Tohatsu 40 TLDI .... Compression 20 PSIG low on one cylinder

[Dave Long]

Hello Boaters;

I am in a questionable situation with a boat we are looking to purchase, that has a 2012 Tohatsu 40 TLDI.

During the sea-trial I noticed the following:
> It started instantly and showed no issues.
> Idling seemed just a touch rough ... much like my current 30 HP Yamaha 2-stroke. Perhaps this is typical of all 3-cylinder outboards? My 9.9 Yamaha idles more smoothly than my 30 HP, and this Tohatsu.
> The boat (Grand 380 RIB) seemed to be slow to plane, with only 2 adults in the boat. When the throttle is pushed to WOT, the engine RPM slowly builds and the bow starts to come down after 10 seconds or so. I had figured it might be a characteristic of the hull, prop selection, or trim of the motor.

When the shop (authorized Tohatsu dealer) did the inspection, they reported that one cylinder was down 20 PSIG compared to the others. So now I wonder if there is a genuine problem with the engine?

I know the current owner, and they took very diligent care of the engine, so I am not worried about abuse. But this engine does seem to have some very subtle issues that might go un-noticed, till the compression test indicated a cause for concern.

So my quesions ...
1) How tolerant are the TLDI's to compression differences?
2) Are my observations indicative of a low cyclinder?
3) What is the repair action? Bore and re-sleeve the cyclinder, new piston and rings ? ... COST ???

Any advice would be greatly appreciated.

Thanks, and take care;

David

 

[Sea Rider]

Welcome to Iboats,

Some Issues :

Being a TLDI engine, probably needs a good scan, fine tune adjustments too. Tohatsu states for 2 strokes engines max 10% difference between cylinders. What did the guy who made compression test told you about ? It could be a simple matter of applying carbon remover over dose squirts through carb while engine is on to clean combustion chambers carbon build ups, then checking latter if compression is even or near even with fact engine specs.

If not, probaly will need to change piston rings, worst scenario pistons and rings and machine shop work ? Is prop in good working condition ? To achieve top combo performance, you need a healthy engine & prop, well height transom seated and trimmed engine and deck weight evenly distributed.

Happy Boating

 

[Dave Long]

Thanks for the reply Sea Rider.

I can offer some further info ...

1) The prop appears fine and has no visible signs of damage.
2) Moving weight to the front of the boat certianly does help it get on plane ... but it still takes a long time to "wind up" the RPMs to get on plane. The impression it gives is thatit is very "over-propped" and under-powered. I find that hard to believe that 40 HP can't make a 600 lbs RIB jump on plane quickly.
3) The engine is trimmed all the way down.


Once on plane, it runs great. I did a speed run and it easily got in the mid to high 30 MPH range and reached 5,800 RPM or so. Excuse my rough numbers, but it was a rough day and looking at the GPS and tach was a lower priority that keeping the boat upright and safe.

So, with that speed and RPM data, it seems to suggest that it is getting into the desried WOT range, which says to me it is propped correctly.

So why so slow to get on plane ? It really feels like it is bogging down and struggles to get the revs up when I punch the throttle.

Of course the seller (my friend) says it has always been like that, and I genuinely trust him. He bought the boat and motor new.


If there genuinely is a problem with the compression being low on one cyclinder ... which the compression test says it is ... what is the repair, and how much will it cost?

David

 

[Sea Rider]

Before venturing in a repair quote, I would perform firstly a prop hub test + chamber spray decarbon.

Some things to know :

-How old is that combo ?
-How's battery condition, is it the right one with correct amp ?
-Are all fuel circuits tank to carb immaculate clean, including fuel separator if having one. How about fuel, was it fresh when tested ?
-How many boaters were on board when water testing ?
-Being a Rib, is it fully infalted to its recommended working pressure ?





-Try again with well balanced deck weight and engine sitting trimmed at 90° on transom. Is it a remote driven rib or tiller steering ? Check correct balance and engine trim settings.

-Paul is an expert on those TLDI engines, he should provide a more appropriate tech answer on the matter.

Happy Boating

 

[pvanv]

20 pounds low is too much to be called "normal". 10% is the limit, so maybe 10-15 pounds difference, depending on the other pressures. Decarbon the motor, check the ECU stats and see if there are any errors logged. If it is still that low after decarboning, there is a mechanical issue. The motor may run well, but it is then time to keep an eye on that cylinder.

Rigging for proper/best running/planing/holeshot is a different topic altogether. If you are too low on the transom jack height, you can get a sluggish hole shot. Are there trim tabs of any kind on the boat? They help launch, and the spring-loaded automatic ones don't need adjustment for running if setup correctly.