502 Mag plug reading
04-06-2022, 07:47 AM
#41
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iTrader: (1)
Yep, still not sure where cuve or advance would be coming from as I only know the module to convert the pulse to something ecm can use.
Now heres my "test" for sake of discussion : If I hook my MSD Ignition 8998 to those 2 pins coming from dist pickup to input side of module (I often do this trouble shooting) using it as a signal generator, I can simulate the engine going anywhere from 10 rpm to 10,000 rpm, a standard brand module will smolder after 30 seconds much above 5000 if the rest of mefi is plugged in, a merc or dui will take 5600 briefly in air, more if mounted to something to pull heat from it i ended up mounting one on aluminum as a heat sink .
Anyways, IF I go in the mefi ecm program and I set timing to say 15 degrees in every map vs rpm table, top to bottom, plug my lqaptop in, set it to scanner pro or plug a merc scan tool in, crank the signal generator between idle and 5000, it will show it commanding 15 degrees of timing top to bottom.
Now, if a mefi motors on my dyno, I again set those timing tables to 15 degrees from top to bottom, I set my actual jumpered timing at 8.1 to correlate those tables (FALLING REFERENCE ANGLE IN REGARDS TO NEXT PHYSICAL TDC" TO 8.1 degrees) rev motor, it will show that 15 degrees thruout the rpm range (it may vary a tiny bit lower from cam deflection, chain stretch as you rev it higher) but in general. So if the ecm dictates when to fire the coil based on timing tables, if the module started doing its own version, where would timing actually be and what would be controlling it?
Now heres my "test" for sake of discussion : If I hook my MSD Ignition 8998 to those 2 pins coming from dist pickup to input side of module (I often do this trouble shooting) using it as a signal generator, I can simulate the engine going anywhere from 10 rpm to 10,000 rpm, a standard brand module will smolder after 30 seconds much above 5000 if the rest of mefi is plugged in, a merc or dui will take 5600 briefly in air, more if mounted to something to pull heat from it i ended up mounting one on aluminum as a heat sink .
Anyways, IF I go in the mefi ecm program and I set timing to say 15 degrees in every map vs rpm table, top to bottom, plug my lqaptop in, set it to scanner pro or plug a merc scan tool in, crank the signal generator between idle and 5000, it will show it commanding 15 degrees of timing top to bottom.
Now, if a mefi motors on my dyno, I again set those timing tables to 15 degrees from top to bottom, I set my actual jumpered timing at 8.1 to correlate those tables (FALLING REFERENCE ANGLE IN REGARDS TO NEXT PHYSICAL TDC" TO 8.1 degrees) rev motor, it will show that 15 degrees thruout the rpm range (it may vary a tiny bit lower from cam deflection, chain stretch as you rev it higher) but in general. So if the ecm dictates when to fire the coil based on timing tables, if the module started doing its own version, where would timing actually be and what would be controlling it?
I’ve worked on several 4cyl 3.0 carbed VP’s that use this, a couple of carbed 5.7’s, and a 502MPI that removed the fi and went carb’d. I’ve hit them all with timing light like i do with all dist equipped engines for disgnostics and/or maintenance.
The following 2 users liked this post by SB:
articfriends (04-06-2022), sutphen 30 (04-06-2022)
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articfriends (04-06-2022)
04-06-2022, 08:06 AM
#43
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Unplug 4 wire connector from ecu to dist, add a carburetor to run engine. Grab a timing light.
I’ve worked on several 4cyl 3.0 carbed VP’s that use this, a couple of carbed 5.7’s, and a 502MPI that removed the fi and went carb’d. I’ve hit them all with timing light like i do with all dist equipped engines for disgnostics and/or maintenance.
I’ve worked on several 4cyl 3.0 carbed VP’s that use this, a couple of carbed 5.7’s, and a 502MPI that removed the fi and went carb’d. I’ve hit them all with timing light like i do with all dist equipped engines for disgnostics and/or maintenance.
