Exhaust Manifold Problems?
 

Sorry for the long post, but I'm new to the performance boat arena. My last boat was a 26 foot cruiser.

I just purchased 1 1997 31' Scarab with twin 502 Mag MPI's. I took it out for a test run yesterday and here are the results.

Once I launched the boat, both motors started perfectly and ran very smooth. I ran at idle or just above for about 15-20 minutes and all gauges looked good on both motors. Once I came on plane, the boat seemed to run nicely. I was running about 5000 RPMs on each engine with just a tiny bit of throttle left. After running for about 5-10 minutes, the starboard engine alarm sounded. At first, I thought that I may have been overheating because it was at a temp of about 180. so I shut it down right away. Port engine was running fine.

I returned home at idle with 1 motor while the other cooled. I immediately checked the raw water pump impeller and it was fine (I just installed it last week). I also replaced the thermostat as a precaution. When I went to start it again, I had difficulty turning it over. When it did turn over, it idled fine but at about 1200 rpms it had a knock and the rpms would fluctuate from 1200 to about 1500 on their own. As a precaution, I started the port (good) engine. didn't have difficulty turning it over, but it cranked for a long time before starting.

I had to return the boat to the ramp about 3 miles away, so I ran slowly back on both engines. I had a hard time running that boat on just one engine. Is this normal? I coultdn't even think of getting on plane with one engine running, the prop just slipped while hardly even moving the boat if I ran only 1 motor.

I'm getting ready to take it apart now. The oil looks clean, so I don't suspect a cracked block. Due to the age of the boat and the fact that I don't know the maintenance history, I suspect that the problem is with the exhaust risers and/or manifold leaking water into the cylinders. I also think the the port (good) motor is probably having the same problem, but on a much smaller scale. That would explain the hard start. I'll probably replace the risers, manifolds, gaskest, etc on BOTH motors.

My big question is how can I tell if I've done any damage to the motor itself? These motors are still covers under the extended Mercruiser warranty, and my shop tells me that this would be a covered repair. Also, if it caused any other damage, that should be covered as well. I want to make sure that it gets done right before the warranty runs out at the end of this season.

Any feedback would be greatly appreciated.

If you really think you did damage then my first step would be to do a leak down check. From there I would just for a added number to go by do a compresion check. I know basically the same thing but redundency is always good. From there I would pull off the risers and look for any sign of water inside the exhaust. If you have alluminum intakes that is the next place I would look. Good luck.

Jon

Thanks for the reply. I plan to remove the risers and manifolds today or tomorrow. I'm just concerned that I may have bent a rod, broken a valve or cracked a piston from the water in the cylinder. I don't have any indication of this. I'm just trying to be cautious.

Will these types of problems occur in a pressure check. I have to pull the motor to get to the manifolds, and I don't want to have to do it twice.

Are these stock manifolds and risers? Or do you have some kind of headers?

Maybe I'm missing something here, but if this is a stock setup with an extended warranty, wouldn't you want to take it to your warranty provider before you start taking things apart? I never mess with anything that is under warranty with the fear that they will say,"its under warranty, but you have already tampered with it." Also did you mean that you ran that boat for 5-10 minutes at 5000 RPM? Wow:eek: :confused:

I agree with Whiteknuckle. If it's under warranty don't mess with it. Also, why do you think there's water leaking in if it's not in the oil? The last motor I had something similar to this happen there was water in the oil from a crack so small in the head you could barely see it!

If you heard a knock at any rpm, don't touch a thing!!! Probably smashed a bearing under partial hydolock. Warranty claim fast!!

Good Luck,

BT :cool:

Here is an update....

After reading through some other spec sheets, I realized that I was NOT overheating at all. The motor was running at about 175 to 180 degrees! My problem is that the previous owner put the wrong thermostat in the port (working) motor. Since this is the first time that I have ever run 502 Mags, I simply compared the two temps and assumed that I was running hot. I pulled the plugs because that certainly shouldn't affect any warranty claims. There was NO water in any of the cylinders. All plugs looked good. I put them back in and off to the shop it went.

Ass soon as the shop started it up it began to knock hard, even at idle. It sounded to them as though it is a bearing and probably more. The oil pressure looks good at idlem but they seem to think that it dropped very quickly at cruising rpm. If that is true, it wasn't vapor lock that was causing the hard start, but lack of proper lubrication.

