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Mercruiser 350 MAG MPI -02 need full trottle to start when warm

R

RobertL

New member
I have problem with my engine (mounted in a Bayliner 2855 -2002);

Symptom:
Engine don't start at idle throttle when warm and have been shut of for approx 3 minutes or more. When I give it full throttle it starts after 5 seconds of cranking. Have to rev it for a few seconds and when it runns fine. Both at idle and at different throttle levels.
Also it consumes around 50% more fuel than other identical boats with same engine.
When starting the engine from cold condition it starts within a few seconds of cranking at idle throttle and it runns perfect from the beginning.

This has been done so far:
1. Fault code check with a ScanTool. No active fault codes related to the motor control system.
2. New Engine Temperature Coolant sensor. There was an fault code in the history related to the engine coolant sensor, so I replaced that one. (the new one gave the same readings as the old one)
3. New Coolant Water Thermostat. Have changed the thermostat as I discovered with the ScanTool that the engine operating temperature was to low (140 F). With new thermostat it is about 165 F. This also fixed my problem with a little to high idle RPM (is now 600).
4. New IAC. Verified to work fine with the ScanTool.
5. The MAPT-sensor give the same pressure as the barometric pressure when the engine is shut off. So the pressure sensor seems to work. Also the temperature reading is the same a the ambiant temperature when the engine is cold.
6. The Throttle Position Sensor seems to work fine. Verified with the ScanTool that it indicate 0% at idle and 100% at full throttle. The number increased fine when moving the throttle handle from idle to full throttle.
7. Have checked the tubing between the throttle house to IAC to manifold. Replaced the tube between the IAC and the manifold. The old one seemed to be a little bit obstructed.
8. Checked if some of the fuel injectors where leaking by lifting the complete set of injectors and fuel rail up. None of the injectors where leaking.
9. The vacuum tube between the manifold and the fuel pressure regulator was worn out and loose at the manifold end. Have now secured it.

I am almost out of ideas. Have been thinking of the ignition system. But as the engine starts fine cold and warm when re-started withing 3 minutes. And also run fine when started at full throttle when warm. I can't think of any problem with the ignition system. Please let me know if I have missed anything here.

I am still on the fuel/air mix ratio track. My guess is it gets to much fuel compared to air during startup. Giving full throttle clear up the wet cylinders and it fires. That might be reasonable due to the fact that it consumes to much fuel during cruising (But of course that could be an other different problem).
Maybe it don't get enough fuel during startup?
What parameters are important for the ECM during the startup sequence?

My last idea is to check the fuel pressure. Maybe it is to high and therefore the engine gets to much fuel. I will try to get hold of a pressure meter and check that. Should the pressure be 43 PSI in the fuel rails at any time?

All ideas are welcome! :)
Thanks,
 
But with MPI,TPI,TBI systems no fuel can get into the cylinder unless the injector opens,and that dosent happen untill the motors turning so fuel pressure
should be a moot point.Only reason to dump fuel into a cylinder constantly is if the injector is in a jammed open condition.That can happen if the fuel contains a quanity of water mixed in.
 
Vapor Lock? Just to rule out VL, run your blower while underway all the time. Prior to shutting down let engine idle for at least 5 min. so that water circulating can cool it down. As has been reported here, guyg suggests a fresh air supply delivered via an exterior source. Look for post. Al W.
 
Last edited:
wickeral said:
Vapor Lock? Just to rule out VL, run your blower while underway all the time. Prior to shutting down let engine idle for at least 5 min. so that water circulating can cool it down. As has been reported here, guyg suggests a fresh air supply delivered via an exterior source. Look for post. Al W.
Click to expand...

This happens even if I rum the engine from cold on idle to warm condition with the engine hatch open.
It is enough to run it for 5 min on idle. Turn it off and wait for 3 min and it won't start unless I give it full throttle.
 
Bt Doctur said:
But with MPI,TPI,TBI systems no fuel can get into the cylinder unless the injector opens,and that dosent happen untill the motors turning so fuel pressure
should be a moot point.Only reason to dump fuel into a cylinder constantly is if the injector is in a jammed open condition.That can happen if the fuel contains a quanity of water mixed in.
Click to expand...

What I was thinking was if the injectors give to much fuel. If the fuel amount is the same as in a cold start situation. But maybe if should start anyway.
 
[FONT=Tahoma, Calibri, Verdana, Geneva, sans-serif]I have now measured the fuel pressure.

The results from the measurement:
Engine off, ignition on: 42-45 PSI
At idle, vacuum hose connected: 35 PSI
At idle, vacuum hose disconnected: 40-41 PSI
0-50% throttle during load: 35-38 PSI
50-100% throttle: 40-41 PSI

I don't know how accurate the pressure gauge is. Maybe it has a little offset error.

I guess the values are quite normal?

