Volvo Penta 151AQ


Old 09-06-04, 07:51 AM
Carpe Deum
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Angry Volvo Penta 151AQ

I have recently replaced the cylinder head and exhaust manifold on my 1992 Volvo Penta 151AQ engine. Due to an inherent fault which the manufacturer subsequently produced a modification, the number four chamber couldnt take the pressure and eventually wore out. The symptoms which alerted me to this were that, under load, the engine would after a warm up period, die back with a dramatic loss of power. Having rebuilt it, I was extremely disappointed to find that on taking it out to bed in the new engine, the same thing happened again!. Although the cylinder head and manifold badly needed replacing, it now seems that its not the only fault. I have changed the plugs, points and leads, have replaced the timing belt and put on new filters and am now thinking that it might be the petrol lift pump which might be the culprit. Can anyone throw any light on this for me. Id appreciate any suggestions especially from someone who has experienced similar problems with their engines.
Thanks in anticipation
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Old 09-06-04, 04:06 PM
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Hello: Carpe Deum. Welcome to our Do It Yourself Web Site and the Boats, Pwc's & Navaigations forum topic.

Have you done a compression check?

Not likely a totally rebuilt engine would drop one of four cyclinders. Drop meaning loss of compression. An engine with only new or rebuilt head can experience a power loss to only one cyclinder. If the existing lower end remained the same. Compression check required. # 4 cyclinder may have internal problem(s).

Not likely a fuel related problem. Would likely effect all cyclinders.

You mentioned "not the only fault." What other problem(s) exist?
Or is it the same problem? Power loss only. Confused.
Kindly explain in more present engine running condition details.
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Old 09-07-04, 02:22 AM
Carpe Deum
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Angry Volvo Penta 151AQ

Hi Sharp

Thanks for the reply - much appreciated. I will try to explain better without being longwinded.
My boat has two 151AQ's and last season they ran beautifully. At start of this season I decided to replace the timing belts, points and filters as well as a full service. I took her out to test the engines and the starboard one blew out an enormouse amount of blue smoke - twice. I came back in to examine it and found nothing untoward but over the next 3 - 4 weeks this ticking noise (exhaust manifold) got louder and louder and I was also now experiencing a loss of power after cruising for 5 - 10 mins with the rev counter dropping dramatically from 4000 down to 1000 and swinging wildly in between. The boat was also now not planing up and at full power, she had gone from a boat which could do 20 knots last season to one struggling to break the 10 knots barrier. On inspecting again, I found a hole in the manifold which had been doctored previously and there was also a crack in the cylinder head which had been badly welded. It was at this point I decided to go for a new manifold and cylinder head, which came as a complete top end.
Now having replaced both, I am still experiencing the same loss of power albeit at a greater speed of up to 17 knots but once it starts to drop away, this continues even after dropping down to a lower speed. This happens in surges which feel like the engine is just doing its own thing - one minute slowing down and the next racing away. In neutral it sounds great and firing on all four cylinders but after a short distance on full power it just dies back.
So this is why I mention "other" problems. Could it be the fuel pump? Could it be water in the filter. (There was a high percentage of water in the filter when inspected at the weekend).
I have tried the following already: changed all the leads and distributor cap (no difference) - swopped coils (no difference) - changed points (no difference) - tearing hair out (no difference)
Your suggestions will be very welcome - regards
Carpe Deum

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