Low Fuel Pressure
06-13-04, 09:48 AM
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Low Fuel Pressure
Hi guys,
Since when I bought my car one month ago, it ran badly over 4.5k, it kinda "pushed". I thought it were due to bad injectors (I'm running stock 550), so I had them checked and they are flowing like new (shop words); I have an almost new MSD 125 psi fuel pump with about 500 miles on it, so maybe the injectors were getting too much pressure. We put a fuel pressure gauge.... and it turns out that I have *low* fuel pressure....
At idle it shows like 32, when I accelerate it to 3-4k (with the car standing, not running) it goes down to 28. As soon as I relase the throttle it goes down to 25, then climbs back to 28 and sloooowly back to 32 psi.... and now I'm really puzzled....
Reading in this forums my first idea was to check the voltage at the pump; now bear in mind that I have kinda basic electrical skills, so when I opened up the plate and saw 3 wires (plus what I guess it's a ground) I was like
; there's a blue, red and white wire. All of them are covered by a black plastic cap. <ahem> How do I check the voltage? Do I have to unscrew also that plate or can I safely remove the black caps?
While checking, we tried also blocking the fuel like, after the gauge, and it went up to 80. All this measures were taken just after the fuel filter in the engine bay.
Does anybody have any suggestions?
In case it turns out the pump is getting 12V, what could it be? Bad FPR/Fuel rail? Vac leak? The tank and the fuel line was completely cleaned 3 months ago, so I doubt it's something clogged.
By the way, mine's a '87 with a TII engine swap, completely rebuilt with all FD internals 700 miles ago, but I have the stock 327 (NA) ECU. The AFM is the 318, turbo one.
Since when I bought my car one month ago, it ran badly over 4.5k, it kinda "pushed". I thought it were due to bad injectors (I'm running stock 550), so I had them checked and they are flowing like new (shop words); I have an almost new MSD 125 psi fuel pump with about 500 miles on it, so maybe the injectors were getting too much pressure. We put a fuel pressure gauge.... and it turns out that I have *low* fuel pressure....
At idle it shows like 32, when I accelerate it to 3-4k (with the car standing, not running) it goes down to 28. As soon as I relase the throttle it goes down to 25, then climbs back to 28 and sloooowly back to 32 psi.... and now I'm really puzzled....
Reading in this forums my first idea was to check the voltage at the pump; now bear in mind that I have kinda basic electrical skills, so when I opened up the plate and saw 3 wires (plus what I guess it's a ground) I was like
; there's a blue, red and white wire. All of them are covered by a black plastic cap. <ahem> How do I check the voltage? Do I have to unscrew also that plate or can I safely remove the black caps?While checking, we tried also blocking the fuel like, after the gauge, and it went up to 80. All this measures were taken just after the fuel filter in the engine bay.
Does anybody have any suggestions?
In case it turns out the pump is getting 12V, what could it be? Bad FPR/Fuel rail? Vac leak? The tank and the fuel line was completely cleaned 3 months ago, so I doubt it's something clogged.
By the way, mine's a '87 with a TII engine swap, completely rebuilt with all FD internals 700 miles ago, but I have the stock 327 (NA) ECU. The AFM is the 318, turbo one.
06-13-04, 03:27 PM
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your last sentence should have been your first. I do not know anything about running that ecu w/ that engine.
to test FPR jumper the yellow fuel pump test connector located near pass side strut tower. look in manual for specs, I dont know off top of my head. when vacuum is applied to fpr the pressure should drop ~5psi
to test fuel pump voltage put voltmeter to pos and neg wires of that connector your talking about. the resistor relay changes voltage from 9v to 12+volts. you want to test it while driving to make sure its getting about 14volts while the car is acting up. common thing is to "rewire" that circuit. ive done it on my car and it helps a lot, search for "fuel pump rewire"
hard to say w/ your setup though. could be many things. I have an s5 TII engine w/ s4 computer and electricals, had to do many modifications for it to run properly. do much reading on this site, someone may have tried your setup
if it were mine i'd check ALL the basics over again and then test ecu operation with lab scope....
