500rpm low at WOT 5.7 merc black scorpion

[monkstr6100]

Well I broke down and got a diacom program of my own. This thing shows EVERYTHING! My question is what should the duty cycle of the IAC be? At idle it's around 30% which I think is normal! but as I throttle up it increases with the throttle. At WOT it the duty cycle goes all the way up to 70-80%? Is this normal? I assume that the duty cycle reflects how far OPEN the IAC is? So it would go down to almost nothing at full throttle or anything over idle for that matter? I am seeing this wrong? Thanks guys. Zach J.

 

[monkstr6100]

Also, according to the RPM history this engine has NEVER gone over the 4500-4999 rpm range. And only logged .87hrs in the range ever!

 

[500dollar744ti]

Are you putting premium non-ethanol 93 octane gas in it?

Modern electronically controlled engines are going to pull timing there's any knock, a few degrees of timing pulled at WOT can easily bring down your max rpm.

I know it sounds elementary but it's something to think about if you're putting the standard 89 octane in it or even worse, the 87 pump gas.

Back in the day, if you ran too much timing at WOT or put in crappy gas, the engine would just blow up.

 

[monkstr6100]

I run 87 ethanol free. That's what the merc rep told me as well as the manual for my engine. Per diacom it runs EXCTLY 28deg timing at WOT. My buddy was a little off when he checked it. At idle and lower rpms it does swing around, but that is the ecm doing what it is supposed to do. I am learning a lot through here. Thanks guys.

 

[alldodge]

Well I broke down and got a diacom program of my own. This thing shows EVERYTHING! My question is what should the duty cycle of the IAC be? At idle it's around 30% which I think is normal! but as I throttle up it increases with the throttle. At WOT it the duty cycle goes all the way up to 70-80%? Is this normal? I assume that the duty cycle reflects how far OPEN the IAC is? So it would go down to almost nothing at full throttle or anything over idle for that matter? I am seeing this wrong? Thanks guys. Zach J.

Just info but for your boat the specified rpm is 4700-5000, only the stern drive package is 4800-5200, and the engine is 320HP. So far as the IAC it has not impact on WOT and there is no information on what it should be doing at that rpm. Everything seems to be working within parameters so finding a fault is more challenging. My next suggestion would be to do an injector balance test. Injector Balance Test
Test Procedure
The injector balance tester is a tool used to turn the injectorONfor a preciseamount of time,
thus spraying a measured amount of fuel into the manifold. This causes a drop in fuel rail
pressure that we can record and compare between each injector. All injectors should have
the same amount of pressure drop. Injector testers are available for variousmanufacturers.
A pulse width that drops the fuel rail pressure to half the normal operating pressure should
be used.
SET-UP
1. Engine cool down period (ten minutes) is necessary to avoid irregular readings due
to heat soak fuel boiling.
2. Relieve fuel pressure in the fuel rail.
3. With ignition OFF, connect fuel pressure gauge to fuel pressure tap.
4. Disconnect harness connectors at all injectors.
5. Connect injector tester to one injector.
6. Use adaptor harness furnished with injector tester to energize injectors. Follow
manufacturer’s instructions for use of adaptor harness.
7. Ignition must be OFF at least ten seconds to complete ECM shutdown cycle.
8. Fuel pump should run about two seconds after ignition is turned ON.
9. Insert clear tubing attached to vent valve into a suitable container and purge air from
gauge and hose to ensure accurate gauge operation.
10. Repeat Step 9. until all air is purged from the gauge.

TESTING
NOTE: The entire test should not be repeated more than once without running the engine
to prevent flooding. (This includes any retest on faulty injectors.)
1. Turn ignition OFF for ten seconds and then ON again several times to get maximum
fuel pressure.
2. Record the maximum fuel pressure reading.
3. Energize tester one time and note pressure drop at its lowest point. Disregard any
slight pressure increase after drop hits low point.
4. Subtract the second pressure reading from the maximum fuel pressure to get the
amount of injector pressure drop.
5. Repeat testing for each injector and compare the amount of drop. Usually, good injectors
will have approximately the same amount of fuel pressure drop.
NOTE: If the pressure drop of all injectors is within 1.5 psi (10 kPa) of the average, the
injectors have proper flow.
6. Re-test any injector that has a pressure difference of 1.5 psi (10 kPa), more or less
than the average fuel pressure drop of the other injectors on the engine.
7. Replace any injector that retests outside the pressure difference of 1.5 psi (10 kPa),
more or less than the average fuel pressure drop of the other injectors on the engine.
Cylinder

As example manual states 21 psi change is to much pressure drop and is running rich, 17 psi drop is to little and runs lean, 19 psi drop is good.

