AD31 Both Engines same problemlow revs

"I Have a CARVER 28 with 2 VOL

"I Have a CARVER 28 with 2 VOLVO PENTA DIESEL AD31 model 1993 (I think)

Last Summer one engine was getting hot during operation without loss of coolant or any other apparent problem. My mechanic suggested gasket problem. He opened the engine. The gasket was OK but he found a hidden hole in the coolant system which he repaired.

Engine now is not getting hot but I run into another bigger problem:
When I run the boat BOTH engines do not go more than 2000 rev each and the speed stays at 15 Km per hour.

I did this test:

With both engines on I get 15 Km/Hour maximum
When I turn off right engine speed falls to 12 Km/hour.
When I turn off left engine speed falls again to 12 Km/hour.

In the fuel filters we found some dirty substances and it was thought the fuel system should be checked and cleaned.

Pumps and were taken out and cleaned tested by professional, turbo system was checked and cleaned. Exhaust system was checked and cleaned. Air filters were removed temporarily.

The problem is not solved. Both engines start and run OK until I reach 2000 revs, then they appear to lose momentum and the boat cannot plane. They have about 600 hours of operation each which means to me they cannot be too old.

Now, at a certain moment when the pump engineer was making some adjustments on board while testing, the engines started going faster and reached about 3000 revs. He told me he had actually increased the fuel supply via a special valve(?) on the pumps.

This worked for about one hour and the boat was cruising at about 40 Km/hour (no smoke). Then slowly it lost power again and went back to 2000 revs (15 Km/h). Please note that at this level(2000 revs) it can go long trips without any additional problems.

The latest test was to bypass the engine fuel system and supply clean fuel from an external tank directly to the pump filters to both engines. Still it did not work but by opening the filters we found that they were not full of fuel which according to my mechanic is an indication that we still have not enough fuel supply.

What makes me puzzled is how can both engines suffer from the same problem?

If we suppose that the problem is related to the overheating of last summer and the change of the gasket then it would be located to this engine only.

Unless if:
Is it possible that one engine has the problem and both show the same revs count during operation?

If the problem was with one engine only is it possible that each one separately produces the same speed (12 Km/hour)?

We have checked props and hull, they are clean. No smoke, no noise, no vibrations.

My opinion is that if the problem was with the compression or the props or the hull or the transmission then it should not go at 40 km/hour for an hour and and then drop again. Am I correct?

I am located in a small city South Greece and all local ‘experts’ seem to run out of ideas.
Does any body have any suggestions??"

Fuel or AIR if it is not overh

Fuel or AIR if it is not overheating then I would suggest you start with air. Did you check the air cleaner? Can you rev the engine with no load to 4000 (just for a couple of seconds only)? The model and serial number is found infront of the fuel pump on the block. These are I/O s right? You need to tell the model so you get the right advice.

"Thank you for the post.
Mod

"Thank you for the post.
Model is Volvo Penta AD31 Diesel, year }1993. Air Filters (if you mean this by 'air cleaner')have been checked and removed for tesing temporarily with no effect. Turbo pressure has been measured and is OK.

Yes, with No load both engines go up to 4000 rev.

Yes They are I/O

I will post Serial Nrs after my next visit to the boat."

"No need for the serial number

"No need for the serial numbers...was looking for the letter following the AD31. I will assume a B or P for now, really little difference.
If you get 4K no load then I would stay with Fuel. Without getting into tank config etc.. I would suggest you simply use a external fuel source on one engine and put a clear return line into the tank. No filter water sep etc. If no air bubbles setup is good and engine should rev under load. If not then move on to more expensive items. Are both engines running off of one tank or do you have separate tanks and lines?"

"We have supplied fuel from ex

"We have supplied fuel from external source (a plastic container) directly to the fuel filters of both engines. This config. bypassed the water separator and connected directly on the fuel filter (which was brand new).

Both engines get fuel from the same tank with separate lines.


Question: is it a problem for both engines or could be only one?

Somebody said that if one engine loses power then the revs of both engines drop. If this is true then it may be that one engine has the problem. But how comes that each engine separately produces the same speed? (Sorry if this question sounds foolish, but I am not an expert

Somebody else says that the low pressure pumps may be clogged or damaged. The person that checked and cleaned the main pumps said the low pressure pumps (that pump the fuel from the tank to the main pumps are OK. Should I open them?"

Something is not adding up her

Something is not adding up here...I do not know about what has been messed with. When you say Air cleaner was replaced... what shape was the old one? Did mechanic mess with Pump timing? Is there fishing line or something rapped around both prop hubs? I think others on this site might jump in with suggestions. With regard to your question I have not run one engine full out while the other is out so I do not know. If you say both engines are getting fuel from the same tank and the problem happened to both engines at the same time then I still feel it is fuel related problem. You say separate lines..Did you clean everything and ensure you have good fresh fuel? Is there a screen on the fuel pickup that might be blocked? Just throwing ideas at you.

"let us go to the basic with t

"let us go to the basic with these engines.The turbo only kicks in about 2500 RPM. Up to that the engine is a 'non turbo' engine.
If you do not reach 2500 rpm on BOTH engines, you have a fuel supply problem.
Normally the engines might smoke a bit up to this rpm, if not it is 'out of fuel'.
The simplest test:
Disconnect fuel return and let it run into a canister. If return disappears, lack of fuel.
Take off the lid of the fuel feed pumps and clean the screens!
Then do a new run with fuel from separate canisters with return into the canister as well and air-filters removed."

"Thank you Sailor and Morten,

"Thank you Sailor and Morten,

Air filters looked OK but have been removed to make sure they do not cause the problem during testing.

Pump timing? The problem happened before the pump mechanic removed the pumps for cleaning and testing. But I will ask him to look to it.

Could the mechanic that changed gasket mess with the pump? He says he did not touch the pumps or the timing of the engine.

Fishing line blocking the props? But as I said after the pump mechanic did some 'fine tuning' on the pumps during testing, the engines worked fine for about half an hour and then dropped again. He said he had increased fuel supply through some valve on the pump and thought the problem was fixed.

It looks like a fuel supply problem. The fact that when I opened the fuel filter yestaerday to change the filters I found that the filter container was half empty confims this I think.

I notice that you both mentioned 'feed pump and screen' and 'fuel pick up and screen'. I understand that you mean what I called 'low pressure pump' i.e the pump that brings the fuel from the tank to the injection pump.

If so, THIS IS a new idea. If this pump has a screen then there is much hope the problem is there.

I will check it and let you know.

Thank you again."

"Just for info, I've had a

"Just for info, I've had a twin installation of those engines myself long ago, and have done most of the 'legal' and 'illegal' tweaks of pumps etc.
What your mech probably did was to turn the fuel amount screw 1/2 turn in! Hopefully he put it back where it belongs, or you will be smoking 'havanas' when problem is fixed!"

"Thanks Morten,

I will keep

"Thanks Morten,

I will keep it in mind. But my problem for now is lack of smoke rather than 'havanas' "

HI very interesting i have similar problem same engines in 28 ft picton spirit starboard wont start did but no revs and died other is fine done everything you have mentioned and same i now intend to cut fuel line (metal) and replace with rubber by passing the primary pump with an electric inline pump is this advisable as as you say sounds like fuel mine will fire on easy start but wont keep running.mickc.