715517

Hi,

just bought a 1998 raw water cooled 454 MAG MPI. It fires up and idles fine, but after few minutes of idling, it gets rough. It smells like gas and water behind the boat gets gas patches. The spark plugs are dry but carbon foul very fast. When new spark plugs in place, motor runs nice for a couple of hours. Operating temperature is somewhere near 120-130F, is this too cold for MPI? The boat planes good and gets about 5200 rpm WOT, have to try a new prop.

Original MEFI-3 had gone bad and now it has a MEFI-1 control unit and wiring harness from a 502 MAG MPI, but it should work right?

I have now replaced or inspected:
-New ECT-sender(the one with two pins)
-New thermostat(tried new 143F and 161F t-stat, 161F now in use)
-New raw-waterpump
-New IAC-valve
-New MAP-sensor

-Fuel pressure is about 35 PSI, had an aftermarket adjustable pressure regulator
-Injectors are tested by professional and they are OK
-Transparent vacuum hoses so i can see if regulators leak gas, but they're OK
-T-stat housing seems rusty, but not leaking
-Had it quickly in Mercruiser shop but they didn't find anything wrong with diagnostic tool. It had a ECT trouble code, but it disappeared when deleted and didn't come back. ECT-sender and wiring harness were inspected with multimeter and they are fine.

So is 120-130F operating temperature too low for MPI and it gets too rich at idle because of that? Why it's running this cold with a new OEM thermostat?

Any ideas what should i do next?

Thanks in advance for the answers, had a pretty ****ty summer with only 2 hours of boating

minxguy

My 99 454 MPI runs 160 water.

Ken

3pointstar

715517-- We need to get you on the water ASAP !!!!

Our 7.4 mpi runs around 165 F per the gauge on the dash. Its a 2000 and has just shy of 550 hours.

If this were my boat BEFORE I start jacking with all the electronics, fuel pressures etc. - I would get to the basics and concentrate on the thermostat and temperature sensor. Check all the connections and the seem good I would replace the thermostat- it is the simplest and least expensive. I know that if the engine is not at operating temperature all kinds of goofy things can happen - 120- 130 is pretty cool to run - Not saying that this is the final solution but I'm always looking for the easiest first.

let us know what happens

3pointstar

sonicss42

^^^^ What he said about changing thermostat and be sure to use the gasket with the 2 little copper contacts in them. Use very little gaskets sealer on this because if it doesn't see a "ground" the temp sender will read incorrectly and ECM will not make correct adjustments.

SB

iTrader: (1)

Mercruiser / Quicksilver # 27-53045Q01

715517

Thank you all for your posts!

The thermostat has been changed with a new gasket, just like in SBs picture.

Thermostat housing in Mag mpi 454 has two temperature senders, one for gauge and another for ECU. Sender for ECU has two pins, and i think another of those pins are for ground connection? I have also replaced this sensor, although the old one was working (measured both new and old senders). The sender for gauge has one pin, and it has ground connection trough thermostat housing(that's why we need this gasket with copper contacts). The gauge and wiring works, if i connect the wire to the ground, gauge goes max 250F.

SB

iTrader: (1)

You have the t-stat housing style in the pic ? And you are positive that you orientated the plastic t-stat spacer correctly and also put t-stat on top of spacer ? ?

715517

Yes that's the housing i have. I have now assembled it three times and checked and double checked it before assembling. Last time it was open was yesterday, and i put it back with a new gasket(with copper contacts) and no gasket sealer at all.

The contact surface between t-stat and t-stat housing is quite rusty, but i did put gasket sealant there and im pretty sure that it doesn't leak water

drumerjb

It sounds like you need to have ECM reflashed? Is it a 502 mag or HO ecm? I'm making an educated guess here. I think this is where you need to start & verify fuel delivery first. IMOP I do not think a cooler running temp will effect the fuel to the degree of the issues you are having.
Read this thread:http://www.offshoreonly.com/forums/g...lp-needed.html
Read this thread:http://www.offshoreonly.com/forums/g...-mefi-3-a.html

dereknkathy

do the 502 and 454 use the same injectors? if so, you are 10% rich right there. across the board. cuz that ecm thinks it is feeding 10% more engine than it is.