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articfriends (04-06-2022)
05-30-2022, 08:08 AM
#44
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Thread Starter
Update:
Basically zero improvement in performance after the following:
New distributors, caps, rotors, wires, plugs
Injectors serviced and flowed by Smitty
New fuel pressure regulator on Stbd motor
Yesterday I really was able to pinpoint the sag in power just past 3/4 throttle. It appears to be equal on both motors. Ran great up until 3/4 throttle, but advancing the throttles more led to a noticeable loss of acceleration. As a reminder, this is a 1996 Sunsation Dominator with mildly modified 502 Mag MPIs, turning Hydromotive 4 blade 26P. At 3/4 throttle I'm seeing about 71-72 mph and 4400 rpm(ish)
I still have not made the effort to see fuel pressure while under these conditions, but since it sure feels like a lack of fuel, I have some of the fuel system apart right now. The pickup tubes are clear, though I need to replace them because the aluminum fittings were seized into the tank (probably haven't moved in 26 years).
The fuel filter heads appear to be the stock Merc stuff, but there's adapters from the filter head to the fuel lines to convert NPT to a flare, and these adapters have an inside diameter of 1/4 inch. Could this present enough of a restriction to reduce fuel flow? I'm looking for factors that are consistent across both motors since they appear to be behaving in an identical manner.
Thanks for any input.
Basically zero improvement in performance after the following:
New distributors, caps, rotors, wires, plugs
Injectors serviced and flowed by Smitty
New fuel pressure regulator on Stbd motor
Yesterday I really was able to pinpoint the sag in power just past 3/4 throttle. It appears to be equal on both motors. Ran great up until 3/4 throttle, but advancing the throttles more led to a noticeable loss of acceleration. As a reminder, this is a 1996 Sunsation Dominator with mildly modified 502 Mag MPIs, turning Hydromotive 4 blade 26P. At 3/4 throttle I'm seeing about 71-72 mph and 4400 rpm(ish)
I still have not made the effort to see fuel pressure while under these conditions, but since it sure feels like a lack of fuel, I have some of the fuel system apart right now. The pickup tubes are clear, though I need to replace them because the aluminum fittings were seized into the tank (probably haven't moved in 26 years).
The fuel filter heads appear to be the stock Merc stuff, but there's adapters from the filter head to the fuel lines to convert NPT to a flare, and these adapters have an inside diameter of 1/4 inch. Could this present enough of a restriction to reduce fuel flow? I'm looking for factors that are consistent across both motors since they appear to be behaving in an identical manner.
Thanks for any input.
Last edited by DrFeelgood; 05-30-2022 at 08:11 AM.
05-30-2022, 08:13 AM
#45
Registered
You need to check fuel pressure under load before you buy anymore parts.
The following 2 users liked this post by underpsi68:
Gimme Fuel (05-31-2022), SB (05-30-2022)
05-30-2022, 08:26 AM
#46
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Thread Starter
Thanks for the input!
06-05-2022, 10:56 AM
#47
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Thread Starter
OK, so today I did some top end runs on the river, with a GoPro aimed at the fuel pressure gauges.
At idle, they're indicating low 30s. At speed, it looks like I'm seeing mid-upper 30s (some vibration making it difficult to read accurately).
At no time did I see the fuel pressure drop below 30psi on either motor.
I'm uploading some video clips to youtube now, if anyone thinks that would be helpful.
At idle, they're indicating low 30s. At speed, it looks like I'm seeing mid-upper 30s (some vibration making it difficult to read accurately).
At no time did I see the fuel pressure drop below 30psi on either motor.
I'm uploading some video clips to youtube now, if anyone thinks that would be helpful.
06-05-2022, 11:38 AM
#48
Registered
Think fuel pressure should be 43
06-05-2022, 11:43 AM
#49
Registered
Thread Starter
My motors are a bit of a hodge podge in the fuel systems. I bought the boat this way -- old style injectors, MEFI 1 ECM, but the Cool Fuel II setup with the fuel pump/fuel cooler/fuel pressure regulator under the port side of the motors.
