Sudden Running Lean Issue

[Yarb]

That’s a good approach. One variable at a time.

[chaseincats]

10 minutes ago, Yarb said:

That’s a good approach. One variable at a time.

Yep - at this point I'm kind of at a loss.  Checked plugs, checked temp sensor, checked all electrical connectors at the ecu, checked injectors - so we'll see if any of these 3 are the ticket.

[Yarb]

Is your temp sensor sub harness nice and clean. It’s the short sub harness going back to the ECU if you’re not familiar with it.

[chaseincats]

Just now, Yarb said:

Is your temp sensor sub harness nice and clean. It’s the short sub harness going back to the ECU if you’re not familiar with it.

That's actually where I tested the temp sensor from, but good recommendation

[chaseincats]

3 hours ago, Yarb said:

Is your temp sensor sub harness nice and clean. It’s the short sub harness going back to the ECU if you’re not familiar with it.

completely unrelated efi question: do all the injectors fire at the same time or do they fire individually in time with each cylinder?

[Zed Head]

They all get 1/2 of their needs every three sparks.

[cgsheen1]

On 2/2/2024 at 3:32 PM, chaseincats said:

Update: the gauge is saying 32psi idling.  When I tap the gas and hold it, it will go up to 38 but drop back down to 32.  Isn't idle PSI supposed to be 36?

If I unplug the vacuum line to the pressure regulator it will sit at 40psi when idling.  Gas is not coming out of the pressure regulator with the hose unplugged.

S30 FPR EF Engine Fuel.pdf

No.

The fuel pressure regulator is supposed to maintain a pressure differential of ~36.3 PSI between the intake manifold pressure and fuel pressure. Hence the "vacuum reference tube" between the manifold and the FPR.

IF you have a MAP of -10 PSI (10 pounds of vacuum), the fuel pressure should be ~26.3 PSI. If you have 10 pounds of BOOST, the fuel pressure should be ~46.3 PSI. IF the engine is NOT running but the fuel pump IS (ZERO MAP), the fuel pressure should read ~36.3 PSI. The same should happen with the vacuum reference tube disconnected.

If your pressure readings are correct above, your engine should be running RICH!

Again, IF your pressure gauge is correct, the pressure reading of 40 PSI with the vacuum reference tube disconnected would show that your FPR is allowing 4 more PSI than it should (40 rather than 36 PSI). (Still, going by your readings, looks to me like you're running 8 PSI of manifold pressure (-8 PSI MAP) which doesn't sound like quite enough vacuum to me... Maybe you should check to see what your MAP actually is. (Well, my L28ET runs about -10 PSI or so at idle - maybe it's not that far off))

Edited February 12 by cgsheen1

[chaseincats]

1 hour ago, cgsheen1 said:

S30 FPR EF Engine Fuel.pdf 598.85 kB · 0 downloads

No.

The fuel pressure regulator is supposed to maintain a pressure differential of ~36.3 PSI between the intake manifold pressure and fuel pressure. Hence the "vacuum reference tube" between the manifold and the FPR.

IF you have a MAP of -10 PSI (10 pounds of vacuum), the fuel pressure should be ~26.3 PSI. If you have 10 pounds of BOOST, the fuel pressure should be ~46.3 PSI. IF the engine is NOT running but the fuel pump IS (ZERO MAP), the fuel pressure should read ~36.3 PSI. The same should happen with the vacuum reference tube disconnected.

If your pressure readings are correct above, your engine should be running RICH!

Again, IF your pressure gauge is correct, the pressure reading of 40 PSI with the vacuum reference tube disconnected would show that your FPR is allowing 4 more PSI than it should (40 rather than 36 PSI). (Still, going by your readings, looks to me like you're running 8 PSI of manifold pressure (-8 PSI MAP) which doesn't sound like quite enough vacuum to me... Maybe you should check to see what your MAP actually is. (Well, my L28ET runs about -10 PSI or so at idle - maybe it's not that far off))

I'm confused, doesn't the fsm say the idling fuel pressure should be 36?

[cgsheen1]

No. Fuel pressure in an EFI engine IS NOT CONSTANT. An EFI L-Series engine DOES NOT have a fuel pressure of 36.3 at IDLE! It has a fuel pressure of 36.3 MINUS whatever the intake manifold VACUUM psi is AT THE TIME... If it doesn't, something is wrong.

FSM:

13. Pressure Regulator

"The pressure regulator controls the pressure of fuel so that a pressure difference of 2.55 kg/cm2 (36.3 psi) can be maintained between the fuel pressure and intake vacuum. This constant differential pressure provides optimum fuel injection in every mode of engine operation."

Isn't that what I just said?

