So, for any of the 82 CFI guys that are running a non-stock, externally regulated fuel system with parallel plumbing and all hard-lines at a higher pressure. There is a problem that I ran into a few years back. If you run the hard-line fuel system you WILL have an issue with what I call burbling or a type of surging at one or more rpm settings. 1. The length of vacuum line for your fuel pressure external regulator will dictate the rpm range you will have the issue at. 2. It sets up a harmonic of sorts and has to do with injector pulses and the inability of the hard-lines to absorb the injector pulses. Since there is no pulse damper in the system like GM put in the third-gen Camaro, Firebird and Trans Am's. You will see to some degree a burble/surge at different rpm ranges. You would think that the compensator side TB would cover this, but it's only for off idle and running the external regulator negates the use of both the compensator and the internal FPR, they both get blocked off with plates.

THE STORY... I had a guy call me from a dyno shop in WA state a number of years ago and asked me about this issue he was seeing on the dyno with an 82 that was running a similar fuel system as mine and wasn't happy with what was going on. At the time, I never had an issue like that or at least not to that extreme. We talked about this for several days back and forth and finally he asked me if I had seen a video on this subject and sent me the link. (I wish I could find this video and post it) It is a really good video and explains why this all happens and why the length of vacuum hose changes the rpm range lower or higher. It's all about flow/fluid dynamics and physics.

Anyway, my 82 does have a small issue, but it doesn't bother me nor I'm I concerned since it is at a low rpm range that I hardly ever drive around at that rpm constant speed. So...fast forward a number of years here on the C4 forum with an 84 CFI vette a guy I know on there who has a thread dealing with his Dynamic EFI EBL Flash II tune which is pretty good and his motor does run pretty good with his 4+3 setup. So he changes a few things on his motor and the motor starts running like crap. He trouble shot the tune, fuel system, ignition etc... with no apparent success. I had been following the thread on and off for a while, but never really read the entire thread to analyze what was going on until lately. I thought, I wonder if... this sounds familiar. I had the owner call me and we talked about the real issue he was having and I told him I think I know what is going on and why your motor is running like crap. I know this "may" sound strange/odd, but check this out and let me know what the results are.

Well, I was right, he played with the vacuum length and saw the rpm ranges change and removed two fuel hard-lines from the "Y" fuel feed block to both TBs with rubber lines and the issue went away. Here is the link to his thread and his fuel issue is around page 3 I think and into 4 and then finally to page 5. So for any of you guys that are running this system and have this issue, change the fuel feed lines to rubber so the injector pulses can be absorbed and the issue will go away or diminish greatly. Enjoy!
84 CFI fuel issue

 

This should probably be moved to the 1982 CF sticky. Can one of the mods move this there? Thank you.
STICKY LINKY

 

That is fascinating.
I was considering running a fuel pressure regulator in the aft part of the car (to recirc the fuel back to the tank back there) and using the old small steel line as a vacuum source to regulate pressure. Now I'm curious how that long of a vacuum line would affect the system? Curiouser and curiouser. Good story and interesting side-effect of trying to make something better creating problems.

 

Wow...talk about chasing a Unicorn......I bet some have sold their damn cars due to this.....not something you would even think of.

Jebby

 

Well, on an earlier year vette, you probably do not have to worry about this even with all hard lines. But since the 82 is a different beast with two throttle bodies and fuel injection, it does make a difference to some degree if you use an external FPR that is vacuum referenced and all hard lines vs rubber or braided hose.

Jebby, this was a head scratcher for sure and at first didn't believe it until I started digging into the issue. WOW! It is a real thing and crazy how stuff happens and works out when you least expect it. I need to see if I can find that video, it is really good to watch.

 

I'm running EZ-EFI. After seeing some of these EFI systems I was wondering if I could put the fuel pressure regulator back near the tank so I was considering installing my regulator back there and then running the vacuum sensing line via the old vent return line. In a well "closed" system the vacuum would respond....this made me wonder whether, or how promptly, the fuel pressure would respond to vacuum changes to adjust the fuel pressure???? And, of course, what other "unicorn" change and/or problems I would be creating.....

 

I would not install the FPG in the rear of the car by the tank. Makes it rather difficult to make adjustments when needed. As a matter of fact, I don't recall ever even seeing one installed in the rear, but maybe I didn't look close enough. I would install it under the hood so you can see it and the gauge, but that's just me. Your vacuum line if used will be much shorter depending on where you mount it as well.

 

I did not mean to mislead. The fuel pressure regulator is mounted on fuel rail in the engine compartment. Since the idea was in the back of my mind this thread brought that I may be creating more problems ...and maybe unicorn problems. I'm mostly curious to see how it would work..that was all.

The EZ-EFI comes with like a 15 foot extension and a fuel pressure sensor that is indicated on the display. Adjusting it would be relatively easy..... I just don't know how it would function.