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Old 21-07-2020, 04:26   #1
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Volvo MD7A not throttling up

I have an early 80s Volvo Penta MD7A (re)powering an O'Day 272.

The other day it started just fine, but wouldn't throttle up. I verified the linkage was fine all the way into the motor. A friend who'd had a similar issue suggested replacing the fuel filters. Of note, they were done at the end of last season as part of winterization. And, I recalled I'd put a few gallons of fresh diesel in the tank after my previous sail. Thought maybe I'd stirred something up?

So I picked up a replacement Racor primary filter. Yesterday morning I went out to the boat. Separately, the 'kill switch' had gunked up so the motor wouldn't start - that's the first thing I addressed. Got that cleaned up and started the motor to test it out. The throttle worked! Ran for while, up and down RPMs. Stopped and started a few times, and after a few I'm back to the same no throttle issue.

I replaced the primary filter, bled the system at the filter with the little pump, and got the motor started. Same problem. I picked up a replacement secondary filter from NAPA.

But before I dive into the secondary filter, are there other things it could be? Bleeding the system after replacing the secondary looks like a complete pain, so I don't want to do it if it stands little chance of fixing the problem.

Water flow out is normal, no black smoke. Motor ran absolutely fine over the past year I've had the boat.

ideas?

thanks in advance!
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Old 21-07-2020, 05:18   #2
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Re: Volvo MD7A not throttling up

Bleeding isn't all that bad. You just run the pump with a fitting slightly open until you take a bath in non-foaming diesel oil. The smell goes away within a few years.

I don't know the answer, but I can think of a few possibilities. Let's assume that you've covered the filters, have flushed the fuel lines, see no debris, and are getting good fuel flow immediately beyond the two filters.

On the fuel side get the engine to fail (run but not rev) and try slightly loosening joints the rest of the way to the injectors to see if fuel is coming through at pressure. If you don't know what to expect, do it with the engine not failing first.

Then there is the air side. Any blockage going in, such as a bird's nest?

Then there is the exhaust side. Any blockage going out, such as a bird's nest?

Then I run out of ideas, and you either need to wait for a response from someone who has had the problem (it's on my list of "gonna get me" items but hasn't happened yet), or get more sophisticated, such as inserting a vacuum gauge just before the pump or inspecting injectors.

Good luck with it. With the engine running fine some of the time and no smoke, it isn't going to be an expensive repair, just a mystery to occupy your mind. Don't let it frustrate you.
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Old 21-07-2020, 06:53   #3
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Re: Volvo MD7A not throttling up

First things first, start at the tank. Could be a blocked vent or pick-up tube. Try running it with the fuel filler cap off.
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Old 21-07-2020, 15:13   #4
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Re: Volvo MD7A not throttling up

Secondary filter is easy to replace. Loosen center bolt, drop filter, clean housing. You can then replace and check to make sure the oring is in place. If you have to hold it use a dab of grease spead thinly over the oring. After it's back together open the bleeder screw (two turns, next to center bolt). Then find the lever on the lift pump and push it down to pump fuel into the bowl and pump until fuel comes out. Another way to do it is buy some Liqui-Moly Diesel Purge and fill the filter bowl and filter with it. Wear gloves. It's pretty nasty stuff. Works amazingly well though



Was the Racor dirty?
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Old 21-07-2020, 15:35   #5
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Re: Volvo MD7A not throttling up

Quote:
Originally Posted by Scubaseas View Post
Secondary filter is easy to replace. Loosen center bolt, drop filter, clean housing. You can then replace and check to make sure the oring is in place. If you have to hold it use a dab of grease spead thinly over the oring. After it's back together open the bleeder screw (two turns, next to center bolt). Then find the lever on the lift pump and push it down to pump fuel into the bowl and pump until fuel comes out. Another way to do it is buy some Liqui-Moly Diesel Purge and fill the filter bowl and filter with it. Wear gloves. It's pretty nasty stuff. Works amazingly well though



Was the Racor dirty?
It did not look particularly dirty, and the bowl did not look fouled. The diesel purge...fill the new filter up with it?

