Fuel pressure adjustments (engine Hot vs engine Cold)

[MyBoosta]

Hello guys,

Can I please get your input on this matter:

Fuel pressure...when do you typically adjust your fuel pressure?

I'm asking because I thought I had this matter pretty much resolved until I saw the fuel pressure go down once I drove for a while. The car felt a little flatter and I noticed during those instances that my fuel press went down from my previous setting. I mean, as much as 5psi was lost due to heat (warm engine...above 164*), when the cooling fan is running. I also noticed that it dropped once I uncapped the dump tube...Is this normal?

I've been chasing this issue for two days now. For example, during warm ups, it showed.... 46psi Vac off, then after a few blast, fuel pressure changes to about 42/43 vac off. With my car, 2 pounds too much or too little makes a huge difference on my car.

Thanks for your help.

 

[Ryan87GN]

My guess would be a junk regulator. I had one once that would fluctuate about 20psi static just from sitting over night. Try a different regulator.

 

[SloGN]

i'm having the same exact problem. i have already replaced the regulator. My next step is to pull the tank and install a new pump.



Also what type of guage do you have?


i have the liquid filled poston one. mounts directly to the fuel rail.

 

[jurassic five 7]

I've read that especially the liquid filled ones WILL NOT be accurate compared to a mechanical one especially when the liquid one is under the hood because of all the heat.

I have a Kenne Bell fp gauge on the hood, its a mechanical one. The fp is at around 50 when cold, but 43-45 when hot.

If you think about it, if you set your fp at cold at 43 psi, then when it warms up, you're at 38. What will be your fp when racing? 38 right, not 43 like you set it for. Set fp when its hot because your car is hot when driving, not cold.

I adjust my fp when the car is warm. Thats what I've read to do it. If you think you're reg is bad, then go put a gauge on an other car (any other car you have access to) and see how theirs is. If it drops when hot then I'd say 'thats just how it is.'

-Adam

 

[MyBoosta]

Thanks guys...at least I am not crazy. I knew I was seeing it move up and down after a few minutes of driving.

Mr. SloGN, I also have a liquid fuel gauge mounted under the hood by the regulator. I purchased mine from Full Throttle, it has a black face and it looks sharp. But I do know that under the hood heat can sometimes screw things up.

Yes, I suspect that the oil in the gauge is getting hot and thus throwing off the psi readings of my gauge. This was causing me to adjust it and thus screwing my tuning all up.

I will double check it in the morning once the car warms up (after T-stat opens up) and then re-adjust it there and leave it.

Thanks

 

[SloGN]

I've read that especially the liquid filled ones WILL NOT be accurate compared to a mechanical one especially when the liquid one is under the hood because of all the heat.

I have a Kenne Bell fp gauge on the hood, its a mechanical one. The fp is at around 50 when cold, but 43-45 when hot.

If you think about it, if you set your fp at cold at 43 psi, then when it warms up, you're at 38. What will be your fp when racing? 38 right, not 43 like you set it for. Set fp when its hot because your car is hot when driving, not cold.

I adjust my fp when the car is warm. Thats what I've read to do it. If you think you're reg is bad, then go put a gauge on an other car (any other car you have access to) and see how theirs is. If it drops when hot then I'd say 'thats just how it is.'

-Adam

Adam what i have noticed tho when the fuel pressure is down. My fast wide band show that it's also running leaner when this occurs so it has me thinking that the fuel pressure is actually dropping thus causing the leaner conditions.

BTW i have never put a fuel pump in the car since i have owned it in. I bought the car back in the end of 1998. it was susposed to have a walbro 255lph pump in it.

Also the fast system seems to show that my injector duty cycle to be @ 85% with a base fuel pressure of 45 with the line off. running 20 psi of boost. this is with 55lb injectors. does this sound right?



thank pat broughton.

 

[RUQWKNF]

Pat,
with the turbo that you are running (6152E), and 20psi boost with 45psi static fuel pressure, (should be 65psi total under WOT) 85% duty cycle seems a little high to me, especially with 55#ers. How much correction factor do you typically see with that tune? One thing I would do, would be to change the factory fuel filter first and see if your duty cycle comes down to around 76-79% range. (If you are still running the factory filter that is) I would do this first before replacing the pump. If your volume is being restricted by a semi clogged fuel filter, I can see where the FAST system would raise DC to compensate.
Another thing, permanently mounting one of the small liquid filled fuel pressure gauges to the rail, has proven that it will become in-accurate over time. Whether it be from heat cycles, or vibration, I've seen it time and time again. This is why I remotely mount my fuel pressure gauge onto the power steering accumulator bracket stud, just in front of the master cylinder. I will snap a couple of pictures and post them up tonight of how I did it. Very easy to do and is easy to see when standing in front of the car.
Just a couple of thoughts.

Patrick

 

[RUQWKNF]

Here's a couple of pictures of how I remotely mount my fuel pressure gauge.
I did this on my old white car as well.

The Autometer mounting cup bracket hole needs to be drilled larger to fit over the stud, and the nut that is already there is used to tighten the mounting base down.

This is a 180* Jegs push lock fitting attached to the fuel rail with the schrader valve removed. The pushlock hose runs over and behind the brake master cylinder.

I chose a Phantom series cause it's easier to read with a flash light for those night time adjustment sessions, as well as the Phantom gauge has more measurement ticks on the face.
HTH

Patrick

 

[SloGN]

thanks for the idea pics. i'm going to go ahead and install the new fuel pump since i have already bought it.


like i said up above the whole time that i have owned the car (end of 98) i have not put a pump in it.

So to be on the safe side i'm gonna go ahead and swap it out that way i now for sure it's one less thing to think about.



Thanks pat b