Starting a Diesel After Rebuild

We’ve learned a lot about diesel engines during this rebuild. On one hand, it’s probably good to know more . . . but some of these things I really never felt the need to know.

Such as what to expect when starting a diesel after a rebuild.

Now, if the rebuild was done in a shop, the shop would have “bench started” it and this won’t apply. Or rather, it will all be done out of sight.

Our rebuild, however, was done in the cockpit and the first start was done right in the boat. What we didn’t know was to expect it to be hard to start the first time. Very hard. To be honest, we were convinced that it wasn’t going to start and all the time and money was for naught.

To be fair, our mechanic (and friend and Gemini owner) kept telling us that it was going to start. Only as it got close to starting did we understand what was going on.

To back up a bit — and I’m keeping this somewhat simple for people like me who don’t know a heck of a lot about internal combustion engines — a diesel runs when the atomized fuel in the cylinder reaches a temperature of about 600 degrees F. To do this on our Westerbeke, the fuel is heated up a bit by a glow plug, while the air is compressed by the piston in the cylinder. As air compresses, its temperature increases — to get it to 600-ish degrees, the pressure needs to be somewhere over 300 psi (the exact numbers depends on the exact diesel fuel, outside air temp, etc.).  The injectors squirt the warm fuel into this hot air and it ignites. Diesel fuel combusts solely due to the temperature (there’s no spark as in gas engines) and thus if there is insufficient pressure to get the temperature rise, it just won’t fire.

To get the necessary pressure for the temperature rise, the pistons are pushed into the cylinders — the shrinking space in the cylinder is the combustion chamber where the fuel is sprayed by the injector. The pistons have to fit very tightly into the cylinders or else the air already in the cylinder will just escape and there won’t be enough pressure to raise the temperature to point where the fuel will ignite. That’s where the “rings” come in (part of what was replaced during the rebuild). There are grooves that go around the piston and the rings fit into them and against the cylinder wall to seal off the combustion chamber so that the pressure won’t just escape.

Brand new rings are said not to be “seated” — that is, not 100% sealed against the piston and cylinder wall. It takes running a diesel some to get everything hot for the parts to mate completely.

And here’s the rub — until the rings are seated, the engine won’t have sufficient compression to start. Sort of a Catch-22, huh? The engine won’t start until the rings are seated, and the rings won’t seat until the engine has run.

The solution: keep trying to start it (in other words, pseudo-run it), using the attempts to slowly seat the rings.

When a rebuild is done in a shop, you never see this part. They typically use a large starter and power supply and keep cranking until there’s compression and the engine fires. Then they let it run a while to fully seat the rings (and probably do some final adjusting on valves and other things) and when the engine is put back into the boat, it just starts.

But for a cockpit rebuild, there’s no shop with a big starter and unlimited power supply. There’s the engine’s own starter and the boat’s batteries. You can’t turn it over forever at one time as you don’t want to burn out the starter . . . and the batteries won’t allow this, anyways.

I know, under normal conditions you’re told not to just keep trying to start an engine. “If it won’t start, figure out what’s wrong and don’t try again until it’s fixed.” How many times have I heard some variation on that? Well, in this one not-normal case, the attempts to start the engine are “fixing” the problem.

With our rebuild, it took several days of trying to start the engine before it fired. At first, the starter was the limiting factor — we cranked less than 30 seconds at a time, but then had to let the starter cool. We watched as compression slowly rose from about 100 psi in each of the three cylinders to 200 (normal is 398). Here’s how it sounded . . . time after time. NOTE: The first two videos are sideways, but there isn’t really anything to see, just hear, so don’t worry.

Okay, good, an increase in pressure is a step in the right direction. The mechanic felt that we weren’t spinning the engine fast enough to build up the heat needed to seat the rings further. We’d been running the generator to give the batteries a bit extra oomph (and to recharge after each effort), but it just wasn’t enough. A load test revealed that one of our three batteries was nearing the end of its life, so we replaced it (we have a single battery bank; the engine is small and does not require a dedicated starting battery).

Now we could crank longer and faster. You couldn’t see it, but you could very faintly smell a bit of exhaust on each try. And occasionally the engine would cough. A bit of oil was added to each cylinder to seal up the minute gap between the ring and the cylinder wall and the compression climbed again. Finally, you could feel the engine heating up as the start attempts continued. And then finally . . .

Okay, it’s running . . . but that sounded pretty rough starting. After letting it run maybe 5 minutes and then restarting it, it started right up.

We let it run about 30 minutes, then changed the oil. This is really important with a freshly rebuilt engine as there may be metal shavings in the engine — and again it’s something that you don’t have to worry about with a shop rebuild as they should take care of it.

Starting up again the next morning (with our mechanic present, thankfully), the engine started vibrating badly and the alternator belt squealed as well. Again, the mechanic expected this as the valves needed to be adjusted as they, too, seated themselves. The alternator had simply loosened slightly due to the vibrations and it just needed tightening.

Again, I want to repeat that if you have an engine removed and rebuilt in a shop where it can be started, you shouldn’t face these problems. But if you — like we — have a cockpit rebuild, it’s important to know that hard starting isn’t a sign that the rebuild was bad. We didn’t know what a bear it would be to start the first time, and really stressed out as it didn’t happen immediately. Knowing that it’s normal brings the anxiety level down several notches.

Barefoot Gal starts easily now, and just purrs!

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  • Diana K Weigel
    Posted at 08 February 2015 Reply

    I would not have know that either. Thanks for the info.

