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5.7 run ruff ( Ecu )

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Markhodges78

Contributing Member
So I have a 1998 mercruiser 5.7 that after it sets for a few weeks it runs ruff the only fix it to ground the purple/white wire to put it into base mode start engine then turn engine off unground the purple /white wire then the engine runs perfect for a week so is this the ecu or knock module? Anyone experienced this?
 
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I'd say its more likely the ignition module than the knock sensor....monitor the voltage on the same wire to tell if the knock sensor is activating...
 
I'm thinking corrosion
Being a 98 it's most likely TBV and "should" have the 3 wire ignition module (serial number would verify)

Remove and clean the connections on the ignition module and distributor pickup.
 
Markhodges78 said:
I did unhook the knock module and the engine was still in base mode.
Click to expand...
Knock sensor, TPS, MAP can't affect ignition and putting it into base timing won't kill spark, either. WHile it was in base timing, did you check the timing advance? Have you checked for codes?

How long since the plugs, wires, cap and rotor were replaced?
 
AllDodge said:
I'm thinking corrosion
Being a 98 it's most likely TBV and "should" have the 3 wire ignition module (serial number would verify)

Remove and clean the connections on the ignition module and distributor pickup.
Click to expand...
I was thinking the same, and I did find and clean corrosion on 1 of the pins on the knock module
Serial number was removed and there is no longer a plastic cover on the breather.
 
I don't think it's the knock module. You can disconnect the wire at the sensor and move it out of the way. Keep it clear of laying along side plug wires, should not change anything
 
The knock sensor and module only cause the ECM to retard timing, they don't interrupt spark.

Why are you grounding the Ref Hi (purple/white) wire?

Is it always the same cylinder(s), or does the misfire change to others?

Again, how long since the plugs, wires, cap and rotor were replaced?
 
jimn said:
Knock sensor, TPS, MAP can't affect ignition and putting it into base timing won't kill spark, either. WHile it was in base timing, did you check the timing advance? Have you checked for codes?

How long since the plugs, wires, cap and rotor were replaced?
Click to expand...
It doesn't have a TPS or Map there was is no spark Killed when its messing up. when it has the problem I check the timing and its in base mode to fix the problem I connect the purple and white to put into base mode and disconnect the purple and white to take it out and the problem is gone until it sets for a week or two
 
jimn said:
The knock sensor and module only cause the ECM to retard timing, they don't interrupt spark.

Why are you grounding the Ref Hi (purple/white) wire?

Is it always the same cylinder(s), or does the misfire change to others?

Again, how long since the plugs, wires, cap and rotor were replaced?
Click to expand...
its not misfiring its going into base timing mode with out connecting the purple and white the only fix is to connect and disconnect the purple and white the plugs cap and button was changed last summer when the motor was put back in. Got the boat with a broken Y pipe and the motor was already disconnected but still setting in the boat.
 
Sorry if I didn't explain it well, When this happened the first time I hooked a timing light up and seen that the timing would not move when the rpms changed. so I put it base timing mode checked to confirm base timing mode turned engine off took it out of base timing mode restarted engine went out the next day and it ran great the whole day. it set for 2 weeks and it did it again. this time i put it in base timing mode turn key on , turn key off, removed jumper started boat and it ran great again the next few trips out. it seems to have the issue when it sets for a week or 2
 
Ya know, look under the distributor cap after sitting and see if it's wet

The TB distributor is known to condensate, this is also where the corrosion will show up and cause issues if you don't have the 3 wire module. The ground lead should have a clean connection to ground
 
AllDodge said:
Ya know, look under the distributor cap after sitting and see if it's wet

The TB distributor is known to condensate, this is also where the corrosion will show up and cause issues if you don't have the 3 wire module. The ground lead should have a clean connection to ground
Click to expand...
Ill check Would this send it into base mode ?
 
Markhodges78 said:
I will check the ground
Click to expand...

Erratic, not 'eradicate'.

When you do this, measure from the IC module base plate to the engine. If you see more than about 1 to 1-1/2 Ohms, clean the base of the distributor. If it goes too high, the black ground wire won't be used as the ground for the ignition. IIRC, it will use the Tan/black Bypass wire- I'll check my manual tomorrow to make sure.
 
jimn said:
Erratic, not 'eradicate'.

When you do this, measure from the IC module base plate to the engine. If you see more than about 1 to 1-1/2 Ohms, clean the base of the distributor. If it goes too high, the black ground wire won't be used as the ground for the ignition. IIRC, it will use the Tan/black Bypass wire- I'll check my manual tomorrow to make sure.
Click to expand...
Ok thanks you .. it will be a few days before I'm able to check it
 
Markhodges78 said:
its not misfiring its going into base timing mode with out connecting the purple and white the only fix is to connect and disconnect the purple and white the plugs cap and button was changed last summer when the motor was put back in. Got the boat with a broken Y pipe and the motor was already disconnected but still setting in the boat.
Click to expand...
Disconnecting the purple/white wire breaks the connection from the IC module to the ECM and when that happens, it can't know the engine speed- it needs RPM to know what to do. <300 RPM means it's cranking, >300 RPM means it's running. Once it's running, the MAP sensor or whatever is used in its place is one of the main sensors used for engine performance- the TPS is used to deliver the proper amount of fuel- when the throttle delta (change) is +20° or more, it goes into acceleration enrichment and when it changes by -20° or more, it goes into deceleration enleanment and these let it know that the boat needs more or less fuel for acceleration or rapid slowing. The IAC follows the throttle in these casess, to add or subtract air.
 
Markhodges78 said:
So I have a 1998 mercruiser 5.7 that after it sets for a few weeks it runs ruff the only fix it to ground the purple/white wire to put it into base mode start engine then turn engine off unground the purple /white wire then the engine runs perfect for a week so is this the ecu or knock module? Anyone experienced this?
Click to expand...
What is the engine serial number?
 
Ok I didnt get to take pictures before it got dark but I did do some voltage readings
Purple 12.2 volts
purple / white 5 volts
gray 10 volts
I did not check the blue I will tomorrow

This is with the engine not running

1741397187604.png
 
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