Yamaha 1999 Saltwater Series II 250-HP Ox66: Loss of RPM and High Fuel Consumption

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Author Topic:   Yamaha 1999 Saltwater Series II 250-HP Ox66: Loss of RPM and High Fuel Consumption

patmcqueen posted 05-25-2014 09:09 AM ET (US)
I have two 1999 Yamaha 250-HP Saltwater Series II Ox66 outboards. After launching this summer I have noticed that the port engine is using significantly more fuel than the starboard engine. Depending on RPM, it is using almost twice the fuel. At first I assumed it was a bad fuel monitor, but the port tank is much lower as well, clearly [an indicator of the port engine] using more fuel
On the last trip when I ran much faster than my typical cruise, the port engine lost power mid-cruise. At [3,200 to 3,500] RPM all is great. But when I stepped up to [4,100-RPM] the boat would run for a while but then stumble. When I pull back the throttle it idles fine. And can even get back up on plane.
I had a Yamaha certified mechanic take a look, and, other than new plugs, they did not see a thing wrong. Unfortunately there are not many mechanics near the boat. The fuel burn and stumble is happening every time we go out and there is clearly something wrong. Any ideas on how to troubleshoot?
One item I forgot to mention is that the port engine is not getting to WOT either. The starboard engine has not problem.
TransAm posted 05-25-2014 02:01 PM ET (US)
When was the last time you changed out the low pressure fuel pumps? There are 3 on the engine When the diaphragms in these pumps become compromised, fuel consumption goes way up and performance and top end is reduced/ Also, the O2 sensor, when clogged with carbon, will increase fuel consumption and decrease performance and top end.
contender posted 05-25-2014 09:46 PM ET (US)
I have an older model of this engine [and] ended up [having to replace] the fuel pumps.
patmcqueen posted 05-27-2014 11:08 AM ET (US)
Thanks for the feedback - I have owned the boat for four years and I have not changed the fuel pumps. This sounds like the right place to start. Is there an easy way to diagnose the pumps vs. O2 sensor? It looks like the pumps are much cheaper!
http://www.simyamaha.com/category_s/1182.htm
Has the pumps at $34 and the O2 sensor at over $300!
thanks,
pat
martyn1075 posted 05-27-2014 07:33 PM ET (US)
The O2 senser may not need to be replaced it may just need to be removed and cleaned up. I would try that first because it should be done almost yearly and its fairly inexpensive to have done or can do it yourself for free. These are real nice engines a little rough on fuel compared to the new ones but they can give a nice overall performance and are reliable.
onlyawhaler posted 05-28-2014 02:19 AM ET (US)
I have the same engine, same year.
Every spring I do the following service by myself and it [makes] trouble free boating. These engines are great, but there are a few things to do
Every spring I do the following:
Change the fuel water separator spin off canister and the small fuel in line filter under the cowling. Very easy.
Change the gear oil and reinstall the prop on a greased shaft.
Remove the 02 Sensor, clean with a soft tooth brush and carb cleaner. Clean out the oil and residue in the small hole in the block leading to the 02 Sensor with a pipe cleaner soaked in carb cleaner. Blow dry with air hose and reinstall with new gasket
Start the engine and burn off the fogging oil with your old spark plugs. Then change the six plugs; you are good to go for the summer
All of this takes an hour and a half. Let the gear case drain completely as you work on the O2 sensor
Every third Year I do the following
Replace the (3) low volume fuel pumps and the internal filter in the VST canister. Reinstall with new gasket on the canister. (Note, the VST canister has a drain screw on the bottom. Drain the canister completely into a cup in the fall so the internal filter doesn't sit in gasoline)
Replace the water pump impeller, housing and gasket
Keeps you trouble free on the water. Use Yamalube in your gasoline when you fill up every time and you are good to go.
Sterling
Onlyawhaler
TransAm posted 05-28-2014 06:02 AM ET (US)
You can diagnose the pumps if you like, but they are over due for service. When you squeeze the primer bulb, does it build pressure and firm up the bulb as it should? If one or more of the pumps is bad, the defective pump can allow fuel to be pushed through the pump directly into the cylinder thereby not creating that firm bulb after several squeezes. If this is happening, pull the safety lanyard so the engine wont start. Then remove all three plugs on one side of the engine; squeeze the primer bulb several times and turn the engine over. If one or more cylinders spits an inordinate amount of fuel out, the corresponding pump is most likely bad. The price you quoted is about right, and the install is straight forward.
As for the O2 sensor, they too can be serviced. And if you have the old style "joint" (its a barbell looking thing about 1" long at the base of the O2 mounting housing), you should replace that with the newer version. This is also an area that needs to be have carbon build-up removed.
I find it odd a Yamaha certified mechanic would not have brought either of these issues up with you. It should have been the first thing out of his mouth.

