496 8.1 Back Firing Through Flame Arrester.

outlaw32

outlaw32

Member
Mar 15, 2014
72
St Petersburg Fl
Boat Info
340
Engines
496 V drives
I have a 2003 340 searay with 496 8.1 engines. I just replaced the exchaust manifolds because of water intrusion which hydra locked the engine. Lucky no damage. Motors run fine but when under load getting popping back through the flame arrester. I have check all wires installed new plugs, did compression check and everything seem good. Still popping just not all the time. Could I have bent a exchaust valve push rod when I hydra locked motor so exchaust valve not opening all the way.
 
How old are those wires ? They have a track record of not lasting that long with that engine.
 
Sounds like a good place to start. Merc wires are stupid expensive. Volvo Penta has a better price. If it was my boat I would make my own. Summit / Jegs
 
Probably is a valve hanging up. Ran fine before this episode ? Leakdown test should pinpoint the sealing issue.
 
Check fuel pressure. Should be 38-43 lbs depending on load. Check if fuel pressure is low when engine is popping. Other than that I had a few that injectors weren't flowing enough, I pulled them out and had a shop clean and test them.
 
water in the fuel, cross-firing thru the dist cap , carbon tracks, weak/broken valve spring
 
had to grab man30 to look , individual coils for each plug. so the crank pos sensor, bad plug, bad intake valve, a injector running lean/clogged resulting in a very lean fuel mix,
a sensor resulting in a lean fuel mix.


2. Install new spark plug wires in proper order.
IMPORTANT: Wire routing must be kept intact during service and followed exactly
when wires have been disconnected or when wire replacement is necessary. Failure
to route wires properly can lead to radio frequency interference, cross firing of the
plugs and/or shorting of leads to ground.
a. Position wires in spark plug wire supports and retainers.
b. Attach plug wires to appropriate spark plug and coil pack. Each end should fit
securely.
 
Last edited:
I would suspect something in the intake track. Most likely a bent intake pushrod, or worse bent valve except the bent valve would have given you low compression reading. Bent pushrod would still allow valve to seal and build compression.
 
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