Welcome to the Corvette Forums at the Corvette Action Center!

Best way to adjust fuel pressure

Apr 7, 2003
Guadalajara, Mx.
1996 LT4 Supercharged and 2002 Z06 656 whp
Best way to adjust fuel pressure?

I just put on an adjustable fuel pressure regulator and I would like to know wich pressure should I take in consideration:

1.- With the vacuum hose conected (42 psi)
2.- Without the vacuum on (47 psi)

Also is there any advices-coments to find the best pressure for my engine-fuel sistem? (94 LT1 no internal mods)

I have been thinking go to the dragstrip and find the best pressure with times but I just changed the rear axle and it is in the break in perid.

I would appreciate any help!

Gee, I believe the vacuum lines remain connected. :confused
Hi Ken,

Thanks for your answer, I always thought that the vacuum lines supposed to remain connected until I read a couple of articles about it in the net and one or two of them said something like "remember to disconect the vacuum line to take the pressure readings" and that make me to doubt about it.

Anyway I'm going to give it a try, I'll put it at 46 pounds with the "v" line connected and see what happens. If you find out something about this matter just let me know and I'll do the same.

When I installed my AFPR a few years back, the instructions said to adjust it with the vacuum line disconnected.
when I did my TPI system a few years ago I am pretty sure it said adjust with the vacuum line off, simular to setting the base timing on a distributer with vacuum advance, the page dealing with that is missing from the manual so Iwill have to print off another copy just to be sure. Anyway that is the way I have been doing it ie. with the vacuum line disconnected and the engine at idle.
Tuna, Bossvette,

Thanks for your advice, I think It will be better to let it as it is right now (42-47) until I find a way to set it at the best pressure for my particular engine. I think I need to look out for a dyno to make the adjustment (they are not very common here in Guadalajara)

The only reason you want to adjust the pressure up or down, is to increase throttle response, and power. The only real way to tell if you have actually done this is to put the car on the dyno.

However, this is a fairly easy way to adjust pressure and ensure you are actually making an improvement.

You need one of two things. You either need a wide band O2 kit, or a full scanner.

I use a www.actron.com CP9100 scan tool.

You set the tool to look at your knock retard circuit (this will show you when the computer detects spark knock (ping), and how much it takes from the overall timing.

Go warm the car up, and drive it hard at WOT. Watch the scanner. Add pressure until you reduce the knock retard to a minimum. Also, take the car to cruise speed, and gently tap into it in overdrive to load it up easy and watch your retard function there as well. Adjust up and down as needed.

Now, park the car, pull the vacuum line, and take a reading off the rail and record that number when you have reached the best blend. This is only for documentation sake so that you know what you are running, and can adjust back to it quickly and easily if you change it or move it around.

Hope that helps.
Steve,....my fault.

It is the 9110. I don't know why I called it 9100.

Yes, I tune with the 9110. It is a very nice tool.

You can see codes, look them up, clear codes, watch an instant data stream from the motor, take a short "movie" of engine data, and pretty much diagnose any EFI related problems.

The cartridge will allow you to scan a variety of other GM vehicles.

I have one for my pre 95 GM cars, and one for all OBDII cars from 96 and up. Between the two scanners, I have a wide range of diagnostic ability.

You see, when the engine is lean, it will ping. When it pings, it reduces timing. When it reduces timing, you have a drop in performance. By adjusting the pressure to the hairy edge of ping/no ping, you will optimize your fuel delivery.
Chris is this scanner user friendly? i've used the snap on 2000 (i think thats what it was) along time ago 12 years or so to be exact on my 82 Z/28 and it was faily easy to use. i think i'd be interested in getting one of these for 220 bucks sounds like a good deal instead of guessing what fuel pressure to run. i hear 47 psi to 42 psi but would rather not guess and go with hear say. BTW does it diagnose ABS also?

The scanner if VERY easy to use.

You plug in, turn the car on, and set up the tool.

You select GM, year model, make of car, then the specific model (it will also let ZR1 guys scan).

After that, you get a main screen that allows you to:

View codes
Erase codes
look up codes
View Data
record data
Field Service Mode

And other neat options.

On the view data, you can see all the data possible, simply scroll the screen up and down as you can only see about 6 lines of text at one time. Or...(my favorite), you select data you want to see....O2 readings, loop status, volts, air intake temp, water temp, RPM, speed, TPS, MAP, idle air motor steps, block learn, etc.

If you are trying to set your TPS, you no longer need to fuss with a voltmeter and test probes, you just set the scanner on that, ignition on, position it where you can see it, and make your adjustments.

You can also set it to start recording data when it sets a code. Very handy if you have a hit and miss problem.

It will tell you on the newer C4, when the fan is requested, a/c request, etc.
CP9110 Scanner


Thanks again for super-informative responses. Sounds like that CP9110 might make fuel injection motors easier to troubleshoot than older carbureted motors.

I was thinking of getting a fancier laptop program/software and interface cable, but based on your experiences with the Actron, I think I will be getting one of these as soon as possible.


Thanks, this is very good information and I have some more questions. I'm very interested in the CP9110 but I don't want to get the wrong one and here is the reason why:
two monts ago I sold my loved 91 L98 I know fore sure they have OBD I because it had the 12 pin ALCL but what about my 94? it has a 16 pin terminal (stock) ¿does it make it an OBD II or it is just some kind of hybrid? should I buy a CP9110 for OBD I with a OBD II cable? I read somewhere that the vettes build for export's have some little differences with the ones for domestic sales (Chevrolet dealers started vette sales in Mexico in 1991)(mine has three O2 sensors and I read some 94s only have two)
The other question is: since I took off the cats my vette its been running in the rich side (most at WOT) and until now I can't fix it. ¿Can the Actron make corrections to the computer or just scan it? I'm asking all this because I been thinking in the past about getting the LT1edit program but I'm not so bright with computers!

I'm sorry if I ask too much but looks like you know very much about this subject! ;shrug


You might want to call Actron direct with your pin questions. I am however very sure it will work. The true OBDII did not hit until 96, but as you have noticed your car has some OBDII characteristics.

The 9110 will not reprogram the computer, it will only clear codes. You will need the LT1 edit stuff if you want to make changes.

I have used it to scan all the way up to 95 LT1s, so I am sure it will do the trick on yours.

If you got rid of your cat and did not install the O2 similator, it will run poor.

See what Actron says.

Thanks Chris,

I'll talk with the Actron people. I really don't want to mess with the computer, but like I told you I took off the cats and let the 3 sensors the way they where (two before the cats and one after in the right side all connected) and after that the vette runs rich on gas. I have been looking for some way to correct this but it seems to me that the only way to do it is through the computer.
There is somewhere in the net (can't remember where) a device called "The Translator air/fuel & spark tuning" that thing works connected between the engine (O2 sensors) and the computer. It may be what I need but I'm going to need more info on this one!
Well I'll keep you informed

See you,

Corvette Forums

Not a member of the Corvette Action Center?  Join now!  It's free!

Help support the Corvette Action Center!

Supporting Vendors


MacMulkin Chevrolet - The Second Largest Corvette Dealer in the Country!

Advertise with the Corvette Action Center!

Double Your Chances!

Our Partners

Top Bottom