I think I have covered it all regarding air supply and fuel supply. What's left? Ignition?
Why would the spak by enough with cold engine and warm engine when restarted within 2-3 minutes. But not when started after 3 minutes or longer?

I will soon go crazy!
Any ideas? [/FONT]
 
How old is your fuel? I'm still of the opinion that you are chasing a Vapor Lock problem. Have you considered putting in a check Valve that will prevent hot water from syphoning down to the Fuel Pump? (Do a search here or Google). The Vapor Lock, (IMHO), is occurring at the fuel pump. Al W.
 
I'm not sure where to start another thread as mine is an inboard. I think I have a stuck injector. All of the sensors check out fine, no codes, good spark. I pulled the plugs (again) to do a compression test and when I cranked it, tons of fuel poured out of #6, and quite a bit out of the others. This is with the spark disabled and the injectors unplugged and no fuel pressure. There must be quite a bit pooled in the intake. Afraid of a fire, I am letting it evaporate before continuing diagnosis!

Does anyone know the injector size for the 315hp version of this motor?
 
sv_1 said:
I'm not sure where to start another thread as mine is an inboard.
Click to expand...
You may want to consider starting a new thread of your own due to your engine symptoms do not appear to be exactly the same issues as RobertL's problems.

To start a new thread of your own, All you need to do is look for Post New Thread on the top left corner area of the main mercruiser page. Or just click on the link below and describe your problems and what you have done so far like Robert has done,

http://www.marineengine.com/boat-forum/newthread.php?do=newthread&f=28

To Robert the original poster, With all the information you have provided thus far, i would suspect you may possibly have a defective one-way check-valve in your cooling system allowing hot water to backflow down to the fuel cooler creating fuel vapor lock.
 
Last edited:
Gixxer6 said:
Hi guys, I'm having the same issues! RobertL, has it been resolved since?
Click to expand...

Hi,

The problem is still there. After my last reply here I have changed ignition coil, rotor, distributor cap, spark plugs, and spark plug wirers.

I now suspect a mechanical problem. Possibly a leaking cylinder inlet valve. Causing an increased pressure in the air inlet manifold. Causing the MAPT sensor to read a pressure value that not correspond to the present throttle position. That could also explain the high fuel consumption.

I will perform a down leak test to verify this.
 
You might try getting that scantool back out and reading all of the sensor values that control the fuel delivery with the engine running. No codes (on the sensors) means no gross fault has been detected and the sensor values are "within expected ranges". If the sensor is not "accurate enough", as read by the ECU, the wrong point on the fuel delivery curve could easily be selected. If you find one that is off, make sure you check the reference and ground leads at the connector...seen more than a few improperly terminated factory harness pins inside of the connectors.
 
You have a stuck injector. Get a fuel pressure gauge, attach to fuel rail, turn the key to the on position to allow the fuel pump to prime and DO NOT START, once the fuel pump runs and the pressure comes up (1-2 seconds) the fuel pump shuts off. Now the pressure gauge should read +- 5 psi from when the pump was running and hold that pressure for at least 30 seconds. If it doesn't, you have an injector that is stuck open and must be repaired or replaced. Keep us posted if you're still having that problem.
 
Just found this post. Any updates?. I read somewhere. That the scan tool only read what the ECM has in memory. (Is this true or false) Also that the ECM only tells the IAC, what position to be in and it can not know if it actually is in that position. Suggestion was to remove the IAC and look at it, clean it, re-install or install new. i may have read it wrong. I will go back and see if i could find that post. It may have not been this site
 
Technically, the ECM can only "read" what is in its "memory" but that is somewhat ambiguous...from a practical point of view, the ECM can "read" its memory as well as various sensor inputs and some outputs...

As far as the IAC goes, most are stepper motor controlled and the ECU calculates a certain number of "counts" for its position. When started the ECU controls the IAC by way of the "counts" and monitors its position via the engine RPM so, yes, the ECM doesn't "know" the exact position of the IAC pintle.
 
RobertL said:
I have problem with my engine (mounted in a Bayliner 2855 -2002);

Symptom:
Engine don't start at idle throttle when warm and have been shut of for approx 3 minutes or more. When I give it full throttle it starts after 5 seconds of cranking. Have to rev it for a few seconds and when it runns fine. Both at idle and at different throttle levels.
Also it consumes around 50% more fuel than other identical boats with same engine.
When starting the engine from cold condition it starts within a few seconds of cranking at idle throttle and it runns perfect from the beginning.

