Volvo Penta AQ171C Overheating

[Dr Michael R. Taylor]

I have owned a Carver Montego fitted with two AQ171C engines for the last 13 years. I overhauled the engine heads 18 months ago. On a recent trip from France to the UK (75 nautical miles) one of the engines overheated immediately after stopping to refuel. When the engine had cooled down I checked that there were no fresh water leaks and the oil was showing no signs of water contamination. These checks were good although some of the fresh water coolent had escaped via the pressure cap.<br /><br />The raw water pump was pumping sea water - proved by cranking engine with distributor disconnected and pipe from oil cooler disconnected to show water flow. <br /><br />When reconnected the distributor and pipe to oil cooler the engine started fine and the thermostate opened and the engine stayed at correct working temperature for less than a minute, then climbed rapidly. The exhaust note from the rubber exhaust connection between engine and outdrive shield sounded louder than normal and the whole area of the exhaust manifild became very hot.<br /><br />What tests can I perform to check if the exhaust manifold needs replacing? Should only the exhaust exit through the central portion of the manifold and NOT water? Should the raw water only exit through the slots in the manifold extension which surround the exhaust pipe? <br /><br />I have turned the manifold through 180 degrees and connected a hose to the drain point and have noticed that water rises from the central portion of the outlet where I would have expected only a path for exhaust gases. I must have both engines running by next weekend.

 

[Ed Polguy]

Re: Volvo Penta AQ171C Overheating

Yes water , water should be coming out of those <br />slots, it could be that corrosion or debri has <br />blocked those passages. If those manifolds are<br />original it may be time to re-place them. I was able to have mine "boiled out" at a radiator <br />shop, lots of rust and silt came out, both engines<br />have been running fine since. <br /> good luck, Ed

 

[Dr Michael R. Taylor]

Re: Volvo Penta AQ171C Overheating

Ed many thanks - but should water be coming out through the central portion? The manifolds are the originals. The slots were clearer than when I rebuilt the heads etc 18 months ago when I did not have an over heating problem. The manifolds cost about $750 each in the UK - if cracked then OK but I am worried this may not be the problem. What were your symptoms?<br />Thanks, Michael.

 

[Ed Polguy]

Re: Volvo Penta AQ171C Overheating

Micheal, I found that the manifolds were getting real hot , real fast ! if you have sufficeint water flow through them they will be pretty warm , but you should be able to touch them. <br />I don't beleive you should be getting water flow through the large axhaust hole , could be a cracked manifold. <br /> I had problems with poor water flow, due to a bad raw water pump on the port engine, and a<br />rotted raw water pick up flange on the starboard<br />side.<br /> Ed

 

[Dr Michael R. Taylor]

Re: Volvo Penta AQ171C Overheating

Ed, thanks for reply. I have checked the manifold with a hose and torch (manifold taken off the engine of course!) and can see water entering the exhaust jacket near the top of the manifold riser just before the manifold joins the rubber coupler. The manifold is therefore scrap. I have ordered a new one and appropriate bits so I hope this manifold corrosion problem was both cause and effect. I assume a piece fell off after the long journey across the Channel but it does seem rather convenient and I am concerned that I may have missed something. The raw water impeller was as good as new by the way. I am trying to figure out the physics of why the hole would cause such an immediate and dramatic overheating of the manifold. I presume the leaking exhaust gases will turn the raw cooling water to steam and therefore will be unable to cool the manifold and the exhaust gases. In fact it will just heat up the manifold. Do you think this is right? I would not normally worry so much but summer holiday boating trips with children need extra care. Any ideas on the physics of the overheating and should I check other possible causes?<br /><br />Michael

 

[Ed Polguy]

Re: Volvo Penta AQ171C Overheating

If I were you , I would re-place the manifolds in both engines, its been my expereince, that when one goes, the other's not far behind. <br /> I guess your lucky no water got into the engine.<br /> Maybe "Dunaruna" can add something, Aldo knows alot about these engines!<br /> Ed

 

[Dr Michael R. Taylor]

Re: Volvo Penta AQ171C Overheating

Ed, thanks for your suggestion about changing both manifolds. I will do that but I will have to wait a month or three until my personal cash flow is up to the challenge. How much do manifolds cost in the USA? With the extension elbow and gaskets the cost for each manifold is about £900 in the UK for each engine (around $1300 I think). I certainly will not be crossing the Channel again until both manifolds have been replaced. I am told by Volvo that I should check that no pitting of the exhaust valve stems for the rear (number 4) cylinder has taken place. I observed some surface rust but if this wipes away then I am told this would be as expected. <br /><br />Do you have any experience of a metal insert ring (retrofitted)for the number 4 exhaust port on the 171C cylinder head? Apparently this was a later modification due to corrosion of the exhaust port where it joins the manifold. The head has to have some metal removed which allows the ring to be inserted pror to refitting the gasket and manifold. Michael

 

[Dr Michael R. Taylor]

Re: Volvo Penta AQ171C Overheating

I have changed the water cooled exhaust manifold on the problem port engine. The old one was so corroded that the exhaust chambers which came together at the exhaust extension (where the manifold extension joins the rubber coupling) had been completely destroyed. They no longer existed - clearly this manifold really did require replacing.<br /><br />On running the engine with the new manifold it was soon clear that the manifold was not being cooled and the engine was overheating again.<br /><br />The final culprit was found to be an impeller that was intermittently slipping on the keyed drive from the engine. The impeller looks perfect but you can hold the rubber impeller and insert a broad screwdriver and turn the inner keyed bushing!! Never had this problem before.<br /><br />Now with a new manifold, new extension and a new impeller the engine runs very smoothly, quietly and delivers more power that the starboard engine. I will now have to replace the starboard manifold as you suggested. If any other AQ171C users are in doubt about changing their manifolds they should take a look at what a new one looks like - not on the outside but on the inside! Amazing.<br /><br />I took the boat for a 60 mile high speed run and it worked brilliantly - unfortunately the sun did not shine on my return trip though but to have the problem cured was just brilliant.<br /><br />Many thanks<br /><br />Michael