1999 5.7 mercruiser (Fuel injected) backfiring issue
 

I developed a hesitation and backfire problem in my port engine this past weekend. The Starboard engine is running fine The boat has been running great since launched a month ago. I've put about 8 hours on it so far this year with no problems til Saturday.
Both engines have 360 hours
While cruising at 3000 rpm's the port engine suddenly lost power and started backfiring then seemed to smooth out some but still wasn't right. Seems to run fine at idle but stumbles when in neutral while throttling up.

I did the following work myself this spring to both engines. (I used to be an auto mechanic in my former life):
New plugs
new wires
new caps
new rotors
Oil and filter change
both Fuel filters
Filled with fresh gas on Sunday (Problem occurred before the fill up and no change after the fill up)

Firing order is correct. I removed the cap and rotor and it all looks fine.
It seems to idle smooth but stumbles under load. My first thought was fuel issue but the starboard engine runs great.

Thanks for reading

Any ideas would be appreciated

Carb issue?
 

Maybe carb on that engine is gummed up? Try running some Seafoam thru it......stuff works great. White and red can.

Water in fuel?

Switch caps and rotors to see if the trouble follows the parts to the other engine.

Check the fuel pressure, should be around 40 PSI when you turn the key on if memory serves. Check it under load too, it should stay around 40 PSI. If it's low, the fuel pump could be the problem, the fuel supply line could be kinked, or the filter might be clogged (not likely with a brand new filter...)

If the pressure is good:

Swap the MAP (manifold air pressure) sensors from one engine to the other and see if the problem follows the MAP. Do the same with the CTS sensors (coolant temperature sender). Perhaps one of these sensors is failed and making the engine lean out. Does not take much of a lean mixture to cause what you are describing when on plane. I doubt it's the crank position sensor, but you could try swapping those too.

Does you hear the classic Merc double beep (it's the boat equivalent of a check engine light)? If so, the ECM (should be a MEFI 3 or MEFI 2) will have code(s) stored and they can be read by a scan tool.

Basics
 

If none of the above pan out you may want to do a compression test which could help you determine if you've got a weak/ broken valve spring or a bent push rod. It would be a PITA to have to take exhaust manifolds off to get to the valve covers otherwise I'd say just pull the covers and do a visual.

Good luck.
CT

Do you have two fuel tanks, or one?

If two, is there a selector lever so you can run both engines from one tank? If you can do this, select the port engine to run from the starboard tank. Then drain the port fuel filter and try again (there could be water in the fuel filter).

I had an issue that sounded just like this, it turned out to be a over 2 gallons of water in one fuel tank. After replacing the fuel filters the problem returned.

Running both engines from the good tank allowed me to take my time to solve the problem while I could still enjoy my boat.

If this is your problem, I can tell you how to see and remove the water in your port tank.

Of course, it could be something else than water in your fuel but don't rule out the water in fuel tank, just because you changed the filter. ;)

Thanks
 

Sorry for the delayed response.
Sometimes that thing called work gets in the way.

The boat only has one tank and my problem shows on only one of the engines so I'm not ruling out a fuel issue but it's not high on the suspect list yet.
I did treat the fuel with the appropriate amount of stabil and Star-tron enzyme treatment last fall and I then ran the engines for a while to get the treated fuel into the engine fuel systems. I don't have any diagnostic tools like fuel pressure testers etc so I will double/double check all my work. I don't want the boat to be down for long with 4th of July just around the corner so I'll probably let the experts have a go at it soon.

Thanks
I've read a lot of scary stuff on line about burned valves and dead cylinders but I'm just not ready to go down that road yet. These are basically Chevy 5.7's (350's) which are an awesome and reliable powerplant. I don't beat the boat up and don't run it wide open often so I'm not sure why I might have swallowed a valve or broken a spring. (Especially where I was simply cruising at 3k rpm's with only 5 of us in the boat) I sure hope this isn't my first experience with a problem like that. I do have a compression tester though and a Mighty Vac vacuum gauge/tester so I will try the mighty vac on a manifold vacuum port and see what it tells me. Should be 17-25 mm/hg at idle.

Thanks again and if you wake up in the middle of the night with the answer, feel free to share it with me:)

Rich
It sure behaves like water in the fuel to me so I will be sure to pull the filters again and dump them in a container to look for water. I guess I would assume with just one on board tank (150 gallons) both engines would behave the same and exhibit the same problem.

