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Blocked exhaust elbow - symptoms

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High Time View Drop Down
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Joined: 04 September 2012
Location: Portsmouth UK
Status: Offline
Points: 798
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    Posted: 21 June 2020 at 19:29
Back in early March I started to notice that High Time would not reach max revs in gear (normally around 3000rpm) when tied to the pontoon. I initially thought it could be a badly fouled prop and hoped that we might clear it on the next outing. Not so, in fact the lack of revs was so bad that we couldn't exit Portsmouth harbour against the incoming (Spring) tide. Still believing it could be the prop, plus hull fouling, I booked a lift out for a few weeks later. Then came lockdown. 

During lockdown I still managed to visit the boat a couple of time and run up the engine. On the second visit I started to notice a loud (and distinctly worrying) clatter from the engine even out of gear. At this point I consulted a Volvo (now independent) 'expert' who thought it was 'diesel knock' (pre-ignition) and therefore must be fuel related. 

Several weeks later and several hundred pounds poorer, the tappets had been checked/adjusted, compression tested (all OK), injectors tested and refurbished (although they didn't really need it after only 800 hours running) and the injection pump was tested (OK). Unfortunately the clatter was still as bad as ever. At this point his next suggestion was to lift the head.
 
In parallel with his investigations I had opened a thread on the PBO Forum about the problem and several responses suggested blocked exhaust elbow. Before he lifted the head I therefore suggested that he should check the exhaust elbow. Surprise, it was almost completely blocked with soot and rock-hard white deposit.
 
I have now purchased and installed a stainless steel exhaust elbow (actually cheaper through Keypart (UK) than a standard Volvo replacement) and the engine runs as sweet as it used to.

The moral of this tale is to remember the exhaust elbow if you have a gradual loss of revs/thrust. If you leave it and it blocks completely, the back pressure causes the exhaust gases to remain in the cylinders and the revs to drop. Then, as the throttle is opened further, unburnt diesel is left in the cylinders that pre-ignites on each compression stroke causing the severe clatter known as diesel knock. The oil also very quickly turns black and need changing as soon as the elbow is replaced.  


Edited by High Time - 21 June 2020 at 20:41
Roger

High Time (415 #038)
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