A friend recently described a problem that they were having with the marine diesel engine in their Elan 37, the symptoms he described were that their Volvo Penta won’t rev, it would struggle to reach 2000 rpm under load but it would freely rev without load, he also said that he was concerned with the black staining on the hull in the vicinity of the exhaust, he attributed this to the black smoke emanating from the exhaust when the engine was under load.

Most of the research he had done implied that the problem was with:
Dirty or blocked fuel filter(s).
Malfunctioning injectors.
Water of other contaminant in the fuel.
Even a dirty prop.
Or any number of other causes.

It occurred to me that I had seen this before on other boats, including the Volvo Penta D1-30 in G-whizz, where it would rev freely in neutral but not in gear. The problem there was a blockage in the elbow at the back of the exhaust manifold where raw water is mixed with the exhaust gases. The offending component is circled in the accompanying picture.

The mechanic who rectified the problem on G-whizz where it’s Volvo Penta would not rev by replacing the elbow explained to me that the problem is a build up of carbon inside the elbow restricting the flow of exhaust gasses negatively affecting the engine’s performance.

He went onto explain that he sees a marine diesel that won’t rev quite regularly, not only Volvo Pentas  but also other engine makes, especially on boats that are raced locally; where the boat is cruised or used for off-shore racing the problem is more with corrosion than with a build up of carbon. His conclusion to this inability for a Volvo Penta to rev under load was that the locally raced boat’s engines are run for a short time to leave the mooring, get to the start area and then turned off, then again started post race for a short time to return to the mooring. Conversely boats that are raced off-shore or cruised have engines that are started and then run for a considerable time, allowing for all components to get to proper operating temperatures reducing the potential for carbon build up. Going on to say that this issue is not unique to a Volvo Penta that won’t rev but also prevalent in Yanmars as well as other engines that have a mixing (Raw water and exhaust gas) elbow.

The conclusion to all this was that our friend had the exhaust elbow replaced and their problem was solved, he said that the mechanic that did the job told him that it was one of many that he had done recently, it would appear that this issue is well known amongst those that fix boats, less known amongst those that own boats. If your Volvo Penta won’t Rev this could be the simple, if not exactly an inexpensive solution.