scoop on fuel injectors?

[BerniniCaCO3]

...actually, I don't think I will do any mods; stock will serve me fine. But you can't look up engine mods without higher # fuel injectors coming up on craigslist, forums, etc., and now I'm curious about some things, have some questions bouncing around in my head.

As I understand it, let's go with stock: I have 19# fuel injectors that can flow 19#s, or 3 gallons, of fuel per hour if running continuously, AND presuming 45psi pressure from the fuel pump.
The mass airflow sensor reads the incoming air. The fact that someone was selling 30# injectors and a 30# MAF suggests that this is programmed into the MAF.
So the MAF is matched to the injectors, it knows how much air is flowing in --more if I'm in a verdant forest at sea level, less if I'm approaching the top of Pike's Peak-- and my stock MAF knows that I've got stock 19# injectors. So it sends a signal either to the ECM, or to the injectors themselves, to fire with a certain duty cycle to achieve a good 12:1-14:1 air/fuel ratio?

Now here are my questions. First, was I right to conclude that you must match the MAF to the injectors? If I just went and put in an 80mm throttle body with an 80mm MAF that says it's for 30# injectors, am I going to find myself running very lean? Or similarly, just put in the wrong overweight injectors, find myself running rich?

Another factor are the O2 sensors on the other end; if they measure lots of latent O2, they know you're running lean... maybe they can compensate for your idiot installation of the wrong MAF?

Another question. I presume that the average fuel pump would be around 45psi, but that might differ manufacturer to manufacturer, weaken with age, drop with a clogged fuel filter, or maybe I put in a new pump and it happens to be 60psi. Now, the same duty cycle of the same fuel injectors will flow a very different amount of fuel, if the pump is weak with age or high pressure because you bought a performance pump presuming it was better.

Will the computer compensate? Does it have a pressure sensor to monitor fuel pressure? Or does it just presume 45psi all the time?

[gadget73]

the ECM is the only thing that determines the injector size. Theres a variable programmed in there someplace with the size of the injector. The "calibrated" MAF meters actually provide a modified signal to the ECM so it thinks its getting a different amount of air than it actually is, which makes it still work with a larger set of injectors. You can do it that way, but its really not optimal.

Fuel pressure is controlled by the fuel pressure regulator. The pump when dead-headed is probably good for 70+ psi, but the regulator keeps it between 32-42 psi so things are consistent. You can fit any size fuel pump you like, and the pressure will remain constant as long as there are no problems with the regulator or a restriction in the line. There is no sensor for the fuel pressure, its just assumed to be correct. If its not, then the motor just runs stupidly.

[BerniniCaCO3]

thanks!
That answered everything.
Where is the regulator located and how would you know if it went bad? (just paranoid now, hahaha)

[gadget73]

back corner of the intake manifold on the fuel rail. Its the round thing with the vacuum line on it. When they go bad (rare), either the fuel pressure drops or the diaphragm ruptures and sends fuel through the vacuum line into the intake. In either case, the motor would run decidedly screwed up. Easy to check with a fuel pressure gauge, and/or pulling the vac line off to make sure its not squirting gas out of the regulator.

[BerniniCaCO3]

now if you have a vacuum leak, or, a leak between the upper intake and the lower for example, you will be getting extra intake that the MAF is not seeing. The motor should run lean at first, get confused: but once the O2 sensors see too much oxygen, will the motor correct anyway despite the fact that the MAF is telling it that it's got less air than it's really getting?

[1990LTD]

no. well, it might try to correct the problem but the air coming in is unmetered so it's not gonna run properly

[gadget73]

it will try and correct, but you'll end up with things not being happy and most likely it will spit out the "excess unmetered air" code, or whatever it is that basically says you have a vacuum leak. The ECM has some amount of correction that it can do before it just gives up. I think its 10 or 15%.

I did that to my truck once. I forgot to hook the tube from the valve cover back up to the intake. It ran, but it ran crappy and gave me a check engine light till I plugged it back in. Never tried it on the Towncar, but mass air cars don't really like vacuum leaks any more than an SD car, though they generally don't run stupid unless the leak gets to be fairly large.

[Crownvicman289]

If the vacuum leak is global, it is possible that it won't affect an SD car at all. If the IACV can correct and all cylinders receive and equal part of the leak, then SD doesn't care. It's as if the throttle were cracked open farther. MAF on the other hand, not a fan myself but it looks like I'll be going that route in the future anyway. I actually unplugged the MAF on my truck and plan to remove it someday, I just have a difficult time visualizing things and MAF's tend to be buttplugs in the intake tract anyway.

[Merc302]

it will try and correct, but you'll end up with things not being happy and most likely it will spit out the "excess unmetered air" code, or whatever it is that basically says you have a vacuum leak. The ECM has some amount of correction that it can do before it just gives up. I think its 10 or 15%.

I did that to my truck once. I forgot to hook the tube from the valve cover back up to the intake. It ran, but it ran crappy and gave me a check engine light till I plugged it back in. Never tried it on the Towncar, but mass air cars don't really like vacuum leaks any more than an SD car, though they generally don't run stupid unless the leak gets to be fairly large.

all of a sudden my 85 302 cfi started bogging itself down at idle...wtf?

[gadget73]

most likely a completely and totally unrelated problem to what mass air or speed density multiport injection systems do with vac leaks. CFI is it's own demon.

[91waggin]

most likely a completely and totally unrelated problem to what mass air or speed density multiport injection systems do with vac leaks. CFI is it's own demon.

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