Powerboat Forums at SpeedWake banner

Base Timing Mode Woes - Falling Fuel Pressure

3K views 12 replies 7 participants last post by  krazzy 
#1 ·
Gen. VI MAG EFI setup

I recently developed a problem on one of myGen. VI 502 MAG EFI motors. Last week, both motors started up just fine in the driveway before the first water test of the season. After a quick ride around the lake, I came back to the boat launch noticing that one engine was running at a rather high idle (1200 rpm).

The next day, I checked all the throttle linkage to make sure nothing was keeping the throttle body cracked opened. Nothing. Next, I checked all the vacuum fittings and ports for any major leaks. Nadda. Finally, I made an attempt to set the base timing thinking that the timing might be too far advance.

This is where things get weird…

I borrowed a MerCruiser diagnostics / service tool from a local marina. I scrolled through the menu's and selected the option that would allow me to set the base timing. What was supposed to happen, was engine rpm's increase to 1200 - 1300 rpm's and the timing to back down to a lesser value (20 - 30 degrees down to 5 - 8). What actually happened was… nothing! The engine behaved no differently. The scan tool was reading like it had successfully entered base timing mode, but nothing had changed.

I tried the second engine, and it's behavior was what I had expected: idled around 800 rpm, timing floating around between 15 and 25 degrees, base timing idle goes up to 1200 rpm's, timing stabilizes around 8 degrees. I am almost suspicious that the first (bad) engine was stuck in base timing mode. However, the high idle caused by base timing mode on the good engine was fluctuating more than the steady 1200 rpm idle of the bad engine.

Thinking something might have been jacked in the ECU, I switched the ECU's between the two engines. Nope, engine still had the same previous high idle.

The next problem I noticed was the falling fuel pressure. When I would first turn on the ignition, the fuel pressure gauge would read about 35 - 40 psi but then quickly drop to nothing once the (electric) fuel pump was shut off. When the engine was running, I had no problems maintaining fuel pressure (about 30 - 35 psi at idle) but the pressure was not holding on "primer" startup. The second (good) engine was holding pressure just fine. I could turn the ignition to ON, watch the fuel pressure build, hear the fuel pump shut off, then watch the fuel pressure stay steady at about 35 psi for minutes at a time.

What could be causing the fuel pressure to leak like that? That same day, I noticed (smelled) a fuel injector leaking gas. The entire upper intake manifold was removed (with blower, intercooler and 2100 CFM throttle body… uhg!) and I replaced a ripped up O-ring on one of the injectors that got damaged from the sharp painted edge of the fuel rail. Once I put the fuel rail back into place, I checked for any new leaks and found everything to be sealing tight. However, the fuel pressure still wasn't holding.

Next, I disconnected all the fuel injector to make sure the ECU wasn't holding one of the injectors open. Nope, pressure still dropping faster than the panties on the prom queen.

At this point, I threw a few tools across the yard, said some colorful words, went inside and pounded a few beers. I am stumped, I have no clue! I have a sneaking feeling that the fuel pressure leak is coming from the regulator somewhere, but I'm not 100% convinced yet. Also, would this be the source of my high idle speed? I don't see why it would be!!!

Please help, I'm very tired, very frustrated, and very broke… all from working on this project.

-B
 
See less See more
#2 ·
Ok, here's where it get's tough.

Not holding psi could be:

Pump - many have check balls/valves to hold psi
Regulator - bad diaphragm could cause fuel to leak by'
Injector - not fully closing and would bleed off pressure


Now - high idle condition. Related? Possibly.
Did you watch the sensor's info while running on each motor to compare?

I'm wondering if the IAC (idle (air)speed motor) is stuck open or not closing down? Not too uncommon for this.
 
#3 ·
OK Bayley, here's what I can tell you about my 3 502 MAG MPI's I've owned.

1) All 3 of them NEVER held fuel pressure after the pump stoped. It would fall faster than panties on prom night.

2) All 3 of them the rpm's wouldn't increase to 1200 rpm when put in base timing mode. Had to raise the Rpm's with the throttle.

These engines all used the MEFI3 ECU, the oldest being a 1999 and the newest being a 2002. As long as you have the steady fuel pressure when the engine is running, You should be fine.
I have a fuel pressure gauge mounted on my dash so I can always see my fuel pressure at the rail. I can tell you that till I got rid of the Merc water/fuel filter and replaced it with a bigger Fram one, I had fuel flow problems. And even that wasn't enough. After spending time with Bret from Aeromotive, I realized that the Merc line feeding the fuel rail was only 1/4" diam :confused1 :eek: :eek: I removed that line, drilled out the rail to accept a 1/2" fuel line, and now I have a steady pressure.
 
#6 ·
cfm,
I had all 16 injectors (both motors) flow tested over the winter by Kinsler Injecting in Troy MI. They all spec'd out OK and don't appear to be leaking. However, I know it's still something I should still be checking for. I'm thinking that there might be something in the regulator that recently let go allowing pressure to leak back. As for the fuel pump, anyone know for sure if there is any type of check valve in the factory fuel pump? I personally doubt it, but I could be wrong.

As for the high idle, I did check out the IAC with the scan tool. I should have checked out the step count on the high idle motor verse the count on the lower (normal) idle motor. I seem to remember the IAC counts being about 32 - 34 on the high idle motor. Sounds a little high to me, but I suppose it could be in the normal range. I also "reset" the IAC with the scan tool with the engine running and it would indeed stall the engine as expected. This indicated that the IAC was moving.

