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Whitecap36
Whitecap36, Marine Technician
Category: Boat
Satisfied Customers: 1543
Experience:  28 years Certified Marine Technician and past FL State Certified teaching degree
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I have a volvo penta 5.7 marine engine. The engine starts

Customer Question

I have a volvo penta 5.7 marine engine. The engine starts fine and continues without problems for quite a while. Having travelled at around 19 knots for half an hour or so I throttled back to approximately 7 knots. After 5 mins or so of going at 7 knots, the engine starts to misfire. The last time this happened I operated the trim tabs (flaps). As I did so, the engine came back to life again. The trim tabs are on a rocker switch so that you can position the tabs up or down. When I released the switch, the engine misfired again. So I travelled about 8 miles at around 11 knots with my fingers on the trim tab switch - this got me back to the marina. Any ideas on what is happening here?
Submitted: 5 years ago.
Category: Boat
Expert:  Whitecap36 replied 5 years ago.
Hello,
You may have a voltage drop at or in the coil for some reason and when you operate the trim tabs the alternator regulator senses it and kick up the voltage(just my theory,don't know without testing)
If you can get your hands on a voltmeter hook it up the battery while engine is running good and see what the voltage is at 19 knots. Now hook it up to the coil positive and ground and see if reading change when engine starts to fail. When it does try the trim tabs to see if voltage increases. Now check alternator voltage output to see if it has dropped from you first alternator reading.
Customer: replied 5 years ago.

Hi Mike,

 

How can I take a reading when doing 19 knots. The engine is below and requires a large hatch to be open (not possible when underway). I have a voltmeter on the dash and this normally shows around 13/14 volts when the engine is running.

 

The trim tabs work independently and directly from the battery - does this mean a ground problem.

 

If having done your tests are you saying that its a coil problem?

 

 

Expert:  Whitecap36 replied 5 years ago.

Ok if the voltage at the dash doesn't change that good, tells me that the problem is not battery or alternator output then.

Coil could be the problem but too early to tell. It's an inexpensive part maybe you just want to put one on.

In order to read coil voltage you will need to improvise then. Run some long wires to your voltmeter so you or someone can be reading this until the engine goes into fail mode.

If it does fail here is something for you to try as a test only. With engine off run a wire from battery+ to coil+ see if this will let you run without activating the trim tabs to do so.REMEMBER THAT THE ENGINE IS NOW HOT WIRED AND WILL NOT SHUT OFF UNTILL WIRE IS REMOVED.

 

Customer: replied 5 years ago.

OK Mike,

 

Sorry to appear to drag this out. What happens if the coil is fine or if I replace it and the misfire continues.

Expert:  Whitecap36 replied 5 years ago.

This unfortunately is a situation where there is no cut and dry answer yet.

I'll stick with you through your troubleshooting, we just need to take it a step at a time until we find something to give us a direction.

Customer: replied 5 years ago.

Hi Mike,

 

Was at the boat today. Ran the engine at high revs (2500) for about 15-20 minutes. The reading from the voltmeter (dash) was just over 13 volts. I attempted to mimick the conditions of the last time I was out when the misfire occured, so I put on the fridge. This caused a slight drop on the voltmeter to just under 13 volts.

 

Once the engine was really warm, I throttled back and the engine misfired (albeit not as roughly as previously). There was a voltage drop on the dash meter down to 12 volts. If I operated the trim tabs, the revs would increase and be more steady - the voltmeter dropped a volt. I also tried other loads such as the actuator (to open/close the engine hatch) and the same thing happened - revs increased. Also tried other small loads that increased the revs slightly.

 

Does this give you any leads?

 

Chris

Expert:  Whitecap36 replied 5 years ago.
Ok Chris, the voltage drop your reading at the dash seems normal and doesn't add any new information.
Please try what I mentioned before if you haven't already,as I need to know if there is a voltage drop and the coil positive.

In order to read coil voltage you will need to improvise then. Run some long wires to your voltmeter so you or someone can be reading this until the engine goes into fail mode.

If it does fail here is something for you to try as a test only. With engine off run a wire from battery+ to coil+ see if this will let you run without activating the trim tabs to do so.REMEMBER THAT THE ENGINE IS NOW HOT WIRED AND WILL NOT SHUT OFF UNTILL WIRE IS REMOVED.

Also important is does the engine run normally with the hot wire in fail mode. If you can try these two thing and report back I can give better direction and or answers.

 

Customer: replied 5 years ago.

OK MIke will do that... probably on Sunday and will then get back to you.

 

I should mention that my boat is in Spain (where I am currently on holiday until next Wednesday) and I live in Scotland. The boat is in a marina and there are electrical engineers there. I had a service at the start of the year but when this problem first occured during my last visit here, we took off the distributor cap and found it had a lot of corrosion - something I would have thought would had been checked at the service. Hence my reason for wanting to know what is going on.

 

Mike, this is probably going to protract a little so I will reflect that in payment later.

 

Chris

Expert:  Whitecap36 replied 5 years ago.
Thank you Chris,

The green ashy deposits found in the dist cap are for the most normal and won't cause any running issues, But I agree and also clean out to look better as well as keep the build up from falling of into the distributor.
I should also mention that I will be traveling from northern US to Southern US starting Sunday, the trip will take about three days of so. I will be checking in with JA at least once a day and as I find WIFI signals.
I look forward to our continued work together to solve the problem at hand
Mike.
Customer: replied 5 years ago.

Hi Mike,

 

Down at the boat today. Started engine, no problems - when engine was warm the reading from the volt meter at the coil was 7.5 to 8.3 volts. I suspect this should have read around the 12 volts. There was no difference in the reading when I applied a load such as the trim tabs..

 

However, with the volt meter connected to the + coil and the - direct to the engine block I got a reading of 15 volts.

 

If I kept the connection of the volt meter to the + and - of the coil and increased the revs, I got a reading of 18+ volts, sometimes off the scale where the meter would not give a reading.

 

When the engine started to misfire, the volt meter was reading 9v at approx 1100 revs however if I operated trim tabs, then the revs would increase to 2000 revs and the reading would increase to 13v.

 

I run a wire direct from the battery + to coil + when the engine was misfiring, it did not make any difference to the misfire but run smoothly when I applied a load from the trim tabs.

 

Is this a coil problem or what is the next test.

 

Chris

Customer: replied 5 years ago.

Hi Mike,

 

Have not heard from you since my last reply. I managed to get hold of a battery load tester this morning. I disconnected all battery leads and got a reading of 9 volts from each of the batteries.

 

I have used the batteries to close and open the engine hatch a couple of times and have not had the opportunity to recharge the batteries. So I suspect that the 9v is a true and accurate reading.

 

I am leaving Spain on Wednesday, so hopefully I'll get a response from you sometime today.

 

Chris

Customer: replied 5 years ago.

I have a volvo penta 5.7 marine engine. The engine starts fine and continues without problems for quite a while. Having travelled at around 19 knots for half an hour or so I throttled back to approximately 7 knots. After 5 mins or so of going at 7 knots, the engine starts to misfire. The last time this happened I operated the trim tabs (flaps). As I did so, the engine came back to life again. The trim tabs are on a rocker switch so that you can position the tabs up or down. When I released the switch, the engine misfired again. So I travelled about 8 miles at around 11 knots with my fingers on the trim tab switch - this got me back to the marina. Any ideas on what is happening here?

 

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Whitecap36
Whitecap36
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28 years Certified Marine Technician and past FL State Certified teaching degree