Volvo AQD32A - Erratic Tickover

[Dave_Seager]

I have twin Volvo Penta AQD32A engines in a Princess 32. A few weeks ago we cruised from Weybridge down to St Kats and back. All was well until the final stages when the idle speed of the starboard engine started to fluctuate while entering a lock.

The engine normally ticks over at about 700 rpm and I can manouvre at close quarters without any additional revs. However, the tickover suddenly rose to 1,500 rpm for about ten seconds and then returned to normal. This kept occurring about every half minute, regardless of whether I was in or out of gear.

When I checked the engine over I found that the throttle cable outer clamp was loose. I tightened it up and assumed that the problem was solved. However, on our next cruise the problem has occurred again but only briefly and after several hours running. Now it happens immediatley on starting the engine.

I recently added an Eberspacher heater, using a tee-junction in the fuel line for the engine in question. Is it possible that an air leak here could give these symtoms?

Otherwise, I imagine that it could be caused by a sticking governor in the injection pump. Is this common?

Has anyone any other suggestions before I start dismantling, please?

 

[MOTORBÅD]

Yes could very well be an air leak and/or a blocked fuel filter.
Michael

 

[scottie]

yes teeing the heater supply WILL cause this problem
It should always be straight from tank

 

[Cliveshep]

Hi Dave,

You've got air in the system m'dear. Check your fuel line connections and bleed out, BUT you really shouldn't have tee'd your main fuel line for the Eber, can't you drill the top of the tank (with powerful magnet and blob of grease to catch swarf) and fit the proper Eber dip-tube pick-up? Alternatively, there is one which fits in the filler hose right at the bottom on the tank fill spigot and dips down the filler tube to the tank. The hole drill one is best though.

Bleed 'err out m'dear!

Clive

 

[Dave_Seager]

Thanks for all of your responses.

I plan to bleed the system tomorrow. It makes sense that the recent change was likely to have caused the problem. However, I am puzzled that air would cause the engine to run too fast. Any fuel shortage would initally slow the engine and although the governor would attempt to compensate I do not see why it would overshoot for such a long time.

I originally intended to fit a separate tube for the heater but the one I bought required a nut to be fitted over the tube, inside the tank. As the only access is through the guage fitting it seemed to risky to use it. If the problem does prove to be an air leak I will certainly reconsider fitting the extra tube.

 

[Dave_Seager]

Re: Volvo AQD32A - Erratic Tickover Update

I bled the system yesterday and found air in the filter and the injection pump. I am now running with the fuel cock to the heater turned off and will change the installation later. I have also tightened the joints in the fuel line that I added for the heater installation. I suspect that air was being drawn back through the Eberspacher pump.

The engine now runs normally on the mooring and we plan a longer test run this weekend. The presence of air in the systems was almost certainly the cause of the problem but the it was intermittant so further testing is needed. This weekend we will be upriver but we plan a downriver trip the following weekend and do not want a breakdown in the tideway.

I am still surprised that air would cause the engine to run faster. My experience with a previous boat and a car was that it caused a loss of power and eventually stopped the engine completely. I would be interested if anyone could explain how it happens.

Thanks again for all the replies. You were all in agreement that it was an air leak. is this a new record for the forum?

 

[Andrew_Fanner]

Re: Volvo AQD32A - Erratic Tickover Update

I asked a more knowledgeable friend about the engine revving up when air got into the fuel lines (and yes, I had run out of diesel). Its because sucking air is easier than sucking diesel, so the "column" of diesel being lifted by the pump weighs less so the pump delivers faster. I thought this to be BS and said so, the illustration was sucking a drink through a straw, its much easier to suck the last of it when the glass is empty and the only drink left is that actually in the straw, the suck required becomes rapidly less as the mass of fluid to lift reduces.