I have inherited a 1996 Boston Whaler RAGE boat with the OMC 115-HP TurboJet engine and jet drive. The boat is in great shape but has a mechanical glitch I can't sort out. Whether under way or in neutral the engine speed automatically drop back to idle speed as soon the throttled is pushed past 3,000-RPM. Pull back the throttle to idle then forward again to 3,000-RPM, and the OMC 115-HP TurboJet purrs all day--but as soon as you push past 3,000-RPM the engine speed falls back.
I figure this has to be something electrical since it happens exactly the same way every time but I don't know how to properly diagnose it.
Someone suggested [the cause of this problem is] the VRO pump: how do I confirm that?
Can the VRO pump be replaced with a "non-VRO" pump?
What is a "non-VRO" pump?
So many questions and zero experience with these engines/ Thanks in advance for any help you can offer.
Mike in MD
RAGE with OMC 115 TurboJet
Re: RAGE with OMC 115 TurboJet
A close and careful reading of the engine owner's guide will explain the behavior of the engine and its alarm system. If you have the original engine owner's guide, it may explain more.
Your inference that the cause of the consistent refusal of the engine to accelerate beyond 3,000-RPM and subsequent return to idle speed is based on an electrical interaction in the engine control system does seem plausible. My basis for that is some familiarity with electronics used in that era on OMC outboard engines. Here I am assuming the TurboJet engine is using similar technology.
The engine spark is generated by an assembly called the Power Pack. The Power Pack receives input from two temperature sensors. In the presence of an alarm input from a temperature sensor , the Power Pack will act to limit engine speed.
The engine speed limit function is called "S.L.O.W" and limits the speed of the engine to 2,500-RPM when an over-heat condition has been detected. Since you report an engine speed of 3,000-RPM as the threshold point, this function could be what you are experiencing,
Also, you have not made any mention of an aural alert sounding. It would be normal for the engine to alert the operator to an over-heat condition with an aural alert sounding going off with a sustained BEEEEEP. If you have the SystemCheck gauge, it should also be signaling visually with an annunciator lamp the onset of the alarm condition.
Let me give you a brief narrative of the alarm system wiring. The alarm sensor wiring is what we call a "wired-OR" logic, meaning that the alert sounder or annunciator will be triggered by the closing of ANY of the sensors; they are all wired in parallel. There are three sensors: fuel suction vacuum sensor, oil tank level sensor, and oil flow rate sensor. These three sensors are all connected to the TAN conductor. The aural alert sounder is also connected to the TAN conductor. If any sensor has continuity to ground, the aural alert sounder (and the visual indicator lamp) will operate.
The oil flow rate sensor is part of the OMS (Oil Mixing System) pump, previously called the VRO (Variable Ratio Oiling) pump. This sensor is designed to detect loss of oil flow, which usually shows up at higher engine speeds. It goes to ground whenever it detects an improper oil ratio. This usually produces a staccato alert when the engine speed is above idle and there is an oil flow problem. The engine speed is not limited to the S.L.O.W. function.
The oil tank level sensor is part of the oil tank, it goes to ground whenever the oil tank level is below threshold. This sensor is unique in that is produces a short to ground about every 20-seconds when its threshold is exceeded, so the aural alert is a periodic BEEP at about 20-second intervals that continues as long as the oil level is low.
The temperature sensor (or sensors, as there may be two, one on each block of a V-block engine) goes to ground when the engine block temperature is above threshold. This produces a continuous tone from the aural alert, and also imposes a limit on engine speed to about 2,500-RPM. This is the only sensor that is allowed to limit engine speed. To achieve this unique function, the sensor is wired directly into the Power Pack (via a TAN conductor) but connected to the alert sounder via a diode. In this way, then the temperature sensor operates it affects both the Power Pack and the alert sounder. The other sensors only affect the alert sounder, and are not supposed to affect the Power Pack.
A explanation for the behavior you describe could be due to a failed diode in the wiring harness. I explain further:
In the alarm system there is a vacuum sensor in the fuel line. This sensor is designed to monitor the suction pressure in the fuel hose delivering fuel to the engine, and to signal an alarm when the suction pressure (or vacuum pressure) in the fuel system exceeds the threshold of the sensor. This condition would normally occur when there was a restriction in the fuel line that was preventing the engine from getting fuel at the rate needed, and that would occur at higher throttle settings. The NORMAL wiring for this sensor is to keep it isolated from the Power Pack input (that initiates the S.L.O.W. function) by a diode. In this way when the fuel restriction condition occurs, there is only an alert produced, and the S.L.O.W condition is not triggered. However, if the diode is bad or if someone previously removed the diode, then the fuel suction sensor would affect engine speed, just as you describe. And, again, just as you described, the engine would be able to immediately resume running at higher engine speeds as soon as the fuel suction pressure dropped, would it would immediately when the throttle was reduced.
