5.7L 350 Merc Back fire ????'s
 

To all you motor experts, trying to help out a friend, he has a pair of 1998 5.7l 350 Merc's in a sea ray, Sorry not high Performance. He has a intermittent backfire problem with one Motor. Usually backfires after heating up and running for awhile. Never consistent.He has changed plug wires distributors, ignition modules etc. swapped parts back and forth between engines to try and duplicate. Still follows original engine. Has had 2 Merc. Mechanics look at it with no resolution. Usually backfires underway at cruise or above. But not all the time. When it backfires several times in a row he of course looses power. He was going to do a compression test on the Motor tomorrow. Any other suggestions. ?? Does anyone know what the compression #'s should be at for that motor.

Thanks for any input.

If you do not find any thing with the compression test, Then you might try doing a leak down test using a tool like what is in this link. http://www.harborfreight.com/cpi/cta...emnumber=94190
On the compression test the numbers will need to stay with in a close range of each other, Now I could not find the specks on the small block but I found them for the 4.3 v6 to be Compression Pressure 180 PSI (1241 kPa) And the big blocks 7.4 - 502 to be at Compression Pressure 150 PSI (1034 kPa). So the small block should be some where in these numbers. Here is some general info to look for while testing your engine. Good luck with it.Engine Runs Poorly At High RPM
Cause Special Information
Also refer to “Poor Boat Performance”
Crankcase overfilled with oil Check oil level with boat at rest in the water.
Anti-siphon valve (if equipped) Restricting fuel supply
Plugged fuel tank vent
Fuel supply Refer to “Carburetor Malfunctions”
(See “Table of Contents”)
Ignition timing
Low grade of fuel or water in the fuel
Spark plugs Fouled, burned, cracked porcelain, incorrect
heat range
Spark plug wires Poor insulation, broken wires
Distributor cap or rotor Dirty or cracked
Coil
Distributor Excessive play in shaft
Engine overheating Refer to “Engine Overheats”
Low compression Worn valves, rings, cylinders, etc.
Restricted exhaust.

Thanks for the reply. My buddy was told this weekend by a GM Mechanic that the distributor he is running had known issues or intermittent problems when the engine was under load. It would loose power and backfire just like he was experiencing. Not sure yet what they are trying next to fix it. The Merc. Mechanics didn't know anything about it.

Long shot but here is some service bulletins to look at. Open it up look for service bulletins, open it and on the left open 1999 and look over a few of them, that might be what they are talking about. And there was one with water getting in side under the cap B/C of a vent or something like that. Good luck with it. http://www.boatfix.com/merc/mainmenu.pdf

My guess would be the distributor or an intake valve. Sounds like you've tried the distributor and it didn't resolve the problem. I have heard of intake valves causing very similar problems and are sometimes difficult to find. Definately do a leak down and hopefully it will show up.

Good luck and keep us posted.

John

PS Is this carbed, throtle body or EFI?