Mercruiser 7.4 MPI - 3,200 Max RPM, High Fuel Consumption

[tomschauer]

[Engines were checked out, and the starboard did have one cylinder with low compression. I got a purchase allowance, and both engines have new manifolds and risers, and the starboard has new OEM heads. Compression good all around. 3,200 rpm max on Starboard is still there.]

Just asking, was the compression actually checked after the heads were replaced, or was it assumed the heads corrected the problem? If you know which cylinder had the low compression, pull the plug and see if it isn't all gummed up. It sounds like you still have one or more cylinders not firing. Could be a simple spark or injector problem. Or if it smokes, a bad piston , ring or cylinder.

[bud37]

After reading all the posts here this sounds just like the engine timing is not going ahead correctly under load, It would make sense in that situation that the fuel usage would be high because the coals are getting poured to it but the engine can't respond.Have to say I don't know much about the fuel injected management systems,but I think that checking the advance on the suspect engine would go a long way to finding out what is wrong.

[tomschauer]

Bud, I would agree if the original low compression diagnosis was a mistake, which it quite possibly was. It seems many marine engine issues are addressed by replacing the heads, and for some reason it doesn't seen to fix the issue??????

[jmagrann]

Thanks for everyone's feedback.

The new heads did fix the compression problem on #6 (burnt valve),

Ignition timing was checked, and all plugs are firing.

I now have a new MAP sensor in hand, and will be able to install this week and will report results. Based on other posts I've come across, it seems like a likely suspect.

[bud37]

Source of the post I now have a new MAP sensor in hand, and will be able to install this week and will report results. Based on other posts I've come across, it seems like a likely suspect.

That could be the issue as that sensor is used to see the engine load and apply ( adjust ) fuel and timing accordingly as the load changes.....hope thats it.

[jmagrann]

Resolved - Replaced MAP Sensor

After replacing the MAP sensor with a new unit, I'm getting 4,200 rpm from both engines, WOT. I'll do one last post after running for a while, but the new sensor made an immediate, obvious improvement.

Some additional notes on symptoms: Before, Starboard would only run at 3,200 rpm, even with throttle at WOT. If I backed off both engines to cruise at 3,200 rpms. Port was about half throttle, Starboard was about 3/4 throttle. Now, throttles are nicely aligned through all rpms.

A tip on replacing the MAP sensor. Manual says you need to pull the plenum, but I was able to get the nut that secures a tab holding the sensor, using a 1/4" drive, 1/2" socket, and a socket wrench that had a hinged head. I had to put the socket on the nut first, and was then able to get the wrench on. It would have been easier if the socket was ground down a bit. Very tight quarters.

I'm now having a strange problem with the Port engine, but that's a new topic!

[Wkuhilltopperfan]

I'm glad the map sensor fixes the first issue. If the boat sat for a while, it's always possible for injectors to stick and cause poor fuel economy and or performance