This has been done so far:
1. Fault code check with a ScanTool. No active fault codes related to the motor control system.
2. New Engine Temperature Coolant sensor. There was an fault code in the history related to the engine coolant sensor, so I replaced that one. (the new one gave the same readings as the old one)
3. New Coolant Water Thermostat. Have changed the thermostat as I discovered with the ScanTool that the engine operating temperature was to low (140 F). With new thermostat it is about 165 F. This also fixed my problem with a little to high idle RPM (is now 600).
4. New IAC. Verified to work fine with the ScanTool.
5. The MAPT-sensor give the same pressure as the barometric pressure when the engine is shut off. So the pressure sensor seems to work. Also the temperature reading is the same a the ambiant temperature when the engine is cold.
6. The Throttle Position Sensor seems to work fine. Verified with the ScanTool that it indicate 0% at idle and 100% at full throttle. The number increased fine when moving the throttle handle from idle to full throttle.
7. Have checked the tubing between the throttle house to IAC to manifold. Replaced the tube between the IAC and the manifold. The old one seemed to be a little bit obstructed.
8. Checked if some of the fuel injectors where leaking by lifting the complete set of injectors and fuel rail up. None of the injectors where leaking.
9. The vacuum tube between the manifold and the fuel pressure regulator was worn out and loose at the manifold end. Have now secured it.

I am almost out of ideas. Have been thinking of the ignition system. But as the engine starts fine cold and warm when re-started withing 3 minutes. And also run fine when started at full throttle when warm. I can't think of any problem with the ignition system. Please let me know if I have missed anything here.

I am still on the fuel/air mix ratio track. My guess is it gets to much fuel compared to air during startup. Giving full throttle clear up the wet cylinders and it fires. That might be reasonable due to the fact that it consumes to much fuel during cruising (But of course that could be an other different problem).
Maybe it don't get enough fuel during startup?
What parameters are important for the ECM during the startup sequence?

My last idea is to check the fuel pressure. Maybe it is to high and therefore the engine gets to much fuel. I will try to get hold of a pressure meter and check that. Should the pressure be 43 PSI in the fuel rails at any time?

All ideas are welcome! :)
Thanks,
Click to expand...

What did you discover. Was this problem resolved? I am having the exact same problems. Thanks
 
RobertL said:
I have problem with my engine (mounted in a Bayliner 2855 -2002);

Symptom:
Engine don't start at idle throttle when warm and have been shut of for approx 3 minutes or more. When I give it full throttle it starts after 5 seconds of cranking. Have to rev it for a few seconds and when it runns fine. Both at idle and at different throttle levels.
Also it consumes around 50% more fuel than other identical boats with same engine.
When starting the engine from cold condition it starts within a few seconds of cranking at idle throttle and it runns perfect from the beginning.

This has been done so far:
1. Fault code check with a ScanTool. No active fault codes related to the motor control system.
2. New Engine Temperature Coolant sensor. There was an fault code in the history related to the engine coolant sensor, so I replaced that one. (the new one gave the same readings as the old one)
3. New Coolant Water Thermostat. Have changed the thermostat as I discovered with the ScanTool that the engine operating temperature was to low (140 F). With new thermostat it is about 165 F. This also fixed my problem with a little to high idle RPM (is now 600).
4. New IAC. Verified to work fine with the ScanTool.
5. The MAPT-sensor give the same pressure as the barometric pressure when the engine is shut off. So the pressure sensor seems to work. Also the temperature reading is the same a the ambiant temperature when the engine is cold.
6. The Throttle Position Sensor seems to work fine. Verified with the ScanTool that it indicate 0% at idle and 100% at full throttle. The number increased fine when moving the throttle handle from idle to full throttle.
7. Have checked the tubing between the throttle house to IAC to manifold. Replaced the tube between the IAC and the manifold. The old one seemed to be a little bit obstructed.
8. Checked if some of the fuel injectors where leaking by lifting the complete set of injectors and fuel rail up. None of the injectors where leaking.
9. The vacuum tube between the manifold and the fuel pressure regulator was worn out and loose at the manifold end. Have now secured it.

I am almost out of ideas. Have been thinking of the ignition system. But as the engine starts fine cold and warm when re-started withing 3 minutes. And also run fine when started at full throttle when warm. I can't think of any problem with the ignition system. Please let me know if I have missed anything here.

I am still on the fuel/air mix ratio track. My guess is it gets to much fuel compared to air during startup. Giving full throttle clear up the wet cylinders and it fires. That might be reasonable due to the fact that it consumes to much fuel during cruising (But of course that could be an other different problem).
Maybe it don't get enough fuel during startup?
What parameters are important for the ECM during the startup sequence?

My last idea is to check the fuel pressure. Maybe it is to high and therefore the engine gets to much fuel. I will try to get hold of a pressure meter and check that. Should the pressure be 43 PSI in the fuel rails at any time?

All ideas are welcome! :)
Thanks,
Click to expand...

Did you or someone solved this issue? Also have the same problem. Have this exact 350 mpi mag engine, though warm start and 45-50 LPH fuel consumption on 3200 RPM on 20 knots cruising with bayliner 2556 (aprox. 2,8 tonnes weight). I think it is to much for such a sophisticated engine with fuel injection technology.

Thank you!
 
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