Acceleration and Overheating Issues *Fixed*

[Ewhaddix]

I bought this boat last winter in Tennessee without test driving it. It had receipts for a new engine and a new wear ring / impeller all within the last 2 years and the boat looked super clean. I got it for a really good price so I didn't feel nervous about the lack of test drive.

Fast forward to this summer. The first time I put it in the water the engine sounded smooth, no hesitation, smooth idle, but it wouldn't accelerate over 15mph and the engine revs to about 4800rpm max. After about 2-3 minutes the engine high temp buzzer comes on. I checked the telltale and it had water coming out but it was scalding hot and I could see steam behind the boat when underway. I let it cool off then idled the boat back to the dock and took it home.

Once home I ran flush water through the cooling system. Water came out clean from the exhaust, the telltale, and the bottom of the jet. I changed the spark plugs. I checked the engine compression and it was 165psi on all cylinders. I checked fuel pressure at idle and it was 35psi (gen 1 motor). I checked for spark on all 6 cylinders. I inspected the impeller and wear ring. No knicks, no scoring. Clearance was checked with feeler gauge and it was in spec and on the tighter side of the tolerance. I installed a temperature gauge on the boat and verified it is indeed overheating. It gets up to 250 degrees if i go over 3000 rpms.

I put the boat back in the water and had 35psi of fuel pressure even at full throttle. I individually pulled plug wires off and each one caused the power to drop off so I believe all 6 cylinders are getting good fuel and spark. The engine still overheats above 3000rpms and the boat will not go over 15mph. RPM's still top off at 4800.

Third time out I replaced the ride plate to hull seal and only had me and my girl on the boat. After 2 minutes it finally planed out and got up to25ish mph at 4800rpm.

Fourth time out with all the kids it only does 10mph and I can't rev over 3000 without overheating. 4800rpms max.

I'm at my wits end. The boat should run great. Air, fuel, and spark are all there and compression is like new. The wear ring and impeller look good. The only thing that looks worn is the stator housing vanes are really chewed up, but I doubt it would cause it to overheat so badly and only go 15mph. Are the stator vanes really that crucial? Any help is greatly appreciated. Thanks.

 

[Tim75]

Compression of 165? Wow! Spec is 135.

It seems you have at least 2 separate problems.
1) Cooling. Since you mention the stator vanes are chewed, it has been in shallow water. A lot of people seem to use them for making gravel and sand out of rocks. You may want to pull the stator and give those vanes some love with a dremel tool. Just use a sanding drum and smooth things up. While you are there, check for any debris lodged in the vanes. Use a dowel rod to tap it out. Make sure all 5 of the cooling water holes in the wear ring are clear. What do the leading edges of the impeller look like? Since all the work is done already, you could pull the wear ring and check that the o-ring in the water passage is there. It is a common item, but needs to be the right cross-section. If it's too small, it might as well not be there at all. There is a second, large o-ring on the front of the wear ring. There is possibility you have sediment in the cooling passages of the engine. The worst thing is that sediment can turn to "cement" and not wash out. If you have the service manual, double check that the cooling water hoses are routed correctly. You may want to disconnect them and back-flush the engine. Including the hose for the pee hole.

2) Lack of rpm.
You need to stand on your head and find the throttle plates in the manifold. There are 4. Are they in fact opening all the way?

Have you checked the oil in the gearbox and stator?

 

[Ewhaddix]

 

[Ewhaddix]

Not sure how much a dremel will fix the vanes. They are pretty chewed up. All the cooling passage holes were clear and I’ve ran water through every hose on the motor.

The throttle plates were a good suggestion. That’s also one of the first things I checked. Throttle linkage and plate opening are 100%. I replaced the oil in the stator when I took it off to inspect it the first time I checked the impeller. The impeller and wear ring are new.

 

[Ewhaddix]

I will check on the o rings. Thanks for the suggestions.

 

[Tim75]

If your stator is still off, I would do a little grinding on those leading edges.

If your engine still has thermostats, many people just pull'em and toss'em.

Note: That hex next to the seal is NOT a nut. It is cast that way.

 

[Tim75]

Check your rubber exhaust hoses. It is rare, but happens. They de-laminate internally and tear. The thin pieces can flop around inside and block the exhaust. Could cause your problems.

 

[Ewhaddix]

The boat has a new exhaust expansion chamber, new adapter plate, new engine, all new hoses, all new electronics. I'm about to just give up and sell it. It really makes me sick. The only thing that isn't new on it is the fuel system, but the pressure is 35psi at the valve at idle and under load so I don't know what could be wrong there. On the bottom of the boat the impeller and wear ring are new. The only obviously damaged part is the stator vanes but I can't get an honest answer from anyone on if that could seriously effect the performance so much. I pulled the interior of the boat out down to the hull and checked for hidden water. I cleaned the bottom of the hull. I pulled the entire intake grating and ride plate off and resealed it. I adjusted the reverse gate up to make sure it was completely out of the jet stream. I've called 5 Merc mechanics and none will touch it.

 

[poppie]

Verify the timing. Too much spark advance will cause the motor to run like it's lean. This could explain the high temp and low rpm.

 

[Tim75]

Looking at your stator vanes again-
There is a company (whose name escapes me) that makes little stainless clips that go on each vane for repair. Another company will weld repair the vanes.

 

[Tim75]

Looking at my service book:
Check water pressure relief valve, starboard side, bottom for clog.
Water pressure: should be 13-17 psi at WOT measured at the fitting behind flywheel.

Gaskets-air intake: missing or leaky gaskets could cause a lean condition
Bleed system hoses- are any missing?
Lean condition can cause overheat and low power.

Fuel injectors- have they ever been serviced? Clogged/dirty causes lots of problems.
"Fuel system" cleaners won't help. Remove and have serviced.

Oil pump- at idle, arm on oil pump should be about 9:00 position.

Any thing else that can cause a lean condition. Anything that can prevent exhaust gas from leaving the engine.

 

[Ewhaddix]

I finally dove into the boat again today. A new motor was installed by a mercury repair facility but apparently they never adjusted the timing. It was stuck at 9° max. I adjusted the idle timing to 9° and the max static timing to 26°. I haven’t taken it back out on the water yet but I’m optimistic

 

[Ewhaddix]

Took her out on the water today. Runs great! 40mph and cool as a cucumber. Moral of the story “assume the previous owner lied about it running right last season.” There is no way that the timing moved 15 degrees sitting in the marina. You could tell the screws were never adjusted properly, they didn’t even touch the stops. Whoever installed this motor didn’t time it apparently.

 

[Ewhaddix]

Side note, having timing marks on the front of the engine makes it really fun to get at with a timing light and get your head low enough to read.

 

[tupperwolf]

Bump for this thread, I'd like to check the timing on my engine, do you have a procedure handy for how to do this? Will a regular timing light (for a 4stroke engine) work? I assume it would, just flash twice as fast.