LT1 FPR question
Thread Starter
Instructor
Joined: Mar 2016
Posts: 184
Likes: 57
From: Bowling Green Ohio
2021 C5 of the Year Finalist - Unmodified
2020 C5 of the Year Finalist - Unmodified
LT1 FPR question
92 LT1 completely stock. I was checking my Fuel pressure tonight and idling it was around 38 psi. As I opened the throttle body the pressure sat around 38-36 until the rpm got around 3500 or so, NORMAL? Then it began to come up. Now, idling with the vacuum off the FPR it made 48psi. And when I'd snap the throttle quickly it would come up into the low 40s. I realize 41-47 seems to be the operating range for this engine. Does this pressure seem on the low side to anyone?
My real oddball question is I noticed while the car was running that the FPR was ticking, as if opening and closing quickly, though pressure was pretty steady. Is the ticking normal? I know that's a weird one but I'm inclined to replace the FPR due to it and to see if it raises the operating range of the pressure before considering the pump.
Sorry for the amateur questions. Car starts and Idles great, just has some part throttle weakness I cant figure out. Also at WOT just before shifting from 1-2 (automatic) it sometimes coughs just before the shift. Wondering if that's fuel starvation.
My real oddball question is I noticed while the car was running that the FPR was ticking, as if opening and closing quickly, though pressure was pretty steady. Is the ticking normal? I know that's a weird one but I'm inclined to replace the FPR due to it and to see if it raises the operating range of the pressure before considering the pump.
Sorry for the amateur questions. Car starts and Idles great, just has some part throttle weakness I cant figure out. Also at WOT just before shifting from 1-2 (automatic) it sometimes coughs just before the shift. Wondering if that's fuel starvation.
Le Mans Master
Joined: Oct 2002
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From: Las Vegas - Just stop perpetuating myths please.
Best just to set it to stock which is 41psi with the engine off. Now if you have an adjustable regulator and the pressure is to high when you turn the key (but not to start) then you of course need to bleed off some press before readjusting. Finally I would start the motor then shut off to measure the fuel pressure one last time engine off to verify you got it right.
Yea so the function of the vacuum operated regulator is to keep the pressure differential across the injector constant as manifold pressure changes. As the throttles open the engine vacuum does go down but that is really absolute pressure going up - as manifold pressure is moving towards atmospheric pressure. That means there is greater pressure resisting injector flow. So the vacuum operated regulator will increase fuel pressure to the injector to maintain the desired flow rate.
When you have are at idle the vacuum inside the manifold is very high but the absolute pressure is very low and there is less pressure to resist injector flow. The vacuum operated regulator adjusts fuel press to lower fuel pressure to the injectors and maintain desired flow.
Well if you change fuel pressure you need to change the injector constants too. That means a new tune and in a '92 that would take a new ECM chip.
Hope this helped.
Yea so the function of the vacuum operated regulator is to keep the pressure differential across the injector constant as manifold pressure changes. As the throttles open the engine vacuum does go down but that is really absolute pressure going up - as manifold pressure is moving towards atmospheric pressure. That means there is greater pressure resisting injector flow. So the vacuum operated regulator will increase fuel pressure to the injector to maintain the desired flow rate.
When you have are at idle the vacuum inside the manifold is very high but the absolute pressure is very low and there is less pressure to resist injector flow. The vacuum operated regulator adjusts fuel press to lower fuel pressure to the injectors and maintain desired flow.
Well if you change fuel pressure you need to change the injector constants too. That means a new tune and in a '92 that would take a new ECM chip.
Hope this helped.
Thread Starter
Instructor
Joined: Mar 2016
Posts: 184
Likes: 57
From: Bowling Green Ohio
2021 C5 of the Year Finalist - Unmodified
2020 C5 of the Year Finalist - Unmodified
Didn't mean to imply I was going to install an adjustable regulator. I would replace with OE part. Now speaking of tuning, isn't the factory flow rate of the injectors based on a 43.5 psi system? That I'm not sure of, and I'm not trying to retune anything. i just thought the ticking in the regulator seemed strange. Still no feedback on that. I understand how it's supposed to work just was looking for thoughts from anyone who had real world experience with this.
Safety Car
Joined: Sep 2001
Posts: 4,157
Likes: 554
From: Glenbeulah Wi
92 LT1 completely stock. I was checking my Fuel pressure tonight and idling it was around 38 psi. As I opened the throttle body the pressure sat around 38-36 until the rpm got around 3500 or so, NORMAL? Then it began to come up. Now, idling with the vacuum off the FPR it made 48psi. And when I'd snap the throttle quickly it would come up into the low 40s. I realize 41-47 seems to be the operating range for this engine. Does this pressure seem on the low side to anyone?
My real oddball question is I noticed while the car was running that the FPR was ticking, as if opening and closing quickly, though pressure was pretty steady. Is the ticking normal? I know that's a weird one but I'm inclined to replace the FPR due to it and to see if it raises the operating range of the pressure before considering the pump.
Sorry for the amateur questions. Car starts and Idles great, just has some part throttle weakness I cant figure out. Also at WOT just before shifting from 1-2 (automatic) it sometimes coughs just before the shift. Wondering if that's fuel starvation.
My real oddball question is I noticed while the car was running that the FPR was ticking, as if opening and closing quickly, though pressure was pretty steady. Is the ticking normal? I know that's a weird one but I'm inclined to replace the FPR due to it and to see if it raises the operating range of the pressure before considering the pump.
Sorry for the amateur questions. Car starts and Idles great, just has some part throttle weakness I cant figure out. Also at WOT just before shifting from 1-2 (automatic) it sometimes coughs just before the shift. Wondering if that's fuel starvation.
Last edited by bjankuski; Jan 28, 2017 at 04:34 PM.
Melting Slicks
Joined: Jun 1999
Posts: 2,949
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From: Bowling Green KY
Didn't mean to imply I was going to install an adjustable regulator. I would replace with OE part. Now speaking of tuning, isn't the factory flow rate of the injectors based on a 43.5 psi system? That I'm not sure of, and I'm not trying to retune anything. i just thought the ticking in the regulator seemed strange. Still no feedback on that. I understand how it's supposed to work just was looking for thoughts from anyone who had real world experience with this.
