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Old 01-02-2009, 01:41 PM
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Jeff Krosse
 
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Fuel Pressure on a EFI Ford

Hello All, I've become caretaker to kind of an interesting Volvo wagon. It has until just recently been a very easy-to-drive sort of car. Lots of double-takes, too.
After warm up:
-the car experiences a fair degree of surge at idle from 900 down to 300 or so -- Possible too lean?
-fuel pressure 25-27 psi (Auto-Meter electric off the BBK regulator)

Question- What should I see pressure-wise for this set-up?

Thanks, Jeff



1993 Volvo 940 wagon w/347ci stroked Ford V8, Ported GT-40 aluminum heads, 10.2 to 1 CR, Track Heat manifold, JBA shorty headers into a single 3" exhaust, Dodge Reidy Custom cam w/ 1.6 to 1 roller rockers, FMS Cobra computer w/ TwEECer RT, 30 lb injectors, and 75mm Pro-M MAF, BBK 73 throttle body, BBK fuel pressure regulator, K&N filter, cold air box, FMS underdrive pulleys, DCC VariableSpeed Fan Relay, 155lph Volvo fuel pump
Tremec 3550II gearbox, SPEC clutch, 3.54 gears, PowrLok LS diff, ipd bars and springs, Koni dampers, 13" Wilwood rotors & calipers in front, 245x45's on 17x9 and 225x50's on 17x7.5, 3375 lbs dry

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Old 01-02-2009, 01:46 PM
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I believe Ford runs their injectors at 39lbs.
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Old 01-02-2009, 06:01 PM
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Quote:
Originally Posted by Jsup
I believe Ford runs their injectors at 39lbs.
Actually, 36 to 39 psi would be good, measured with the vacuum hose off the FPR nipple...

What kind of vacuum do you have at idle??

K
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Old 01-03-2009, 01:24 AM
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Hey, thanks for the response. With the KAM cleared, engine was run at idle. After initial warmup at idle, idle surge began 900 down to 500 and return to 900. I adjusted the fuel press up to 38 psi vacuum line off. Approx 5 min later idle smoothed out steady at 900.

I'll have a look at the vacuum in the morning.

Jeff
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Old 01-03-2009, 12:39 PM
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You need 28 at idle with the regulator hooked up,
and 38 with the regulator unhooked.
Measured at the shrader valve on the main fuel rail.
GET A GOOD/accurate GAUGE.
If you are not sure, then take your gauge to a local aircraft Certified Repair Station that has a gauge test manifold and check yours. $ 20.

Further adjustments are per specific tuning for that individual engine.
If 39 is required at idle, then you should be searching for other problems, since higher running pressures might max out the duty cycle of the injectors.
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Old 01-03-2009, 01:12 PM
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After adjustment we're seeing 38 and 28. The idle has smoothed out-
Vacuum at idle is a steady 17"

So far so good.
Thanks guys.

Jeff
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Old 01-03-2009, 04:44 PM
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Quote:
Originally Posted by k19driver
After adjustment we're seeing 38 and 28. The idle has smoothed out-
Vacuum at idle is a steady 17"

So far so good.
Thanks guys.

Jeff
Excellent.
The only thing that I also should have noted is that the pressures I gave are at idle.
The fuel pump should be producing enough volume that at maximum rpm you still have the 38 psi at the fuel rail.
I have seen pumps that do it at idle but not at rpm.

Hooking up a gauge that you can see while driving is a pain.

Actually something in the back of my head keeps saying that a 347 with 30# injectors seems a little small. It might take more than 38 psi at RPM to supply the necessary fuel through a 30# injector.
Of course, the computer chip reprogram might have the injector pulse width widened for more fuel delivery. It might take some research to find out the requirements if you detect any problems at high rpm power.

Last edited by ScoTFrenzel; 01-03-2009 at 04:50 PM.
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Old 01-03-2009, 07:07 PM
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Understood. All pressure readings were taken at idle. What I have is a 12v fuel press gauge taken at the regulator. I keep it in the glove box. It'll have to do.

Thanks.
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Old 05-19-2010, 08:35 PM
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Normally fuel pressure should be about 35 to 45 pounds, depending on the make and model of car. If it is significantly higher than this, then you most likely have a bad fuel pressure regulator or a possible bad fuel pump. I would also check the vacuum line going to the regulator and make sure it is getting vacuum at idle.

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