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Author	Topic: Yamaha 1999 Saltwater Series II 250-HP Ox66: Loss of RPM and High Fuel Consumption
patmcqueen	posted 05-25-2014 09:09 AM ET (US) I have two 1999 Yamaha 250-HP Saltwater Series II Ox66 outboards. After launching this summer I have noticed that the port engine is using significantly more fuel than the starboard engine. Depending on RPM, it is using almost twice the fuel. At first I assumed it was a bad fuel monitor, but the port tank is much lower as well, clearly [an indicator of the port engine] using more fuel On the last trip when I ran much faster than my typical cruise, the port engine lost power mid-cruise. At [3,200 to 3,500] RPM all is great. But when I stepped up to [4,100-RPM] the boat would run for a while but then stumble. When I pull back the throttle it idles fine. And can even get back up on plane. I had a Yamaha certified mechanic take a look, and, other than new plugs, they did not see a thing wrong. Unfortunately there are not many mechanics near the boat. The fuel burn and stumble is happening every time we go out and there is clearly something wrong. Any ideas on how to troubleshoot? One item I forgot to mention is that the port engine is not getting to WOT either. The starboard engine has not problem.
TransAm	posted 05-25-2014 02:01 PM ET (US) When was the last time you changed out the low pressure fuel pumps? There are 3 on the engine When the diaphragms in these pumps become compromised, fuel consumption goes way up and performance and top end is reduced/ Also, the O2 sensor, when clogged with carbon, will increase fuel consumption and decrease performance and top end.
contender	posted 05-25-2014 09:46 PM ET (US) I have an older model of this engine [and] ended up [having to replace] the fuel pumps.
patmcqueen	posted 05-27-2014 11:08 AM ET (US) Thanks for the feedback - I have owned the boat for four years and I have not changed the fuel pumps. This sounds like the right place to start. Is there an easy way to diagnose the pumps vs. O2 sensor? It looks like the pumps are much cheaper! http://www.simyamaha.com/category_s/1182.htm Has the pumps at $34 and the O2 sensor at over $300! thanks, pat
martyn1075	posted 05-27-2014 07:33 PM ET (US) The O2 senser may not need to be replaced it may just need to be removed and cleaned up. I would try that first because it should be done almost yearly and its fairly inexpensive to have done or can do it yourself for free. These are real nice engines a little rough on fuel compared to the new ones but they can give a nice overall performance and are reliable.
onlyawhaler	posted 05-28-2014 02:19 AM ET (US) I have the same engine, same year. Every spring I do the following service by myself and it [makes] trouble free boating. These engines are great, but there are a few things to do Every spring I do the following: Change the fuel water separator spin off canister and the small fuel in line filter under the cowling. Very easy. Change the gear oil and reinstall the prop on a greased shaft. Remove the 02 Sensor, clean with a soft tooth brush and carb cleaner. Clean out the oil and residue in the small hole in the block leading to the 02 Sensor with a pipe cleaner soaked in carb cleaner. Blow dry with air hose and reinstall with new gasket Start the engine and burn off the fogging oil with your old spark plugs. Then change the six plugs; you are good to go for the summer All of this takes an hour and a half. Let the gear case drain completely as you work on the O2 sensor Every third Year I do the following Replace the (3) low volume fuel pumps and the internal filter in the VST canister. Reinstall with new gasket on the canister. (Note, the VST canister has a drain screw on the bottom. Drain the canister completely into a cup in the fall so the internal filter doesn't sit in gasoline) Replace the water pump impeller, housing and gasket Keeps you trouble free on the water. Use Yamalube in your gasoline when you fill up every time and you are good to go. Sterling Onlyawhaler
TransAm	posted 05-28-2014 06:02 AM ET (US) You can diagnose the pumps if you like, but they are over due for service. When you squeeze the primer bulb, does it build pressure and firm up the bulb as it should? If one or more of the pumps is bad, the defective pump can allow fuel to be pushed through the pump directly into the cylinder thereby not creating that firm bulb after several squeezes. If this is happening, pull the safety lanyard so the engine wont start. Then remove all three plugs on one side of the engine; squeeze the primer bulb several times and turn the engine over. If one or more cylinders spits an inordinate amount of fuel out, the corresponding pump is most likely bad. The price you quoted is about right, and the install is straight forward. As for the O2 sensor, they too can be serviced. And if you have the old style "joint" (its a barbell looking thing about 1" long at the base of the O2 mounting housing), you should replace that with the newer version. This is also an area that needs to be have carbon build-up removed. I find it odd a Yamaha certified mechanic would not have brought either of these issues up with you. It should have been the first thing out of his mouth.