Some days ya just can't get ahead. Here's a good one.

I've got a 2001 312 Fastech with 6.2's Mercs and bravo 1's pushing 26 inch props...just a standard set-up. The boat used to run 64.5 MPH at WOT (on GPS) with 1/2 tank of gas, 80 degree ambient temp, smooth water and two people on board. She would max out at about 4800 RPM on both engines. I had the engines serviced; new plugs, wires, oil change, filters, pulled and serviced the outdrives and I had to replace the stock exhaust due to rust around all the joints. I went with Stainless Marine exhaust manifolds for the warranty. I boat in the brackish bay water around Destin and I flush the engines religiously after every use.

Well, since the boat was serviced she will only run about 4600 RPM on both engine and only hit about 60 MPH!!! She runs great, no problems, starts right up, idles great (700 RPM), throttles up great, but will only go upto 4600 RPM. Both engines are running identical to each other so whatever happened appears to have happened to both.

The service guys ran the engines on a scan tool and on a lap top diagnostic and everything comes back as normal! No trouble codes and everything is supposed to be within nominal settings! "Both engines are running at 100% power at 100% throttle setting." Not sure how they know the engines are making 100% power.

I can't understand it. I got rid of about 200 pounds off the back end and have a more free flowing exhaust, and lost RPM.

Has anyone experienced something like this??!!!

I know these engines do not respond to minor upgrades, but loosing RPM?! I didn't expect that.

Hmm, can't give you an answer but I added stainless marine manifolds and risers to my 06' 6.2 and gained maybe 80-100 rpm's with little to zero speed increase on the top end. I did notice a much better midrange accleration, much better fuel economy, and a deeper grawl but don't understand why you went the other way either. Steve

A couple of things that can go wrong with minor service -

• a cracked spark plug
• plug wire not on all the way
• firing order
• a cut in a plug wire boot arcing to manifold

None of these will show up on a scan tool and believe it or not can be hard to detect. I would expect more than 200 RPM drop however.

Back to the basics - assume nothing

I think what they mean is that the Engine Gaurdian is not reducing the Available Power. The engines could still have reduced power output due to some other cause.

Mbam

If it were a cracked plug or cut wire I would expect problems on just one engine, but both engines are running identical to each other. The firing order may be a good item to check. If they tech mis-wired one engine he may have copied it on the other.

Also, my gas mileage seems to be a little worse.

firing order check first, but the order is so different it doesn't seem plausible for that kind of loss.

18436572 old small block
18726543 new small block

I would suspect weather and water conditions to be the culprit. Maybe the bottom of the boat has gotten a bit dirty. The only way to know for sure is to put it on a prop shaft dyno and see if the power is where it should be. I've never seen an aftermarket exhaust cost horsepower, but I have seen it not gain anything.

I would assume the 6.2s are mefi if so a freer flowing exhaust in theory could make engines run lean since there is no way for the efi to adjust the mixture to compensate .Easiest way would be to pull the plugs and see if they look lean.

Ditto! Thats where I would look also.......

Wouldn't lean buy you rpms?