350 mag mpi surging under load

[alldodge]

You don't need marine lifters (basically no such thing)

 

[silver_power]

So if I finally need lifters I will buy aftermarket and not original from mercruiser. Many thanks.

 

[ghunttwo]

Please keep us posted as to what you found.
I have the same problem with my 2003 350 mag mpi.
I'm with you in that there is no way I'll be able to get to the cool fuel regulator and or pump.
I've got a Donzi 18 classic and there is no way to get my 6' 3" body in the eng compartment. Where is that trained monkey we talk about?
Good luck
George

 

[Bt Doctur]

measure the railk pressure first. if you have 40 psi at idle and it drops to 20 or less at power the boost pump is no good, not the high pressure pump

 

[silver_power]

Well , the problem still exists...the fuel pressure at rail is ok with a gauge . The lifters are ok and still the ecm has no stored faults! This is the bigger problems. Till now, new plugs, new cap, new wires and the problem is still there. Besides if it was a fuel issue , the engine would starved at high rpm. The boat can reach about 4600-4700pms with a hesitation . Thw hesitation exists at all rpm after get on plane.The boat normally with the same setup reach about 4950-5050 rpm. I am thinking to suggest my mechanic to try another cam position sensor and another ecm 555. ...we can not find what can cause the problem......

 

[alldodge]

Things that tell the ECM to vary power, MAP/MAT, TPS, CPS and all need a good steady 5V supply
Measure the 5V line and see if it varies, if not then put the meter on the sense leads one at a time on the sensors

 

[ghunttwo]

I took mine to the marina with the same problem. The computer indicated that there is a fault at 2400 in one of the shift switches. They call.it the shift antisapation switch, not the shift interrupt switch. I ordered both and will.have them installed Thursday. The mechanic says that he has replaced many for this same reason.
Ill let you know the results.

 

[silver_power]

well, the problem still exists. The lifters are ok as you very right insisted, swapped the ECU with another with the same results.
I will change a rotor inside the distributor (as i only changed the cap) and change with total new spark plugs NGK ITR4A15 iridium ( as i now do not have iridium) and check it but i doubt this is my problem.
The mech can not think something else as the fuel pressure is correct, no faults exist . But he is sure that this is an electric issue.
The next i am thinking to do is to check for grounding issues......can you please tell me what exactly and how to check it with a VOM meter?

 

[alldodge]

Put the red lead of the meter on any of the sensors listed above on the Black/Pink and the Black lead on ground. With the key ON and running, you should read 5V.

 

[silver_power]

I will check it. Thank you very much.

 

[Fun Times]

If I recall correctly, you have a scan tool correct?
Next time your out, watch all the live data numbers you can and see if any of them seem to be varying around to much while it's happening. Things like the Throttle Position Sensor, available power, any of them that show a % or voltage that would typically stay put at one throttle position setting.

Since some people are thinking it might be an internal engine issue, you may want to consider connecting up an engine vacuum gauge to see if the engine is doing any odd things.

Though the following guy had an intermittent spark issue, he ended up finding a bad connection to one of the wires at the ignition coil "Module"....Maybe something to inspect on yours too....If not, just carefully wiggling wire around sometimes helps find electrical issues.

http://www.ybw.com/forums/showthread.php?242512-Intermittent-spark-on-Mercruiser-350-mag-mpi

Did you ever move or remove the complete distributor assembly especially before you noticed this surging issue? Is the distributor loose by chance?

Doubtful you'll have this issue but might be worth a try anyways....There is a test on page 3 of 3 that you could try doing while out on the open water...Just be careful as you won't be able to shift into other gears until you reconnect the wire connector at the shift plate on the exhaust riser area. http://www.google.com/url?sa=t&rct=...ooGwBA&usg=AFQjCNHUdvqBYKGkyHF9hZalwTRD-p2YZw

Your throttle body/plate, flame arrestor, IAC and IAC muffler are clean?

 

[QBhoy]

Ok...so having had this issue and more with my MPI...hopefully I can pass on some useful info.
Firstly...don't mess about with normal plugs...these need Irridium plugs..pure and simple. Either the NGK you mention or the AC delco superseded 993 model.