So my best guess for the cause of the described behavior: in the wiring of the alarm sensors, there is either a bad diode or a missing diode in the wiring.
Also see
WARNING HORN SIGNALS
http://continuouswave.com/whaler/reference/warningHornSignals.html
Your inference that the cause of the consistent refusal of the engine to accelerate beyond 3,000-RPM and subsequent return to idle speed is based on an electrical interaction in the engine control system does seem plausible. My basis for that is some familiarity with electronics used in that era on OMC outboard engines. Here I am assuming the TurboJet engine is using similar technology.
The engine spark is generated by an assembly called the Power Pack. The Power Pack receives input from two temperature sensors. In the presence of an alarm input from a temperature sensor , the Power Pack will act to limit engine speed.
The engine speed limit function is called "S.L.O.W" and limits the speed of the engine to 2,500-RPM when an over-heat condition has been detected. Since you report an engine speed of 3,000-RPM as the threshold point, this function could be what you are experiencing,
Also, you have not made any mention of an aural alert sounding. It would be normal for the engine to alert the operator to an over-heat condition with an aural alert sounding going off with a sustained BEEEEEP. If you have the SystemCheck gauge, it should also be signaling visually with an annunciator lamp the onset of the alarm condition.
Let me give you a brief narrative of the alarm system wiring. The alarm sensor wiring is what we call a "wired-OR" logic, meaning that the alert sounder or annunciator will be triggered by the closing of ANY of the sensors; they are all wired in parallel. There are three sensors: fuel suction vacuum sensor, oil tank level sensor, and oil flow rate sensor. These three sensors are all connected to the TAN conductor. The aural alert sounder is also connected to the TAN conductor. If any sensor has continuity to ground, the aural alert sounder (and the visual indicator lamp) will operate.
The oil flow rate sensor is part of the OMS (Oil Mixing System) pump, previously called the VRO (Variable Ratio Oiling) pump. This sensor is designed to detect loss of oil flow, which usually shows up at higher engine speeds. It goes to ground whenever it detects an improper oil ratio. This usually produces a staccato alert when the engine speed is above idle and there is an oil flow problem. The engine speed is not limited to the S.L.O.W. function.
The oil tank level sensor is part of the oil tank, it goes to ground whenever the oil tank level is below threshold. This sensor is unique in that is produces a short to ground about every 20-seconds when its threshold is exceeded, so the aural alert is a periodic BEEP at about 20-second intervals that continues as long as the oil level is low.
The temperature sensor (or sensors, as there may be two, one on each block of a V-block engine) goes to ground when the engine block temperature is above threshold. This produces a continuous tone from the aural alert, and also imposes a limit on engine speed to about 2,500-RPM. This is the only sensor that is allowed to limit engine speed. To achieve this unique function, the sensor is wired directly into the Power Pack (via a TAN conductor) but connected to the alert sounder via a diode. In this way, then the temperature sensor operates it affects both the Power Pack and the alert sounder. The other sensors only affect the alert sounder, and are not supposed to affect the Power Pack.
A explanation for the behavior you describe could be due to a failed diode in the wiring harness. I explain further:
In the alarm system there is a vacuum sensor in the fuel line. This sensor is designed to monitor the suction pressure in the fuel hose delivering fuel to the engine, and to signal an alarm when the suction pressure (or vacuum pressure) in the fuel system exceeds the threshold of the sensor. This condition would normally occur when there was a restriction in the fuel line that was preventing the engine from getting fuel at the rate needed, and that would occur at higher throttle settings. The NORMAL wiring for this sensor is to keep it isolated from the Power Pack input (that initiates the S.L.O.W. function) by a diode. In this way when the fuel restriction condition occurs, there is only an alert produced, and the S.L.O.W condition is not triggered. However, if the diode is bad or if someone previously removed the diode, then the fuel suction sensor would affect engine speed, just as you describe. And, again, just as you described, the engine would be able to immediately resume running at higher engine speeds as soon as the fuel suction pressure dropped, would it would immediately when the throttle was reduced.
So my best guess for the cause of the described behavior: in the wiring of the alarm sensors, there is either a bad diode or a missing diode in the wiring.
Also see
WARNING HORN SIGNALS
http://continuouswave.com/whaler/reference/warningHornSignals.html
Re: RAGE with OMC 115 TurboJet
MikeeH wrote:Someone suggested [the cause of the problem is] the VRO pump: how do I confirm that?
