of money or effort into adding a lot of power (383). That hull isn’t ever going to be fast, as you noted even with all 260 HP on 8 holes / 42 mph you’re only down a couple running on 7. Another 30-40 HP might get you to 45 -46.
Curtis73 (Forum Supporter) said:
On the amperage/compression... I think you're getting an X-ray when you need an MRI. By that I mean - real compression test. I would imagine that something as simple as a different resistance plug wire or smaller plug gap could send those amperage spikes in any direction.
We are talking about 50kV in carbon-trace ignition wires with an intentional gap in the circuit that it has to cross.
The nice thing is, it's a 350 with no special internal parts. It's a bone-stock GM goodwrench longblock off the Chevy assembly line that Mercruiser finishes assembling. If there is something that is out of whack and isn't fixed by just running the cobwebs out, it shouldn't be hard to fix. Its rare that a stuck ring can't be fixed with a little kerosene or diesel or kroil. A valve will mean pulling a head. Partially collapsed lifters aren't out of the ordinary, but it's a roller cam motor so you could swap out one pair of lifters.
Anything bigger than that (hole in a piston, scored cylinder bores, etc) would likely show up as a misfire or a noise.
They're measuring starter current with the ignition off. The trace is strictly a function of how hard it is for the starter motor to turn over the engine. The only thing I can think of off the top of my head that would produce the results shown that's not an internal engine issue would be a leaking or loose spark plug.
That's not to say that they shouldn't do a compression and leakdown test. That's definitely the next step.
Yeah....as much as big cubes running out of through hull exhaust is the song of my people, this ain't that boat. Anything over 40 gets less and less enjoyable anyway. Cruising between 25 and 30 is really plenty.
In reply to ultraclyde :
So now you got me all interested in boat SBCs. Can you vortec swap them without marinization* issues? How about aftermarket aluminum heads?
*I had to scroll up to copy and paste Curtis' fancy word.
As I understand it, the 2000 model in this boat IS a Vortec. Aluminum heads are common on fast freshwater boats, even aluminum exhaust manifolds, but saltwater eats them fast.
I think that you might get away with aluminum heads on a salt motor if you are running closed cooling. That system only runs coolant through the block, not raw water. The raw water is only in the manifolds and the raw water pump.
Edit: Enough newbie knowledge to be dangerous: The actual engines used in the Mercruisers are identical to the car engines after about 97. Before that there were differences in the valvetrain but the castings were the same. Boats run longer hours at constant higher RPM ranges than most cars so it's harder on valves and springs. Cam grinds focus more on torque than top end HP unless you're in a racing application. A multi-engine boat would run half of them in reverse rotation, which has more differences. Most of the stuff bolted to the long block has differences, either minor or major depending on the component. Cooling system is different of course, but not inside the block.
In reply to ultraclyde :
Actually, I think you'll find since about 1975, all the engines are forward rotation regardless of position. With the advent of the Bravo and Alpha, they were designed to do the reversing in the lower unit. A Bravo is actually made with straight-cut gears so it can operate either direction. Alphas in twin screw applications have helical gears that are opposite cuts and the upper end of the case has the ring gear on the other side so the downshaft is what gets reversed.
Many inboard/velvet drive ski boats kept a reverse rotation engine, usually a Ford 302 or 351, but somewhere in the late 80s most of the inboards primarily went forward rotation.
Tons of Vortec 350s out there in boats with a carb. Little known secret, some of the manufacturers used the GM 820 vortec/carb intake and recast it with the Mercruiser or Penta emblem instead of the checkered flag. Great intake used on up to 430 hp crate engines. Buying a Vortec carb intake could set you back $450 because it's a specialty thing, but hit up a boat junkyard and you can likely find one for $50. At one point I owned about a dozen of them but I either sold them all for a pretty penny or used them in builds.
Interesting. In looking at parts I've seen Mercruiser 5.7 EFIs offered in Reverse rotation so I thought it was still a thing. Maybe they were for repowering older craft.
In reply to Curtis73 (Forum Supporter) :
Amp clamping the starter is a remarkably accurate way to check relative compression.
Not sure what you are getting at with ignition. Ignition won't affect a relative compression test, and anyway the ignition is disabled when cranking the engine.
The problem may be a slightly burnt valve that seals up better in a running engine.
Pete. (l33t FS) said:
In reply to Curtis73 (Forum Supporter) :
Amp clamping the starter is a remarkably accurate way to check relative compression.
Not sure what you are getting at with ignition. Ignition won't affect a relative compression test, and anyway the ignition is disabled when cranking the engine.
The problem may be a slightly burnt valve that seals up better in a running engine.
This is my bet since it's likely run for nearly a decade as is. Only an issue when cold testing it or trying to ge the last few hundred RPMs out of it.
