Mercruiser 5.0 backfire up intake

[johnkom]

What were the final symptoms that made you decide it was the #6 valve??

JK

 

[ibrw1]

Bigg Jimm suggested pulling spark plug wires off the distributer one at a time, it narrowed it down to one cylinder. If it would have narrowed down to two cylinders, next to each other, then head gasket blown between cylinders. Because the backfire is blowing flames out the carburetor then somehow flames are getting past the intake valve. Removing the spark plug wire fixed it, no more flames. It's possible that the head is cracked? Not sure, spring? I bet Valve. I will certainly post photos in about 4 weeks once I get the boat placed in the garage so the wife can park her car in my man cave during the winter. I have borrowed a color fiber optic camera that I'm going to attempt to view the valves from inside the combustion chamber going in through the spark plug opening and through the intake manifold to see other side of valve. Only reason I'm going through the effort with the camera, if it proves fruitful, then it adds a certain amount of pizzazz to the story. Thanks for asking.

 

[johnkom]

Thanks spades for sharing. I learned a lot. AND put a lot of tips into my head for future trouble shooting. Sooo, as I understand, you would remove one plug wire then run in the rpm range of trouble. One way it only missed. With the suspect cylinder wire off it missed AND backfired?? Do I have the picture right?

JK

 

[FreeBeeTony]

Just want to add my $0.02......
I had a similar problem a while back.
The engine didn't backfire through the carb but @ 3200RPM it would begin to miss and what seemed like unburned fuel coming out of the carb.
It happened at 3200RPM like clock-work. I narrowed it down to the #3 intake valve just as you did.
To make a long story short.....the engine finally blew, intake stuck open, piston hit he valve and you can guess the rest.
I attributed it to a lifter hanging up (I replaced the lifter prior to the blow-up) caused by a possibly bad lifter bore.
I replace the engine w/ a Vortec long block from RapidoMarine......Just passed 520hrs and runs great.

Hope this helps......might try adjusting the valve lash on the suspected valve and see if anything changes....to see if it's adjusted too tight.
PS....not sure I would run it much more before you fix it.

 

[ibrw1]

Thanks spades for sharing. I learned a lot. AND put a lot of tips into my head for future trouble shooting. Sooo, as I
understand, you would remove one plug wire then run in the rpm range of trouble. One way it only missed. With the suspect cylinder wire off it missed AND backfired?? Do I have the picture right?

JK

I removed wires one at a time until the backfire issue stopped. Another note. About a year ago, I had a spark plug wire fall off. Engine ran pretty well, so much so that I didn't notice until I went to change the oil, saw it hanging there. Back to the procedure, every plug that I removed gave a noticeable difference in power. Once I unplugged the bad cylinder, the engine still ran quite well, that and no backfire. Anyway the boat is winterized and ready for dissembly which should start in 2 weeks. I can't wait.

 

[johnkom]

Thanks for all the troubleshooting tips. Looking forward to know what you find in the engine.

JK

 

[ibrw1]

No Title

Was real slow to investigate issue with the backfire. Anyway got it. I removed intake manifold. Shoved a camera in there. And saw an anomaly, this photo shows the crack in the valve seat, the crack continues on. Glad it's over. I'll post more pictures shortly

 

[alldodge]

Looks like a piece of the valve broke off

 

[Bondo]

Anyway got it. I removed intake manifold. Shoved a camera in there. And saw an anomaly, this photo shows the crack in the valve seat, the crack continues on.

Huh,..?? that picture is of a head, off the motor,....

Also looks like the head gasket was about to puke too,....
'n the cylinder is to clean, it's been passin' water,...

Probably been runnin' lean, 'n detonated itself to death,...

 

[ibrw1]

Sorry, to clear things up. I used a camera shoved inside the intake port of the head with the intake manifold removed. Unfortunately due to my 15 hour days at work I have been unable to upload the actual photos I was on about. That photo I did upload was after the head was removed. The exhaust valve was eroded away due to me running the boat with a cracked head. I will do my best to get pictures up tonight.

 

[ibrw1]

No Title

null

 

[ibrw1]

No Title

null

 

[ibrw1]

Anyway, all cylinders are the same. No milkshake in the oil. Carburetor was rebuilt by some outfit in Florida. Calibration of carb was base timing and followed procedure. Was working on trying to fine mystery backfire since spring. Didn't really care if engine exploded or not. Was thinking I would put in new 5.7 if it did. Last boating trip engine was sooooo weak that we couldn't trailer it, we had to use crank to pull it up and out. I did compression test early on, it passed. Guess that continued use burnt more and more material off the exhaust valve.

 

[ibrw1]

No Title

 

[ibrw1]

Anyway, I cleaned head with steel wool to see if it was repairable. Answer is no, it's got a small crack from valve seat out about one inch.
Anyway, I might just fix this engine. It's a newer 5.0 liter 1999 8 bolt intake with casting #059

I was offered 2 new heads by World Casting. Casting I 052 with 1.94 intake valves, and with matching 4 bbl intake with 4 bbl carburetor. $500.00

I see the crosshatch is still visible with no noticeable ring ridge along the top. In other words. I'm thinking go for it. Only unknown is these heads. They a direct fit? Anyone have experience with word casting?

 

[ibrw1]

Photo inside using fiber optic camera

 

[Bondo]

Anyway, all cylinders are the same. No milkshake in the oil. Carburetor was rebuilt by some outfit in Florida. Calibration of carb was base timing and followed procedure. Was working on trying to fine mystery backfire since spring. Didn't really care if engine exploded or not. Was thinking I would put in new 5.7 if it did. Last boating trip engine was sooooo weak that we couldn't trailer it, we had to use crank to pull it up and out. I did compression test early on, it passed. Guess that continued use burnt more and more material off the exhaust valve.

Ayuh,..... Sounds, 'n Looks like ya detonated it to death, probably from a lean condition,.....

 

[alldodge]

Anyway, I cleaned head with steel wool to see if it was repairable. Answer is no, it's got a small crack from valve seat out about one inch.
Anyway, I might just fix this engine. It's a newer 5.0 liter 1999 8 bolt intake with casting #059

I was offered 2 new heads by World Casting. Casting I 052 with 1.94 intake valves, and with matching 4 bbl intake with 4 bbl carburetor. $500.00

I see the crosshatch is still visible with no noticeable ring ridge along the top. In other words. I'm thinking go for it. Only unknown is these heads. They a direct fit? Anyone have experience with word casting?

Haven't dealt with them but there are very few foundry's and they don't sell direct that I'm aware of. World castings is probably the third part retailer, don't think there should be any issue.

 

[ibrw1]

I want to thank everyone for their input! I can't you how helpful your information is. I'm going to go for the new World Castings and will post results so we can start a bench mark. In addition I'm upgrading to a 4 BBL. Why not. My vendor offered cash for my old intake and newer 2 BBL. So the 4BBL Upgrade cost me $150. This was part of negotiations.

 

[Captin Bly]

Thanks to everyone for the help with my 470 mercrusier.