Mer cruiser black scorpion no power and backfiring

[Ian-r]

I have a 2010 mer cruiser black scorpion 350. Issue is no power under load, backfiring, rough idle when hot. Have done all the usual, fuel, fuel pump, injectors 'plugs leads distributor etc. scan tool shows all sensors ok. May possibly be ect but this is a bit scary due to cost. Anyone have any ideas that may assist?

 

[Bt Doctur]

you don't list a location but have you checked your fuel. put a sample in a clear container, add the same amount of water, shake well, look for a separation line.
should be at the first level unless you have ethanol and water mixed in.

 

[Ian-r]

Totally flushed fuel system fresh tank of 98 octane. Still the same.

 

[Bt Doctur]

the backfiring could indicate a lean fuel mix or intake valve not sealing , cross-fire from the plug wire or cap.bad "crab cap"
With all the plugs removed and the throttle plate wide open, what are the compression readings.
A leakdown test would help isolate the problem too.

 

[Ian-r]

Compression all ok. New plugs and leads. Engine top end is all fine. Only has 220hrs on it.

 

[Scott Danforth]

Did you replace the crab cap?

 

[Ian-r]

What is the crab cap?

 

[Ian-r]

If you mean the distributor cap, it is a hei, not crab style. Checked and cleaned.

 

[Bt Doctur]

whats the fuel rail pressures with and without the vac line connected, not just an ok, the actual numbers

 

[alldodge]

Howdy

The black scorpion is a 377 (350 with a 400 crank) or a 6.2L
Do you have an engine serial number?

 

[Bt Doctur]

One has to assume that this motor did at some time run correctly. Something has happened and all the original tests and reading are out the window.
You need to start from scratch. New compression readings, new leakdown readings. TDC of #1 verified with a piston stop tool to verify mark on harmonic balancer the correct.spark plug firing order, fuel pump pressures and flow.
You may have to test with a remote gas tank to rule out a fuel line/fuel tank issue.

Does this motor use a crank position sensor or not?

 

[Scott Danforth]

If you mean the distributor cap, it is a hei, not crab style. Checked and cleaned.

HEI is an automotive cap. Post pictures.

I suggest you replace cap vs cleaning it

 

[Ian-r]

Fuel rail 34 psi at wot 26 psi at idle pressure reg working ok but changed to make sure. High pressure fuel pump has been changed and all filters.

 

[Ian-r]

It is a black scorpion 330hp s/n 1A092588 350 chev

 

[Ian-r]

Motor did run fine until just before it went to local mercruiser mechanic for a service. Told them it was running a bit rough. They did the service and told me to put mer cruiser fuel treatment through it and charged me $800. Never ran properly since then. I pulled the plugs and they were filthy, so they have never serviced it properly. Went back to basics, all compression checks etc, engine back to tdc firing order ok plugs leads replaced still no good. Replaced crank sensor. Only thing I have have not tried is remote gas tank but I have totally cleaned all fuel lines and emptied tank.

 

[thumpar]

I would check the firing order again and timing also. Have you called the shop to see if they will fix what they broke?

 

[alldodge]

Fuel rail 34 psi at wot 26 psi at idle pressure reg working ok but changed to make sure. High pressure fuel pump has been changed and all filters.

Your fuel pressure should be 43psi at 1800 rpm. As BT mentioned above, at idle check fuel pressure and while still at idle remove and vacuum line from the regulator, what is the pressure change.

 

[Fun Times]

Have done all the usual, fuel, fuel pump, injectors 'plugs leads distributor etc.

What did you do to check/inspect the injectors? Were the spark plugs fuel fouled?

Looking up Ian-r s fuel pressure regulator part number 807952A1, the specifications suggests his pressure should be in the 30 PSI range.
http://forums.iboats.com/forum/engi...ives/656714-350-mag-mpi-fuel-pressure-problem

Here's the correct parts diagram for this engine model. This model does use a crankshaft sensor but not the crab style cap.
http://www.mercruiserparts.com/selectDocs.asp?doc_nbr=835003007

Motor did run fine until just before it went to local mercruiser mechanic for a service

Call them and ask them (the servicing tech) if they by chance re-adjusted the ignition timing by turning the distributor shaft assembly. If they did, they weren't supposed to on this model but being the design of the distributor, I could foresee someone moving it since it looks like the older distributor assembly found on the carbureted engine models. Sometimes looking at the paint on the distributor hold-down bolt helps indicate if the bolt was disturbed by a wrench.

 

[Ian-r]

Mercury service guy said not their fault. I am not spending any more money with them . Dodgy outboard mechanics, do not do much inboard work and take no responsibility for their stuf ups. Only dealer in town so I have to sort it myself. They had played with the distributor and it was loose so I have reset it to no1 tdc with rotor in line with no 1 pin. Fuel pressure is correct for this model engine 30-34psi. Plugs lead firing order is correct. Have double checked it several times. I think it's down to an ecm555 fault but just want to know if I am missing anything before I send it away for test and possible replacement.

 

[Ian-r]

Injectors cleaned and tested in injector booth. Spark plugs were fouled with buildup so I replaced them.