good luck
Mike
to test FPR jumper the yellow fuel pump test connector located near pass side strut tower. look in manual for specs, I dont know off top of my head. when vacuum is applied to fpr the pressure should drop ~5psi
to test fuel pump voltage put voltmeter to pos and neg wires of that connector your talking about. the resistor relay changes voltage from 9v to 12+volts. you want to test it while driving to make sure its getting about 14volts while the car is acting up. common thing is to "rewire" that circuit. ive done it on my car and it helps a lot, search for "fuel pump rewire"
hard to say w/ your setup though. could be many things. I have an s5 TII engine w/ s4 computer and electricals, had to do many modifications for it to run properly. do much reading on this site, someone may have tried your setup
if it were mine i'd check ALL the basics over again and then test ecu operation with lab scope....
good luck
Mike
06-13-04, 07:01 PM
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At idle the fuel pressure should be approx 28psi
The fuel pump pressure with the line blocked off should read b/t 71 to 92 psi.
If you are idling, and jumper the ORANGE FPR solenoid...the pressure should be approx 34 to 39 psi. An alternate or jumpering the ORANGE FPR solenoid, is to just pull the small vac hose off the FPR itself.
To look at the fuel pump voltage.....follow that fuel pump harness back into the car where it ends at a connector on the back side of the left rear strut tower. It's the BLUE wire on the chassis side of the connector that you want to back probe. It, the Blue wire interfaces with a black/white wire on the fuel pump side of the harness. That's for reference.
A normal car would read approx 9.5 volts at idle and when the car is under load, will see the output voltage of the alternator or close to 14 volts.
If you suspect a fuel problem......replace the fuel filter that is located near the engine, below the brake master cylinder AND remove the fuel pump from the tank and clean the mesh filter with carb cleaner or acetone and reinstall.
I'm not sure the fuel pressure figures you mentioned indicate a bad pump. Try tying that fuel pressure gauge to the wipers and go for a ride. Boost the engine and see how high the thing goes. I've seem close to fifty psi during boost. Normal driving, to the best of my memory was around 38 psi. I said normal driving, not under boost and not idling. Actually at idle I've always seen closer to 32 than 28. Several gauges and three different cars.
If you suspect the fpr (should not be that) put a MittyVac on the FPR vac hose fitting and put pressure on there. If you put about 10psi on that nipple the fuel pressure should have risen to close to fifty psi. Probably a few psi lower. Not worth worrying about.
The fuel pump pressure with the line blocked off should read b/t 71 to 92 psi.
If you are idling, and jumper the ORANGE FPR solenoid...the pressure should be approx 34 to 39 psi. An alternate or jumpering the ORANGE FPR solenoid, is to just pull the small vac hose off the FPR itself.
To look at the fuel pump voltage.....follow that fuel pump harness back into the car where it ends at a connector on the back side of the left rear strut tower. It's the BLUE wire on the chassis side of the connector that you want to back probe. It, the Blue wire interfaces with a black/white wire on the fuel pump side of the harness. That's for reference.
A normal car would read approx 9.5 volts at idle and when the car is under load, will see the output voltage of the alternator or close to 14 volts.
If you suspect a fuel problem......replace the fuel filter that is located near the engine, below the brake master cylinder AND remove the fuel pump from the tank and clean the mesh filter with carb cleaner or acetone and reinstall.
I'm not sure the fuel pressure figures you mentioned indicate a bad pump. Try tying that fuel pressure gauge to the wipers and go for a ride. Boost the engine and see how high the thing goes. I've seem close to fifty psi during boost. Normal driving, to the best of my memory was around 38 psi. I said normal driving, not under boost and not idling. Actually at idle I've always seen closer to 32 than 28. Several gauges and three different cars.
If you suspect the fpr (should not be that) put a MittyVac on the FPR vac hose fitting and put pressure on there. If you put about 10psi on that nipple the fuel pressure should have risen to close to fifty psi. Probably a few psi lower. Not worth worrying about.