Logic:

Under LOAD the intake vacuum DECREASES - that means MORE FUEL  (eg. MAP -2psi (36.3 - 2 = 34.3psi)). You want more fuel under load (you have your foot in the firewall OR you're climbing a hill...). Under less load (you're cruising straight and level at a constant speed) there is MORE intake vacuum (less load) and fuel pressure to the injectors DECREASES - because you need less fuel (eg. MAP -8psi (36.3 - 8 = 28.3psi)). And if your foot comes OFF the accelerator, the intake vacuum MAXES OUT and you have the LOWEST fuel pressure. (eg. MAP -13 (36.3 - 13 = 23.3psi))  Why do you need fuel when you're rapidly decelerating?

Changing the fuel pressure to the injectors controls the fuel flow and is used in conjunction with continually changing the injector pulse width by the ECU. BUT - the ECU IS ALWAYS expecting the fuel pressure to be: "a pressure difference of 2.55 kg/cm2 between  the fuel pressure and intake vacuum"! If you screw with that pressure, you've just screwed over your stock ECU...

 

Edited February 12 by cgsheen1

[chaseincats]

Gotcha, that makes sense.  So mine reading ~32 is fine then?  At what psi would you say is a red flag for low fuel pressure?

[cgsheen1]

 

15 hours ago, chaseincats said:

...  So mine reading ~32 is fine then? 

Do you think you only have 4 pounds of vacuum (I'm assuming at idle?)? Maybe you should check. I have much more vacuum than that at idle.

15 hours ago, chaseincats said:

 At what psi would you say is a red flag for low fuel pressure?

At less than 36.3psi MINUS current vacuum pressure.

(you seem to still act (forgive me if I'm wrong) as if fuel pressure is a static (constant) value - and it's not - not with a running engine. It changes by the millisecond with changes in the manifold pressure (which changes every millisecond). You can't really catch it without sensors and equipment and probably data logging - and you don't really need to. (I can watch it on my boost gauge (MAP gauge) or on my laptop with the Megasquirt software and I can datalog MAP and FP - most people can't, and don't really need to)

IF you turn on the fuel pump - while the engine IS NOT running - and you have fuel pressure of ~36.3psi, you're jake, move along. OR if you pull the vacuum reference tubing OFF the FPR and you have fuel pressure of ~36.3, Bob's your Uncle. move along. You've never said you get those values.

I can't help but think that you're focusing on the wrong thing here (while still feeling that your fuel pressure is all kinds of wrong - BUT - IF your fuel pressure is actually what you state above, your engine should be running RICH, not lean...))

Have you checked the filters on the injectors themselves for debris? You pulled the fuel rail? The injectors all squirt and squirt the same? Have you tried  isolating a troubled cylinder by removing the injector lead and noting the change or lack thereof in engine operation? (which you should have done very first) That's just a start.

I really only entered this thread to RANT about the misinformation surrounding EFI FUEL PRESSURE and I'm sure everyone agrees that I've already ranted way more than enough...

 

[chaseincats]

32 minutes ago, cgsheen1 said:

 

Do you think you only have 4 pounds of vacuum (I'm assuming at idle?)? Maybe you should check. I have much more vacuum than that at idle.

At less than 36.3psi MINUS current vacuum pressure.

(you seem to still act (forgive me if I'm wrong) as if fuel pressure is a static (constant) value - and it's not - not with a running engine. It changes by the millisecond with changes in the manifold pressure (which changes every millisecond). You can't really catch it without sensors and equipment and probably data logging - and you don't really need to. (I can watch it on my boost gauge (MAP gauge) or on my laptop with the Megasquirt software and I can datalog MAP and FP - most people can't, and don't really need to)

IF you turn on the fuel pump - while the engine IS NOT running - and you have fuel pressure of ~36.3psi, you're jake, move along. OR if you pull the vacuum reference tubing OFF the FPR and you have fuel pressure of ~36.3, Bob's your Uncle. move along. You've never said you get those values.

I can't help but think that you're focusing on the wrong thing here (while still feeling that your fuel pressure is all kinds of wrong - BUT - IF your fuel pressure is actually what you state above, your engine should be running RICH, not lean...))

Have you checked the filters on the injectors themselves for debris? You pulled the fuel rail? The injectors all squirt and squirt the same? Have you tried  isolating a troubled cylinder by removing the injector lead and noting the change or lack thereof in engine operation? (which you should have done very first) That's just a start.

I really only entered this thread to RANT about the misinformation surrounding EFI FUEL PRESSURE and I'm sure everyone agrees that I've already ranted way more than enough...

 

I remember testing it a while back and it was on the low end of normal on atlantic z meaning it had ~17 lb at idle at 800 rpm - "An engine in good condition should have a gauge reading of 17 to 21 and hand should be steady".  When the car is running and I pull the FPR vacuum hose, it sits at 39 psi.

Coincidentally I tested the injectors by pulling the fuel rail, sticking all of the injectors in cups and cranking it over the weekend - the spray patterns and volume were identical which was great news.  I did just change the fuel filter today so we will see if that does anything but it was only a few years old and there already is a clear filter between the tank and fuel pump.

Edited February 13 by chaseincats