454 Horizon - loses power after 3900 rpm

[West32]

OK, this has been a battle for a friend of mine with the Exact same engines I have and we have tried everything to fix his problem. 1998 Sea Ray 340 Express Cruiser with 454 Horizons. We take it out, it jumps on plane and runs great. He throttles up to 3800, runs great and sounds great. He goes to 4000 and the port motor has nothing more to give. Starboard goes to 4400 and the port stays at 4000, 4100 with throttles all the way down. Fuel Flow on the Rinda is 1.4 GPH higher on the port than the starboard motor at 3400, and 2.4 gph higher on the port than the starboard at 3800
We've replaced the fuel pump, fuel pressure regulator, plugs, cap, rotor, wires, sent in the fuel injectors for cleaning and flow tests. I've swapped coils, ignition modules, and Map Sensors. We replaced all the Fuel lines from the Tank to the Fuel Pump. Replaced the Fuel Pressure Regulator as well. Started motor and fuel pressure was 38 psi. Revved motor and went up to 43. Base timing was checked at 8 degrees.
Ended up doing a compression test and found all cylinders low, and a few around 100 - 110. Did a leak down test and found intake valves were leaking. Pulled the heads to find extreme carbon buildup in the heads holding the valves open. Took the heads in and had them gone through. No deformation of seats or valves, just Heavy carbon. They went through everything and we got them back. Put everything back together, timed it to 8 degrees and took it out. SAME exact problem. over the course of 2 weeks, we swapped computers, sensors, throttle bodies to no avail. For shits and grins, I arbitrarily advanced the distributor to see what would happen. BIG difference! Fuel flows were inline with each other right off the bat. AND we got both motors up to 4550 RPM. Both engines sounded great. I then checked the base timing. 18 Degrees. I then checked the total time at 3000 rpm at 48 degrees. WTH? With my Rinda connected, we took it back up to WOT and there was NO knock, no knock Retard No pinging. Everything ran great. Once we were back in the slip, we checked the vacuum at idle 12 inhg. At 2200 rpm it settled in to 20 inhg. I am absolutely baffled with this one..... Why is it running so good...? What are we missing?

 

[Thornton69]

Is it possible there is a problem with the dampener? Believe the key should be inline the 0 mark.

 

[West32]

Is it possible there is a problem with the dampener? Believe the key should be inline the 0 mark.

We verified #1 TDC and the Dampener position was correct...

 

[scoflaw]

Did you check compression after the valve job?

 

[West32]

Did you check compression after the valve job?

Not yet.

 

[ttmott]

Since you set base timing at 8 degrees (ECM does not control timing advance) then I assume it's a mechanical / vacuum advance distributor.
Then either the mechanical advance isn't advancing enough or the vacuum dashpot is ruptured; the two advance mechanisms are additive. That would explain why the problem engine fuel consumption is greater than the other.

 

[scoflaw]

ECM controls timing on his engine.

 

[Thornton69]

We verified #1 TDC and the Dampener position was correct...

Curious how you did that?
Back in the 80's, we were repowering off highway equipment with 6 cyl Perkins. Don't remember the model. They didn't have a TDC indicator on the crankshaft. To find it, we pulled an injector and dropped a dial indicator into the hole. Rolled number one back and forth, marked the dampener +/_ and split the difference.

 

[Misterhelms]