Oh, i have gloves! Which i don't always remember to put on...
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Old 21-07-2020, 15:36   #6
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Re: Volvo MD7A not throttling up

Quote:
Originally Posted by jimbunyard View Post
First things first, start at the tank. Could be a blocked vent or pick-up tube. Try running it with the fuel filler cap off.
heading back out to it in the morning...i will try this first!
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Old 21-07-2020, 15:38   #7
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Re: Volvo MD7A not throttling up

Quote:
Originally Posted by tkeithlu View Post
Bleeding isn't all that bad. You just run the pump with a fitting slightly open until you take a bath in non-foaming diesel oil. The smell goes away within a few years.

I don't know the answer, but I can think of a few possibilities. Let's assume that you've covered the filters, have flushed the fuel lines, see no debris, and are getting good fuel flow immediately beyond the two filters.

On the fuel side get the engine to fail (run but not rev) and try slightly loosening joints the rest of the way to the injectors to see if fuel is coming through at pressure. If you don't know what to expect, do it with the engine not failing first.

Then there is the air side. Any blockage going in, such as a bird's nest?

Then there is the exhaust side. Any blockage going out, such as a bird's nest?

Then I run out of ideas, and you either need to wait for a response from someone who has had the problem (it's on my list of "gonna get me" items but hasn't happened yet), or get more sophisticated, such as inserting a vacuum gauge just before the pump or inspecting injectors.

Good luck with it. With the engine running fine some of the time and no smoke, it isn't going to be an expensive repair, just a mystery to occupy your mind. Don't let it frustrate you.
thanks for the input! Exhaust side looks fine...normal flow from outlet. And when it ran great for a few minutes, full flow. I'm actually not sure where the air is sucked in from. Hm. I will find that.

i'm getting more confident in tackling the bleeding.
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Old 21-07-2020, 15:59   #8
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Re: Volvo MD7A not throttling up

One: what did you discover INSIDE the filters. Were they clean or dirty?


If there was gunge at the bottom of that tank ... you got your answer.


Otherwise:



Primary filter? Secondary filter? What did you find inside them?



Priming is an easy think. You are panicking it a bit. If you have Bosch, it is easy. If you have CAV it is easy - unless you failed the order (filter top screw - by manual pump, then the 3 screws (in their own odd order), only then - injectors. EASY and not pita as you said. Mind part is done by that manual pump, part is done by the starter - and do not fill the neck with water - if cranking long time CLOSE the bottom valve (and do NOT forget to open it when the donkey gets going)



NOW -



NOW -


;-)


Yes now you are ready to take off the accel. cable from its arm on the engine. Also disconnect the stop cable.


Start the engine. Cut your hair navy way. Leave your Gaultier shawl at home, remove the Armani tie (any of these can get caught in that fly wheel).


As the engine is running in low speed, grab the accel lever by hand and SLOWLY accelerate.


If it accelerates swiftly, you are fine.


If it is slow to accelerate, the pump (high pressure pump, not the manual one) may need cleaning.


If it does not accelerate or goes slow and then suddenly top speed - the pump DOES need cleaning.


If your pump is being cleaned, pull out these injectors and have them checked and cleaned at the same time.


What you are describing sound either dirty fuel, or else the hp pump. Less ofte - air in the system (in the hp pump).





barnakiel, MD7A here.
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Old 21-07-2020, 16:10   #9
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Re: Volvo MD7A not throttling up

Quote:
It did not look particularly dirty, and the bowl did not look fouled. The diesel purge...fill the new filter up with it?
Yes, clean bowl, put filter in, fill with Diesel Purge. Sounds like it may not be filters but change them as a diagnostic tool anyway. Save the Diesel Purge can and use it next time you have to fill either primary or secondary filter. Or fill it with diesel next time you fill up.

With the secondary filter off try pumping the feed or lift pump by the lever. You should be able to get a steady stream of fuel coming out. Capt. Obvious would point out to use a bucket or coffee can under the filter housing when the bowl is off. If you can not then you either have an air leak (loose hose or fitting) or a blockage (inlet pipe clogged maybe).