I sure am glad that I bought a used performance boat with a factory warranty!

They seem to feel that with the Mercury Extended warranty I am entitled to a short block at the very least. Has anybody had any experience with this type of warranty claim? What can I expect them to offer and can I get anything else. The shop is checking for corrosion to see if he can get a full engine replacement. If that is the case, wouldn't it just be a matter of time for engine #2 to do the same thing?

Thanks for the feedback guys. I've owed a boat for about 15 years. But this is my first performance boat. Now I'm concerned about my other motor doing the same thing.

I'm curious about the 5-10 minute 5000 RPM run myself. That might explain why you lost the bearings.

5-10 minutes shouldn't be a problem for a healthy engine. The surveyor that surveyed my scarab said that part of the engine survey would be to run the engine at WOT for 5 minutes. This of course shortens the life of an engine so the more you do it the shorter the expected life of your engine will be. I prefer short burst, say less than 1 minute.

BT :cool:

I'm a bit confused why the fuss over the 5-10 minutes at 5000 rpm's. What is the safe operating range for these motors? I got the boat with 24" Bravo I props. The drives are running a 1.50:1 ratio. I suspected from day 1 that the props were a bit undersized and was planning on putting on 26" labbed props. My thought process was that the higher pitch would lower the rpm's, but the blueprinting would gain rpm's. I figured that this would put me right back at about 5000 rpms. I was under the impression that this was the normal WOT range. If it isn't please let me know before I cause major problems.

Thanks

Maybe I don't understand this performance boating thing, but shouldn't I be able to run at WOT? I had a 26' cruiser with a 7.4MPI and ran at WOT all the time.

Is it possible that I need to go from factory 24" Bravo I's at 5000RPM to 28" labbed Bravo I's to run safely?

I gotta agree with Blue Thunder. Being a mechanic I'm familiar with all those expensive moving parts going up & down inside that engine at 5,000 rpms and I prefer to keep my wide open blasts to a minute or two at best! But .... If its any consolation, and this is certainly a testimony to the durability of the mercruiser engine, I ran in a poker run last year, at 1,000 islands, in my non mechanically inclined friends boat, a 32 foot Fountain with twin 502 mag MPIs, and for 135 miles we ran at WOT - 5,000 rpms ! At one point about 1/2 way thru the run I looked over and said " Are you gonna slow down for a while and give the motors a rest ?" He gave me a funny look and said " Why ? These motors are built to take it. " He then informed me that he runs wide open in all the poker runs he enters. I just kept thinking about the beating those poor motors were taking and all those moving parts going up & down up & down !! But they never skipped a beat !! He also had an extended warranty on the motors, I don't. Maybe that makes a difference.

Bo

On the 5000RPM for 5-10 minute thing, I guess I am just more conservative than most. I feel that is very demanding for a production pleasure boat engine and drive. Even if it is under warranty, it won't always be and you will have to pay the price for your earlier habits. Now, if you are comportable with that, and willing to accept shorter life on your components than have at it. I guess it is a comfort level on how you run your equipment. MY WOT blast are usually just a minute or two. Not only is putting the equipment to the max, but you really have to be on your game as a operator.

You would not believe what they do at the motor assembly plant here in Western N.Y. when they test motors off the assembly line!

Dave,

I would really like to hear that story!!

Bo

I run my 502's WOT during a
poker run. At Lake Cumberland
one leg is 38 miles. Factory
engines can take it. Most home-made engines won't.
That's why factory engines
cost twice as much. I'd rather go 70mph every Saturday and Sunday than 90mph once. Next time you
have one engine running hot,
use both engines to get on
plane, then shut off the hot
one. One will keep you on plane.

Hey Dave;

Thanks for the M.S.D. information. I will put it to good use.

Vinny

SEE-YA ---- It is VERY hard on an engine / drive to keep a boat on plane if the other engine is shut down. Also I would bet that the properly built (home-made ) engine will easily outlive a stock factory engine if both are run at the same rpm and HP. I would put money on that. Factory engines are twice as costly because the factory is gready and covers there poor work with a warrenty. If you are an engine builder than you would already know this.

TinkerBoater-
I am an engine builder. Check
my 604ci 755hp engine for sale in the OSO classified.
Nuff said.