OK, this has been a battle for a friend of mine with the Exact same engines I have and we have tried everything to fix his problem. 1998 Sea Ray 340 Express Cruiser with 454 Horizons. We take it out, it jumps on plane and runs great. He throttles up to 3800, runs great and sounds great. He goes to 4000 and the port motor has nothing more to give. Starboard goes to 4400 and the port stays at 4000, 4100 with throttles all the way down. Fuel Flow on the Rinda is 1.4 GPH higher on the port than the starboard motor at 3400, and 2.4 gph higher on the port than the starboard at 3800
We've replaced the fuel pump, fuel pressure regulator, plugs, cap, rotor, wires, sent in the fuel injectors for cleaning and flow tests. I've swapped coils, ignition modules, and Map Sensors. We replaced all the Fuel lines from the Tank to the Fuel Pump. Replaced the Fuel Pressure Regulator as well. Started motor and fuel pressure was 38 psi. Revved motor and went up to 43. Base timing was checked at 8 degrees.
Ended up doing a compression test and found all cylinders low, and a few around 100 - 110. Did a leak down test and found intake valves were leaking. Pulled the heads to find extreme carbon buildup in the heads holding the valves open. Took the heads in and had them gone through. No deformation of seats or valves, just Heavy carbon. They went through everything and we got them back. Put everything back together, timed it to 8 degrees and took it out. SAME exact problem. over the course of 2 weeks, we swapped computers, sensors, throttle bodies to no avail. For shits and grins, I arbitrarily advanced the distributor to see what would happen. BIG difference! Fuel flows were inline with each other right off the bat. AND we got both motors up to 4550 RPM. Both engines sounded great. I then checked the base timing. 18 Degrees. I then checked the total time at 3000 rpm at 48 degrees. WTH? With my Rinda connected, we took it back up to WOT and there was NO knock, no knock Retard No pinging. Everything ran great. Once we were back in the slip, we checked the vacuum at idle 12 inhg. At 2200 rpm it settled in to 20 inhg. I am absolutely baffled with this one..... Why is it running so good...? What are we missing?

Ummm - can we be friends?

 

[West32]

Curious how you did that?
Back in the 80's, we were repowering off highway equipment with 6 cyl Perkins. Don't remember the model. They didn't have a TDC indicator on the crankshaft. To find it, we pulled an injector and dropped a dial indicator into the hole. Rolled number one back and forth, marked the dampener +/_ and split the difference.

We had the heads off and rotated the crank until we saw #1 at TDC after the valves sequenced and closed.

 

[West32]

Update.
We took the boat out fishing on Saturday with 4 people on board. Powered out 15 miles or so at 3600 RPM. Engine sounded great. Also dad 2 gear heads on board and they thought everything sounded good too. We noticed at 3200 RPM fuel flows were identical on each motor. At 3600, they were about .8 GPH off (Problem motor using more). On the way in, we got a few miles from port and powered up to WOT. 4500 RPM, engines sounded great and the fuel flows were exact on each motor.
Why is it doing this.....

 

[ttmott]

At this point I would send the props in and check to make sure they are identical.

 

[kap2]

When you set the base timing did you set your Scan Tool to "Service Mode"? For MEFI versions 2 & 3 Service Mode will preset the idle to the correct rpm to set the base timing. For MEFI 1 versions you need a shop tach and adjust the idle to 1800 rpms.

Using your Scan Tool you may want to compare the Port engine timing number to the "good" Starboard engine.

The module inside the distributer maintains the base timing and provides some timing advance. Have you tried swapping the modules between the two engines?

 

[West32]

I have the Rinda Scan tool. Yes we set the ECM to service mode when I set the base time. We have swapped Modules and he even bought and replaced the entire distributor.
I spoke with an Engine builder yesterday who has seen the 454 Horizon Plenum not seal well due to warpage on the 4 port that it sits on. He recommended pulling the plenum and resealing with with a Bead of RTV instead of the gaskets. Then go an reset base time and see how it performs. He thinks there is a major vacuum leak and the ECM is trying to compensate and we are compensating it with timing advance. All sounds logical.

 

[scoflaw]

Mefi 3 gets timed at 1800 rpm in base mode. Your scanner will tell you what the IAC count is, that will tell you if it's trying to compensate for a vacuum leak.

I thought it was running good?

 

[West32]

Its running great at 18 degrees advanced and 48 degrees total time. Why? that is the question

 

[ttmott]

A vacuum leak becomes less of an impact to performance as an engine builds power and RPM. That is unless the engine air flow is MAF sensored. As these use manifold absolute pressure, have you looked at the MAP sensor output voltage and verified it is linear with manifold pressure and doesn't have some weird hiccup at low manifold pressure ranges?

 

[PlayDate]

I just read through this. I would check the compression again so you have a baseline. Additionally, the 454 damper issue is a known problem. All it has to do is slip a quarter of an inch to create the very problem that you are experiencing. I would use a boroscope or a piston stop to make sure that the dampener has not shifted. Easy to do if you are doing a compression check with the plugs out.

 

[scoflaw]

If the cam is retarded, for whatever reason, advancing the timing would put the spark in sync with the late valve events. If this was so, your compression would be lower than normal.150