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CapnSantiago 04-09-2012 02:18 PM

Why does my diesel not rev when in gear?
Don't have a lot of experience w/diesel's but understand if they have fuel and air they'll run. My 1982 volvo/penta MD7B (2 cylinder/13HP) has gotten progressively harder (longer) to start over the last 3 yrs after setting for 1-2 weeks until the weekend use. (Think maybe I've been a bit impatient with it amd maybe have been flooding it trying to start). The fuel seems clean and I've changed filters regularly & used stabilizer additives over the winters and don't believe there's air in the fuel lines. Once started and warmed up it restarts quite easily. Doesn't seem to like to idle though and will sometimes die so either idle is too slow or something else.

Also, the engine will rev with throttle input while in neutral (although sometimes has quite a delay...if it were gas engine I'd say it was flooding/stumbling some with too much throttle input too fast). But sometimes when I put it in gear it will not rev at all with throttle input but just maintain idle (starting to do this more and more). When it does this I can sometimes nurse it to rev in neutral and then keeps the revs up slightly shifting into gear and it will have throttle response after that (try not to do this too much so I don't mess up tranny or shear the prop pin).

My batteries are starting to get a little weak (going on 5-6 yrs old) so it does turn a little slow but don't think that accounts for the lack of throttle response. Am hoping for something simple to get it more trustworthy (fun docking sometimes when the throttle doesn't respond) but may have to bite the bullet and get the mechanic to look at it ($$ouch).

Any usefull help would be appreciated...Thanks.

eherlihy 04-09-2012 02:23 PM

Re: Why does my diesel not rev when in gear?
Dirty/fouled injectors.

rugosa 04-09-2012 02:27 PM

Re: Why does my diesel not rev when in gear?
I'm no diesel expert either, but it sounds like your injectors need cleaning/rebuild/replace. You should be able to get a diesel injector repair shop to deal with it without paying marine prices. It's a part, not a marine part.

5-6 years on the batteries, likely need some new ones too.

olson34 04-09-2012 02:47 PM

Re: Why does my diesel not rev when in gear?
Based on our experience, it's a good idea to remove the injectors and have them tested (and often also go ahead and replace the tips) about every 1000 hours, give or take.
And on the subject of needing "fuel and air" I have found that when our engine has problems with starting or throttling up, it's always the fuel system -- miniscule air leak (vacuum side) around a fitting or at one of the gaskets in my Racor. Tiniest trace of a air leak may cause symptoms like that.


asdf38 04-09-2012 02:47 PM

Why does my diesel not rev when in gear?
I don't know about your starting problem but the revving problem sounds like it must be your shifter/throttle linkage. When it's in gear its not pulling the throttle line.

Faster 04-09-2012 03:08 PM

Re: Why does my diesel not rev when in gear?
Injector overhaul is a good place to start.. figure on $100 per... The stumbling might be a timing issue too, and that's usually an hour or two of good mechanic's time to set straight.

Difficulty starting can be other things... slow cranking, inadequate preheat (if appl) and poor compression. Battery health is one thing, long small gauge wire runs between batteries, panel and starter is another. Keeping a small heat source in the engine area will keep the oil warm and make for easier cranking esp in cool weather.

Not revving in gear is often prop related.. over pitched or really foul with marine growth...

larrytwo 04-09-2012 03:21 PM

Re: Why does my diesel not rev when in gear?
You might get a compression check. Diesels with lost compression, relying on compression to fire are hard starting when cold and just fine when warmed up. Check your tack idle speed to that recomended in your manual and see if they match. Allso as mentioned by asdf38 you may have a linkage problem. start at the engine/gear connections and work to the control lever, Cheers

shogan50 04-09-2012 04:00 PM

Re: Why does my diesel not rev when in gear?
I have much experience with this engine and at least half of your symptoms. Disclosure: in the end, I raised the figurative radiator cap, removed my Volvo and inserted a Beta.

The starting problem and the reving problem may or may not be related. The starting problem is common and is most commonly a symptom of low compression. My MD7a (yours is also 7a if only 13 HP) wouldn't start on its own unless warm without starting fluid on all but the warmest summer days. (yes opinions vary on the advisability of this - not looking to start a thread)

After a $463 injector rebuild, I saw no improvement in performance in any measure. Not saying it applies to your case, just a data point.

