Fuel pressure

[Vettefan87]

Okay I just recieved my fuel pressure gauge from Mid American and decided to check my car. I put it on and started it up and I am getting 49 pounds. I hooked it up to my brothers car, and his is reading 38 pounds. I did not set mine the shop that put the motor back in did. Should I take the pressure down to about 43 pounds or so? I think I have been getting bad gas mileage. It seems like I am running out more often, and when I changed the plugs they were all wet, I dont know if that was due to the car not running good, or the fuel pressure. Any suggestions?

 

[WhalePirot]

FP changes with vacuum

Changing the fuel pressure will change the air/fuel mixture. The ECM will compensate, as well it can, to achieve the optimum. A fair bit has been written on other threads about tuning, as regards fuel pressure, but my 'guys' recommend the factory setting, with properly sized injectors; then programming the ECM accordingly. It is all related.

Think of a an ECM pulse to each injector as a varying-length time period when the pipe is open. With higher pressure behind the valve, more fuel squirts through, when opened. The ECM has a limited range that it can decrease the pulse, with a higher pressure (and vice versa).

Wet plugs are not good. I'd bet your tailpipes are black and you could be plugging your CAT.

With an adjustable fuel pressure option, pressure is set to the spec with no vacuum applied. That unit uses vacuum to sense engine load; lower vacuum means open throttle, means more fuel needed, means higher pressure. See:
http://www.corvetteactioncenter.com/forums/showthread.php?t=57587&highlight=adjust+%22fuel+pressure%22

You changed the engine, as I recall. It may draw less vacuum than your brother's engine and could well need adjusting. It seems a bit of a crap shoot, though, as by changing fuel pressure, you're messing with the mixture through most areas of the fuel tables. It sounds like you really need less fuel now, but will need a proper tune, perhaps a properly programmed chip, from someone who really knows what they're doing. From your prior posts, I am not sure 'your shop' fits that bill.

 

[Vettefan87]

So when I check it it should be with the vacuum line off, which is the line going into the AFPR right? And that should read 40, with that line off correct?

 

[Vettefan87]

I took the line off and it read 50. I adjusted it down to 40, and we will see how it runs. If it runs worse can I move it up a little, or down a little?

 

[Vettefan87]

Well I just took it around the block with it set at 40 with the vaccum line off and it is running terrible. Any suggestions?

 

[Vettefan87]

Nobody out there plays with the fuel pressure? The car should not be running like this, or at least I don't think so...

 

[NT Vettman]

I have a 90 vett and I just bought an adjustable fuel regulator from TPIS and will be installing it shortly. The factory manual states that with the ignition on but not running the pump will run for 2 seconds the fuel pressure should be 40.5 to 47 psi and hold steady when the pump stops. When starting and idling the pressure should be 3-10 psi lower.

[font=&quot]TPIS says set it to 49 to 52 psi. My buddy has one and has no problems at all. I think you have other problems. I’ll let you know how I make out.[/font]

 

[WhalePirot]

The factory manual states that with the ignition on but not running the pump will run for 2 seconds the fuel pressure should be 40.5 to 47 psi and hold steady when the pump stops. When starting and idling the pressure should be 3-10 psi lower.

Chart A-7 in the driveability section of the '87 manual agrees with this. The pressure drop referred to is due to about 10 psi of vacuum from the idling engine.

HOWEVER!
Is your engine fairly stock?
Do the injectors flow the same as stock, or are they resized for a 'bigger' motor?
Was your ECM programmed to run the engine at a non-stock fuel pressure?

This tuning, is a somewhat complicated, yet critical process, to attain a properly running engine. I know and sense your frustration. Sorry about the problems, but this is all interrelated. Simply stated, a given engine will produce a certain amount of power. To do that, it takes a specific amount of fuel and a specific amount of air, under every combination of factors (speed, load, temp.(outside air and water), altitude, etc.)

The ECM looks to its fuel table to determine how much fuel to squirt based upon these inputs and feedback from the oxygen sensor. The pulse length it sends to open the injectors is the only means for it to adjust the fuel flow. Those fuel tables are based upon specific flow-rate injectors and specific fuel pressure. (Hole size; oomph behind it). If these things are mismatched, the ECM MIGHT be able to adjust. If it can, it will take some time to 'learn' the adjustments to the fuel tables; more than 'around the block'.

I think you will be lucky if you can get the engine to perform as you wish simply by adjusting the fuel pressure. You will first need to know if the car is running too rich or too lean.

I installed an adjustable fuel pressure regulator, but once set, I leave it alone. Others may have different needs/techniques.

 

[Vettefan87]

Okay I think the fuel pressure might be where I want it. I might move it up another pound and see. My brother drove it tonight, and he says it runs like to me that the timing is not advancing enough down low, but from a roll it seems to run fairly well. Could I have an ECM problem? When I put the chip in I did notice that it is a reproduction unit. Could I swap out ECM's from my brothers 86, into my 87, and vice versa?

 

[Vettefan87]

Anybody?

 

[Vettefan87]

I am completely at a lost fellas. I changed the ECM's and it is not running any better. If you punch it from 40 I think it rolls out nicely it jumps by twos and threes on the speedo, but off the line it stumbles really bad. Anybody want to buy an 87?? :cry

 

[WhalePirot]

Man, I feel for ya, but have spoken my piece. It could be so many things, and I have no faith at all in the shop that did your engine..

 

[Vettefan87]

Yeah well when you have very little cash, and an even lower income what can you do? He suposedly builds drag car motors, and sounded like he knew what he was doing, more so then the other retards that were doing it. I don't think it is his fault. I blame more the shop that put the motor back in if there is any blame.

 

[WhalePirot]

Hanging the motor back is requires a bit more car than the pre-ECM cars. There are sensitive electric parts and vacuum lines galore; all of which can cause running problems.


I asked before. How close to stock is this new engine?

 

[Vettefan87]

All that is done is it is bored 30 over, and it has racing type main bearings. and a higher volume oil pump.