Mercrusier 350 MAG MPI - RPM issue under a load

[wbas1]

The TPS should be 4.5V at WOT. Disconnect the linkage and see if you can get it to increase. If it will go up to 4.5V then check into the throttle linkage as to why its not moving that far

How do the readings on the MAP look? Are they ok

 

[alldodge]

The MAP 1.5V is good for idle, ignition ON not running should read 4.5V
Should also show 4.5V at WOT

 

[wbas1]

The MAP 1.5V is good for idle, ignition ON not running should read 4.5V
Should also show 4.5V at WOT

View attachment 366343

This might be a stupid question, but at idle I am at 650rpms and the volts on the map are 1.8. When I increase the throttle to 1k rpms the map volts go down to 1.5. Is that right?

 

[wbas1]

The TPS should be 4.5V at WOT. Disconnect the linkage and see if you can get it to increase. If it will go up to 4.5V then check into the throttle linkage as to why its not moving that far

I disconnected the linkage and I get 4.5v. I adjusted the linked and I can get to 4.2 due to some slack in the cable.

 

[alldodge]

Hey were getting some where
When the rpm's are increased in neutral the vacuum can increase do to more CFM being drawn in. It all goes according to the graph above. At WOT there is next to no vacuum

 

[wbas1]

Hey were getting some where
When the rpm's are increased in neutral the vacuum can increase do to more CFM being drawn in. It all goes according to the graph above. At WOT there is next to no vacuum

Lol! At least you have an idea. I am clueless.

 

[alldodge]

The ECM controls the motor based on inputs from sensors. If you open the throttle 3/4 then the ECM says you want 3/4 power. The TPS was not opening all the way, so motor is not getting full power.

The MAP vacuum is not going to near zero so ECM says things agree, 3/4 throttle and 3/4 power. When the MAP is disconnected it tells ECM you are at WOT so let me give motor a bit more fuel based on the tables. This is why it increases a bit more power but not all like before.

Only way to get full power is to tell the ECM the TPS is also at WOT. So cable was adjusted so it gets much closer to that point. I'm betting the motor will run better because the TPS will be open more, and the MAP will show less vacuum, so then the max area of the fuel and spark tables will be where they should be

Otherwise it going to be

 

[wbas1]

The ECM controls the motor based on inputs from sensors. If you open the throttle 3/4 then the ECM says you want 3/4 power. The TPS was not opening all the way, so motor is not getting full power.

The MAP vacuum is not going to near zero so ECM says things agree, 3/4 throttle and 3/4 power. When the MAP is disconnected it tells ECM you are at WOT so let me give motor a bit more fuel based on the tables. This is why it increases a bit more power but not all like before.

Only way to get full power is to tell the ECM the TPS is also at WOT. So cable was adjusted so it gets much closer to that point. I'm betting the motor will run better because the TPS will be open more, and the MAP will show less vacuum, so then the max area of the fuel and spark tables will be where they should be

Otherwise it going to be
View attachment 366344

Lol! Thanks a lot. As soon as I can get out I will try it. Maybe I can just fly you out to NY to fix it for me.

 

[nola mike]

When the MAP is disconnected it tells ECM you are at WOT so let me give motor a bit more fuel based on the tables.

Don't know about this particular system, but usually the pcm can tell that the MAP is disconnected and will default to a stock fuel map based on other inputs (tps and rpm for example)

 

[wbas1]

Update:
I finally had a mechanic come by yesterday and put a laptop to the motor. All the readings were normal at the dock & under a load. He mentioned that the boat just seems to be working hard trying to get the boat on a plane. He also thinks the boat is over prop'd. He did mention that maybe the original motor had a larger bore and stoke setup and that is why it had the 6.2L ECM on it. I know people have said the 6.2L ECM works fine on the 5.7L I have now. I am going to see if I can get the original props re-pitched to 18's or I might just need to by new 4 blades at 18 pitch. More to follow.

 

[alldodge]

A 4 blade 18 is like a 3 blade 20

 

[wbas1]

Update: I received the new props 4 blade 18 pitch. Put them on today and took the boat for a ride. I was barely able to get the boat on a plan and at WOT I was only at 3400 rpm’s.. not sure what else I can do at this point.

 

[alldodge]

Update: I received the new props 4 blade 18 pitch. Put them on today and took the boat for a ride. I was barely able to get the boat on a plan and at WOT I was only at 3400 rpm’s.. not sure what else I can do at this point.

Post 1 list

Duo Pro 20 Pitch

Post 91

A 4 blade 18 is like a 3 blade 20

now

Update: I received the new props 4 blade 18 pitch. Put them on today and took the boat for a ride. I was barely able to get the boat on a plan and at WOT I was only at 3400 rpm’s.. not sure what else I can do at this point.

So if you originally had a 3 blade 20 and changed to a 4 blade 18, for the most part you didn't change anything

 

[wbas1]

Post 1 list


Post 91


now



So if you originally had a 3 blade 20 and changed to a 4 blade 18, for the most part you didn't change anything

Really? That is what the prop guy was telling me to go to? Any suggestions? I am going to call them on Monday because they said they would exchange for another type.

 

[wbas1]

Really? That is what the prop guy was telling me to go to? Any suggestions? I am going to call them on Monday because they said they would exchange for another type.

Should I go 3 blade 18 pitch?

 

[Scott Danforth]

For every blade you add, you are adding about 2 inches of pitch

4-blade 18p = 3-blade 20p

 

[Scott Danforth]

Your original motor was most likely a 6.2 (and yes the block will be a 5.7 block)

The 6.2 ECM will dump more fuel

 

[alldodge]

Should I go 3 blade 18 pitch?

Yes 3B 18 but 3B props normally come in odd numbers. If that's the case get 3B 17. If rpms get to high and your on plane go with 3B 19, but start lower first

 

[wbas1]

Your original motor was most likely a 6.2 (and yes the block will be a 5.7 block)

The 6.2 ECM will dump more fuelTh

Your original motor was most likely a 6.2 (and yes the block will be a 5.7 block)

The 6.2 ECM will dump more fuel

I thought the same thing but I called Mercury and gave them the serial # and they said it was a 350.

 

[wbas1]

Yes 3B 18 but 3B props normally come in odd numbers. If that's the case get 3B 17. If rpms get to high and your on plane go with 3B 19, but start lower first

It looks like the company has 4 x 3 not 3 x 3. Will those be any good?