# Why does my diesel not rev when in gear?



## CapnSantiago (Jul 17, 2007)

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.


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## eherlihy (Jan 2, 2007)

Dirty/fouled injectors.


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## rugosa (Aug 30, 2011)

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.


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## olson34 (Oct 13, 2000)

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.

LB


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## asdf38 (Jul 7, 2010)

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.


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## Faster (Sep 13, 2005)

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...


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## larrytwo (Aug 31, 2011)

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


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## shogan50 (Jan 2, 2011)

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.


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## klem (Oct 16, 2009)

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.


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## steel (Sep 1, 2010)

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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## mitiempo (Sep 19, 2008)

One item nobody has mentioned is the exhaust elbow. If a diesel will rev without load but not with load a clogged exhaust elbow is one of the most common causes. It is also the least expensive fix. Diesels need clean fuel, compression, and air - both in and out. A clogged elbow will restrict the revs as much as a clogged air intake and over a few years it is a common place for build up crud to cause a problem.

A barnacle covered prop will do the same.

As far as starting the batteries probably contribute to the problem, and may along with too small wiring and a bit of corrosion on connections be the bulk of the problem. I re-wired my Yanmar YSE-8 with new larger cables - 1/0 vs the original 4 awg - and my starting time went from about 15 seconds or more to about 5 seconds.


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## shogan50 (Jan 2, 2011)

I forgot to mention one other thing that sometimes contributes to not reving - plugged exhaust. I've not experienced it, but a friend suffered it last summer. It would restrict air flow effectively causing a super rich condition, which would be accompanied by black smoke or at least a very distinct diesel odor.


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## INMA (Sep 13, 2011)

Check the tappet adjustment. Comments on blocked exhaust are relevant.

Also small engines idling for most of their life cause rings fouling.

If you have a clear exhaust and tappets adjusted, run the engine flatout tied to a dock for 8 hours to burn the rubbish out of the cylinder piston and rings. Cheapest service ever is to run flatout for a day and burn the rubbish out of the engine. 

After the hot run give it an oil change.

If it won't run flatout for 8 hours tied to the dock then its not seaworthy.

Try to do it when the marina is empty, not on a Sunday afternoon.


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## GMFL (Jun 9, 2010)

INMA said:


> If it won't run flatout for 8 hours tied to the dock then its not seaworthy.


Sorry, gonna have to disagree with that statement, my sailboat has an AUXILIARY engine on it.

It might be more accurate to say that a Captain that can't sail his boat (without using the engine) in and out of a harbor/slip/mooring isn't seaworthy. Though I'd disagree with that statement as well.

Maybe you wanted to say the engine wouldn't be reliable, IDK.


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## CapnSantiago (Jul 17, 2007)

Thanks for all the good input...following is additional info per various above suggestions:

I'll checkout the injectors (though not sure what they're supposed to look like dirty or clean)...
Yes, new batteries are in order although I'm not sure this has anything to do with the lack of reving in gear (and check the wire gauge/connections)...
I've checked for air leaks but not detected any (maybe they're too elusive for me)...
I'm on lake Texoma (in Texas) so no engine preheat...
Had the same engine/propr setup for the 6 yrs I've owned it so don't think that would be the problem...
I'll have to get the marina to do the compression check but it's probably worth doing...
I'll recheck the linkage...
It is am MD7B as stamped on the engine (maybe I'm wrong about the 13hp?)...
I've long suspected the tank could stand to be emptied/cleaned and filled with fresh fuel but not quite sure just how to do all that...
I'm in the process of changing filters and will check all that out...
I believe the exhaust to be black (never really understood the visual blue to look for) and I'll clean the filter (it's reusable type)...
The engines worked fine in past years but got progressively worse last summer and now has my attention...
When I say along time to start I'm talking 10-15-20 minutes (not seconds), when it does start it starts catching as I continue to crank until eventually it "catches up" and I can stop cranking, usually dies 2-3 times but restarts fairly quickly (10-20seconds)...after it gets up to temp it restarts for the remainder of the day as it should (but doesn't always rev in gear)...
Is it true you can't flood a diesel?
I'll check the governor (didn't know I had one)...
Replaced the exhaust elbow/water muffler last summer so do think that's clogged...

Thanks...


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## INMA (Sep 13, 2011)

_"but got progressively worse last summer" "Replaced the exhaust elbow/water muffler last summer"_

Check the exhaust since issues started when you put the new exhaust bits on. Did the exhaust installer check the back pressure after installation? Blocked exhaust causes excess soot which fouls the exhaust more.

If it has a glow plug check its working.


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## BarryL (Aug 21, 2003)

Hello,

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

Barry


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## klem (Oct 16, 2009)

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.


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## dabnis (Jul 29, 2007)

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


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## steel (Sep 1, 2010)

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.


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## cas206 (Mar 25, 2011)

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?


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## MarkSF (Feb 21, 2011)

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.


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## cas206 (Mar 25, 2011)

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.


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## cas206 (Mar 25, 2011)

Oh and I did sail it for a few hours. It sailed fine which seems to eliminate the bottom, but not the prop.


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## CapnSantiago (Jul 17, 2007)

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 hits...it more or less readily restarts throught the weekend.

Hope this helps...


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## Faster (Sep 13, 2005)

Thanks for the followup....


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