1999 5.7 mercruiser (Fuel injected) backfiring issue

[Bear Islander]

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.

[dippasan]

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!

[8gv]

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.

[BroadHopper]

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.

Originally Posted by dippasan

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!

[Dave R]

Originally Posted by dippasan

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!

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.

[dippasan]

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

[Rich]

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.

[ITD]

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.

[Rich]

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!

[Dave R]

Originally Posted by ITD

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.

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.

[dippasan]

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!!!!!

[dippasan]

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.

[ITD]

Originally Posted by dippasan

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.

[8gv]

I see the valve and agree it us a problem.

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

[Blue Thunder]

Originally Posted by 8gv

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

[8gv]

Originally Posted by Blue Thunder

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.

[dippasan]

Originally Posted by Blue Thunder

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

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

[Rich]

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