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Discussion Starter #1
ive been upgrading my fuel system with a rising rate fuel pressure regulator and a walbro 321. i noticed while mapping the car (with an apex powerfc and no O2 sensor adjustment) that the fuel pressure and AFR would gradually drop as the car sat idling for 10 mins with a hot engine.

so ive spent the last few evenings looking at this. ive moved the fpr to a cooler location and insulated it, this made no difference, fuel pressure would still drop to ~0.5bar from ~2.5bar, when hot, but not too hot to touch. the fpr is being returned for assessment.

ive gone back to the stock fpr (man, that was a bitch to get back on) and a similar thing happens. let the warmup phase finish and the idle is showing 14.7:1 on the wideband. go for a hard run, to get everything hot, park up with the engine idling and AFR will start to drop. alot slower with the stock FPR but it will still gradually drop to 16:1.

is this normal?

anyone experienced something similar?

thanks in advance..
 

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What you are seeing with regard to AFR is pretty common on cars with aftermarket fuel and a VPC (I'm not sure about cars on stock fuel). Several of such cars I've seen tend to shift the AFR around a bit at idle -- kinda cycling over a period of 10-15 seconds. My car will idle at around mid-14s for a few seconds, then it might lean out to 17:1 or so, then drop back down to 13:1 or so, then back to the mid 14s for a few seconds. Going lean at idle will not harm anything. I'd rather it idle lean than rich.

I'm not sure what's up with your fuel pressure shifts though. The cars I'm talking about above all hold steady fuel pressure while the AFR shifts.
 

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I've got the same freaking problem. I can lose up to 10 psi at idle when everything is hot. I put in new pumps, new filter, have a new regulator and checked the voltage to the pumps. So then I put in a fuel cooler. It helped a lot at cruise, only lose a few psi, but come to a stop and it starts to drop. I took a can of cold spray and cooled the regulator off one day and the pressure came back up. My next move is to move the regulator down in the bumper away from the engine. My theory is the hotter the spring in the regulator, the more it relaxes allowing the pressure to drop.
 

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Discussion Starter #4
thats a relief, to find someone else with the same problem. i was begining to think i was going mad or just missing some stupid point. i agree with the spring theory, what i dont understand is why everyone else doesnt suffer the same problem? i guess u would only see it if u have fuel pressure gauge. i still would have expected more reports though.

i started to think along the same lines as u regarding moving the regulator, but i decided it would b too dangerous to have fuel lines running right at the front of the car. a minor front impact could spell disaster.

which regulator r u using?

stock ecu?
 

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I took a can of cold spray and cooled the regulator off one day and the pressure came back up.
That sounds like pretty powerful evidence that the regulator spring can't take the heat. Sounds like a piss-poor spring selection by the manufacturer if it varies that much with a relatively few degrees in temperature change that a FPR would see.

Just curious.... What brand FPRs are you guys using?
 

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Never enough
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I know for sure that myself and a few others I know have had the same problem. I figured it was from the fuel heating up because I have both pumps running all the time. I never had a problem with the car running lean though, no matter how hot the car was, so I didn't really worry out it. I usually see about a .3-.5 bar drop from cold to fully warmed up after driving around quite a bit. I never did think about the regulator being the cause of the problem though, thats interesting. BTW I'm running the Paxton FPR.
 

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Discussion Starter #7
im running an FSE (fuel system enterprizes) valve, from the UK. the stock FPR does the same thing , but it just takes alot longer to drop.

boost junkie, what FPR's have u used (seen) that havent done this?

i think the pressure drop i gave before (2bar) in the intial post is exagerated by the gauge. its an underbonnet thing that i dont think is good enough to stand the temp changes.
 

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Discussion Starter #9
im running an FSE (fuel system enterprizes) RR FPR, from the UK.
 

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My regulator is an SX, mounted along side the intake manifold where most people put them.
 

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Quick question guys - what pressure will a std pump produce as a PSI without the FPR in situ?

eyefi and I have been discussing this - as it has happened to me too


:(
 

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i've posted a new post on the mkiv section about fuel pressure problem.

guys i think it can only be a few things. fuel pressure regulator (which might not be the case), wiring, or the fuel pumps.

three things i think we should look into, replace or correct.

i think this problem is more common to a lot more people than they'd think or are aware of.

glad this has been brought up to in topic..
 

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Posted a topic on a slightly different issue I'm having. Previously while working on the fuel system I found some interesting things - mainly that most FPR leak vaccum! Put a pump on them and you'll see. They leak around the adjuster and they leak around the vac nipple. Make sure these two things are sealed with Teflon goop! The SX regulator seemed to be a little better than the Paxton and Aeromotive FPRs I also tested which leaked like sieves!

Hoe are you guys measuring pressure? Are you doing it with just a pressure gauge or with a differential gauge that also looks at manifold pressure\vac? Differential pressure is what you want to look at and in my case I have two pumps flowing enough fuel that the Aeromotive FPR cannot bypass enough and pressure off load at idle etc. is too high. My pumps didn't used to move this much fuel, over time thye have apparently worn in and begun producing MORE volume:eek: I expect I'll know in a day or three if my FPR also does the same thing you're seeing with yours. So far I don't think my FPR has had to do too much work except under WOT so I guess we'll see!
 

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BLKMGK said:
...They leak around the adjuster and they leak around the vac nipple. Make sure these two things are sealed with Teflon goop!...
Fwiw, teflon is not a sealant...it is a thread lubricant that allows you to smoothly tighten the fitting enough to seal. Iow, the only way to seal an NPT fitting is by tightening it more. If you want a true sealant on NPT threads you need to use jb-weld or the equivalent...either that or convert everything to -AN lines that seal perfectly without goop.

Btw, great post - glad to hear we're tracing down the root causes of some of these problems.
 

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Discussion Starter #15
paul, the pump would produce pretty much no pressure without the fpr, cos the fuel will just flow thruogh the feed line and back throught the return with no restriction to provide any pressure.

for ref. sleeknsilver's post on the same topic can b found here
 

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Another factor is the fact that on aftermarket fuel systems (most all of the utilize 2 in tank pump) the pumps super heat the fuel in the fuel tank causing the FP to drop. I have 3 walbro in tank pumps and if i'm not too careful it'll super heat the fuel so much that the car won't run. I am setting up my third pump to come on at a pre-determined boost level via AEM. That should help.
Peter
 

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What's it take to superheat fuel to the point where fuel pressure is affected?

FWIW, I've done tons of pulls in an hours time (tuning my AEM on the street) with 1/4 tank of gas or less, and my fuel pressure is still the same at idle as if I just turned the car on. I'm not sure I agree with this superheating up the fuel theory. I run the Aeromotive FPR and twin Walbros in the back.
 

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If you guys have 6-speeds don't forget your motor will pull more vacuum at idle under no load then it does while you are driving it. I know my auto pulls like 51cm/hg at idle when warmed up in neutral vs. 44 or so in drive at idle when it is under the load of the torque converter/trans spinning in gear. Also as your car warms up the rings seal as they expand with heat and this will produce more vacuum causing the fuel pressure to be pulled down slightly. I would suspect that you could loose 3-4psi just from your car warming up. Now 10psi, well that is a different story.

Steve K.
 
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