Above said, I'm not thinking this is the problem. Given your symptoms, it most likely lies with the throttle position sensor.
You also need to replace the rotor as well as the cap. Pointless doing just the cap, for the sake of it and given this is a well known weak point.
Failing the TPS...IAC is a less likely culprit (but another known weak point) and will usually show up on faults.
Also most likely not to blame would be the SW pressure sensor....but this will put the MPI into a 90% or less gaurdian mode and make things play up.

I really do think this may be a TPS issue, also worth thinking about is a problem I discovered by messing around.

These engines take a fair bit of electrical power when running...if your battery isn't up to much it can be an issue. Even when the alternator is pumping out 14v dc, a failed battery cell can loose this to void and not give the required power to the engine.
Unlikely your problem, because this issue I found was causing the engine not to fire up....and not in the usual way.
To explain...the engine cranked over perfectly, making me think the battery was good...and would only try to fire when I released the key.
Turns out that the battery had enough power in it to turn her over quick enough, but the cranking amps required tool just enough power away from the coil(really high demand) to cause her not to fire up.
This was proven by cranking and when releasing the key (just when she tried to fire upon release) quickly turning key back on...she fired straight away because there was no longer such great ampage taken away from coil.
This also suggests a weak coil. I replaced battery and haven't had the issue since.
Anyway, I digress. As mentioned, likely a TPS called for, along with iridium plugs and a rotor.
Let me know how you get on.
PS....all the above is assuming that you aren't picking up crap from the fuel tank pipe and first line of defence metal gauze on the pick up isn't blocked !

 

[silver_power]

But wouldn't the TPS throw a fault to the ECU? That is why I think that my problem is something else than a senor or sender.....according my opinion the problem is something that does not cause a fault to the ecu.....something like loose ground, spark plugs, rotor maybe.......i will try a rotor and checking the grounds tomorrow and I wait the iridium plugs to change and see if the problem will be solved. I will post as soon as I habe news.....bad or good.
But something else. Is it possible , or maybe happen to somebody an MPI to have surging problems because of the use of non-iridium plugs. ? I mean someone just to change them and the problem went?

 

[tpenfield]

My MPI engines run well without iridium plugs :noidea: I just use the AC/Delco marine plugs.

Iridium . . . what is that anyway :noidea: . . . sounds like kryptonite

 

[alldodge]

Don't see it being the plugs. The TPS should throw a code if there is an issue, but if something is loose/corroded inside it may signal the ECM that the throttle is being moved back n forth due to vibration. Cannot say for sure, but faults are thrown if voltage is to high or to low

 

[silver_power]

First thing i am going to do this weekend is to change the distributor rotor and check for corrosion to the harness and grounds. I will post the results. I hope to solve the problem somewhen.......btw the consuption is bigger about 30%!!.....

 

[scoflaw]

Read post #31 again

 

[Dave-R]

I had a similar problem with a 2011 Volvo penta 5.7 300-C engine. My fix was a new MAP sensor. Mine also did not have any codes to suggest what was happening. I bought a GM one for about 50$. All my injectors tested fine, and when the spark was removed from each cylinder, I had a RPM drop. Dave-R

 

[silver_power]

I aleady changed the map sensor and I retested it's values. The output voltage is correct.

 

[silver_power]

As my mechanic told me that the distributor rotor will be available next week, he gave a used one from another engine. So I test it and the situation now is about 50-60% better. There is still a surging but not so bad. The engine has more power ,the response is much better and the fuel economy is better as well. The problem still exists but now is much better that seems that the problem is for sure electric and I search at the correct path. Next week I will have the new rotor and new ngk iridium spark plugs and I hope that this solve totaly the problem. I also checked the resistance of the spark plug wires and all except one was about 4,5-5kΩ/ft which is very good and so the wires is not the problem. The wire I changed was measured at 13kΩ/ft which is bad. I also checked the voltage at the sensors listed above and there was a steady 5v supply. Something I noticed as I took off the distributor cap, was that eventhough I changed it recently, about 4 operation hours ,there is points of burn and little corrosion at the points that the rotor meets the metals inside at all the 8 cylinders. I think that this shows there is for sure an issue with the spark and that is why I have the surging. Is it normal for 4 operation hours the cap to have points of cossosion and burning??