7.4mpi just started running rough and with gas coming out exhaust

[QBhoy]

Hi. Couple of thoughts here. From the simple perspective. A couple of simple things that can make these run bad would be;
Your alternator failing or a bad connection somewhere causing a low voltage or no charge voltage. This means your ignition system won’t have enough to run the thing properly. Check the voltage when it’s running to see.

On all mpi…they run rich when they are cold. They over fuel a fair bit until they warm up. If you have a bad temp sensor, it may think it’s cold and be running rich even when warm.

Other simple thing would be a failing IAC valve.

 

[Ozshadow]

UPDATE - I was able to get to the #2 injector so I could get a NOID light on it while running. The NOID light does not blink. It stays on.

The wiring that I can see looks to be good.

 

[dubs283]

The wiring that I can see looks to be good.

Test for a short to ground. Both key on and off

 

[Ozshadow]

Test for a short to ground. Both key on and off

How exactly? Test light from battery to the injector connector when the key is off? Anything else?

 

[alldodge]

How exactly? Test light from battery to the injector connector when the key is off? Anything else?

Use the noid of meter, turn key ON, motor not running and the light should be OFF. Or with a meter measure both pins to ground. Both side will measure 12V. If one measured 0V then there is a ground being applied

 

[dubs283]

How exactly? Test light from battery to the injector connector when the key is off? Anything else?

I would use an ohm meter on both leads, key on/off

 

[Ozshadow]

I'm still researching and will go back for more testing with the meter.
What I noticed yesterday with the NOID light on the #2 connector was:
Key off, light off.
Key on, light on but dim and solid,
Running light on very bright and solid.

I gather all even injectors have a common connection. Have you ever seen one injector internally short to ground which makes them all stay on?

I don't want to send my ECM out until I test all alternatives.

Thank you.

 

[dubs283]

Have you ever seen one injector internally short to ground which makes them all stay on?

Not that I recall however it is quite possible.

This is why an ohms test on the harness is your next step. Check for a short to ground first on all injector leads, with injectors connected/unconnected, key on/off, etc....

As stated yes, the ecm uses "batch fire" for the injectors, one side/four injectors per "fire". It's a very simple system and normally the only time a reflash is necessary is when repowering non naturally aspirated

 

[alldodge]

All evens should not be tied together, only way this can happen is if someone moved the connectors. The below pic is from a 555 ECM but does illustrate batch firing better due to the colors.

Injectors are seated with rubber type seals which keep it from grounding out against the intake manifold. If one of the ground side wires is chaffed and grounding out this will keep them ON. When the motor is idling the NOID flashes around 300 times a minute (5 times second).

 

[Ozshadow]

Use the noid of meter, turn key ON, motor not running and the light should be OFF. Or with a meter measure both pins to ground. Both side will measure 12V. If one measured 0V then there is a ground being applied

Doing this, the odd bank shows 12v on both connectors at an injector. The even bank shows 12v on the power connector and 8v on the wire that goes to the ECM.

All disconnected reads 12v on one side and none on the ECM wires. Plug any injector in and the first paragraph happens.

All injectors read 12.9 ohms.

 

[dubs283]

Doing this, the odd bank shows 12v on both connectors at an injector. The even bank shows 12v on the power connector and 8v on the wire that goes to the ECM.

All disconnected reads 12v on one side and none on the ECM wires. Plug any injector in and the first paragraph happens.

All injectors read 12.9 ohms.

This is good practice. Keep going, next is key off check continuity on injector leads to ecm/ ground.

It's rare for an ecm to go bad. It does happen but you need to rule out all possibilities besides spending big $ just to find out there's a broken/melted/unconnected wire or something else causing issues

I forget, have you investigated fuel pressure/regulatory stuff?

 

[alldodge]

Looking at wires going to the injectors, can you see 2 colors? Can you see if all colors are on just one side or are they divided up on both sides?

 

[Ozshadow]

This is good practice. Keep going, next is key off check continuity on injector leads to ecm/ ground.

It's rare for an ecm to go bad. It does happen but you need to rule out all possibilities besides spending big $ just to find out there's a broken/melted/unconnected wire or something else causing issues

I forget, have you investigated fuel pressure/regulatory stuff?

