Volvo AQ171 Performance Problem

[matto]

Hi Folks, I have recently aquired a Carver Montego 27' with twin Volvo Penta AQ171 on 290 legs. One of the engines refuses to pull beyond about 4000 RPM. It can just about get to 4500 with the other engine screaming at 6000RPM! I have cleaned the carbs, replaced the spark plugs, new fuel filters, oil and oil filters and I have checked that the choke is not stuck on. Despite many hours in the engine compartment I still cant get the engine to pull under load. It revs fine in neutral but underway it lacks power. Any suggestions would be greatfully received! Thanks in advance - Matt

 

[DepSol]

Have you checked the props are ok??

Dom

I am boating again ;-)

 

[tr7v8]

Have you done a compression test or a cylinder leak down test?
These will show overall engine condition and possibly the next steps to
take. The other thing is do the carbs open completely on that engine?
As a follow on from the bit about props, they are both the same and the one on the low revving engine has no damage.

Jim
----

 

[matto]

Thanks for the advice. The carbs both open correctly and I have syncronised and checked them fully (as far as I can) I put new props on the boat at 14x19 (from the original 16x15) the idea being to get better top end speed. I will take your advice and get a compression tester and try that this weekend. I had heard that these engines were prone to valve failure but there does not appear to be any rattling or clanking as it sounds very smooth. The only other minor points are that the engine oil pressure gauge goes well over the 100 mark and the engine runs on slightly after switch off. - Ah the joys of boats! Thanks again for your advice - will let you know. Matt

 

[DepSol]

These engines were only prone to valve failure because of salt water dripping on the pulleys and rusting them causing the belts to fail.

Dom

I am boating again ;-)

 

[matto]

Hi Dom - Yes the props are OK as far as I know. The boat was origionally used on a river and had 16x15 props. I have just changed them for new Volvo high speed props at 14x19. Both props are new and I figured that if one engine can pull at up to 6000RPM then the other should be able to do the same with the same (but opposite handed prop) Maybe life is not as simpe as that?

Thanks - Matt

 

[summerwind]

The engine running on after switch off might not be a minor point. Is it running on or running backwards?

This is usually caused by a build up of carbon in the compression chambers burning after the electric ignition is switched off, so keeping the engine running.

The high oil pressure may be a result of a sticking oil pressure relief valve not able to get all the way back in its housing. This again can be caused by a carbon build up in the oilways. This could be a worse problem if the valve sticks open rather than not being able to open properly as you will then have insufficent pressure for your big ends and main bearings and knock them out. A clue to how much carbon is in the engine would be the state of the spark plugs that you removed. Other than that you will have to whip the head off!!!

As well as doing the compression test, I would be inclined to check the ignition timing. This could cause your symptoms.

Finally, for this post, I would be inclined to check your running gear. How free to turn is the gear the gear box and legs?

Best of luck. Let us know what you find.

 

[matto]

Yes it is running backwards, my first reaction was the timing but its not adjustable on this engine. I checked it anyway and it is OK and the advance is working correctly at higher revs. The boat was used for the last 3 years on the river - you know the score 6 kns max speed, little use with old petrol, probable carbon buildup. When I changed the plugs they looked ok but then for all I know they might have been changed a week before I bought the boat.

Assuming there is carbon in there is it worth tying one of those decarbonizing fuel additives? Or is that a waste of £ and I should just bite the bullet and take the head off?

I will check the plugs again as I gave the boat a good blasting last night and invest in a compression tester. Then I will check out the running gear. Is it a case of seeing if the prop can turn freely when in neutral?

Thanks very much for your help, all good advice and very much appreciated. I will let you know the outcome after tonights efforts and another go at the weekend.

Cheers - Matt

 

[tico]

I had the same on my sunfury with twin 171's. Neither would pull more than 4500rpm at times. I think that the root cause is that they are so highly tuned that they are very sensitive to anything including fuel quality, valve timing etc that once anything is a little out they dont pull properly.
Eventually after getting fed up with them, i replaced them with new diesels, but it's not a job for the faint hearted.

PS 14x19 props are correct, do you want a spare set??

Beware the timing belt.... if it slips, you get the valves hitting the pistons.