Kanookstr,
I find it odd that you've never been able to hold fuel pressure after the prime up. I was able to do this on both engines prior to the first sea trial, and I'm still able to do this on the one engine that is still idling normally. I also have the fuel pressure gauge installed on the back of the fuel rail so I don't think out instrumentation is any different. Would something in the regulator cause the pressure to drop? I've got the cheap - o Livorsi Fuel PSI gauges that have very small movements, so it's difficult for me to tell if the fuel pressure is changing as the engine fires up. It doesn't appear to, yet I expected it would.

Does anyone have a schematic of the fuel lines for a 1999 MAG EFI setup?

The base timing mode is a weird one. I've head many people say the ECU will raise the engine speed, while other people tell me I will need to do it myself. I guess I've got one of each. :rolleyes: I'm wondering if the engine was at a lower idle speed, would it raise the RPM's in base timing mode like the other engine? I tried both ECU's on the "good" engine and they both raised the RPM's on the engine while neither one did a thing on the high idling engine.
 
#7 ·
cfm,
I had all 16 injectors (both motors) flow tested over the winter by Kinsler Injecting in Troy MI. They all spec'd out OK and don't appear to be leaking. However, I know it's still something I should still be checking for. I'm thinking that there might be something in the regulator that recently let go allowing pressure to leak back. As for the fuel pump, anyone know for sure if there is any type of check valve in the factory fuel pump? I personally doubt it, but I could be wrong.

As for the high idle, I did check out the IAC with the scan tool. I should have checked out the step count on the high idle motor verse the count on the lower (normal) idle motor. I seem to remember the IAC counts being about 32 - 34 on the high idle motor. Sounds a little high to me, but I suppose it could be in the normal range. I also "reset" the IAC with the scan tool with the engine running and it would indeed stall the engine as expected. This indicated that the IAC was moving.

Kanookstr,
I find it odd that you've never been able to hold fuel pressure after the prime up. I was able to do this on both engines prior to the first sea trial, and I'm still able to do this on the one engine that is still idling normally. I also have the fuel pressure gauge installed on the back of the fuel rail so I don't think out instrumentation is any different. Would something in the regulator cause the pressure to drop? I've got the cheap - o Livorsi Fuel PSI gauges that have very small movements, so it's difficult for me to tell if the fuel pressure is changing as the engine fires up. It doesn't appear to, yet I expected it would.

Does anyone have a schematic of the fuel lines for a 1999 MAG EFI setup?

The base timing mode is a weird one. I've head many people say the ECU will raise the engine speed, while other people tell me I will need to do it myself. I guess I've got one of each. :rolleyes: I'm wondering if the engine was at a lower idle speed, would it raise the RPM's in base timing mode like the other engine? I tried both ECU's on the "good" engine and they both raised the RPM's on the engine while neither one did a thing on the high idling engine.
 
#8 ·
I guess we have a different setup then Bayley. I don't run the stock Merc fuel return. Merc returns fuel to the filter/separator instead of the tank, and all the fuel injector guy's I deal with says that's a No..No... I run an Aeromotive pump with an Aeromotive Regulator back to the tank. I've been told that the reg will leek down all the pressure back to the tank within seconds of the pump not running. You might have a fuel line reversed in the Merc filter/separator and that's why one of your engine's pressure drops after your pump stops. Com pair your fuel line's with the engine thats holding pressure. I think that Merc filter housing has something to do with keeping it a closed loop system and hold's pressure. But if one line is reversed......system acts real wierd :lick: :yak:
Good luck
 
#9 ·
Back to the high idle.

Are we 100% sure there are no air or vacuum leaks?

Air getting into the system (other than thru the throttle blades or IAC) can cause this high idle.

Vacuum line to the reg slip off or loose connection? Vacuum plug on intake manifold missing?
PCV line routed to behind the throttle body?

Apparently the computers are doing okay. Something else is up.
 
#10 ·
to: cfm tech

We have boost pressure gages on this setup which also read vacuum. Will there be a significant difference in the vacuum reading if there is an intake leak? In the old days engine vacuum could be monitored to help diagnose problems like leaks. It doesn't seem like that is possible anymore with all the computer stuff. Is this true? Are there any other basic methods to look for intake leaks other than visual?
 
#11 ·
Spraying carb cleaner. It will change idle when sprayed over leak.

Just don't spray it near the front of the t-body or throttle shafts for it will suck in there and give you a false symptom.


Go for intake to head sealing first. Than at where the intake halfs meet. Then t-body to intake seal. Then where injectors go into intake. Vac lines, etc, etc.

You get the point.

You'll know when you've found the leak - if there is one.
 
#12 ·
Not sure what to make of the high idle except for a vacuum leak somewhere up top. The sudden fuel pressure drop I'm assuming is new to you this year on startup. Your motors should have a cool fuel system. You may want to make sure that the fuel cooler on that motor is not cracked. If it's a fairly large leak you may be able to use a cup to collect some water from the exhaust while it's running and see if you have some raw fuel in the water that exited. Disconnecting the feed and exit side of the cooling hoses on the cool fuel may give you some help too. Take them off and prime the system of fuel. See if there's any fuel coming into the cooler on the water side. Hope that helps a bit. Good luck!
 
This is an older thread, you may not receive a response, and could be reviving an old thread. Please consider creating a new thread.
Top