 

[monkstr6100]

@alldodge, my reasoning behind thinking that the IAC being almost wide open at wide open throttle is that it would be the same as a large vacuum leak correct? I will get a tester for the injectors and do that test. The plate right on the BFA list my serial, RPM range, and HP, so that is what I was going off of. Thanks for informing me that's it's time to buy MORE diagnostic tools! Awesome. I think my diacom can only run one bank of injectors at a time. That could narrow it way down though I suppose?

 

[monkstr6100]

Did the balance test with my diacom program, on this engine you have to do one whole bank at a time, but was exact same drop on both sides. Did a resistance test on my coil today and unless I am doing something wrong I think my coil is weak. The primary was within pec but the secondary was only 5,700, and spec on that coil is 9,400-11,700. Also kinda makes sense then that as the engine warms up I loose MORE power/rpm, caused that coil heats up and breaks down even more? Is my thinking off here? Weak coil=weak spark=less power and unburnt fuel? This is not a cheap coil so don't want to buy one till I know. The manual gave ZERO info on proper test for the coil, but I font see why it should be any different than any other coil. Thanks guys!

 

[alldodge]

Did the balance test with my diacom program, on this engine you have to do one whole bank at a time, but was exact same drop on both sides. Did a resistance test on my coil today and unless I am doing something wrong I think my coil is weak. The primary was within pec but the secondary was only 5,700, and spec on that coil is 9,400-11,700. Also kinda makes sense then that as the engine warms up I loose MORE power/rpm, caused that coil heats up and breaks down even more? Is my thinking off here? Weak coil=weak spark=less power and unburnt fuel? This is not a cheap coil so don't want to buy one till I know. The manual gave ZERO info on proper test for the coil, but I font see why it should be any different than any other coil. Thanks guys!

I don't see it, if it was the coil it would start to miss. You need to get a scanner put it in base timing and check the timing. See if your getting full advance, you know your just talking about 500rpm's.

Edit: It could very well be there is nothing wrong. You friend has the same boat and turns more rpm, but his prop even though it looks the same maybe be different

 

[monkstr6100]

I have DIACOM and when I run it and record live data I get 28deg at WOT. I called merc to ask what he spec is for full advance and they told me to take it in to a merc dealer or buy the service manual. I don't see where in my DIACOM program that I can put it into base timing mode? And it does have a slight miss to it, not enough to pull a code though. I know I am just beating myself up over this and it really is more than likely NOTHING! Thanks again, it helps tons talking to other people that know more about these engines. Now if you have a question about an OMC outboard that was made in the 70's, I am your guy and I more than likely have what ever part you need too!

 

[old islander]

Try swapping props with your friend and run it. Chances are his prop is slightly different then yours. There is a lot of difference between props of the same pitch and diameter when made by two different manufactures.

 

[alldodge]

Your boat does 4600-4700 rpm and does 43-44 mph. Buddies boat does 5200 rpm and does 50mph.
Two things to consider, props are different even though they say they are the same. Are the tachs on both boats accurate? Are the speed readings used by both boats accurate? Don't see any boat picking up 5 mph by turning prop 500 rpm more (1 mph per 100 rpm).

 

[DaveG55]

I hope you find out what's going on, you've certainly put in enough effort.

You probably don't want to hear this but its true. No two boats are identical, even identical boats are slightly different.
My friend just bought 2 Kawasaki top of the line, supercharged jet skies. Identical in every way except for decal color. Both are mind bogglingly powerful and fast, 0 to top speed approaching 70mph in seconds. And yet, one is faster than the other. They recieve the same maintaince, are fueled at the same time from the same pump, and yet one is faster.
I also agree with AllDodge, it's a stretch to believe 500 rpm is going to net 5 mph.
Going back to the weight question, have you weighed the boat? It's possible that there could be extra water weight. Does your boat and your friends have the same draft?