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

[jmagrann]

Greetings,

I have a 1998 Carver 400 CMY with twin Mercruiser 7.4 MPI engines, MEFI-2. I've had the boat for about 6 months. Starting with the sea trial, it was noted that the Starboard engine would not rev to much more than 3,200 RPM under load. The Port revs freely to about 4,400. Starboard will hit 4,400 in neutral. The engine isn't showing any fault codes, my engine guy is using Rinda's Diacom software. Props, standoff bearings, shafts all check out fine.

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.

I also note that fuel consumption on the starboard engine is about 30% higher than the port engine. 3,200 is a good cruising speed, and I'd probably leave things along, but the fuel consumption is a problem.

Based on my internet research, it's likely a sensor issue. 3,200 is the 'safe' rpm for limp home mode, and there are a few posts out there with similar symptoms, but often end before a conclusion is published.

My question to the group: anyone heard of similar problems, and a resolution? I think the next step for me is to swap sensors between the engines, and see if the problem moves to Port, thereby identifying the problem sensor.

Thanks!

John Magrann

[Viper]

In my opinion, you should not be revving your engine that hi in neutral.

How low is "low" on the one cylinder? It sounds like heads were replaced and the low cylinder is okay now?

Did the tech not find any anomalous readings with the Diacom such as engine temp, MAP, etc.? MEFI doesn't give you much but it does give the basics to steer you in a direction. Does it run well at idle? Did he put a fuel pressure gauge on it while it's running under load? Did he hook up a vacuum gauge? I assume the plugs, wires, cap and rotor were all replaced to eliminate an ignition issue? Was the fuel filter replaced? Does it start okay?

If you're going to swap sensors, do only one at a time and then retest before swapping the next part.

[g36]

I think you might be mistaken about rpm in guardian mode being as high as 3200. It's more like 2800. Anyway the causes for that would involve high engine temp and oil level or pressure out of normal. I second the fuel pressure Guage and ignition parts. Pickup in distributor might be a thought.
Good luck
Like you I have always wondered why people almost never post a resolution to a problem when they have asked for help so that others could benefit. I hope you will post your findings when you fix you problem

[jmagrann]

Thanks for your responses.

After the heads were replaced, compression is good on all cylinders.

Engine starts right up, idles well, behaves exactly like the port engine, until the throttles are brought up. Port will continue to WOT, 4,400 rpm, Starboard will max out about 3,200 consistently. When cruising with both engines at 3,200, the starboard throttle is significantly 'higher' than port. Starboard throttle cable and linkage were checked, and has full range of motion.

Fuel pressure was checked, but not under load. During engine survey/sea trial, the Diacom was hooked up, but no fuel pressure or vacuum gauge.

My engine guy left a coil for me to try, and the distributor caps and wires are about a year old (but I realize that doesn't mean that they are good).

I think another sea trial with my engine guy, with fuel and vacuum gauges, would be a good next step. If that doesn't turn up something obvious, I can swap ignition parts between engines, as well as sensors.

I'll get a checklist together and spend an afternoon in the harbor, swapping one item at a time then test, before progressing to the next.

I will definitely post progress.

[Viper]

I'd have to give this some more thought but as a quick check, check to see if it is stuck in base timing mode either by having been jumped with a wire or left in Service Mode by a Scan Tool rather than switched back to Normal Mode before the tool was removed. It's a common oversight and would cause timing related issues. Has the timing been checked and was the proper timing procedure for this application followed? Then was it returned to Normal Mode? You may not be getting full advance. Something to add to your list.

[Danno66]

Have you checked the carbs to make sure they are working properly? Had a similar issue with my Ford and the Power valve (operates the secondary on the carb) was corroded and stuck open. Made that engine use a ton of fuel and the throttles were way our of whack from each other. Rebuilt the carb and its all fixed.

[Viper]

"7.4 MPI" and "MEFI-2".....it's fuel injected...no carb.

[Danno66]

Source of the post "7.4 MPI" and "MEFI-2".....it's fuel injected...no carb.

oh...da der....

[jmagrann]

Thanks for everyone's input. I able to confirm over the long weekend that the timing is in 'normal' mode, and did my first test by swapping the ECM units between the motors. No change, both engines perform the same as before, with the Starboard having issues.

Next up, I'll swap out the ignition coil. I think a long shot, but my mechanic had a spare, and it's easy to do.

The MAP sensor is what I suspect, but I'm still working out how to remove it. If anyone has some tips, I'd appreciate it. I found it easy enough, but it looks like I need to remove the heat exchanger/thermostat housing to get at the bracket holding in the sensor. Hopefully I'll be able to do both the coil and MAP sensor this weekend

[jmagrann]

I swapped back the ECM's, then replaced the Starboard ignition coil with a new unit. No change. I'm now going for the MAP Sensor. I found a manual, and it says I need to pull the plenum to remove the sensor, so I'll buy a new one (not worth pulling two plenum's to try a 'swap').

If any one knows of a way to pull the MAP Sensor without removing the plenum, it would be greatly appreciated.