Mercruiser 7.4 mpi

[Cruis]

Hi everyone,
I have a 1999 -2000 mercruiser 7.4 mpi serial# L061906 engine will turn over strong but will not start. I sprayed carb cleaner and it started right up and runs great but will not start if I turned it off and back on. I changed the map sensor, idle air control value, TPS. The fuel pressure is 35 psi. I did a leak down test after 10 min it went down to 32 psi. when cracking the engine the pressure jumps up to 40 psi and back to 35. also compression is all 150 psi.
Any thoughts also, I might invest I a scanner.

Thanks

 

[Scott Danforth]

how old is your fuel?

 

[Fun Times]

Technically you're low on fuel pressure as the regulator shows it should be 43 psi for your engine model and combined with possibly old to older fuel left in the system? might be a reason you need to help get the engine started with an additive. See your fuel regulator item number 15 here, https://www.mercruiserparts.com/bam/subassembly/31918/2705/110

Though rare, this engine also has another item mentioned as a fuel regulator by Mercruiser but is supposed to be used more of a fuel pressure damper to the injector's vs regulate like it does on the GM truck engines becuase your engine has a regulator down under the portside, front engine mount area / next to the oil pan... so as time and use go by it's been slightly known to have an effect on the overall conditions of the fuel system operation.
This part is very expensive from Mercruiser but it is a GM part found on the Automotive side of the GM L29 - 7.4L engine used in that timeframe... See item number 5, https://www.mercruiserparts.com/bam/subassembly/31918/2705/130

The damper one is mounted on the fuel rail and depending on fuel condition, etc. it might be time for either cleaning/inspection or replacement too.

When you get the engine running, while watching the fuel pressure gauge, remove the vacuum hose that runs to the lower fuel regulator and see if the fuel pressure goes up about 5 psi as the pressure should move up and down a bit as you remove and reinstall the vacuum hose.

 

[Cruis]

how old is your fuel?

I added fresh fuel also I changed my fuel filters and add new fuel before installing.

 

[Cruis]

Technically you're low on fuel pressure as the regulator shows it should be 43 psi for your engine model and combined with possibly old to older fuel left in the system? might be a reason you need to help get the engine started with an additive. See your fuel regulator item number 15 here, https://www.mercruiserparts.com/bam/subassembly/31918/2705/110

Though rare, this engine also has another item mentioned as a fuel regulator by Mercruiser but is supposed to be used more of a fuel pressure damper to the injector's vs regulate like it does on the GM truck engines becuase your engine has a regulator down under the portside, front engine mount area / next to the oil pan... so as time and use go by it's been slightly known to have an effect on the overall conditions of the fuel system operation.
This part is very expensive from Mercruiser but it is a GM part found on the Automotive side of the GM L29 - 7.4L engine used in that timeframe... See item number 5, https://www.mercruiserparts.com/bam/subassembly/31918/2705/130

The damper one is mounted on the fuel rail and depending on fuel condition, etc. it might be time for either cleaning/inspection or replacement too.

When you get the engine running, while watching the fuel pressure gauge, remove the vacuum hose that runs to the lower fuel regulator and see if the fuel pressure goes up about 5 psi as the pressure should move up and down a bit as you remove and reinstall the vacuum hose.

Thanks for the reply. I know the fuel pressure is a bit low but why doesn’t the engine try to at least start. I added gas in the filters and the tank.

Are you saying I need to change both
regulators?

 

[dubs283]

Thanks for the reply. I know the fuel pressure is a bit low but why doesn’t the engine try to at least start. I added gas in the filters and the tank.

Are you saying I need to change both
regulators?

Possible need to change one or both regulators, you can try testing as described to ensure the pump is good, i.e. can produce the proper pressure. IME the pump should produce upwards of 80 psi hence the need for regulator(s).

Most times the regulator(s) fail causing excess pressure and a rich running condition, which you state isn't happening, the engine runs great once running.

You mention replacing a number of efi parts/sensors but not tune up parts? You can start by inspecting dist cap/rotor, plugs, wires, etc. Most often replacing these items helps immensely with hard start issues

 

[Cruis]

Possible need to change one or both regulators, you can try testing as described to ensure the pump is good, i.e. can produce the proper pressure. IME the pump should produce upwards of 80 psi hence the need for regulator(s).

Most times the regulator(s) fail causing excess pressure and a rich running condition, which you state isn't happening, the engine runs great once running.

You mention replacing a number of efi parts/sensors but not tune up parts? You can start by inspecting dist cap/rotor, plugs, wires, etc. Most often replacing these items helps immensely with hard start issues

Hi, yes i forgot to mention I changed the ignition coil, distributor cap, rotor I changed the plug wires last year. Also I started the engine and I didn’t change the plugs because I was waiting for the fogging gas to burn out.
Since I was having the same problem I changed the plug and found that old ones were really black like it was burning rich.

 

[tpenfield]

All those parts that you replaced . . . those were not the problem.

Do you have spark? Have you tested that you have spark while cranking? Is the spark while cranking consistent or erratic?

IIRC, the distributor is in a base timing mode while cranking and the ECU takes over at/above 300 RPM. Your 7.4 probably cranks at about 180-200 RPM.

The injector pulse is probably 12ms while cranking, which should be plenty even with slightly low fuel rail pressure

 

[tpenfield]

Other things to check are the various relays and fuses for the injector system. Reference the wiring diagram for your engine by serial number to see the exact fuses & relays.

 

[Cruis]

Other things to check are the various relays and fuses for the injector system. Reference the wiring diagram for your engine by serial number to see the exact fuses & relays.

Hi, I got a scanner today and I’m getting one code. This is the code “Low oil pressure telltale has been set”. Does anyone know what it means?

