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Old 04-05-2008, 12:21   #1
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Engine Won't Rev in Gear

I just rebuilt my volvo penta md7a. It runs great in neutral but does have some white smoke. It rev good but in gear it will only go to 1200 rpm and has some white smoke. Any ideas would be appreciated.
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Old 04-05-2008, 22:37   #2
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What was the extent of the rebuild?

Did you futz with the injectors?
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Old 04-05-2008, 23:58   #3
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Does it sound smooth or ruff like a cylinder is not firing?? Have you double checked the tappet/valve clearance? Sounds like a cylinder is not firing, but reasons why are many.
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Old 05-05-2008, 02:31   #4
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Exhaust, filter...

Are your exhaust or fuel filter or air filter partially blocked?
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Old 05-05-2008, 07:34   #5
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engine won't rev

well I replaced the rings, surfaced the head ground the valve seats, replaced the valves, new valve seals. I also noticed that it seems fuel may be coming out the exhaust unburned. I can see a sheen on the water. It revs to 2600 out of gear but like 1200 in gear. I repaced the fuel filter but not the fuel water seperator filter.I had the injectors rebuilt as well.
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Old 05-05-2008, 19:20   #6
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Sounds like an injector is not working well.

Or air in an injector line

Or a valve problem.
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Old 05-05-2008, 20:07   #7
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or a cylinder is egged out and a hone and rering was not enough. What are the piston to wall clearances?
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Old 06-05-2008, 01:52   #8
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You can help narrow down your problem by checking a couple of areas. First, do you have severe blow by if you remove the oil filler cap on top of the engine. That will be a sure sign pressure is seeping past the rings to the sump.
Try cracking the pipe to the each injector and allowing the fuel to bleed. This causes the injector to not biuld enough pressure and won't fire in the fuel dose. If the cylinder has been firing, it will drop out and you will notice the engine will run (at idle) a little ruffer. The cylinder that is not firing at all will make no change when you crack the injector fuel line. That will at least narrow down what cylinder/s is causing the issue. Then you can check the injector and valve clearance. Tappet clearance can be an easy mistake.
If you suspect and injector, swap over another cylinders and see if the miss now swaps too.
Apart from that, you will have to run a compression test and see what the pressures are. A leakdown test may help pinpoint where the pressure is going. You will hear the escaping air going to the place of suspect.
White smoke is a sign that the cylinder is not firing but fuel is entering. So a bleeding problem should not be the issue. One of the worst case scenarios is that the head gasket has not sealed or damaged and you are getting water in a cylinder. It doesn't take much to stop the cylinder from firing. But normally shows it's ugly head when you go to turn the engine over a few days later and it locks solid.
That should give you some starters anyway.
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Old 06-05-2008, 07:36   #9
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engine won't rev

Thanks for all the info. I will again be working on it this weekend.This is only a 2 cylinder and seems to run like a sewing machine . That is what throws me. I may have the valves not ajusted correctly. I think you turn the flwheel till exhaust and intake valve rock the turn one full turn to top dead center and adjust both intake and exhaust valve on that cylinder. Does that sound correct? Thanks n8
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Old 06-05-2008, 15:26   #10
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The easiest is to turn the engine through till the cylinder reaches TDC of compression stroke. Both valves will be fully closed. However, cams can bet cut slightly differently between makes and models and it always pays to read the manual for the manufacturers advice on the position.
Quote:
It revs to 2600 out of gear but like 1200 in gear.
Is 2600 Max RPM for this engine??
Check timing, that could also be a possible, mostly if it is firing slightly too late.
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Old 06-05-2008, 22:45   #11
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I am not familiar with the particular model engine so this may be a load of rubbish that I say.

But if you are unsure as to setting up the valve clearances are you confident that you set up the timing as between the crankshaft, camshaft and injection pump correctly. The fact that you say that the engine runs smoothly indicates this is likely not the problem but I mention it just in case as it can cause improper combustion and so power loss and smoking.

However, I'm no diesel mechanic.
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Old 07-05-2008, 09:38   #12
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Prop Pitch?

One more check for your troubleshooting list...If you are getting good performance in nuetral, but can't get her to rev under a load, you may have a problem with your propeller pitch. Do you have a variable pitch propellor, or have you done anything recently to change the prop pitch? If you are pushing too much water with the prop (high prop pitch angle), you will not get the engine to rev very high, and you will see black smoke and fuel coming out of your exhaust, as the struggling motor is getting more fuel than it can burn. I ran into this last month, and was relieved that my engine was fine, but p@*##ed off that I had to pull my boat out again.
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Old 07-05-2008, 18:52   #13
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engine won't re

I put a new prop on last year but it ran fine.I had a diver put mine on though.I thank everyone for there idea and advice. I have been scratching my head for a whille now.
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Old 16-06-2009, 05:06   #14
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Hi, I have exactly the same problem with a Volvo MD7b. Engine has not been worked on, ran OK last outing. Removed intake silencer, replaced and bled fuel filter and cleaned strainer on lift pump and dismantled and inspected exhaust; clean!
I suspect a problem with a feathering prop(maybe damaged or heavily fouled) or possibly old fuel (3years) ?
What was the outcome of your investigations, Cheers Doug
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Old 16-06-2009, 08:07   #15
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Sounds like timing. When in gear, same revs but more smoke with increase in throttle? Also check firing sequence ( may be irrelevant to diesels, my experience with gas engines).
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