Mercruiser 3.0 seems short on top end power-gctid354084

[Guest]

Hi All,

Any thoughts or advice would be appreciated.

My 175 has a 3.0 Mercruiser with Alpha One sterndrive (all standard for the 2003 model), when using a Piranaha 4 blade composite prop with 18" pitch the WOT is 4800 rpm with a GPS speed of 28kn, the WOT range for the engine is 4400-4800 and this is the fastest its ever run with the 18" prop.

if I swap the blades to make it a 20" pitch the WOT only gets to 4000 with a GPS speed of 29kn which is less than it was a few months ago, back then I would get about 34kn GPS speed with WOT of 4500 rpm.

The boat load hasn't changed and performance seems abaout the same as it was with the 18" prop but with the 20" there has been a noticable reduction in top end performance. I'm thinking that it may be an engine power issue and I've removed the fuel filter from the standard fuel pump and put a high flow filter with water bowl inline instead (on the pump suction side, close to the tank) but this has made no difference.

The spark plug colour looks OK so I think that the fuel/air mixture is good but I'm not sure what to check next.

[Chief_Alen]

http://www.boatingmag.com/maintenance/understanding-propeller-pitch

Give it a read.

Compression is what ?

[Guest]

erm.... thanks for the link but I understand the pitch issues, a few months ago the 20" pitch 4 blade prop had a WOT rpm of 4500 (within the 4400 - 4800 range) and the top speed was 34kn on GPS and I was very happy but now its only 4000 rpm and 29kn with nothing else changing on the boat.

The engine seems to have lost top end power and I don't know why. I guess I'll have to take it for a service and see what they can do for me.

[Mocoondo]

It's a 3 liter. It was born short on power.

[Guest]

true, pity I can't shoe-horn a V8 into the engine bay but its good enough for day trips on the bay and is very economic to run.

I forgot to mention that it hesitates when accelerating, especially when cold and the manual has a huge list of possible problems like compression issues, carb problems, spark plugs, breakers, ignition timing...etc..etc basically it lists every part of the engine that makes it go so I think a visit to a professional is going to be needed.

[Chief_Alen]

Check compression on a warm motor, all the plugs out and the throttle fully down.

A dirty bottom is always the first thing i would look for.

[Guest]

Do a plug change and a cap and rotor as well. I had this problem last summer gained power back on my 3.0. And clean your airester filter.

[yachtman]

JWebber wrote:
Do a plug change and a cap and rotor as well. I had this problem last summer gained power back on my 3.0. And clean your airester filter.

+1

[Guest]

rc42a wrote:
true, pity I can't shoe-horn a V8 into the engine bay but its good enough for day trips on the bay and is very economic to run.

I forgot to mention that it hesitates when accelerating, especially when cold and the manual has a huge list of possible problems like compression issues, carb problems, spark plugs, breakers, ignition timing...etc..etc basically it lists every part of the engine that makes it go so I think a visit to a professional is going to be needed.

I'm sure someone else will be able to provide the details, but I seem to remember that there was an adjustment for the accelerator pump to eliminate the hesitation when accelerating.

Not sure if that will help with the top end issue though.

[Guest]

JWebber wrote:
Do a plug change and a cap and rotor as well. I had this problem last summer gained power back on my 3.0. And clean your airester filter.

Thanks, I'll do that.

I already cleaned the air filter or 'flame arrestor' as the manual seems to call it but it looked OK to start with.

I'll get new plugs but is there an option to convert it to contactless electronic ignition? I did that on an old Land Rover with a similar petrol engine many years ago and remember that it did help it to run better but I don't know if that's suitable for a marine environment.

I'll have a look at the carb adjustment too, its likelythat the hesiitation and lack of top end power are related but it could also be two separate problems.

[Chief_Alen]

" contactless electronic ignition " standard in the early 90's to present.

[Guest]

At this time don't bother with carb adjustments yet, just to the tune up and report back.:livid:

[Guest]

Mocoondo wrote:
It's a 3 liter. It was born short on power.

Don't think so.......

[Guest]

Chief Alen wrote:
" contactless electronic ignition " standard in the early 90's to present.

That's good, no contact points to adjust then

The Land Rover engine that I upgraded was a 2.25 single carb model from 1969, solidly reliable and about the same technology level as the 3.0 Mercruiser, I'll get the distributer cap off and give everything a good clean in there anyway. The plugs should be easy to source replacements but the cap and rotor will be more difficult here in Australia.

[Guest]

OK, I cleaned the rotor and distributor cap contacts, they were quite corroded but otherwise undamaged so hopefully they will be OK

I got new HT leads, the old ones had slightly corroded ends and were very oily and dirty and I got a new set of plugs.

The compression test results are as follows (this is without any prior running of the engine so oil was not distributed to the rings and it was not up to temperature though)

Cylinders 1,2,3,4 in PSI - 165, 165, 170, 150

I inserted a small amount of oil into cylinder 4 and re-tested and it was up to 175 PSI so the valve seating looks OK, I'm assuming that it was just lack of prior running that caused this lower result and that the piston rings are OK but comments would be welcomed.