Can't go over 3200 RPM's

[TonyG23]

I have a 1999 MCM 7.4L Bravo, immediatly after it heats up I cannot go over 3200 rpm's. I replaced cap, rotor, plugs, wires, coil, distributor sensor, and ignition module. I did this after eliminating fuel problem by adding fuel through carb. Any help would be appreciated.

 

[Don S]

Re: Can't go over 3200 RPM's

Here is a list of items that can cause the problem. Time to start eliminating and narrowing the problem down.<br /><br />

 

[KRS]

Re: Can't go over 3200 RPM's

Has it ever? How long have you owned boat?

 

[TonyG23]

Re: Can't go over 3200 RPM's

Sorry, I should have explained that the motor cuts in and out as if in a total stall. It is not a power problem. I purchaced the boat last fall and tried to do the mentioned repairs over the winter

 

[Don S]

Re: Can't go over 3200 RPM's

Do you have a serial number for your engine? Merc didn't use carbed 7.4's in 99.

 

[TonyG23]

Re: Can't go over 3200 RPM's

The boat is a 1999 Checkmate Convincer 253. It is the original engine. I guess they may have used an earlier model. Model MCM 7.4 Litre/Bravo<br />Serial Number: 0K281397.

 

[Don S]

Re: Can't go over 3200 RPM's

It's a 97 engine, and that boat was originally purchaced new in July of 98. <br />You have the Thunderbolt V ignition, look at the connectors on the ignition control module and the knock control module on the back of the distributor. If those terminals are corroded it can cause that exact problem.

 

[TonyG23]

Re: Can't go over 3200 RPM's

I replaced the ICM those connections were clean I'll check the connection on the knock sensor over the weekend. I kind of doubt it though, it looks like the previous owner changed the knock sensor.

 

[TonyG23]

Re: Can't go over 3200 RPM's

checked the connections but all looked good. I put 12 volts directly to the coil and eliminated the problem. It would help if I had a wiring diagram of the engine harness

 

[Don S]

Re: Can't go over 3200 RPM's

I put 12 volts directly to the coil and eliminated the problem.

You didn't "Eliminate" the problem, you only covered it up.
Why didn't you have 12 volts to the coil, how many volts did you have. What else is NOT receiving full voltage.

You really lucked out, your engine is one of the few that has an OEM Mercruiser service manual online.
Here is the link.
http://www.boatfix.com/merc/Servmanl/16/16cov2_1.pdf
Section 4F has all the wiring diagrams you will need.

 

[JackCoWo]

Re: Can't go over 3200 RPM's

I'm assuming your talking about the ignition coil, and if so you don't want to have 12 volts there too long, (ONLY for starting) it typically runs around 9 volts, after the engine is running, or in the igntion on, engine off. You may just have a ballast resistor, or some other kind of voltage limiting device going bad, or a bad connection somewhere in the circuit going to the positive end of the coil. Double check the service manual it'll tell you want you need to have, but its usually not 12 volts for ANY engine.

 

[Don S]

Re: Can't go over 3200 RPM's

Hold on Fastpony, he doesn't have a ballast resistor or wire. He also doesn't have a points type ignition that uses the ballast resistor or wire. He has a Thunderbolt V electronic ignition.
He better have 12+ Volts to the coil or there is a problem.

 

[smilinpirate]

Re: Can't go over 3200 RPM's

hey Admiral Don S. --
could these solutions also help with a 1993 - 4.3 merc with Thunderbolt V electronic ignition -- i have the same problem as TonyG23 -- Thanks!

 

[Don S]

Re: Can't go over 3200 RPM's

Best to start a new thread with your problem smilinpirate, with a 93, you have a Thunderbolt IV ignition, not V.