I will recheck continuity but I am fairly certain each check returned infinity. I don’t recall any test I attempted show resistance. Some appeared to change the numbers for half a second before showing infinity.

Should I do this with the ECM connected and only 1 injector lead disconnected at a time? Key off then same with key on?

Fuel pressure was where it should be. Can I disconnect the relay and the injectors still pulse(or supposed to)? I am doing this now with the upper plenum off. Originally I was only able to test with the noid light only on #2 with the engine cranking or running and it stayed solid. I would like to see how the other 7 respond.

 

[Ozshadow]

Looking at wires going to the injectors, can you see 2 colors? Can you see if all colors are on just one side or are they divided up on both sides?

Yes, pink power to all and different colors for the ECM side connections.

 

[alldodge]

Yes, pink power to all and different colors for the ECM side connections.

Thanks, still is weird one side is to much fuel and other is lean

 

[dubs283]

Thanks, still is weird one side is to much fuel and other is lean

This rehashing makes me think there is a regulator/damper issue. Merc used a few different fuel supply rails/regulator/damper setup on these fantastic engines (seriously, love isn't a big enough word for my feelings about the 454).

Fortunately merc used a fuel supply map in the mefi ecm to provide the engine with enough fuel (at times too much) for its healthy appetite. This allows for the proper performance and power output the 454 can provide. There is one fuel rail that I believe is a GM part that has a regulator installed oem that merc didn't abate and claims it as a "fuel pressure damper". It is used to regulate the pulses of the injectors to eliminate runnability issues. The damper can fail and allow excess fuel past it and cause a rich running condition most notable at idle/low rpm

Unknown if your engine is equipped with the mentioned fuel rail/damper setup

 

[Ozshadow]

This rehashing makes me think there is a regulator/damper issue. Merc used a few different fuel supply rails/regulator/damper setup on these fantastic engines (seriously, love isn't a big enough word for my feelings about the 454).

Fortunately merc used a fuel supply map in the mefi ecm to provide the engine with enough fuel (at times too much) for its healthy appetite. This allows for the proper performance and power output the 454 can provide. There is one fuel rail that I believe is a GM part that has a regulator installed oem that merc didn't abate and claims it as a "fuel pressure damper". It is used to regulate the pulses of the injectors to eliminate runnability issues. The damper can fail and allow excess fuel past it and cause a rich running condition most notable at idle/low rpm

Unknown if your engine is equipped with the mentioned fuel rail/damper setup

Mine has the unused regulator but I don’t recall any fuel in the vacuum hose.

 

[Ozshadow]

Thanks, still is weird one side is to much fuel and other is lean

I think the L29 fires all evens and all odds doesn’t it?

I am particularly focused on why the voltage on the even side wires from the ECM is almost 5v lower when only the key is on. That is a unique voltage drop.

 

[alldodge]

I think the L29 fires all evens and all odds doesn’t it?

I am particularly focused on why the voltage on the even side wires from the ECM is almost 5v lower when only the key is on. That is a unique voltage drop.

Please clarify the even and odd? Is that cylinders 2,4,6,8 and 1,3,5,7 ?

If it is what I just said, then that shouldn't happen. Would ask if the injector plugs can be moved around to the pic in post 29

Agree the difference in voltage might be the static state of the ECM, don't know, just guessing. Would be interesting test to remove on of the injectors and see if the injector would fire just with the key ON. Might try on/off a few times to see if things change

 

[Ozshadow]

Please clarify the even and odd? Is that cylinders 2,4,6,8 and 1,3,5,7 ?

If it is what I just said, then that shouldn't happen. Would ask if the injector plugs can be moved around to the pic in post 29

Agree the difference in voltage might be the static state of the ECM, don't know, just guessing. Would be interesting test to remove on of the injectors and see if the injector would fire just with the key ON. Might try on/off a few times to see if things change

My injector plugs are numbered and connected correctly. 2, 4, 6 and 8 on the starboard side.
I will see if the voltage readings ever change.
I will also look to see if any colors match. Maybe the banks are really different than just evens and odds. The spark plugs were definitely only noticeably black on the even side.