Last edited by HAILERS; 06-13-04 at 07:08 PM.
06-16-04, 09:29 AM
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Hi guys, thx for the info!
I checked the voltage this morning: I'm getting 12.5 dcV at idle; when I accelerate (car not moving) it goes up to 13.2 (3,500 - 4,000 rpm). Is this normal?
Remember that mines is a '87 NA with TII turbo swap (running the NA ECU)
I checked the voltage this morning: I'm getting 12.5 dcV at idle; when I accelerate (car not moving) it goes up to 13.2 (3,500 - 4,000 rpm). Is this normal?
Remember that mines is a '87 NA with TII turbo swap (running the NA ECU)
06-16-04, 09:53 AM
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I still think we're on the "why are you running the n/a ECU" question....The pump voltage looks good...Like Hailers said, I don't think the pump/ pressures are your problem...The boost pressure signals you're sending to the stock ECU might be tripping it out (and it thinks it's scheduling fuel for a 9.4 CR combustion chamber)...
06-16-04, 10:33 AM
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Originally posted by WAYNE88N/A
I still think we're on the "why are you running the n/a ECU" question....The pump voltage looks good...Like Hailers said, I don't think the pump/ pressures are your problem...The boost pressure signals you're sending to the stock ECU might be tripping it out (and it thinks it's scheduling fuel for a 9.4 CR combustion chamber)...
I still think we're on the "why are you running the n/a ECU" question....The pump voltage looks good...Like Hailers said, I don't think the pump/ pressures are your problem...The boost pressure signals you're sending to the stock ECU might be tripping it out (and it thinks it's scheduling fuel for a 9.4 CR combustion chamber)...
If you still have questions about the fuel.......tack the fuel gauge to the wipers and go for a ride. I doubt it is the fuel pressure. Another way to do it without going for a drive is to put a source of pressure on the fpr and watch the fuel pressure rise as your idling in the driveway. It should go to forty five to fifty psi.
So....are you using the stock non turbo pressure sensor?
06-16-04, 10:36 AM
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I have a S4 Turbo ( N333 ) ECU and boost pressure sensor Id sell if you are interested. I suspect that is your problem as well. Also have the S4 T2 AFM , if you are still using the N/A one. PM me if you are interested.
Last edited by Rxmfn7; 06-16-04 at 10:40 AM.
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06-16-04, 10:58 AM
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domyalex, you'd better hurry and pm him, because if you don't I wiil and will buy it if it is a reasonable price. I need that for another little project.....tubo on a non turbo engine.
06-16-04, 12:27 PM
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I alredy have the Turbo (318) AFM, but I have the NA Boost/pressure Sensor (327) and Wiring harness.
Do i just need the T2 ECU and the Boost/Pressure Sensor or the wiring harness too?
My engine is a 318, will a 333 ECU work without problems?...
Do i just need the T2 ECU and the Boost/Pressure Sensor or the wiring harness too?
My engine is a 318, will a 333 ECU work without problems?...
06-18-04, 01:09 PM
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As the story goes on: oh well, seems like trickfc was right....
We tried with another fuel pressure gauge and this one showed a steady 45psi, at idle as well acelerating; we switched the vacumm source of the FPR to one we *do* know that works (BOV and Boost Gauge are on that one) and... problem solved!
The car still has some little hesitations at 6,500, but nothing that a wideband can't diagnose....
Anyway, thx a lot for all the infos guys!
I have a strange grin on my face that I can't make go away today....
We tried with another fuel pressure gauge and this one showed a steady 45psi, at idle as well acelerating; we switched the vacumm source of the FPR to one we *do* know that works (BOV and Boost Gauge are on that one) and... problem solved!
The car still has some little hesitations at 6,500, but nothing that a wideband can't diagnose....
Anyway, thx a lot for all the infos guys!
I have a strange grin on my face that I can't make go away today....
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