Thanks, we'll keep at it

If it's water, here is a hint on how to easily see water in your fuel.

Go to a grocery store and buy a bottle of black food coloring (or some other dark color if they don't have black).

Put a couple of DROPS of the black food coloring into the gasoline. If there is no water in there, you will see the black drops sitting on the bottom of the tank, or container.

If there is water, it will be tinted black and be easy to see as the black food coloring is water based and will easily mix with the water.

It may be hard to see into your tank, but if you can get access to the fuel level sender, removing this allows you to see into the tank with a flashlight. Before removing your sender, get a gasket kit, which should include a new gasket as well as sealing washers. Use the kit when reinstalling the sender.

If should go without saying but please use the utmost caution, all fuel vapors are very VERY flammable!! Don't do any of this if you don't know what you're doing. You don't want fumes going into your bilge as any static electricity or any spark could start a big fire. Be careful.

You need to find someone with an analyzer, not sure if marine use obd, if so it makes it easier. See if the efi is throwing any codes. Work from there. Start simple and work to the more drastic. It's probably something simple, like a bad sensor or a wiring problem.

look at the main wire harness that plugs in at the engine to see if its loose.

fuel pump
 

I am having a similar problem with my 2004 Volvo Penta. I have narrowed it down to what Volvo calls the fuel cell: It is a lift pump, high pressure pump filter and heat exchanger all in one convenient (translation expensive) unit. I am tearing mine down soon, as I have read that the inside of the unit was painted, and the ethanol peels the paint. When enough paint blocks the internal screens, the pump starves. As it sits now, I can idle in gear all day long, but if I try to get up on a plane, it lasts a few hundred feet before backfiring, sputtering and dying out.

Most of the advice is around fuel, but the first thing that comes to my mind is spark. If the coil is working intermittently you will absolutely get a backfire. And the coil is more likely to fail at high revs.

The test is simple, swap coils and see if the problem moves.

Problem found
 

I performed a compression test on the engine this past Saturday.
7 cylinders showed 175 psi but #4 cylinder has 45 psi.

In talking to my mechanic, in the 90's for some reason, it was a common problem to "tulip" a valve. Probably an intake valve hence the backfiring through the throttle body. I removed the valve cover and ran the motor and saw nothing out of the ordinary with the spring, push rod or rocker (as you might expect because the problem is on the combustion chamber side).

So it looks like I have a valve job in my immediate future. I'm just a little stumped. I've worked on cars my whole life and I've never had a valve fail on a motor. Especially a Chevy 350 which are typically bullet proof. These motors have 360 hours and run as sweetly as can be with no sign or indication that a valve would fail. Strange!

While the news is not so good at least you now know where the problem is.

Here's hoping it can be repaired quickly so you don't lose much of the season.

Tuliped valves on 350 GMC engines are quite common! I and buddies of mine had the problem on fairly new 350 Mercruiser engines. If the engine is within the warrantee period, Mercruiser will replace free of charge. After the warrantee service, depending on who you talk to at Mercury, they may or may not replace the valves. In my case, my insurance company paid for the replacement. Check your insurance company.

My uncle has a fleet of GMC trucks with the 350. I remember him telling me it was a pain dealing with GMC regarding tulipped valves.

45 PSI could be a valve for sure, but it can also be a broken piston ring, cracked valve seat or a cracked head. Put a little oil in the affected cylinder and try again. If the number goes up, it's likely a ring issue. If you do a leak down test, you may be able to hear where the air is escaping by listening at the throttle body (intake valve), exhaust manifold (exhaust valve), thermostat housing (cracked head) or dipstick tube (rings). That said, the back firing sure points to the intake valve...

Valves tulip for two reasons, they are defective (not hardened enough when manufactured) or they overheat and get soft. Once they stop sealing well, they don't transfer heat to the valve seat very well and they get hotter and softer. The valve can also crack and so can the valve seat.

If you pull the head yourself and bring it to a machine shop, you can save a fortune. Valve jobs are not terribly expensive when done this way.

Thanks!
 

I'd love to be able to pull the heads myself. If it was in my driveway at home there'd be no question. The boat stays in the water all summer. It's a 28' and I'd have to pick up the trailer, pull it, get it home, disassemble, reassemble, re-launch etc etc, all with limited time around work.

I think trying to do the work while parked at my dock would be a pain in the rear.

Decisions, decisions

Good luck with this!