The reving problem is likely fuel related. Start by checking your lift pump. Diesels need a way to get the fuel from the tank to the injector pump. They do this with what is called a lift pump, which is similar to a diaphram fuel pump driven from the cam shaft on most gasoline vehicles up to the 80's. To check is, put a hose on the output port going into a containiner and operate the priming lever. If nothing happens it is in your fuel system including possibly the lift pump itself. If you get something, release the compression lever, and crank. See if you get anything now. If not, you've confirmed it is upstream fuel system. If you do get regular pulses of fuel, then it is likely the injector pump, injectors. If it is down stream, try bleeding your injectors.

If up stream, replace fuel filters. Pour the fluid from them (there should be two or more) in a clear container if it isn't already the see through type. Check for water. Cut the filter in half with a pipe cutter or hack saw. Check for slime.

Check that your fuel pick up isn't plugged by above mentioned slime. Buy an outboard priming bulb and try to suck fuel from the tank. Water in the fuel, even if it doesn't make it to the filter, is fertile soil, so to speak for bacteria that eat diesel. That bacteria dies, forms a goo of bacteria carcasses, and plugs things up.

klem 04-09-2012 08:47 PM

Re: Why does my diesel not rev when in gear?
If you could provide some more information on your exhaust color, it would be very helpful. Blue exhaust means that you are burning oil (bad compression), black means you aren't getting enough air (plugged air filter), white is water or timing and grey is poor combustion (a good sign of bad injectors)

As mentioned, the older volvos can get very hard to start with a lack of compression, air in the fuel lines will make it extremely difficult as well. For starters, you need to get the engine cranking at the appropriate speed and it sounds like your batteries are old and worn out. A slow cranking engine will not build as much pressure due to a longer time for the air in the cylinder to leak out and a longer time to loose heat to the cylinder walls. To rule out air in the fuel lines, try pumping the hand primer for a long time before starting the engine on a cold start sometime. If it fires up like it used to, then you have an air leak but if it doesn't show any improvement, then you probably don't have one. Many people insist that they don't have an air leak when they actually do. The air leak can be either on the send or return side and they can be very difficult to find because they are often invisible. If batteries and looking for an air leak don't solve your problem, then I would look into running a compression test and hooking up a fuel pressure gauge.

When an engine fails to rev up in gear, it either isn't making enough power or there is too much load being put on it. The easiest things to check as to whether you are making sufficient power is to make sure that your shutdown cable is properly adjusted and your throttle cable is properly adjusted. You should be getting full travel on the arms of both of these, if you don't, you need to fix it. The next place to look would be fuel pressure if you have a fuel pressure gauge. Fuel pressure can be inadequate due to an air leak in the suction side of your fuel line (this could be related to starting), a bad lift pump, a plugged fuel filter or a bad overflow valve. A pressure gauge at the inlet to the injection pump is really the easiest way to go through this but you can trouble shoot it without it. Finding an air leak would be easiest to find when trying to sort out your starting issue. A bad lift pump will be very difficult to diagnose without a pressure gauge. The fuel filter is cheap and your should change it and cut the old one open if you haven't done so already. To check your overflow valve, try pinching off the fuel return line with a pair of vice grips for a very quick test and see whether things improve. Also, if you haven't looked at your air filter, it would be worth doing.

It is also possible that you are putting too much load on the engine although this seems unlikely if it revs sometimes and it doesn't rev others. Were you overpropped to start with? Is your prop or hull dirty? These are the most obvious culprits but there are plenty of others such as a worn out cutlass bearing.

If I had to take a wild guess based on your symptoms, I would guess that you have an air leak in a fuel line on the suction side of your lift pump which makes the engine hard to start and means that the injectors are not getting a good supply of fuel. Someone who really knows engines can often tell this by sound and exhaust color alone.

Good luck.

steel 04-10-2012 12:49 AM

Re: Why does my diesel not rev when in gear?
Don't worry about flooding diesel engines don't flood. The fuel air mixture doesn't ever prevent combustion in a compression ignition engine.

Sounds like a throttle linkage problem as was said above or a problem with the governor on the engine. Try to find out what moving parts control the governor and see if anything there is getting stuck. Maybe the throttle output of the governor is getting stuck and not going over a certain point.

The starting thing may be related to having the throttle stuck at a low position and not putting out enough fuel when the fuel pressure is low during starting, but then becomes sufficient to run the engine once the injector pump comes up to full pressure. Or it could be the injection system or engine is wearing out.

If it cranks for a long time and then catches and starts suddenly it's more likely low compression. If it cranks for a long time and starts to fire a tiny bit and then a little more and a little more and slowly gets enough power to run without the starter then it's a fuel system problem.

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