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Old 08-11-2000, 12:07 AM   #1
ARG
Scooby Specialist
 
Member#: 386
Join Date: Oct 1999
Chapter/Region: SCIC
Location: Upstairs in the bedroom
Vehicle:
1999 Impreza RS-S
Rally Blue & Supercharged

Thumbs up Too much fuel pressure at idle? I have the fix.........finally

As most of you know by now, I have been having problems with my Mazda RX7 twin-turbo fuel pump. The issue is too much fuel pressure at idle. If you are still using OBDII, then you WILL have idle problems.

If anyone out there plans on using the RX7 pump, then be aware of the fact that the stock Sube FPR can't relieve enough fuel pressure back to the tank. You will have about 40 PSI at idle. Ideal is 37 PSI.

I have spent the last three days in junk yards pulling off stock FPRs from every Japanese car I could find. I am now the proud owner of 18 different FPRs. All shapes and sizes. I have cut five of them in half, to see how they are constructed. The problem is that the return orifice is just too small.

Today, while walking through "Pick-A-Part", I spotted a wrecked XT6. You don't see many of those nowdays. I looked at the FPR and it was the exact same style and fit as the one on the 1999 RS. The XT6 engine is a flat 2.7 liter 6 cylinder. I got the FPR and installed it, without any modifications, onto my RS. My fuel pressure at idle went from 40 to 33 PSI!!!! Yeah!!!!!! Thank you Lord!!!

My supercharged/intercooled RS idles PERFECTLY! No more stalling when cold. I have finally solved all the drivability problems with the Rimmer kit and the RX7 fuel pump.

So, if any of you out there are running a WRX, Supra TT, Mazda TT, or Holley high volume pump, this is the answer. Go find yourself a used Subaru XT6 fuel pressure regulator. Or, if you are really anal about used parts, you can order a new one from Subaru for $143.00

The used one cost me $3.00

ARG...the endless pursuit of perfection.
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Old 08-11-2000, 12:13 AM   #2
ARG
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Join Date: Oct 1999
Chapter/Region: SCIC
Location: Upstairs in the bedroom
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1999 Impreza RS-S
Rally Blue & Supercharged

Cool

One more thing! Anyone out there interested in a factory high volume pump for their Sube??? I got the one out of the XT6!!! HA! And guess what, it is a perfect fit for the RS. The pump in the XT6 is the same one for the SVX. The problem is, when is the last time you saw a wrecked SVX?

Anyone like to make me an offer on this factory high volume pump????? If not, it's going into my RS and I will sell the RX7 pump.

ARG....the modification maniac.
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Old 08-11-2000, 12:23 AM   #3
8Complex

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04 FXT
Red

Thumbs up

Not only do you have way more time on your hands then I do, you use it really wisely. Nice going! Now when I finally nudge myself into FI-land, I'll have the confidence knowing there is someone that has already solved the more major problems. Now if you could only solve the MAP sensor under FI problem...
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Old 08-11-2000, 05:25 AM   #4
SteveS
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2007 BMW Z4 M Coupe
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Post

ARG ~~ I've been reading about your problems with fuel pressure at idle, but I really didn't have a solution to offer up. I thought other people had been using the RX7 fuel pump without any such problems. I'm using the Supra TT fuel pump and fuel pressure at idle was not a problem with the stock FPR. I added a Bell rising-rate FPR to help get the fuel pressure up to almost 80psi at around 8psi of boost.

Anyhow, glad you got it straightened out. Just find it strange that nobody else has run into this.

SteveS
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Old 08-11-2000, 08:04 AM   #5
Diz
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2004 Mazdaspeed MX-5

Cool

ARG -

I'm in the same boat as SteveS. Holley FP, Bell FPR. No problems at idle before the FPR. However, I am running a turbo instead of a supercharger.

PS - Great research info on the XT6, though.

-Paul

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Old 08-11-2000, 08:30 AM   #6
Trey
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Location: Austin, the Republic of Texas
Post

ARG - Good info! I've been putting on an independent 1:1 adjustable FPR and just gutting the stock regulator.

8Complex - There's a problem with MY00s and FI? I just finished tuning a MY00 w/ turbo yesterday and we're running 7-8psi with no hickups or stumbles. I monitored everything on our wide-band lambda and all is good. Using the stock ECU w/ MAP too, though a stand-alone would be MUCH better. E-mail me and I'll give you the details.

Cheers,
Trey
CobbTuning.com
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Old 08-11-2000, 10:32 AM   #7
edekker
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Supercharged - and STOLEN

Post

Great Info ARG. Thanks a lot

Ed.
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Old 08-11-2000, 11:56 AM   #8
Tim Prudence
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Post

What's so bad about having a higher fuel pressure at idle? I've been running my car with 45 psi at idle for several months now... (this is with an adjustable fpr, but I need the extra fuel pressure under boost).
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Old 08-11-2000, 12:21 PM   #9
ARG
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1999 Impreza RS-S
Rally Blue & Supercharged

Post

Tim, are you still OBDII? The ECU senses an over rich condition at idle and tries to cut the injector pulses. (At least on my car) If you are running an AFC or an aftermarket ECU, it most likely won't concern you.

ARG
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Old 08-11-2000, 05:13 PM   #10
Shik
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2006 GT Ltd 5spd sdn
2015 WRX 6spd CWP

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ARG, according the the SVX FAQ, the pump flows 222lbs/hr. Is the part number from SOA the same between the two? Very interesting to say the least.
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Old 08-11-2000, 07:55 PM   #11
Eric SS
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Location: 2013 335i, 2011 G37 coupe
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2000 2.5RS w/ EJ22T
swap and N20. gone. : (

Post

I'm interested in the pump. What do you want for it???

Eric
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Old 08-12-2000, 01:03 AM   #12
Tim Prudence
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Post

I have an AFC connected to the stock ECU, however, I'm not doing any kind of idle corrections. I played around a little with negative settings at idle, but all it did was cause my idle to die.
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Old 08-15-2000, 01:12 AM   #13
Red-Imp
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2000 Imp 2.5 RS, SRP
& a fast red car ;)

Question

Trey,

Are you using an AFC or something to clamp the MAP voltage within normal limits when it sees boost? Also, how are you handling fuel enrichment on the high end? You said you're using a 1:1 FPR (not rising rate?)

ARG,

I've been drooling over your setup. If Rimmer ever surfaces again (and if my stocks ever go up) I may get that sidewinder. If you have time, would you post a summary of what you've had to do to perfect your SC'd RS? I'm sure I'm not the only one who'd greatly appreciate it.

Thanks,

-Red
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