Why is working in your driveway easier than in the water? I assume the engine is in a engine bay of some sort. So you'll be working inside the boat no matter if the boat was in the water or in your driveway.

If the engine was an outboard, then I could understand how the driveway would be a lot easier to work on.

Of course I do understand that having all the proper tools handy can make the job go easy or bad.

Let us know how it goes! I always like to hear the end of the story.

I would think the problem with pulling heads in the water would be keeping the water from filling up the boat once the water jackets were exposed.

Thanks for mentioning it, I never thought about this point. I wonder how many people have sunk a boat without thinking about such things! I'm glad it wasn't me!

I fixed my similar issue today. I found that the gasket surface for the fuel filter had been painted, but some paint was missing, causing it to suck air instead of fuel.


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That would be easy to avoid, the water passes through water pump, and if equipped: the power steering cooler, fuel cooler, and oil cooler; before it goes into the engine via the thermostat housing. One could simply remove the port side inlet hose at the thermostat housing (and plug it, though it's very likely high enough that plugging won't be necessary) to prevent any issues. One would want to drain the water from the motor regardless, otherwise cooling water will end up in the cylinders when you pull the head.

Boat heads are typically MUCH easier to remove than car heads because the exhaust system does not get very hot and comes apart easily. That said, access to the head is a function of boat design and that can vary wildly...

Removing the head is not very daunting, there are 8 exhaust manifold bolts, 1 distributor bolt, 2 thermostat housing bolts, 12 intake manifold bolts, 4 valve cover bolts, and 17 head bolts. One would also need to disconnect the fuel line and some electrical connectors, as well as a bracket or two for the throttle/shift cables and perhaps the ECM, slave solenoid and circuit breaker, but it's all pretty straight forward.

I made an appointment to drop the boat off on Sunday the 12th for repair.
I'm having both heads re-done on the port engine. I was able to use the boat quite a bit over the last 2 weeks without any issue except for a very slight noticeable vibration. As long as I stayed below 3000 RPM's I didn't get any backfiring.

I'll be sure to post the final results after repair.

Thanks for all the help folks!!!!!

Here's the culprit!!
 

Crack in valve in #4 cylinder. Crack goes approx 60-70% of the way around. Had I run the engine much more the entire valve face would have broken away and ruined the motor.

Ouch, glad you figured it out, hope it didn't cost too much to fix.

I see the valve and agree it us a problem.

Can someone help me understand how that valve caused the symptoms?

Sure. Without getting too overly technical a 4 stroke engine does just that. Intake, compression, power, exhaust. When the piston was coming up for the compression stroke, it wasn't really compressing much, hence the 45 psi of compression referenced earlier in this thread. The spark plug fires within a few degrees of top dead center. When that fuel/air mixture ignited it would blast right by that huge crack in the intake valve and backfire out through the throttle body. I'm assuming the valve pictured is the intake. If it's the exhaust valve the same thing would happen but the backfire would go out the exhaust port...
BT

Thanks for the explanation.

What has me wondering is the inconsistancy of the issue as reported in the first post. It seems to me that a valve like that would give a more consistant symptom.

Good explanation.

4 stroke:

1. Intake- Piston travels downward creating a vacuum in the cylinder and the fuel air mixture is drawn into the cylinder through the open intake valve.
2. Compression-Intake and exhaust valves are closed creating a sealed chamber where the fuel/air mixture is compressed by the upward traveling piston, almost to the point of igniting the fuel/air mixture on it's own. (How a diesel motor works)
3. Power-Both valves are still closed, The spark plug fires and the rapidly expanding combustion drives the piston downward creating power. (In my motor all the power that was supposed to be generated in this cylinder was lost through the broken valve and forced back up into the intake mainifold)
4. Exhaust-Piston traveling back up in the cylinder forces the spent gases back out through the open exhaust valve. (Some of the hot exhaust gases would pass through the broken intake valve during this stroke as well possibly igniting more fuel in the intake manifold)

Then the process starts all over again

Each cylinder in a 4 stroke motor only makes power during one of the 4 strokes (power stroke). In a V8, V6, etc, the power stroke is happening at different times in different cylinders thus the motor is producing power all the time. In a 1 cylinder "lawn mower" motor they need a weighted mass (flywheel) to rotate/carry the piston through the other 3 strokes

Glad you found the problem before it turned into a large expensive problem! I'm sure you are also!