 

[tpenfield]

It means that the engine has been running at low oil pressure (< 10 psi) for too long of a time. Maybe clear the code(s) and try to start the engine to see if you are getting oil pressure at the gauge.

Otherwise shut down . . .

 

[Cruis]

It means that the engine has been running at low oil pressure (< 10 psi) for too long of a time. Maybe clear the code(s) and try to start the engine to see if you are getting oil pressure at the gauge.

Otherwise shut down . . .

I cleared the code but it’s still coming back I can getting 40 psi at the gauge when I turn the ignition on.
Can this be the reason the engine won’t start on its own?

 

[Scott Danforth]

If you are getting 40 psi on the gauge when running, however the ECM is seeing 0, then plumb in a mechanical gauge to see which sender is bad.

you either have oil pressure and the ECM sensor isnt working, or you do not have oil pressure and the gauge/sensor is faulty

 

[Fun Times]

What scan tool model did you happen to get?

Are you able to see oil PSI in live data while the engine is running? (When you get it running with additive help that is)

I don’t recall this engine model type utilizing an oil pressure sensor that the ECM reads and looking over the parts diagram for your engine, there doesn’t seem to be one either… plus you should be hearing a warning horn usually under the dash should the oil psi go under 4psi.

SENDERS AND HARNESS BRACKET

SENDERS AND HARNESS BRACKET

www.mercruiserparts.com

WIRING HARNESS(EFI)

WIRING HARNESS(EFI)

www.mercruiserparts.com

Does this engine use a bravo stern drive or is there a transmission type drive system?
It may be a false code depending on engine/drive application.

Is there any numbers to the code or any other wording?

 

[Cruis]

What scan tool model did you happen to get?

Are you able to see oil PSI in live data while the engine is running? (When you get it running with additive help that is)

I don’t recall this engine model type utilizing an oil pressure sensor that the ECM reads and looking over the parts diagram for your engine, there doesn’t seem to be one either… plus you should be hearing a warning horn usually under the dash should the oil psi go under 4psi.

SENDERS AND HARNESS BRACKET

SENDERS AND HARNESS BRACKET

www.mercruiserparts.com

WIRING HARNESS(EFI)

WIRING HARNESS(EFI)

www.mercruiserparts.com

Does this engine use a bravo stern drive or is there a transmission type drive system?
It may be a false code depending on engine/drive application.

Is there any numbers to the code or any other wording

I didn’t try starting and looking at the live data. It as a bravo 3. No number with the code. The scanner I got is a techmate pro.
Also I am only seeing the oil pressure sensor with the single wire on it so I believe you’re right.

 

[Fun Times]

In first thoughts that one is interesting though I’m pretty sure it won’t have an effect in this situation… I believe when you get the engine started again the live data may show a oil psi reading but it won’t be in true accordance (though close) with actual oil pressure because the MEFI 3 ECM does have a provision for oil psi and the updated tech mate is able to read it’s default values in turn setting a fault but keeping in mind there’s no sending unit to read.., To add to that plus your question of engine starting, technically EFI’s don’t have a safety feature that would keep the engine from starting if the oil psi stays under 4 psi.

At this point I’d recommend staying on the lower fuel pressure to see what you can determine what it’s doing with a bit more testing with the vacuum hose, inspection, etc..

Normally we here at iboats try not to have others throw a bunch of parts at a situation without test but unfortunately the fuel rail regulator on this one would be like taking a trip to Las Vegas as there’s not much testing to know if it’s an issue or not for your seemingly unusual not starting without help but will run. < Remove and inspect at the very least to start. That mentioned again, it is a GM part that should be able to find in the $100.00 range vs Merc in the $400.00 range.

Have you tried opening the throttle anywhere from 1/4 to full open to see if the engine will start easier?

Is all this testing being done on the trailer vs on the water testing under load?
The engine runs and sounds normal at RPM under load?

 

[Cruis]

In first thoughts that one is interesting though I’m pretty sure it won’t have an effect in this situation… I believe when you get the engine started again the live data may show a oil psi reading but it won’t be in true accordance (though close) with actual oil pressure because the MEFI 3 ECM does have a provision for oil psi and the updated tech mate is able to read it’s default values in turn setting a fault but keeping in mind there’s no sending unit to read.., To add to that plus your question of engine starting, technically EFI’s don’t have a safety feature that would keep the engine from starting if the oil psi stays under 4 psi.

At this point I’d recommend staying on the lower fuel pressure to see what you can determine what it’s doing with a bit more testing with the vacuum hose, inspection, etc..

Normally we here at iboats try not to have others throw a bunch of parts at a situation without test but unfortunately the fuel rail regulator on this one would be like taking a trip to Las Vegas as there’s not much testing to know if it’s an issue or not for your seemingly unusual not starting without help but will run. < Remove and inspect at the very least to start. That mentioned again, it is a GM part that should be able to find in the $100.00 range vs Merc in the $400.00 range.

Have you tried opening the throttle anywhere from 1/4 to full open to see if the engine will start easier?

Is all this testing being done on the trailer vs on the water testing under load?
The engine runs and sounds normal at RPM under load?

Thanks for all the help everyone, I found the problem behind the engine upper left there is a oil pressure sensor. In my case the ground wire was loose. In other cases it can be a bad pressure sensor.
Soon as I secured the ground she fired right up.

 

[Fun Times]

Thanks for all the help everyone, I found the problem behind the engine upper left there is a oil pressure sensor. In my case the ground wire was loose. In other cases it can be a bad pressure sensor.
Soon as I secured the ground she fired right up.

For memory check, Any chance of getting a photo of the sensor and wire?
Thanks