Mercury 175 EFI Boggs down under load

[KAMEagle]

Recently installed thermostats to see if Engine would idle better at low speeds. Not sure if it had anything to do with my current issue. Engine runs great using muffs at idle and higher speeds, in and out of gear. However it boggs down horribly under a load in the water. I can put it in neutral and it will rev up just fine. A couple of times I played with the trim and it kicked in and ran fine. I replaced the primer bulb and it had no effect.

 

[Faztbullet]

Need to test spark and fuel pressure

 

[Dukedog]

year?..

 

[KAMEagle]

It’s a 91. I did find one bad coil. Will be doing a deep dive latter to check fuel pump I did a cold compression check and average about 90 psi in all cylinders.

 

[KAMEagle]

Fuel pressure is 39. Could 1 bad coil cause the issue?

 

[Faztbullet]

Check spark ..should jump a minimum 3/8 air gap. Sounds like you got a stator or switchbox down.

 

[Dukedog]

do ya have access to another gauge?.. 90 lbs is tired/low.. how are ya testin' comp?

 

[KAMEagle]

I bought a new one and will recheck those. One thing a bit confusing is that they say the plugs should have a high meg reading. All my plugs read extremely high so I purchased new ones along a new compression tester. All the new plugs read less than 5 ohm. They are of course the flat spark type

 

[KAMEagle]

Cylinder compression averages between 115 and 120. Coils 1 and 4 are not working. 2 and 5 are firing great, and 3 and 6 are firing but not as good as 2 and 5. All but 1 and 4 have a good blue spark. Fuel pressure is 39. It’s sounding like four bad coils. Could not read primary input voltage on the coils

 

[Dukedog]

swap switch box leads from 1 with either 3 or 5.. see if fire fire pattern follows? sounds like a coupla switch boxes are in ya future.. maybe stator also..

wouldn't worry 'bout plugs.. i can say i've never had a surface gap plug go bad with any merc...

 

[KAMEagle]

swap switch box leads from 1 with either 3 or 5.. see if fire fire pattern follows? sounds like a coupla switch boxes are in ya future.. maybe stator also..

wouldn't worry 'bout plugs.. i can say i've never had a surface gap plug go bad with any merc...

That rules out the coils. The problem stayed with the wires. Is there a way to test both the switch box and stator?

 

[Dukedog]

send me an email addy and i'll send ya an OEM manual (pdf)..... your motor should have tha water cooled rec/reg?.. i may not have tha test for it?

 

[KAMEagle]

send me an email addy and i'll send ya an OEM manual (pdf)..... your motor should have tha water cooled rec/reg?.. i may not have tha test for it?

My trigger coil is definitely bad. It’s a open circuit on all wires. The stator wires between the blue and the red on one side are 170 ohms less than a minimum of 3500. The other side is within the limits. Should I replace the stator?

 

[KAMEagle]

send me an email addy and i'll send ya an OEM manual (pdf)..... your motor should have tha water cooled rec/reg?.. i may not have tha test for it?

I appreciate your help. Replaced the switch boxes and trigger and she runs great. Today was the the first we were able to take the boat back out on the lake!