Port Engine RPM's cycling Up and Down

[Searaybio270]

My boart engine is fine at idle but from 1000 to 1800 RPM's it will cycle up and down. Meaning the RPM's will go from 1800 to 1000/1200. At higher RPM's it does it but not as much.
any suggestions - I have already replaced the Racor Fuel Filters.

 

[Hampton]

Is there an electric fuel pump and a mechanical one? Maybe the electric one has gone out or is failing. At idle - not much demand. At high RPM, mechanical pump has enough power to supply fuel. In the middle, need both. Just brainstorming.

 

[Searaybio270]

John,
Thanks. Headed to the boat after work to check a few things. I will see if I can find someone who knows something about the two pumps.
I plan to check the MAP sensor. the IAC Valve and the AGI.
Joe

 

[sdleo26]

That's an interesting one. If it was the IAC valve it would stall at idle. Switch MAP sensors like you mention, maybe fuel regulator, or one of the fuel pumps (high or low pressure)?? Let us know what you come up with

 

[Searaybio270]

So here is what I have done so far.......
Replaced both the fuel filters and water separators. Put Star Tron in both tanks. Checked hoses, connections etc. All good. Started the engines both at idle were great. Pushed both up (separately) to 1200 RPMs. Both stayed at 1200 and no wavering.
So I decided to take her out for a spin. Let the marina at No Wake Speed and no issue. Felt good. Pushed her up to 1400 RPM doing about 9MPH and no issues. Actually, is was smooth and felt good. Ran at that speed for some time just to see what she might do with regard to the fluctuations. Nothing. Ran Awesome.
On my way back to the Marina I pushed her up to 3200rpm's and she jumped up there but then started the fluctuation in the rpm area. She also felt like she was starved for fuel in a way as it felt like I was asking for 4800rpm's instead of 3200.
So tonight I am going to drain the fuel separator into a jar and see if there is water in there. then - maybe the MAP, fuel pumps, I say fuel pumps as it was happening on both engines not just the port.
Chime in!

 

[Hampton]

Water in fuel...

 

[sdleo26]

I am surprised you can see the water in the fuel; I am told its nearly impossible now with all the ethanol in fuel these days. I didn't realize it was both motors surging; I would say fuel issues, no doubt. You would be pretty unlucky to have both motors with failing fuel pumps and/or MAP sensors. Glad you found it.

Scott

 

[Searaybio270]

Scott,
You are correct. I couldn't see if it had any water in it or not. On Saturday I decided to go across the bay with our group to hand out with them. At idle sounds excellent. At 2000RPMs it is still awesome. At 2600RPM it is still ok. But at 300)+ it cycles. Both engines at first then only, now get this, just the Starboard engine. The Port seemed to be fine. Not great but nothing like before and nothing like the Starboard.
Once I came back down to under 2600 no problem. except you can hear the engine (Port) surging a little. Not smooth but with a little <100 rpm move. But you can hear it.

I have asked my mechanic to find someone that has test equipment to put on both engines to measure fuel/air/electrical etc. Any commnets/suggestions please send them.

 

[sdleo26]

Put a remote fuel tank on the input to the water separator filter of one motor. A little 3 gallon tank should do. The fact that both of your motors are behaving the same way (relatively) is a MAJOR indicator that you have bad fuel. If the motor with the remote fuel tank runs fine, you have found it.

 

[ziekman@cox.net]

Joe, I was curious what the outcome of this was?