You might want to look at the linkage and make sure the shaft that the lever arm clamps on to is moving when the lever moves. Especially if you have the CAV injection pump.

You could also try running the engine off a separate fuel supply I keep hoses and a jerry jug on my boat to be able to bypass the fixed tank and lines. Simply hook it up to the outlet of the Racor and the return line.

You can find the service manual online for free on this motor Might be worth looking through it. It also covers bleeding the injection pump.
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Old 21-07-2020, 20:17   #10
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Re: Volvo MD7A not throttling up

Quote:
Originally Posted by Scubaseas View Post
Yes, clean bowl, put filter in, fill with Diesel Purge. Sounds like it may not be filters but change them as a diagnostic tool anyway. Save the Diesel Purge can and use it next time you have to fill either primary or secondary filter. Or fill it with diesel next time you fill up.

With the secondary filter off try pumping the feed or lift pump by the lever. You should be able to get a steady stream of fuel coming out. Capt. Obvious would point out to use a bucket or coffee can under the filter housing when the bowl is off. If you can not then you either have an air leak (loose hose or fitting) or a blockage (inlet pipe clogged maybe).

You might want to look at the linkage and make sure the shaft that the lever arm clamps on to is moving when the lever moves. Especially if you have the CAV injection pump.

You could also try running the engine off a separate fuel supply I keep hoses and a jerry jug on my boat to be able to bypass the fixed tank and lines. Simply hook it up to the outlet of the Racor and the return line.

You can find the service manual online for free on this motor Might be worth looking through it. It also covers bleeding the injection pump.
I've got the 'Instruction Book Owner's Manual" with a simple bleeding procedure, and poor resolution graphics. Diagrams are tough to see! Just reviewed in preparation for tackling it tomorrow.

I checked the linkage - when I move the mechanism at the motor by hand, it moves the lever in the cockpit.

It might be worth noting the entire fuel system was replaced when the Volvo went in...about 4 seasons ago. So it's not decades of crud in the tank, at least.
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Old 21-07-2020, 20:55   #11
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Re: Volvo MD7A not throttling up

All it takes is a few months sometimes to grow a ton of crud. You can download the workshop manual from capedory.org and a few other places. Grab it now while it's still available. You want to check if the lever at the injection pump is moving the shaft it it bolted on to. Best of luck. Let us know what you find.
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Old 22-07-2020, 06:05   #12
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Re: Volvo MD7A not throttling up

morning everyone!


I'm on the boat working through priming. Replaced the secondary filter, no problem. I opened the bleeder 4 complete turns, as specified, and the air hole was completely visible. I then used the priming lever until fuel spurted out at a higher arc than I was expecting (oops!). Tightened screw.

I loosened the two nuts above the injectors - the instructions say "Loosen", not 'remove' - and cranked. No fuel came out. I then removed the locknuts, sliding them out of the way on the rails. With the rails hanging loose and cloths/rags around, I cranked. For a while. Still nothing. Shouldn't fuel be coming of there? I will double check the stop control...
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Old 22-07-2020, 06:25   #13
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Re: Volvo MD7A not throttling up

some time back, I had a problem like that.....found out that some "mud doppers", a wasp that builds its nest from mud, had chosen the little shield over the fuel tank vent to build it's nest....and had completely blocked this little vent..

another time, was helping another boater with a similar problem.....we finally removed the vent hose from the side of the hull to the tank, stuck it in his mouth and told him to blow like crazy...sure enough, all sorts of crud came flowing out the primary fuel line.

diesel fuel tanks have tendency to have a lot of crap inside on the bottom, which can get picked up by the main feed line....and block that line...

just some suggestions..
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Old 22-07-2020, 06:26   #14
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Re: Volvo MD7A not throttling up

Stop is now fully disengaged. I put the rails back on loosely and cranked. Same result, unfortunately.


How long should I crank? I gave it a while.
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Old 22-07-2020, 06:35   #15
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Re: Volvo MD7A not throttling up

pressurize the tank as explained above...
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