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Carb to EFI conversion questions?

Old 04-08-2003, 09:47 AM
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Default Carb to EFI conversion questions?

There are a couple of complete Mercruiser EFI setups on EBAY, how hard is the conversion from a carbed motor to the Mercruiser EFI setup? What sensors are involved? Is the fuel system issue? Are there any wiring issues? Most of the auctions state that everything is included, has anyone done this and compiled a complete list of required parts? My current motor is a brand new 454HO which has retangular port heads, are the Mercruiser EFI intakes retangular port intakes? Will the ECM need to be reprogrammed for my motor?

Is it worth changing? I have the alum intake that came with the motor from GM and the stock carb on it now. Can I expect a performance gain with the EFI system?

1994 Baja 260
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Old 04-08-2003, 11:03 AM
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Default If everything is there...

No problem changing it over if everything is there but you won't get a performance increase. It will idle a little better and give you better fuel economy though. I don't think you will find the slight improvement worth the effort involved.
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Old 04-08-2003, 01:45 PM
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1. ECU will need reprogramming if it is from a 502 if going on a 454, from 454 to 454 is ok. If you have a 425hp 454 crate motor though you will need custom programing to match your cam/set up.

2. You will need both wireing harnesses, the efi harness and the main engine harness, most of the time the seller does not include the engine harness just the efi harness.

3. The common sensors missing will be a knock sensor usualy left in the old block and the water temp sensor that goes to the ecu usually left in the thermostat housing, this a second water temp sensor in addition to the water temp sensor that goes to the guage. Other sensors are TPS, MAP,MAT but they are usually still attached to the plenum and throttle body

4. You will need the the electric fuel pump/regulator/relays and fuel lines(and VST tank depending on the year it is) may or may not be with a "complete unit"

5. The ignition coil and distributer are different if your carb engine has a the Thunderbolt ignition may or may not be with a "complete unit"

6. Lifting brackets/eyes which also mount the wireing harness plug are different, the ones from the carb engine won't work and you need a way to lift the engine into the boat

7. The injectors will need professional cleaning from sitting .

8. do not skip 7 or you will be rewarded with one or more burnt pistons.

9. the metal bracket holding the 50 amp breaker is different , the carb one is on the wrong side of the engine for the new wireing harness

10. you will need the ecu bracket which also mounts the fuel pump relays, may or may not come with a "complete" unit

11. various bolts may be missing which you have to buy from merc because they are odd ball lengths, 3/8-16 x 5 3/4" to mount the intake manifold comes to mind.

12. the merc intake is a sort of inbetween the large oval and rectanguar port for shape but it is meant to be used with rect port heads

When I did this I was installing the efi on a new 502 long block, I probably would not do it as a changeover, gains would be minimal

Last edited by ursus; 04-08-2003 at 02:02 PM.
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Old 04-08-2003, 04:27 PM
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Stick with the carbs.
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