502 mpi running on 4 cylinders
 

Ran the boat today made a couple laps at WOT, headed back and it hiccuped so I backed out of the throttle then started running like s*** after putting a new cap and rotor and a coil still running the same

After some trouble shooting 2 cylinders are down on th port and starboard side

Port side cylinders 1 and 7 are down
Starboard side cylinders 4 and 6 are down

Any ideas? I'm thinking a distributor but not sure

sound like an injector wire feed from ecu went bad.

So is that the injector harness that runs cylinders 1746?

sounds like it.there are 2 driver circuits from the ecu.also check the 12v feed to them also.

Ok will do

How would I test these? And where

at the injector w/ a noid light.

I am looking at my wiring harness for another problem but you might check your grounds. There are 3 grounds on the injection harness alone. Randy

It's weird it was running great then it hiccuped and has run on 4 cylinders...they just all of a sudden go?

I just checked the spark and it does have spark on all cylinders

I don't have a meter on hand so it will be a bit to check the grounds. A friend of mine has a noid tester so ill try that tomorrow

Does the mercathode have anything to do with that?

no

Just to shine a little light on the subject. Our 502 efi engines are "batch fire". Meaning the ecm fires 4 injectors at one time unlike sequential were there all individual.

So reading you lost 4 all at once tells me you broke a wire in the harness some where. only 2 wires from the ecm fires 4 injectors. iirc there is a fuse under the ecm box lid for the injectors. since 4 still work im guessing its good. Google the mercruiser #16 manual. the wire diagrams and trouble shooting guide is section 5 I believ for the fuel.injection. I just got done reading it cover to cover ;-)

Hope this spreads some light on the subject

http://www.maxumownersclub.com/forum...cruiser/16.pdf

Im guessing J2-5 (on the ecm) wire (color Dark Green) is broken somewhere in the harness or at the ECM. 468 drives injectors 1, 4, 6, 7. Same ones you have that are dead ;)

Look at page 501 on the above link. It is the injector wire diagram.

First check the wiring at the ecm and harness (that is the easiest). After that you will have to remove the top intake manifold and start tracing j2-5 (dark green wire) from ecm to injectors.

does the rev limiter shut off 4 injectors or 4 plugs?

At idle it wil rev past 3k
But when you try and take it on plane it is 2-2500rpm max like its under a big load

^^^??? So now you have all 8 injectors working?

Look at the merc manual #16. "troubleshooting". It will walk you threw step by step of what to check.

^^^still only firing on 4

check to see if theres at least 12v at the injector w/ the key on.then we'll go from there.

Will do thank you for the help I apreciate all the help you guys have given me

had a 95 272 that went into limp mode when outdrive iol was low. just a thought could it be somthing along those lines?

I had the exact problem with my 350 mag mpg. Ended up being one of the injectors shorting the driver in the ecu out. After many days of trouble shooting I started engine on hose and started unplugging injectors and once I got to the bad one engine went to running on 7 cylinders. Once plugged back in it went to missing all for 1,7,4,6. Once that was figured out I replaced all injectors.

My resivor is low so this could be the problem...I hope it's that easy
But I do see a wire going back to the ECM...would that tell the ECM go go into limp mode if my drive oil resivor was low?

fixx
 

like mentioned,,if one of the injectors have a low ohme reading it will tell the quad driver in the ecu to shout them off..unplug them and ohme them all out,,i think they should be around 12.3 tp 13.2 ohmes cold..

If you lost those 4 injectors, then more than likely either there is a break in the blue or green(whichever goes to the bad injectors) wire going to the injectors, or the driver in the ecm is bad. You can check with a noid light at the injectors. Unplug all 4 of them and check each one. As mentioned, also check the ohms of each injector. They should be about 12.2-12.3 ohms. The pink wire is 12+ volt. It should have power since all of the injectors are tied together and 4 are working, so i don't think that is your problem.

If you don't have a noid light, you can use an analog voltmeter. It's easier to see what's going on with an analog instead of a digital meter.
Eddie

Just for an update I took each injector connector and turned it over till I found what injector wasn't working...I disconnected cylinder 4 connector and it started running on 7 plugged it back in and back to 4

I then pluged the noid into the connector and pulses just fine so I ordered a new oem injector

But another issue is I pulled the dipstick and the oil has turned a hazy brown milky color? Moisture or water in oil? Just changed the oil on Saturday with valvoline 20-50 vr1 racing oil
I never had this happen

well the badly running engine is sucking water back in.I'd wait till you get it running right.

Yeah that's what I was thinking

I had the same problem with my last boat (1994 454 Mag EFI--385hp). Only running on 4 cylinders. Tested EVERYTHING and it was finally determined that my ECU was bad. Replaced it and good as new. If replacing your injector doesnt do the trick you might want to consider a new ECM (buy one used on OSO for a couple hundred).