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fegettes
06-16-2010, 11:16 PM
I have a 95 Blackhawk with a 502 CI, 415 HP, EFI Mercruiser engine. I ran the boat last weekend for the first time since last September when it ran great, but this last weekend I could not get the engine to increase above 3,000 RPM's. Other than the reduced RPM's, the boat ran great. Also, I used a fuel stablizer when I let the boat sit since last September. Any ideas what the problem would be?

Steve

Tidbart
06-17-2010, 07:24 AM
Steve,
Are the secondary plates opening up on the throttle body?
Bob

gero1
06-17-2010, 07:34 AM
check the throttle positioning sensor. i had to replace mine

Phil S
06-17-2010, 08:18 AM
Try it on a fresh tank of gas. I had the same problem. Used stabilizer, filled it all the way up, etc. last fall. A fresh tank of gas cured it.

fegettes
06-17-2010, 08:52 AM
Steve,
Are the secondary plates opening up on the throttle body?
Bob

I am not sure.


check the throttle positioning sensor. i had to replace mine

I am clueless where this throttle positioning sensor is, but I will investigate.


Try it on a fresh tank of gas. I had the same problem. Used stabilizer, filled it all the way up, etc. last fall. A fresh tank of gas cured it.

The tank was almost empty when I stored it, with the stabilizer in the remaining amount of gas, but I filled it up before I went out this weekend.

Thanks for the responses.

Steve

glashole
06-17-2010, 09:33 AM
where are the tabs

mine won't go over 3500 rpm until the tabs are up

Tidbart
06-17-2010, 11:06 AM
I am clueless where this throttle positioning sensor is, but I will investigate.

Steve,

It is located on the underside of the throttle body along with IAC motor.

I would take off the air cleaner, engine off, open the throttle all the way and make sure both butterflies open all the way. They will be sequential, primary then secondary. If they are ok, which they most likely will be, the next would be to check the TPS. There are two ways to check the sensor. First and easiest is to hook it up to a Merc Scan tool and check the voltage readings. The second is to remove it and test it with a multimeter, not easy to get to the terminals to test it, and if I went this far, I would just replace it. Sensor is about $50. If the part is original and not the cause of this problem, eventually it would be the cause of a future one.

Another cheaper alternative to scan tool is the code reader. It does two things, reads any error codes that were set, if there were codes set, but also allows you to put the engine in service mode to set base timing. Setting the timing of an MPI is different than a conventional V8. I have a code reader that I don't need if ever want one.

If you don't have access to a scan tool the MPI can be a pain in diagnosing some issues. Others are diagnosed conventionally such as fuel issues, checking pressure,etc.

Bob

PS. Only use Merc sensors.

fegettes
06-17-2010, 03:39 PM
Thanks Bob. I will do some checking.

Steve

glashole
06-18-2010, 07:18 AM
is it just under load?
will it rev up on the trailer

Tidbart
06-18-2010, 07:21 AM
I would put it in the water and rev it in neutral, but wouldn't do it on a garden hose, especially to 3000+ rpms. In fact, merc says not to go past, if I remember correctly, about 1600 rpms.

B

fegettes
06-18-2010, 08:57 AM
is it just under load?
will it rev up on the trailer

I ran it all of last weekend at the Clear Lake event. It ran great, idled and excellerated, with the exception that it could not get over 3,000 RPM's. And at approximately 2,900 RPM's I was still cruising at 50-55 MPH.

glashole
06-18-2010, 09:54 AM
lift the tabs

fegettes
06-18-2010, 10:45 PM
lift the tabs

At 2,900 RPM's at 55 MPH, the tabs are approximately are 75% up from the bottom, and any higher the boat was coming too far out of the water.

gero1
06-19-2010, 12:03 AM
they really do ride out of the water

Tidbart
06-21-2010, 03:14 PM
Steve,
Just checking in to see if you have had a chance to check anything out.
Also, have you replaced the fuel filter lately? Could restrict flow.

B

fegettes
06-21-2010, 07:22 PM
Steve,
Just checking in to see if you have had a chance to check anything out.
Also, have you replaced the fuel filter lately? Could restrict flow.

B

No, not yet. The boat is in Texas and I am working in Phoenix. My time is short when I get home, and I wanted to ask questions so that when I do get back home, I can target the symptoms without having to scratch my head on what to do, or try next. I will ad the change the fuel filter to my list. Thanks Bob.

Steve