Many people think that any problem with an OMC engine is due to a problem in the OMS pump. The OMS pump is blamed for problems it could not create. See
THE MYTH OF THE MIXER
http://continuouswave.com/whaler/reference/VRO.html
As I mention above, an alarm condition from the OMS pump is not supposed to affect engine speed limit. If the OMS pump were signaling an alarm condition, and if there is a bad diode and this is causing the S.L.O.W function to occur, you could test for this by temporarily disconnecting the TAN conductor from the OMS pump from the other TAN conductors in the wired-OR wiring.
Note that as mentioned in the THE MYTH OF MIXIER, if an OMS pump is failing, it tends to use too much oil. It does not fail into a no-oil state.
Due to the use of blended fuels containing ethanol and gasoline, fuel system components that have rubber internal parts are often degraded. OMS pumps used with blended ethanol-gasoline fuels for 10 to 15-years often have failures and need replacement. On my 1992 Evinrude V6 engine the OMS pump needed replacement after about 20-years of operation.
Re: RAGE with OMC 115 TurboJet
MikeeH wrote:Can the VRO pump be replaced with a "non-VRO" pump?
Yes, but there is really no good reason to do this. If the OMS pump is bad, you should replace it with a new OMS pump. You can replace it with a non-OMS fuel lift pump, but then you will have to always pre-mix your fuel to the proper ratio of oil and gasoline.
Re: RAGE with OMC 115 TurboJet
MikeeH wrote:What is a "non-VRO" pump?
The pump you describe is just a fuel lift pump without the oil mixing system (OMS). It is a standard Evinrude part.
Re: RAGE with OMC 115 TurboJet
AN ALTERNATIVE THEORY: the cause of the problem is just a failed Power Pack. Failure of the Power Pack is common, particularly in engines that are more than 20-years-old. The Power Pack may be just unable to operate the engine at speeds above 3,000-RPM.
My 1992 Evinrude V6 outboard engine required replacement of the Power Pack in c.2005 after 13 years of service.
My 1992 Evinrude V6 outboard engine required replacement of the Power Pack in c.2005 after 13 years of service.
Re: RAGE with OMC 115 TurboJet
Thank you for all the great responses. Just to be clear, there is no audible alarm sounding and the events I describe occur before the engine even reaches normal operating temp. I haven't taken the boat off the trailer yet and have only run it on the trailer, on the ramp with the intake sufficiently in the water to support safe operation. So, since the previous owner described this problem as appearing suddenly without any warning and based on the input here I'm going to suspect the power pack or one of the diodes failing. Sound right?
Re: RAGE with OMC 115 TurboJet
I have managed to get a copy of the appropriate OMC TurboJet service manual. It makes no mention of the diodes so I'm assuming they are included in an assembly. Would that be correct and, if so, which assembly? At this point I'm considering R and R of the power pack unless I can confirm a failed diode with my multimeter.
Re: RAGE with OMC 115 TurboJet
Recall that I prefaced my reply above with this comment:
That the TurboJet uses some different configuration of the sensors, alarms, and S.L.O.W function seems quite reasonable.
Since you now have "a copy of the appropriate OMC TurboJet service manual" you are in a far better position than I am to have insight into the workings of the engine and its sensors. If the wiring diagram shows no diode in a wired-OR alarm circuit, then there likely is no diode.
Regarding the Power Pack as the source of the problem: when I replaced the Power Pack on my engine, before buying the new one I methodically tested a number of less expensive parts as possible causes. When none of those efforts cured the problem, I bought a new Power Pack; it cured my problem. I summarized that repair in an article in the old forum.
jimh wrote:...[I have] some familiarity with electronics used in that era on OMC outboard engines. Here I am assuming the TurboJet engine is using similar technology.
That the TurboJet uses some different configuration of the sensors, alarms, and S.L.O.W function seems quite reasonable.
Since you now have "a copy of the appropriate OMC TurboJet service manual" you are in a far better position than I am to have insight into the workings of the engine and its sensors. If the wiring diagram shows no diode in a wired-OR alarm circuit, then there likely is no diode.
Regarding the Power Pack as the source of the problem: when I replaced the Power Pack on my engine, before buying the new one I methodically tested a number of less expensive parts as possible causes. When none of those efforts cured the problem, I bought a new Power Pack; it cured my problem. I summarized that repair in an article in the old forum.
Re: RAGE with OMC 115 TurboJet
Well, I'm 90% certain its the Power Pack that is the cause of my problem. I did find one on-line, NOS for $120 but I haven't ordered it yet because I want to pull the old one first and make certain the NOS one is the same P/N as mine. However, for the life of me I have no idea how you did an R&R of the PP with the flywheel in place. You must have some very nimble fingers! Since I wear a size 13 ring and my 70 year old digits are experiencing some symptoms of arthritis from years of besting on them I'm going to find a nice person with a gear puller to lend before I try it.