1996 502 mpi rpm issue

[bornfree2]

First want to say Hello, glad to be here...And i thank any advice i receive as im at a loss at the moment.. I have a problem with a 1996 mpi 502. The issue it drops rpm at apx 4400. When it drops it goes to 3200 apx and doent move.. I have to pull back on the throttle and move it forward again... but it does it every time at apx same rpm. I have changed all the spark plugs, changed rotor and cap, had all the injectors cleaned professionally.. I had the computer scanned and there is no flags... I have ran the motor up to 4000 rpm wile the other two were shut off...no issues no extra noise...Its in a 419 triple engine formula. I have electric hi volume fuel pumps that build 35 psi to 55 psi... There is no fuel delivery problem. I have had the tec scan the motor in real time as it happens ,,, still no flags...Can the coil or module cause this issue... iv only seen them completely fail. Its like it goes into guardian mode...but it isn't by the computer readout. Iv even open the hatch to increase air flow...no difference.. Any ideas..

 

[HT32BSX115]

I have had the tec scan the motor in real time as it happens ,,, still no flags...Can the coil or module cause this issue... iv only seen them completely fail. Its like it goes into guardian mode...but it isn't by the computer readout.

Howdy,

Welcome aboard!!

Well, you are in a unique position in that you can swap the ECM/coil etc from one of the other engines to the problem engine and see if the problem follows.

Other than fuel starvation (filters/pick-up screen/injectors/induction blockage) it sounds like ignition.


Regards,


Rick

 

[Fun Times]

Hi there, did you remove the VST tank? (Mercruiser's Vapor Separator Tank)

Is your engine serial number handy?

If the fuel system and engine is stock, 55 psi is to high.

Some engine models in and around 1996 used power reduction mode but not all.

What engine out of the 3 is causing the problem?

 

[alldodge]

First want to say Hello, glad to be here...And i thank any advice i receive as im at a loss at the moment.. I have a problem with a 1996 mpi 502. The issue it drops rpm at apx 4400. When it drops it goes to 3200 apx and doent move.. I have to pull back on the throttle and move it forward again... but it does it every time at apx same rpm. I have changed all the spark plugs, changed rotor and cap, had all the injectors cleaned professionally.. I had the computer scanned and there is no flags... I have ran the motor up to 4000 rpm wile the other two were shut off...no issues no extra noise...Its in a 419 triple engine formula. I have electric hi volume fuel pumps that build 35 psi to 55 psi... There is no fuel delivery problem. I have had the tec scan the motor in real time as it happens ,,, still no flags...Can the coil or module cause this issue... iv only seen them completely fail. Its like it goes into guardian mode...but it isn't by the computer readout. Iv even open the hatch to increase air flow...no difference.. Any ideas..

:welcome: to iboats

Check the routing of the knock sensor wire coming off the starboard side of the block. If its running along side the spark plug wires it can sense a knock and go into power reduction mode. Your systems doesn't have guardian mode, and as FT mentioned your engine serial number could help make sure we are look at what you are

 

[bornfree2]

engine number is f772141. the reason the higher fuel pressure is it has pro chargers on all 3 engines. The other engines work well...so did this one until this happened, the problem engine is the port side.. https://www.youtube.com/watch?v=0yNuyDTkqUQ

 

[alldodge]

engine number is f772141. the reason the higher fuel pressure is it has pro chargers on all 3 engines. The other engines work well...so did this one until this happened, the problem engine is the port side.. https://www.youtube.com/watch?v=0yNuyDTkqUQ

Well while it was a nice video it was just wasting my time, it had nothing to do with your issue as stated, looked more like a commercial for the ,com site.

Your engines don't have VST they have been removed and you have a custom setup. The engine dropping to 3200 rpm is what mine did when the knock sensor was activated. What you can do to verify, is wait for it to do it, then pull the wire off the knock sensor. If the issue goes away I would recommend stop running the engine until you find out if you have a knock or the wire sensor is defective.

 

[bornfree2]

The video was to show the rpm on the one motor before it stopped working and the setup in the engine department.. I will try what you suggest...The tec did say the flag did not come up on his computer wile we ran it in real time wile the rpm drop...im also going to swap the computers and will report back... thank you for your time..

 

[alldodge]

The video was to show the rpm on the one motor before it stopped working and the setup in the engine department.. I will try what you suggest...The tec did say the flag did not come up on his computer wile we ran it in real time wile the rpm drop...im also going to swap the computers and will report back... thank you for your time..

The MEFI1 computer is not that great, it doesn't store the knock sensor flag, mine didn't do it either. I would suggest before you swap computers try the knock sensor wire first. You have a very clean engine room and there was another on OSO which did it to him self when he used tie wraps to make everything look nice.

 

[bornfree2]

Ok... This is what i have done... I have inner swapped the computers...the problem stayed with the original motor. i then disconnected the knock censer.. did not change.. i traced the knock censer wire and made sure it was shielded...did not change... i disconnected the out drive oil fill censer... still cut out around 4000 rpm. i tested the alarm by crossing the 2 censer wires from the fill tank for the oil out drive... alarm sounded at helm...so that works.. I then changed the coil by the distributor, keeping in mind i also changed the rotor and cap earlier.. no change in problem ...still kicks out at 4000 apx.

At this point im at a loss...my oil pressure is 50 to 75 psi under load... I run a 50/50 synthetic... oil is topped up. there is no blow by at all...i opened the hatch and ran it with out any air restriction...no change..can a bad oil sender switch give a false reading to the computer... just wondering if that is a possibility...

Any ideas

 

[alldodge]

I'm thinking its still electrical. Computers swapped no effect, Knock sensor wire removed not effect. Will say that by removing the knock sensor wire the computers will retard the timing by 3 to 4 degrees, but those few degrees will not cause that amount of reduction in the rpm, which is good to know. The next thing I would try is swap the knock module, and if no effect, then the distributer.

You wouldn't happen to have an engine synchronizer installed?

 

[bornfree2]

All Dodge, thank you for your input... it is very much appreciated. there is no synchronizer on the boat.. mb possible the pick up on the distributor... is that what your thinking...i havent changed it... mb the computer losses its position at high rpm... just a thought...

 

[alldodge]

I'm still thinking along the line that something is causing the engine to retard timing. Could be something wrong with the pickup but that is a bit far outside of possibilities. The distributer is like if all else is coming up empty

If the computer is not able to sense the knock sensor is working, as mentioned before it will retard timing by 3 to 4 degrees. If the engine has a knock or the sensor is bad at is signaling the ECM there is a knock it will remove most all advance. So swapping the knock sensor is a possibility along with the knock module. There is also a 10 amp fuse for the knock module

Just thought of ringing out the wire connection between the ECM and the knock sensor.

You have a MEFI1 ECM and there are only 7 possible codes. If the knock circuit fails it will show something, but if it is telling the ECM there is a knock it will not set a code.