Breezy13x

Hi, I have a 94 lt1 in my 78 camaro. I just got it put back together and it has been running rich especially at idle. I’ve researched and can’t figure it but I think it might be my FPR. It’s key on engine of it is around 47 psi but when idling it jumps to 52psi. Then when i pull the vacuum line of it will jump to 55 psi. I know the idle and vacuum off psi is not in spec. I also noticed after the motor is is warmed up the I check the fuel pressure with the key on engine of it primed to 48 psi and starting climbing slowly over a few mins it was to 53 psi. Does this sound like a bad fpr or something else? There is no fuel in the vacuum line. I appreciate any help you guys can give me.

Injuneer

Factory spec, engine running, no vacuum is 43.5 PSI. GM tolerance accepts 42 - 47 PSI. So your pressure is too high. And unless you have a huge cam, reattaching the vacuum line show drop the pressure by 8 to 10 PSI below the no vacuum pressure. So that's higher than expected as well. Since a stock cam will usually produce 33-35 PSI with vacuum compensation attached, your 52 PSI (with vacuum) vs. typical 34 PSI is causing the injectors to flow more the 20% extra fuel. The PCM can only pull out 15% of the fuel flow, based on O2 sensor feedback.

What fuel pump are you running? Do you know the rated liters/hour? Is it possibly a “high pressure” rated pump? Sounds like it may be a higher flow than the FPR can handle.

Breezy13x

It is a inline fuel pump but I can’t remember off hand the specs on it but It’s not anything crazy. But I’ll go back through and double check the specs on it. I guess I should of listed my mods. I have le1 heads and Cam, le ported intake, 58mm bbk tb, 36# bosh injectors, 1.6 rr, long tube headers and a tune. It just seems like the fpr is the only thing I haven’t replaced and could be the weak link with the the fuel pressure being so for out of spec. I’ll try and throw the gauge on it again tomorrow because I didn’t have a lot of time last time I did. Is there anything else I should look for when testing the fuel pressure? Should I let us sit for a certain amount of time to see if it’s climbs or how low it bleeds off? Thanks again for the help. I’m still learning and just trying to figure it out rather than throw parts at it. Thanks again for the help!

Injuneer

Are you sure it’s a “stock” fuel pressure regulator?

Shoebox’s photo of a stocker:

http://shbox.com/1/fp_reg.jpg

What size is the fuel return line from the regulator to the tank? The fact the “rich” condition exists (primarily?) at idle may be indicating the return line is too small, crimped, etc. That's when the fuel flow through the return line is highest.

You do have the supply line on the larger (3/8”) hard line on the fuel rail assembly, and the return line on the smaller (5/16”) hard line coming from the regulator? I suspect you do, but I have to cover all the bases. Dealing with a conversion, with no possibility of looking directly at how it was done makes problem solving more difficult.

Breezy13x

I do know it’s not a stock fpr, it’s an adjustable one but I didn’t see any markings on it to see what brand. It was on the motor when I got it 15+ years ago. I check the pressure with when the motor was install and set it to 45psi. The supply and return line are hooked to the correct size on the fpr. I notice it running rich more at idle but it is still rich going down the road, just not as bad. I’m assuming because I need more fuel as the rpm’s go up. But, with the vacuum off I’ve read that it’s suppose to simulate WOT? If that’s the case then It would be out spec too I think? Again I really appreciate the help especially the simple things I could easily have overlooked.

Injuneer

Does the adjustable fuel pressure regulator (would have been good to have that info in first post) look like this:

https://aeromotiveinc.com/product/gm...-994-97-f-body

How did you set the fuel pressure at 45 PSI? Was the engine running? Did you remove the vacuum line? If so, have you tried resetting the pressure?

Removing the vacuum line simulates what happens at WOT - intake manifold pressure rises close to atmospheric pressure. But it isn’t equal to atmospheric pressure, because of the pressure loss in the air filter/intake path. There is always a small vacuum in the intake manifold when the engine is running.

The function of a vacuum compensated fuel pressure regulator is to maintain the DIFFERENTIAL pressure across the injectors. That fuel rail gauge pressure minus manifold vacuum (which is minus a negative number). The injector is an orifice, Flow is proportional to the square root of the DIFFERENTIAL pressure. Removing the vacuum line at idle removes the engine vacuum from the equation. You want the rail pressure to be equal to 43.5 PSI.

Breezy13x

Sorry, I didn’t even think about it. And thanks for that explanation that’s some very good info. Yeah I set it idling no vacuum line. It’s looks more like the accel model than the areo. I reset it down a month or so ago when messing with it and it was still running rich so I through the gauge back on when I had time to mess with it. I’m wondering if I would just be better off buying ac delco . I know I don’t need a adjustable fpr. It just seems like my issues are point toward it being the problem, whether it be user error or not working properly.

Injuneer

Just to follow up on my questions, do you know the capacity of the pump (or a brand/part #)? What size is the return line? Is it possible it has a kink, or is routed too close to hot exhaust?

When I bought my Formula new in 1994 I fell into the “AFPR is a performance enhancer” trap. We all learn from our mistakes. With that AFPR in place (don't remember the brand, but similar to the Accel) I added an NOS 5176-EO 125 HP dry nitrous kit. The kit sprayed only nitrous into the intake manifold. The extra fuel was supplied through the injectors by raising the fuel pressure. It accomplished that by applying a pressure regulated slipstream of nitrous to the vacuum port of the fuel pressure regulator, raising the fuel pressure to about 90 PSI.

The kit didn’t work right. Being in California, and close to the NOS factory, I had the inline booster pump tested - it was fine. They were at a loss, but knew the fuel pressure regulator was not stock. Suggested I try reinstalling the stocker. System worked fine. We had no idea why the aftermarket regulator didn’t work. (Note that the kit also required switching to a pintle style injectors, because the stock GM Multec injectors didn't like any more than 60 PSI. The injectors were changed when the kit was installed, so they weren’t causing the problem).

The Aeromotive unit I linked is almost always the source of LT1 fuel pressure problems.

Injuneer

Another question - how stable is the car's system voltage (I.e. - does it stay in the 12-13.5 volt range?)? Higher the voltage the more fuel it pumps.

Breezy13x

Sorry been super busy. So the return line is 5/16 and it has plenty of distance from the exhaust so heat isn’t an issue, it is also link free. I haven’t found the receipt yet for the fuel pump and haven’t had time to crawl under it and look for a part number. Hopefully I’ll be able to next week. And that’s wild the issues you had with yours. The voltage looks great, I had actually ran into that issue years go. It’s pretty crazy how that can make everything go crazy. Again I appreciate you taking the time to help me and sharing the info you know/experienced.

Injuneer

When you take the fuel tank fill cap off, is there a significant pressure release?

How does the pump take suction on the tank? Sump?

It's really hard to troubleshoot these engine swaps, when there are so many unknowns.

Breezy13x

Hey man, sorry for the late response, it’s been a crazy few weeks. But I finally got a chance to go over the car pretty good. Everything thing that you mentioned looked good and I checked again for vacuum leaks, which there wasn’t any. I was trying to re adjust the regulator but the adjuster bolt broke, not sure how. But anyways, i went bought a new ac delco regulator and slapped it on and it seems to have fixed the problem. So I don’t know if it was bad or i had it adjusted wrong or what but we are good now! Thanks again for your quick responses and all the help trouble
shooting. I really do appreciate it!!

Injuneer

Glad you got it sorted out. Thanks for the follow up.