Mercruiser 7.4L High Fuel Usage

[Fireman431]

Hope someone can help me. I have a 2001 Carver 374 with twin gas 7.4L Mercruisers (velvetdrive trans/shaft). The engines have 275 original hours on them and have been cared for with an open checkbook by the previous owner. My problem is the starboard engine using up to 50% more fuel than the port engine.
The engines sound perfect, idle perfect, both run @ 175 degrees. The port side makes 4500 RPM at WOT but the starboard will only make 4000 RPM. Out of gear, I cannot find anything wrong with it (will rev to 4600 RPM in neutral, same as the other). But in gear, the starboard engine is working harder and using much more fuel than the port (I have to keep the starboard throttle a little higher to keep the engines sync'ed). Fuel use on the port is 9 GPH @ 3200 RPM and the starboard engine is 14 GPH.
Is it possible that the bearings on the shaft/final drive are degrading, causing it to work harder? The props have been checked at the survey last month and both are great as well as the same pitch.
Any help is greatly appreciated.

 

[Haut Medoc]

Re: Mercruiser 7.4L High Fuel Usage

What do the plugs look like?......
You did not mention if it is EFI or carbed......​

 

[QC]

Re: Mercruiser 7.4L High Fuel Usage

Propeller? Are you positive it is the same pitch? Condition? Possibly damaged . . . If the marine gear was the issue I would expect it to be seriously howling

 

[Fireman431]

Re: Mercruiser 7.4L High Fuel Usage

Sorry...I tried to think of everything on the first shot.

Yes, the props look new and the surveyor put a pitch guage on them and they are the same.

The engines are Multi Port Injected.

 

[Fireman431]

Re: Mercruiser 7.4L High Fuel Usage

Darn it...the plugs are new. I changed all 16 of them myself and even the original ones looked good. All cylinders appear to be firing well, and as I mentioned, out of gear both engins sound identical.

 

[QC]

Re: Mercruiser 7.4L High Fuel Usage

Intake restriction? Exhaust restriction?

 

[dcg9381]

Re: Mercruiser 7.4L High Fuel Usage

I just had a similar problem on my 7.4L Mag MPI.. I've only got a single motor... I had a power drop, could get to about 3900 RPM. Plugs looked good. Replaced all the plugs and wires, a mildly crapped up distributor cap.
No change.

Turns out I had a bad injector. These motors typically operate on non-circulating fuel systems, so any crap that gets sucked up is going straight into the injector bank.

Those engines also have a diagnostic.. It's not wonderful, but it's better than nothing. It can be checked via buying a diagnostic connector which simply jumpers two wires and provides an LED for code flashing.. $25

I asked the Merc guys how they found my bad injector. They needed a merc diagnostic tool that measured fuel pressure. Then they fired each injector individually and recoreded the fuel pressure drop. Couldn't be done at home - at least not easily.

 

[Fireman431]

Re: Mercruiser 7.4L High Fuel Usage

Thanks DCG. That's the next thought I had, but I was wondering if there was any way to troubleshoot that on my own. I guess not. Stupid boats.

Thanks for the infor. Any other thoughts if that doesn't work?

 

[QC]

Re: Mercruiser 7.4L High Fuel Usage

I am not clear on this, but would a bad injector cause 50% more fuel use?

 

[Tail_Gunner]

Re: Mercruiser 7.4L High Fuel Usage

Very doubtful, all he would have to do is check his oil and if it is badly contaminated then yep. But id suspect he has a combination of tuning issue's and prop or powertrain issue

 

[dcg9381]

Re: Mercruiser 7.4L High Fuel Usage

I am not clear on this, but would a bad injector cause 50% more fuel use?

I wonder about that too.. but the Mercs are non-closed loop EFI, meaning that they've got a fixed fuel map for RPM and throttle position - meaning a bad injector could possible push that motor into a portion of the fuel map where it's eating fuel. It doesn't take much blockage to throw off a cylinder's usefulness when the A/F is off....


I'd probably put a fresh set of plugs in it.. Run it hard, then pull the plugs - see if you find any differences. This will give you a big indication of if you've got a fuel/spark issue.

Compression test while you're in there maybe?

 

[Fireman431]

Re: Mercruiser 7.4L High Fuel Usage

Thanks again. The fuel MAP'ing is over my head. I quit working on engines when carbs went the way of the dinosaur.

Compression is 149-151 psi on all cylinders. All plugs looked the same, more or less. No definite differences (burnt/white/soaked, etc).

Someone else had mentioned that a bad injector would/may not spray above an idle, which would account for the plugs looking similar, because it begins working normally once you're off the throttle. Also, the injectors on the remaining cylinders would spray more fuel than normal to try to keep the RPM's and power output where the throttle is set. This would mean more fuel use and less power out (RPM's). Again, I have to keep that engine throttle advance further than the other to keep the engines in sync.

No fuel in the oil (that I could tell). No odor in the engine room (no leaks).

Thanks for all the help. Still trying to sort all possibilities.

 

[JustJason]

Re: Mercruiser 7.4L High Fuel Usage

did you check to see if you had a filthy flame arrestor?

 

[QC]

Re: Mercruiser 7.4L High Fuel Usage

^^^^ Yeah, and/or exhaust restriction ^^^^^ It matters

 

[Fireman431]

Re: Mercruiser 7.4L High Fuel Usage

I'll check those, too. Thanks for the input.

 

[dcg9381]

Re: Mercruiser 7.4L High Fuel Usage

Engines haven't changed much from when you started working on them.
They still require 3 things: air, fuel, and spark.

Fuel MAP is the EFI equivalent of jetting - except for Mercruiser makes some assumptions about the engine and doesn't rely on the feedback of an 02 sensor like most automotive engines do.

My suggestion would be to change the plugs to new plugs, run that motor at a pretty decent pace, immediately shut it off and pull the plugs. When I had a bad injector, my plugs looked OK also - probably because they were discolored from normal use and I couldn't read lean on a plug that had been run fine for a year or more. Changing the plugs is a good idea anyway as you can't always tell when they go bad internally.


Other things to check - put a timing light on each plug wire individually - see if you have a good inductive pickup on all 8.

 

[thrasher]

Re: Mercruiser 7.4L High Fuel Usage

Why does everyone forget the 2 other things that engines need.

Compression and MONEY!! Lots of both..

Gary

 

[180shabah]

Re: Mercruiser 7.4L High Fuel Usage

Compression was stated as 149-151 so that's good

He didn't mention the size of the check book though


This has been mentioned before, but you need to check for exhaust restrictions.

 

[Fireman431]

Re: Mercruiser 7.4L High Fuel Usage

I will check for the intake & exhaust restrictions. Plugs were replaced last week (first thing I tried). Compression is good on all cylinders. Money.....whew! The checkbook looked just fine until I bought a 37' boat. I know, I know...a hole in the water....

Well, at least gas is cheap.......