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  Topic Review (Newest First)
11-12-2012 06:49 PM
Re: Why does my diesel not rev when in gear?

Thanks for the followup....
11-12-2012 06:10 PM
Re: Why does my diesel not rev when in gear?

While I've not solved everything completely it is working much better. Turns out there were several contributing issues and I've sorted most of them out.

The local mechanic noted that the exhaust manifold had a short distance of slightly upturned section before it sweeped down to the water muffler...this was apparenatly allowing condensation (with some rust from blackiron exhaust) to trickle back down into the engine thru the valves. The manifold was removed cut apart and rewelded to have the proper angle down...apparently this is a critical issue to keep spent exhaust water from getting back to your valves/engine.

The mechanic also noted the engine was idling too slow and increasing the idle to the proper RPM fixed the engine rev issue immediately.

I completely drained the tank and put in fresh fuel...the diesel in the bottom was quite reddish/brown and smelled bad.

There's apparently a small air leak in my fuel delivery system which given enough time (a week or so) will make the engine a bit difficult to start...thankfully cranking the engine eventually expells this air and when fuel hits it starts.

To clean up the valves/engine I added some Marvel Mystery Oil to both the fuel and the crankcase. After getting the engine started with this in it there was some nasty stuff coming from the exhaust that over a period of time cleared up. The engine gradually ran smoother as this ran thru it and it became easier to start (if this hadn't worked a valve job would be in order).

I still have the air leak issue so starting after setting a week still takes a bit until it flushes and the fuel more or less readily restarts throught the weekend.

Hope this helps...
11-12-2012 05:43 PM
Re: Why does my diesel not rev when in gear?

Oh and I did sail it for a few hours. It sailed fine which seems to eliminate the bottom, but not the prop.
11-12-2012 05:39 PM
Re: Why does my diesel not rev when in gear?

OBTW, I have the MD7A of the 13 hp flavor, not the 7B of the OP. There are no glow plugs on this engine.

Thanks for the reminder regarding fouled prop. I missed that on my todo. The boat was out of the water last winter and bottom painted this past spring. I'll still take a look though.
11-12-2012 05:30 PM
Re: Why does my diesel not rev when in gear?

The first step is still the same : have you eliminated the cleanliness of the propellor and bottom as the cause of the low boat speed?

Secondly hard starting is often the glowplugs.
11-12-2012 05:24 PM
Re: Why does my diesel not rev when in gear?

Reviving thread from the dead because I have same issue and found this thread to have lots of useful info.

Problem Statement: 1) Hard starting. Crank about 2-5 minutes. The engine will sound like it's trying to start, cranking sound will increase in pitch, engine slowly chugs to life and I can let off the starter. Once warm, engine starts right up again.

2) Idles fine. Revs up to max on tac with no load (and throws off alternator belt). Under load, I get a max of about 1500 RPM with boat speed on GPS of about 3.7 knots. I have either a light white or grey exhaust, can't tell which.

Additional info: A PO removed the fixed tank and installed bladder tank in locker. The feed from this tank includes an electric fuel pump which is switched on before starting (to overcome a faulty fuel pump elsewhere?)

I spent a lot of effort trying to get cooling water through the engine. The reverse gear water jacket was completely plugged up and required acid solution to get water flowing again. Although I eventually got a good water flow, the exhaust elbow could still be an issue as mentioned in thread.

So my todo list appears to be changing fuel filters, checking fuel quality, fuel flow, and exhaust problems.

Did I miss anything?
04-12-2012 11:51 PM
Re: Why does my diesel not rev when in gear?

This has to be a fuel system / throttle control problem.

When you said that you can put it in neutral and get it to rev up a little and then put it back in gear and it stays revved up whereas you can't put it in gear first and and then rev it up is the biggest piece of evidence that suggests there is some throttle / governor problem.

There's no need to check compression or starter speed or anything. It's not getting fuel! When starting it is taking a very long time to build up enough fuel at cranking speeds to do anything.

I think the governor may be built in with the injector pump, which is probably where all the problems are.
04-11-2012 06:25 PM
Re: Why does my diesel not rev when in gear?

Perhaps your tank vent is plugged. Try it with the fuel filler cap removed. Probably too easy of a fix, but easy to do.

Paul T
04-11-2012 05:34 PM
Re: Why does my diesel not rev when in gear?

That is an unbelievable amount of cranking.

Since you are not jumping at the idea of hiring a mechanic, I assume that you are cost sensitive. If this is the case, start with the stuff that you can easily do yourself for very little in parts costs. Check the throttle linkage and shutdown linkage again. Change your fuel filter and cut the old one open. Check your air filter. Check your exhaust hose visually by simply taking it off. I would also recommend cracking your fuel bleeder while cranking and seeing whether you get solid fuel or air and fuel.

Once you get through all of the visual checks, you want to make sure to start by doing tests that will tell you the most about the most likely culprits. Since you are not seeing blue smoke, a compression test would not be the first thing that I tried. The first thing that I would do is to check your fuel pressure. You can do this easily yourself with a gauge or you can get a mechanic to do it. If your fuel pressure is low (meaning <15psig), then you need to fix it before you do anything else. Low fuel pressure will cause many of the symptoms that you describe because the injectors fail to pop off. Next, I would check compression, again you either need a gauge or a mechanic. After that, I would think about pulling the injectors and testing them.
04-11-2012 12:13 PM
Re: Why does my diesel not rev when in gear?


Considering how many possibilities there are, and your lack of experience, my suggestion is to find a mechanic.

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