330 black scorpion low power

[Falky]

I have a 3330hp black scorpion that is chugging at idle, down at least one cylinder. When I take off its sluggish and wont rev above 3000 rpm. The engine will surge every now and then.
I have done the following so far:
checked water fuel separator = all good no water or debris
tested injectors = spray pattern appeared ok
tested engine compression = all cylinders ok
checked/cleaned the distributor cap = appears normal
tested fuel pressure = 30 psi on injection rail as per specs for this motor SN L679844
tested spark plug leads = between 12.8k ohms and 17.5k ohms, ht lead 750k ohms
Does anyone have an idea where to proceed from here ?

 

[Scott Danforth]

What are the compression numbers?

Check your neutral safety switch

 

[dubs283]

Posting test results as "ok" and "appears normal" doesn't tell us very much.

If you removed and inspected all 8 injectors and observed the spray pattern that again doesn't say much as a dirty injector spray pattern can appear visually good but still lack proper flow

There is a service bulletin regarding fuel pressure regulators on mpi engines including the scorpion models. IIRC the spec for scorpion models is 34 psi. You need to verify constant fuel pressure under a load along with a fuel system vacuum test.

Ohms tests on plug leads is good but a true test uses a kV meter to measure the voltage passing through the leads during engine operation

It's quite possible there are one or more fouled plugs causing the issue, be curious to know when the last full tune up was performed

Neutral safety switch would have little/no effect on engine running performance. The switch is part of the starter system, not the ignition or fuel supply systems

 

[Scott Danforth]

If neutral safety switch malfunctions, and motor thinks it's in neutral, RPMs are limited to 3000 rpm

 

[Falky]

Posting test results as "ok" and "appears normal" doesn't tell us very much.

If you removed and inspected all 8 injectors and observed the spray pattern that again doesn't say much as a dirty injector spray pattern can appear visually good but still lack proper flow

There is a service bulletin regarding fuel pressure regulators on mpi engines including the scorpion models. IIRC the spec for scorpion models is 34 psi. You need to verify constant fuel pressure under a load along with a fuel system vacuum test.

Ohms tests on plug leads is good but a true test uses a kV meter to measure the voltage passing through the leads during engine operation

It's quite possible there are one or more fouled plugs causing the issue, be curious to know when the last full tune up was performed

Neutral safety switch would have little/no effect on engine running performance. The switch is part of the starter system, not the ignition or fuel supply systems

I have a constant 30 psi under load at the fuel test point. I did not do the vacuum test. I replaced all plugs and leads as well as the ignition coil. The spark coming off the plugs is very small. I agree about the injectors and have ordered a new set, also got a new distributor on the way which has the Hall effect sensor and amplifier built in.

 

[alldodge]

Your throwing parts when the problem is yet to be found. The motor needs to be scanned for codes. You have a MEFI 3 ECM so checking for codes is easy

Might be the MAP sensor or a bad sensor causing the 5V supply to drop below 5V. Measure the MAP or TPS to see if voltage is exactly 5V with key ON and running.

What is the MAP and TPS voltage when running?

To check codes, find the DLC connector and use a 12Vresistor LED or a digital meter with a tone function. Place paper clip and connect LED or Meter. Turn key ON and count flashes or beeps.

Example
Flash, pause, Flash, Flash longer pause = 12
If there are more codes there will be a longer pause and another set of Flashes will start. The Flashes/Beeps will repeat 3 times

 

[Falky]

Your throwing parts when the problem is yet to be found. The motor needs to be scanned for codes. You have a MEFI 3 ECM so checking for codes is easy

Might be the MAP sensor or a bad sensor causing the 5V supply to drop below 5V. Measure the MAP or TPS to see if voltage is exactly 5V with key ON and running.

What is the MAP and TPS voltage when running?

To check codes, find the DLC connector and use a 12Vresistor LED or a digital meter with a tone function. Place paper clip and connect LED or Meter. Turn key ON and count flashes or beeps.

Example
Flash, pause, Flash, Flash longer pause = 12
If there are more codes there will be a longer pause and another set of Flashes will start. The Flashes/Beeps will repeat 3 times

View attachment 392261

Your throwing parts when the problem is yet to be found. The motor needs to be scanned for codes. You have a MEFI 3 ECM so checking for codes is easy

Might be the MAP sensor or a bad sensor causing the 5V supply to drop below 5V. Measure the MAP or TPS to see if voltage is exactly 5V with key ON and running.

What is the MAP and TPS voltage when running?

To check codes, find the DLC connector and use a 12Vresistor LED or a digital meter with a tone function. Place paper clip and connect LED or Meter. Turn key ON and count flashes or beeps.

Example
Flash, pause, Flash, Flash longer pause = 12
If there are more codes there will be a longer pause and another set of Flashes will start. The Flashes/Beeps will repeat 3 times

View attachment 392261

Turned out to be the coil all along, thanks for the help.

 

[QBhoy]

If neutral safety switch malfunctions, and motor thinks it's in neutral, RPMs are limited to 3000 rpm

A good call Scott and crossed my mind too. But the mention of being sluggish before those rpms too…made me think otherwise. Can’t remember if it’s 2500 or 3000 rpm the neutral switch would cause. Only happens on some mpi from about at least the mid 2000’s onwards too, as most will know. My 2003 has no limiter in neutral at all. But aware of plenty from the late 2000’s that do. Certainly the case for all the modern outboards too. My own mercury outboard on the other boat is exactly at 2500 rpm.
Anyway…back to this post…I’d be likely ruling out the distributor cap properly, by replacing it entirely. Can never assume these are good at all, certainly not by cleaning or even visually looking ok. Learned that lesson years ago. Many have since too.
I’d also be looking at the fuel delivery. Although it may indicate decent numbers on the gauge at the time of testing…unless it’s monitored constantly and in particular, at the moment the issue starts to appear…I wouldn’t rule out the suspicion towards fuel pick up and check valves. Fuel regulator issues too.
I’d also never rule out the chance of it being electrical related. Battery isolation switches, main harness, terminal post connections and continuity…earthing or shorting perhaps, should all be considered.

 

[alldodge]

Sure wish more learned to use the return key

 

[QBhoy]

Sure wish more learned to use the return key

Was that a wee dig at me ? Not sure I get you, if so. Typing on phone, as usual.

 

[alldodge]

Anyone who doesn't break up the paragraph, makes it harder to read

 

[dubs283]

The short lived MEFI controlled small blocks mercury used for mie did not utilize the neutral safety switch as an additional "in gear" switch to control rpm in neutral. That wasn't introduced until the 555 series ecm was used and in such case would set a fault along with an audio alarm warning but also would not affect engine running performance from idle to approx 2500 rpm

The MEFI controlled mpi engines used the neutral safety switch to control the ground side of the salve solenoid primary circuit and nothing else.

Sounds like OP figured it out with a new coil which ime can cause the issues noted previously