7.4L MPI (L29) MEFI 2 Fuel Pressure issue

[Lg101a]

I have a 1998 Nordic with the 7.4L MPI. No other issues than a stumbling issue around 3500 rpm. New plugs, wires, cap, rotor, ign module, coil, fuel filter. No issues with start up. Gets up on plane, pulls boards/toys, etc. If we get up over 3500 rpm it will stutter bad until we drop down to 3500 rpm and its smooth again. On the rare occasion it will get up over 3500 (up to 4000) then will stutter a few times but gradually increase in stuttering until we bring it back down to 3500.

While sitting in marina, in neutral, I have attached a fuel pressure gage. Ignition on produces 36 psi. Engine on, at idle 36 psi. Increasing the rpm slowly will bring pressure down to 32 psi. Near 3000 it will start stuttering bad and the gage needle bounces greatly but will not peak over 32

Boat has two gas tanks. Problem exists regardless which tank is used. I'm thinking fuel pressure regulator is the issue, not the fuel pump (I could be wrong). I live 250 miles away from the boat. I have to purchase parts to take with me to work on it. I am posting this issues looking for some guidance what the problem could be. I don't want to drive out there and find I need more parts. . Any advice would be greatly appreciated

 

[tpenfield]

:welcome: to iBoats . . .

32 psi may be OK, I do not recall what the exact spec if for your 7.4 is . . . My engine (7.4L MEFI 1) was 36 psi +/- 2 psi . . .

The problem with MPI engines is that you really need the computer program to diagnose them. Troubleshooting an MPI engine is a different process than a carburated engine. You really need to see what the various sensors are indicating to the engine controller. Not sure you have access to the computer program (Rinda Diacom) but that is probably what you need.

 

[alldodge]

Howdy
First your fuel pressure is low, it should be 43 psi just running, so first thought is the regulator is an issue or the pump. It bogs because pressure is not supplying enough fuel

Your serial number is between 0L010003 THRU 0M024330

Item 15 is the pressure regulator
https://www.mercruiserparts.com/bam/subassembly/31918/2705/110

 

[Lg101a]

Thanks for the input. My serial number on engine is 0L068416

 

[Lg101a]

I don't have access to a Rinda Diacom. I'll have to ask some shops in the area. I've done some reading and see they can be somewhat expensive

 

[dubs283]

a scan tool won't tell you much seeing as how your engine is mefi and in your situation i don't think it'll do you much good anyways

your problem is definitely fuel delivery, as stated your pressure is low and the gauge bounce is a bad sign that the pump is struggling especially under a load

some of the older mefi (1&2) mpi big blocks had a vst located at the back of the plenum and would collect all kinds of debris from the fuel system and cause runnability issues

start cheap and work your way expensive, clean vst (if equipped), replace regulator, then pump if issue persists

 

[Lg101a]

Another question... How can I tell what is the correct MEFI.? Per Manual 23, the MEFI 1 and 2 should have 34-38 psi. MEFI 3 should have 41-45 psi. I THOUGHT i had an MEFI 2..?

 

[tpenfield]

You may have to get a P/N off the MEFI unit. Otherwise the manual for the engine may reference it.

 

[Lg101a]

I went on the Mercruiserparts.com site. Entered my engine serial number. Scrolled down to "Electrical and Ignition Components". Located the ECM in the exploded view. In the list below the exploded view it shows two ECM listed with different part numbers. If I put my cursor over the "Notes" feature for each, this is what it says...

Part number 861736R1 MEFI-2 SN# 0L085005 & Below
Part number 861720T2 MEFI-3 SN# 0L085006 & Up

Based on my serial number (0L068416) I should have MEFI-2. I will check ECM part number next time at boat.

Regardless of MEFI -2 or MEFI-3, the fuel pump and fuel pressure regulator (in Fuel Pump and Fuel Cooler section) is the same for both per exploded view and parts list.

 

[alldodge]

This is what they look like

 

[Lg101a]

So based on the above info, it appears my fuel pressure should be 34-38 psi. I'll be taking a fuel pressure regulator AND a fuel pump for my next trip. I will keep you all updated

 

[Lg101a]

Thanks for the photos of MEFI.

 

[tpenfield]

So based on the above info, it appears my fuel pressure should be 34-38 psi. I'll be taking a fuel pressure regulator AND a fuel pump for my next trip. I will keep you all updated

OK, so MEFI 2 . . . I don't think 32 psi on a 34-38 psi spec is going to cause huge problems, but it is a process of elimination. Just be sure to check the new regulator to see that you are getting 36 psi +/-2 psi. When I put in a new regulator in my engine, I tested the regulator using compressed air of know psi and increased it to see when the regulator would start to 'dump' pressure.

I had previously gotten a new regulator off eBay and it would not hold pressure beyond 28 psi . . . So, I bought the OEM Merc/Quicksilver version and it held 36 psi, started to dump pressure above 36 psi.

 

[Lg101a]

Thank you tpenfield. Good to know about the quality issue with pressure regulators. I will keep this page updated when I get back to the boat. ETA unknown at the moment.

 

[Lg101a]

Update. Replaced fuel pump and pressure regulator. The boat seemed to run slightly better but still had we miss at 3500. Took it to a real mechanic and he found the ignition module did not have a required heat shielding material at the distributor. He believes the ignition module was failing. The one I replaced did not have this, so I wasn't aware of this. He installed a new mercruiser ignition module with heat shielding. The issue seems to have gone away when running it in neutral. Have yet to put it on the lake. Another item he found was the main wiring harness connector between motor and boat. He disconnected this to eliminate any errors due to failing gauges. During this operation he noted one prong was loose. He suspects this may have also been the problem but because we had fix this issue several years ago he thinks the ignition module was the culprit.

 

[Lg101a]

Final Update. Been on the colorado river three times this year. Boat runs great after the new igntion module WITH heat shield. No hickups, sputters, etc.