Just some quick questions regarding fuel pressure. I recently changed my cam, heads, chip (for new 24# injectors) and I'm wondering how to go about resetting fuel pressure. I have an AFPR and it has a new diaphragm. The rail has new o-rings and the system holds pressure.....it just seems a little low. At first, it was around 15 psi. I then turned the AFPR bolt to the right a few turns, purged the fuel line, and primed the system again (key on, engine off). This time it rose to about 26psi, again steady. Finally, I bottomed out the adjusting bolt (and dialed it back a couple of turns because that's a no-no) and it got to right around 32psi, but no higher. (1) Is this normal without the engine running? Keep in mind that it would jump to around 42psi on priming up until the time I began the tear down a couple of months ago, and was running fine. (2) When adjusting FP, can you do it with the engine running? Also, must the vacuum feed and port be plugged while you do it? I'm trying to set my FP to around 45 psi for initial start up. Any help would be greatly appreciated. Thanks to all who respond.

 

Sounds like you are doing it right but you need a longer adjusting bolt, but I'm not sure why changing the diaphram would change the adjusting range of the regulator.

Priming without cranking yields the same pressure as engine idling with vac line disconnected from FPR. Personally I prefer to set it during priming.

Plugging the vac port and feed won't change the reading.

Why is bottoming out the bolt a no-no?

 

As always, I neglected to clarify exactly what was going on. I changed the diaphragm, and when I lifted off the housing, the bolt loosened, so my frame of reference changed. Had the bolt not moved, the calibration probably wouldn't have changed. As for why allowing the bolt to bottom is bad, that's a good question. I remember vaguely reading that in the instructions at some point in the past (1995??). Since that posting, I have adjusted the pressure up to around 45 psi. When I finally ran the car, it went up higher than it was during priming.

Since I have your attention though, here's another question for you and all who might know the answer. Is it normal for fuel pressure to fluctuate at idle? I don't mean by a large amount, but within 2-3 psi? The needle on my gauge "dances" a little bit and it got me to thinking about something. My AFPR has a vacuum port that isn't brazed or welded into the housing. It threads in. It has no wrench flats to tighten it, and I've found it loose on a couple of occasions (including now). This, I theorize, could cause at LEAST 2 problems: (1) a vacuum leak, (2) erratic fuel pressures (3) a boost leak if an FMU for a blown car gets its boost/vacuum feed off of the same line, and (4) screwed up fuel pressure under boost because of reason (3). I know this is a lot to consider, but since my first posting, I've discovered a couple of pre-tuning gremlins for my new combination. Thanks again for your response, it helps. Thanks also to all who can shed light on this subject.

 

Yes, it does fluctuate, I assume due to the fuel injectors opening and closing.

I had the same problem with my AFPR, and I Locktited the port to the body to create a seal and keep it in place.

 

Thanks for clarifying that. I don't check that often enough to have a good handle on all the subtleties. Loctite is probably a better choice than silicone for sealing that little vacuum fitting. It turns out I did use silicone in the past, as there was some left on the small flange. I may very well try the loctite. Thanks again.

 

Fuel pressure with power applied, but engine off, should be the same as with the engine on and the vacuum line disconnected and plugged.
I find it easier to jumper the ALDL "G" terminal to 12v to power the pump. I use a pressure gauge with a purge line to purge the air out of the system before setting the pressure.
The ECM must be reprogrammed because 24#ers are at the upper limit of it's conpensation span which is ±10%.
If you lower the pressure instead of reprogramming the ECM the injector spray pattern will be incorrect.

 

I know about reprogramming the ECM. My new chip was set up for the larger injectors, and there's no indication that I'm running rich. Since my first posting, I have set my fuel pressure at 44 psi (idle). It seems to be fine. Next, it's just the basic tune-up procedure (TPS, minimum air) and a test drive. Thanks.