4.3 GL SX-M Backfires at 3~4 grand - Exhaust Bellow Torn

[jimi]

A week ago I thought I had a bad exhaust bellow because I heard a loud "pop" and when I investigated the exhaust bellow was torn. This also happened last summer but I attributed it to a defective exhaust bellow.
Well I had the boat out again this weekend and under the same conditions, steady running at 3,000 to 4,000 rpm, and again there was a loud popping sound. I know it wasn?t the bellow tearing as it was already torn. Also, it seemed like the boat may hesitated for a moment during the pop.
Assuming it is backfiring, any ideas what would cause it?

SX-M 1.79 prod
4.3GL PEFS

 

[jimi]

Re: 4.3 GL SX-M Backfires at 3~4 grand - Exhaust Bellow Torn

Anyone?

 

[Bondo]

Re: 4.3 GL SX-M Backfires at 3~4 grand - Exhaust Bellow Torn

Ayuh,... Check the fuel filter for Water,.... It sounds like it's goin' Lean,...

 

[jimi]

Re: 4.3 GL SX-M Backfires at 3~4 grand - Exhaust Bellow Torn

Will do!

A friend suggested double checking the exhaust valve to make sure it hadn't broken free and gotten stuck in the lower unit.

 

[The_Other_Dave]

Re: 4.3 GL SX-M Backfires at 3~4 grand - Exhaust Bellow Torn

Will do!

A friend suggested double checking the exhaust valve to make sure it hadn't broken free and gotten stuck in the lower unit.

I just found both of mine stuck in the lower (i.e. they went through the exhaust bellows) 2 weekends ago.

 

[dtescher]

Re: 4.3 GL SX-M Backfires at 3~4 grand - Exhaust Bellow Torn

Check the fuel pump. I had the same issue(loud pop/backfire at 3,000-4,000 RPM). It was the fuel pump. Installed new one and problem solved.

 

[jimi]

Re: 4.3 GL SX-M Backfires at 3~4 grand - Exhaust Bellow Torn

I'm going to have a busy weekend.

Is there anyway to check the fuel pump or do you just replace and see if it makes a difference?

 

[dtescher]

Re: 4.3 GL SX-M Backfires at 3~4 grand - Exhaust Bellow Torn

Yes you can test it. Check out this reply to another post.

Maclin

Default Re: Volvo 4.3 GL Fuel Pump Problem

"You can perform a vacuum test on the pump to see how much it can draw, there are vacuum gauges designed just for that, and the test can show if the pump is healthy or not. I do not know the specs for the pump, but if it is low on the draw then it is time for a new pump. If the draw is within specs then there may be a problem between the pump and the pickup in the tank, like cracked or leaking lines or hoses, plugged anti-siphon valve (this is usually the fitting on top of the tank), or plugged intake."

 

[jimi]

Re: 4.3 GL SX-M Backfires at 3~4 grand - Exhaust Bellow Torn

I was rained out this weekend.

The_Other_Dave;

I managed to remove the prop between downpours and poked around with a wire. I don't think there is anything in the bottom half of the lower unit. Geting at the rest will be tough without removing the leg.

Other Dave, where exactly did you find your flaps?

 

[The_Other_Dave]

Re: 4.3 GL SX-M Backfires at 3~4 grand - Exhaust Bellow Torn

I was rained out this weekend.


Other Dave, where exactly did you find your flaps?

Jim,

I found them in the top of the lower when I separated the drive from the gimbal plate to change the bellows (both the
drive and exhaust ones). They were stuck in the passages where the exhaust splits to go down around the driveshaft housing.

So basically they were just after the exhaust bellows in terms of flow out of the drive.

Hope that helps!

Davd

 

[jimi]

Re: 4.3 GL SX-M Backfires at 3~4 grand - Exhaust Bellow Torn

Removing the lower unit means taking the boat out of the lake which is a pain. So I borrowed a bore scope from work and had a look. I went in through the torn exhaust bellow (both ways) and up from the prop. When I looked towards the engine I got to the Y and it didn't look like there were any holes for a pin shaft to hole the flappers so I suspect my boat never had any (see Don's post http://forums.iboats.com/volvo-pent...l-pbyc-flapper-valve-y-pipe-issue-601052.html ).

Also, based on my engine being a 4.3 GL PEFS this parts diagram doesn’t show exhaust flappers Volvo Penta Exploded view / schematic Exhaust 4.3GLPEFS, 4.3GiPEFS - MarinePartsEurope.com

Going upstream from the prop was a trick but I'm pretty sure there isn't anything in the bottom 50% of the LU. Going from the exhaust bellow downstream to the LU I didn't see anything either. This was the hardest to see but it didn't look like there was anything there.
Also, the engine backfired again (big pop out the back end) so I'm leaning toward Bond-o's theory. I'll change the fuel filter this weekend. I’m almost finished running off the gas that overwintered so it will be good timing.

PS All, sorry for the slow pace of this thread. My boat is 3.5 hours north of where I live and it has been raining biblically so I don't get a chance to check things out very often. Thanks again...

 

[Bondo]

Re: 4.3 GL SX-M Backfires at 3~4 grand - Exhaust Bellow Torn

Also, the engine backfired again (big pop out the back end) so I'm leaning toward Bond-o's theory. I'll change the fuel filter this weekend. I’m almost finished running off the gas that overwintered so it will be good timing.

Ayuh,.... Don't just pull it, 'n toss it,....

Inspect it's Contents for water, 'n crud,...

 

[jimi]

Re: 4.3 GL SX-M Backfires at 3~4 grand - Exhaust Bellow Torn

Well the flash flooding in Toronto slowed down my progress a lot but ... I got back to it last weekend.

The motor still “pops” every other outing. It seems to happen between 3,000 and 4,000rpm. The motor will hesitate notably, a loud pop (un-burnt fuel/air mixture detonating in the exhaust path, and then the motor resumes working as if nothing happened.

The Fuel filter and the gas in the tank are all new. I kept the contents of the fuel filter in a glass jar and never saw any water separate out. . I think that rules out water as the cause.

The exhaust isn’t blocked in anyway.

I found a marine carburetor re-builder but that will take the boat out of use for two weeks so I have it scheduled for the September long weekend.
Until then the only thing I can think of checking this weekend is the anti-siphon valve. Any other ideas?

 

[jimi]

Re: 4.3 GL SX-M Backfires at 3~4 grand - Exhaust Bellow Torn

dtescher;

You said that the same issue(loud pop/backfire at 3,000-4,000 RPM) and that it was the fuel pump.
Did you also have a bit of rough idle and did it require priming with the trottle to start unless it was really warmed up already?