Resolved: 1996 5.7L Scorpion Backfire

[jwaymer]

Just started helping a friend with his boat problem....1996 Maxum with V-drive 5.7L Scorpion MPI. He told me that it idles fine, but when attempting to accelerate, that it backfires thru the arrestor. He bought the boat used, but has never been able to run it properly. So far has replaced the MAP sensor with no change.<br /><br />This week I am going to take it on a water run with the laptop hooked up to get some data. I also suspect it may be a timing problem, so will check this also.<br /><br />Any other suggestions or anyone with similar problems?

 

[Don S]

Re: Resolved: 1996 5.7L Scorpion Backfire

Has he done the basics, tuneup (cap, rotor, plugs), checked compression, checked plug wires?

 

[jwaymer]

Re: Resolved: 1996 5.7L Scorpion Backfire

Cap and rotor only...I am going to check the plug wires (for correct order too) and plugs. I can check compression for him also.

 

[Don S]

Re: Resolved: 1996 5.7L Scorpion Backfire

See if you can get a serial number for the engine also.

 

[Reel Poor]

Re: Resolved: 1996 5.7L Scorpion Backfire

Also check fuel pressure and fuel vaccum between tank and pump.

 

[lilmandavis]

Re: Resolved: 1996 5.7L Scorpion Backfire

i was inspecting my motor after backfires started. they started when i hit the gas to go then cleared. i could detect a miss. it was a bent pushrod. did not backfire all the time. turned out to be a lobe ground off the cam

 

[saltwater_cowboy]

Re: Resolved: 1996 5.7L Scorpion Backfire

Just out of interest is it a maxum 210ss / 200ss skimax?

 

[jwaymer]

Re: Resolved: 1996 5.7L Scorpion Backfire

I believe it is the 210ss skimax

 

[saltwater_cowboy]

Re: Resolved: 1996 5.7L Scorpion Backfire

The skimax was never offered with MPI, just 2bbl, 4bbl and TBI.<br /><br />If someone has retro fitted a MPI engine then this mabe be the cause of your troubles. Check all the wiring and connectors.<br /><br />I have the same boat. They are quite rare.

 

[jwaymer]

Re: Resolved: 1996 5.7L Scorpion Backfire

Not sure if it is a "skimax", but it is definetly a 210ss with "ski" written after it. It's a 1996.

 

[jwaymer]

Re: Resolved: 1996 5.7L Scorpion Backfire

Summary of what has been done:<br /><br />Plugs/wires replaced - OK<br />Firing order checked - OK<br />MAP sensor replaced - new, diagnostic tool shows OK<br />Fuel pressure and vacuum checked - OK<br />Base timing checked - OK<br /><br />Water test today:<br />Smooth, gradual acceleration does not backfire, but hesitates during accel. More aggressive acceleration...hear "sucking" sound at air intake, hesitates and then "catches" and starts to accel. Full throttle acceleration back fires almost immediately.<br /><br />Data from diagnostic tool during full throttle accel shows:<br /><br />Spark advance: 31 degrees<br />Power enrichment: Off, then ON, at about 31 degrees of spark advance.<br /><br />Question: What is "Power Enrichment"?

 

[Don S]

Re: Resolved: 1996 5.7L Scorpion Backfire

Were you able to locate a serial number?

 

[Reel Poor]

Re: Resolved: 1996 5.7L Scorpion Backfire

Were there any codes in the system?<br /><br />The injectors may be cloged on the inlet side. There are small screens there that will stop up with sludge and restrict fuel flow. A lean condition will cause the sucking noise and the backfire condition.

 

[jwaymer]

Re: Resolved: 1996 5.7L Scorpion Backfire

Sorry Don S, I looked at the S/N, but forgot to write it down. I'll get it soon.<br /><br />Reel...the other day there was one code, 34 or 33, it was MAP sensor related. I reset it, but it didn't come back.<br /><br />As far as fuel injectors...the fuel flow reading on the slow accel steadily rose with RPM and throttle position. On the full throttle, it stayed at 1.2gph and did not rapidly increase as I would expect it. At the backfire it jumped to 10gph, but we backed off the throttle right away. Maybe this would indicate an injector problem?

 

[Reel Poor]

Re: Resolved: 1996 5.7L Scorpion Backfire

The backfire could have set the map sensor code.<br /><br />Can you do an injector balance test? This should reveal possible injector blockage.<br /><br />Also do a vaccum test on the engine. May reveal possible weak valve springs or sticking valves.

 

[jwaymer]

Re: Resolved: 1996 5.7L Scorpion Backfire

I don't know how to do an injector balance test. Any advice?

 

[Reel Poor]

Re: Resolved: 1996 5.7L Scorpion Backfire

Its done with a scan tool and a fuel pressure guage.<br /><br />With the fuel pressure guage and scan tool installed you manually activate the injectors for a determined period of time and watch the fuel pressure drop. The amount of pressure drop is relative to each injectors open time and ability to flow fuel.<br /><br />If you don't have a scan tool you can pull the fuel rail and visibly check the injector inlets for blockage. Also check the fuel rail itself for rust. The rail can rust on the inside and start flaking off little pieces which will clog the injectors.<br /><br />BTW visual inspection will not tell you if the injector is mechanically able to flow the proper amount of fuel. It will only let you see crud build up.

 

[jwaymer]

Re: Resolved: 1996 5.7L Scorpion Backfire

I have the Rinda Diacom software. It doesn't allow for me to activate the injectors on the MEFI 1, only the PCM555. Any other way to do it?

 

[Reel Poor]

Re: Resolved: 1996 5.7L Scorpion Backfire

OTC makes an injector tester for about a hundred dollars.<br /><br />Seriously, a visual inspection may reveal the problem.<br /><br />Be sure to check all pin connectors for corosion. Check all ground also.

 

[newport dave]

Re: Resolved: 1996 5.7L Scorpion Backfire

Any chance of looking at the data you recorded? I have Diacom also and if you could email me a copy of the data file, I would like to take at look at it. <br /><br />Also. I have an injector balance pulse tool I would sell cheap if you are interested.<br /><br />Dave