    • The Boat Galley
      Posted at 08 February 2015 Reply

      I found a few mentions on the internet about diesels being hard to start after a rebuild, but only a tiny bit on the why. So wanted to put it out there for others facing the same questions. Hope you never need the info!

      • Candy Williams
        Posted at 14 February 2015 Reply

        Thanks soooooo much for this info! I didn’t know this either!!

  • Connie Smith
    Posted at 08 February 2015 Reply

    So happy for you that all is well now!

  • Ed Robinson
    Posted at 08 February 2015 Reply

    Very informative. Thanks.

  • Nicki Reineck
    Posted at 08 February 2015 Reply


  • Mike Boyd
    Posted at 08 February 2015 Reply

    Yea! Glad you got it going. Hadn’t really thought about it, but what you describe totally makes sense.

    A question for you, though. What did you do to prevent hydrolocking (filling the muffler up with water and then having it back up through the exhaust into the engine)? Did you shut off the raw water and pull the impeller until you thought it was close to starting?

    Hopefully won’t have to do this for a long time myself, but always good to have the knowledge.

    Again, congratulations!


    • Carolyn Shearlock
      Posted at 08 February 2015 Reply

      Our engine is a hi-line, which means it sits above the waterline, so we didn’t have to worry about that. Otherwise, the mechanic said that the procedure is to close the seacock but have it accessible so you can open it immediately after the engine starts (a few seconds, maybe up to a minute, of running dry is okay I’m told — but remember I’m not an expert!)

  • Michelle Rene
    Posted at 08 February 2015 Reply

    Terrific write up!

  • ChrisW
    Posted at 08 February 2015 Reply

    Very useful post!

  • Colin Mombourquette
    Posted at 08 February 2015 Reply

    Excellent information; excellent write up.

  • John
    Posted at 08 February 2015 Reply

    Great write up. Thanks for the info. Best of Luck now

  • Dave Skolnick (S/V Auspicious)
    Posted at 08 February 2015 Reply

    Hi Carolyn, I’m glad you are running again.

    I’m a little confused. I’ve never had a problem starting an engine, diesel or gas, after a rebuild. Did the cylinders get honed progressing from course to fine and running fine hones in both directions? Were the rings buffed a bit (bronze wool is plenty) before the cylinders were installed again? A hone is not terribly expensive in the grand scheme of things and can be run in a handheld drill motor. You would want one with interchangeable hones or three fixed ones. Oil the cylinders before inserting the cylinders (as you would for all bearing surfaces) and a spritz of oil into the cylinder before installing the injectors. It should start up in one or two tries.

    • Carolyn Shearlock
      Posted at 08 February 2015 Reply

      Gas is no problem after a rebuild because of the spark plugs causing the ignition. Glad you haven’t had a problem with the diesel — yes, we (with our mechanic) did the cylinder hone, oil, etc. It was still tough.

  • Bruce Comeau
    Posted at 08 February 2015 Reply

    Glad it was not a hand crank. LOL

  • Dan
    Posted at 10 March 2015 Reply

    I have a Bachelors in Diesel Science and currently work for and have worked with several industry leaders in diesel engines. I have rebuild or been involved in 10,000’s of diesels being rebuild both in a shop and (in the field, mostly due to the size) and I have never had this starting issue you describe occur once. Most common issue is air in the fuel thus causing a hard start. I can ensure you if the work was done correctly and the engine doesn’t have a issue it will NOT take more than a few start attempts.

    Based on your last post on symptoms that I read a little while ago:

    -I would ensure the fuel system was looked at closely especially the injectors, fuel pump and timing.
    -Have exhaust back pressure checked and the rise on it inspected.
    -Ensure you operate your engine at rated RPM, I can send you the ranges if needed for your engine.
    -Take oil and fuel samples at least once per year.
    -Use high grade oil and filters (this is often over looked and will make a big difference on a marine diesel). Don’t be afraid to use a good additive.


    • Carolyn Shearlock
      Posted at 10 March 2015 Reply

      Thanks Dan. The injectors were replaced. Timing is by gear and Westerbeke tells us there is nothing to adjust. Exhaust has been inspected by two different mechanics. We know the rated RPM and cruising range and do operate it there (of course, we don’t know what prior owners did).

      The engine now starts right up and sounds great. It was just tough getting that first start. But once it did, it’s first time, every time.

  • Dan
    Posted at 10 March 2015 Reply

    There is timing via a gear and there is also fuel timing which for a W30 engine is 19 degrees + or – 1 degree BTDC Static.

    Cruise RPM should be 2500 to 3000 RPM and you should see 1.2 to 1.6 GPH.

    Also the other thing I forgot to mention in my last post is ensuring that the engine makes rated RPM (3600RPM) at full throttle when the boat is loaded down with all supplies, full tanks and spares, i.e. its not over propped – Nothing will kill a engine quicker than being over propped. If you do any of the things I suggested this should be number one (its free to check) on your list. If the engine is over propped it needs corrected before the boat is taken anywhere as this will make certain your rebuild will not last as long as your new engine did before failing again. If you find it over propped which I suspect you might you will need to send your prop in to have the pitch adjusted to gain whatever RPM is needed. If it is really far off or you have a damaged prop you might have to buy a new one.

    I am not trying to scare you, just don’t want you to end up with a engine that needs rebuild again in 300-400 hours. I have seen over propped engines on large boats and small last as little as a 100 hour due to this, its a big deal.


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