Help with 502 Power Problems

[Greg Maier]

I had the Blackhawk out in the C&D Canal last weekend testing the top speed for the first time this year. I could not get above 72 MPH. I'm trying to track down what is wrong. Here are the changes that were made to her this winter:

1. Stainless Marine Exhaust (should not reduce speed)

2. Gaffrig two handle throttle I checked the air intake and both butterfly valves are wide open when the throttle handle is wide open. One hypothesis is that my throttle position sensor is malfunctioning.

3. Amsoil Synthetic Oil (should not reduce speed)

4. AC Delco Rapifire Plugs . Maybe a cylinder or two is not firing because spark plug wires are not making proper contact. I did not change spark plug wires but I have some new ones on the way. Would the engine still run well (72 MPH) with a cylinder not firing?

5. Redline High Shock Lube in the Blackhawk Drive

Does anybody have any ideas why the 502 is not making the proper power?

[boldts]

Greg,

Something that has seemed to pop up after exhaust manifold work is damage to a fuel injector itself. Check all your cylinders and make sure each one is firing with fuel in it. (plug will look brand new with no burn marks if no fuel is entering the cylinder and the computer compensates for the missing cylinder which isn't noticed until trying to add power to the engine)

Just a thought since the BlackHawk boats are capable of much higher speeds than your posting. Did you use a GPS? Do a search on Bryan Tuvelle posts where he talks about his recent problems with his 502.

[Greg Maier]

Scott,

Yes, I do use a GPS. Yes, I did follow Bryan's saga, I just hadn't thought I would have damaged an injector. Question: If I hooked the engine up to a Mercruiser Computer, would it tell me if I had a bad injector? Would the engine still be able to produce decent power (72 MPH) with a bad fuel injector and a cylinder not firing?

Thanks

[boldts]

Greg,

Sorry, but I can't tell you what if anything the Mercriuser Computer will tell you about the injectors. I was just relaying information that I had read and seen in the field. I witnessed a gentleman with a new 502 that had been built up and headers added in a Fountain. Boat ran fine until they tried to open her up. Upon further inspection, a cylinder with a bad injector was found.

I put 2 + 2 together. 72 MPH with a BlackHawk is about 8 to 10 MPH slower than that boat should be. So, I'm assuming something isn't firing somewhere? I'm not a mechanic, but was just trying to give you some past experience and thoughts. Hope it works out for you.

[Greg Maier]

Thanks Scott,

I had her up over 80 mph before the Winter, so I know something is definitely wrong. I guess this will teach me to only make one change at a time so it is easier to track down the root cause of any new problems.

[Lil Grizz]

I had some bad plug wires that somehow didnt slow the boat down much until one completely grounded out on the manifold. After I replaced them I got a few hundered RPMs that I never had in the first place.

Now besides the loss of speed are you also losing RPMs too? If the RPM is where it was when you were running fast last year I would look for something like a bent prop or anything that could possibly be scrubbing off speed. If the Stainless exaust was replacing the stock mercury units it is quite possible that the 125 lbs you took off the stern (stock exaust= 150lbs, stainless = 23lbs) could be affecting the ride attitude enough that new driving habits are needed. Sounds like a stretch but you never know.

[MOP]

Something we all do or should in our autos is to use fuel injector cleaner. On boats I feel it is much more important the gas sits in the system for weeks at a time and over the layup season it has for years screwed up carbs, I am sure it does a number on injection systems. I have talked most of our clients into putting an over dose in the fall and using it periodicly over the boating season. We all take good care of our rides we should think about the same for our boats.

[Greg Maier]

Well, my girlfriend was sitting on the transom dragging her legs in the water when I was making a top speed run, maybe that could be it, but honestly, the props are fine, and nothing is dragging. I am leaning towards a problem with the new throttle. With the old throttle, to get my top speed, I would actually have to back off a tiny bit. This makes me think that the throttle position sensor was not calibrated properly. This new throttle might have exagerated the problem. Or, maybe I did damage a fuel injector when I installed the new exhaust. We will see. To address Lil Grizz's point about the weight of the exhaust, the boat does seem to handle better (less porpoise)with the new exhaust on. Doesn't go any faster, but does handle better. Thanks for the help.

[Bryan Tuvell 33ZX]

Greg, my issue was a much larger loss of power, the injectors on the 502 are banks of 4, two centers/front - back cylinders the other side. The Merc Diag tool won't show them as bad, my guy noticed an OHM drop on one bank and called Merc, they actually helped him get through a test he had not done before.

Mechanic and Merc said they should not have failed, my theory is we have bumped them etc during exhaust install.
Merc sent 4 freee ones! She is as good as new now.
I know your frustration, hang in there.

Keep us posted on how the AMSOIL works out, I am curious.

Just for GP, try the recommended plugs, they put NGK's in ours, not MR43T's according to Merc.

Good luck, Bryan

[Ed Donnelly]

Are you running with last years gas? Even with additives the gas gets a little skunky.A real pain in the a** but drain out the old and put some Sunoco in with injector cleaner..If that solves your problem it will be the first time one of my brilliant ideas has worked. PS I never make just one alteration, usually 3 or 4 at a time. That way it is just so much more fun trying to find what is causing a new problem. Then the famous last words " That is the last time I make over 1 change before testing"...Ed

[blackhawk]

Greg, were your rpm's down too?