PS Dont consider taking the head off unless as a last resort!! I speak from experience!!! You will end up cursing Volvo designers from now until next year.
PM me if you want any help

 

[DepSol]

Soltron will certainly help with clearing your carbon build up and it will clean up that old fuel you have and make it more combustable. At the thought of spending £30 to see if that works it is a much cheaper option than getting an engineer down or buying any other parts. I know my AQ171 went much better using Soltron than not.

Dom

I am boating again ;-)

 

[summerwind]

As the engine runs backwards, your problem is almost certainly build up of carbon. Try the compression test to ensure you have good sealing in all cylinders. That being so, use the Soltron or other fuel additive to attempt de-carbonisation. You might be lucky and get rid of enough to have your engine start to run properly. Once you've got it running somewhere like normal, give it a long, hard workout. That should blow out some more of the deposits, but you won't get rid of it all and sooner or later you will have to have the head off.

Don't forget the oil pressure relief valve!

Good luck.

 

[DepSol]

I am willing to bet that with constant use of Soltron it will clean it all up and there wont be the need for the head to come off to get rid off the carbon.

Fancy taking me up on it.?

Dom

I am boating again ;-)

 

[matto]

Hi - Back again after a few more hours in the bilge! I just checked the compression and all cylinders are fine or at least within a gnat's whisker of each other. A dose of de-carbonising STP treatment and three hours of running the boat at about 2500 RPM. I also swapped the carbs from one engine to the other and the fault is not with the carbs as the poor performance stays on the starboard engine.

Still progressing with the other pointers that people have kindly posted.

Regarding the oil pressure relief valve - where is it and is it easy to check / clean / replace? Also how can I check to see if the gearbox is acting as a brake instead of a gearbox? Is it just a case of turning the prop by hand with the engine in neutral?

I know - a thousand questions but any advice is appreciated

Thanks again - Matt

 

[matto]

Hi!

Yes you are absolutely correct. After about an hour of running the engines they seemed much smoother and the run on was only very brief. I still have a few gallons of fuel left in the tank with it in as I only managed 3 hours yesterday on the water.

The thing still lacks the power to pull much above 3500 RPM so I am progressing with some more ideas from this post. More hours in the engine bay to come me thinks!

Cheers - Matt

 

[chippie]

I know you have checked the (ignition) timing; is it feasible to swap one ignition unit for the other as you did with the carbs. It could be a problem in the unit rather than just ignition timing.
Has any work been done on the engine that could have upset the valve timing?

Good luck with the search for the problem.

 

[matto]

Hi,

Valve timing might be an idea as it looks like someone has had the head off at some point in the past. I am running out of ideas now so it may well be that. Even the local Volvo Penta service center is stumped :-(
The list so far...

Checked fuel lines – OK
Changed Oil and Filters – OK
Changed Fuel Filter – OK
Stripped cleaned and syncronised carbs – OK
Swapped carbs from port to starboard engines – Still the same
Checked auto choke is going off - OK
Changed spark plus – OK
Checked plugs after a run – good colour all even
Checked ignition timing – Non adjustable but advancing correctly
Replaced Distributor and rotor arm – OK
No coolant loss from self contained coolant.
No serious oil leaks
Compression test – All OK
Checked inlet manifold for leaks – OK with vacuum gauge
Checked timing belt – old and needs replacing but seems aligned with the correct marks.
Fuel quality is good – Shell Optimax
Used a decarboniser STP to clean valves etc


Matt - Puzzled of Devon

 

[BarryH]

I know it sounds silly, but have you cheched the fuel pump for correct delivery pressure, or a blockage in the exuast system thru to the out drive. The thing is obviously being resticted somewhere.

OK, to hell with it. Unbolt it and we'll use it as an anchor!

 

[BarryH]

Just had another thought. You say swapping carbs had no effect. Have you checked the throttle cables at the lever end. Normally the contols have 2 attachment points for the different travel requirements. If its on the wrong one then maybe its just the cable isn't opening up the throttles fully. As I said just a thought.

OK, to hell with it. Unbolt it and we'll use it as an anchor!

 

[chippie]

Thats a pretty comprehensive list of things you have checked on the engine.
have you eliminated the driveline from your queries?
With each gearbox in neutral is it possible to compare the effort required to turn the props? Perhaps even compare the effort to turn the engines over in gear with the plugs out (if possible) to see if it is an internal gearbox problem. I'm afraid I am shooting in the dark a bit now.

Good luck with your continuing efforts.

 

[castaway]

Mat.. Something to add to yr list. Check that the exhausts are clear.

Regds Nick