Debugging fuel pressure loss with rpm
05-26-17, 10:17 PM
#1
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Thread Starter
Debugging fuel pressure loss with rpm
Just analysing some of my logs from a recent track day. Think I know what the issue was, but wanted another opinion just to confirm I am interpreting this right.
Basically, in the final session with about 1/4 tank fuel my fuel pressure engine protection function tripped on what I believe was fuel surge.
But going back and looking over the logs, I've noticed that the fuel pressure seems to fall short of target (calculated by base fuel pressure (300kPa) + manifold pressure (gauge)). This error seems to increase with rpm but not linearly - it hits target at ~4500-5500rpm, then quickly falls away, but the delta doesn't really cahnge.
Would I be correct in assuming this is a fuel pump limitation? I am still on the stock fuel system apart from the fuel pressure regulator, but I have a T04S running 0.9-0.8 bar (as you can see from the logs, it's tapering a fair bit).
Between the two logs below there is a 10kPa difference in boost pressure, however the delta in fuel pressure doesn't seem to vary much.
Look at the bottom chart in each image. Green is calculated target fuel pressure, white line is measured fuel pressure.
Engine Trip Event
Boost 90kPa and tapers to around 78kPa at redline
Fuel tank 1/4 full
Ambient temperature hot
Previous Session with lower boost target and higher fuel tank level
Boost hits max 80kPa and tapers to around 68kPa at redline
Fuel tank full
Ambient temperature cool
So would everyone agree it is the fuel pump? Or FPR? Wiring?
Basically, in the final session with about 1/4 tank fuel my fuel pressure engine protection function tripped on what I believe was fuel surge.
But going back and looking over the logs, I've noticed that the fuel pressure seems to fall short of target (calculated by base fuel pressure (300kPa) + manifold pressure (gauge)). This error seems to increase with rpm but not linearly - it hits target at ~4500-5500rpm, then quickly falls away, but the delta doesn't really cahnge.
Would I be correct in assuming this is a fuel pump limitation? I am still on the stock fuel system apart from the fuel pressure regulator, but I have a T04S running 0.9-0.8 bar (as you can see from the logs, it's tapering a fair bit).
Between the two logs below there is a 10kPa difference in boost pressure, however the delta in fuel pressure doesn't seem to vary much.
Look at the bottom chart in each image. Green is calculated target fuel pressure, white line is measured fuel pressure.
Engine Trip Event
Boost 90kPa and tapers to around 78kPa at redline
Fuel tank 1/4 full
Ambient temperature hot
Previous Session with lower boost target and higher fuel tank level
Boost hits max 80kPa and tapers to around 68kPa at redline
Fuel tank full
Ambient temperature cool
So would everyone agree it is the fuel pump? Or FPR? Wiring?
Last edited by Jarik; 05-26-17 at 10:24 PM.
05-28-17, 10:25 AM
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it is interesting that is seems to hit the target in the first boosting, but not the second or third. is it literally 2nd gear, 3rd gear and then 4th?
i agree you need a bigger fuel pump, i'm a little surprised that the stock pump is keeping up. common lore would have you believe you need a fuel pump before you even buy the car...
i agree you need a bigger fuel pump, i'm a little surprised that the stock pump is keeping up. common lore would have you believe you need a fuel pump before you even buy the car...
05-29-17, 07:36 AM
#3
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Thread Starter
It's related to rpm. The first boosting I barely got to 5000rpm if you look carefully. So it's likely related to fuel flow.
Here is another chart:
X axis - Calculated fuel flow = ([Injector Stage 1 Duty Cycle]/100 * 550 + [Injector Stage 2 Duty Cycle]/100 * 850)*sqrt([Fuel Pressure]/300kPa)
Y axis - Fuel Pressure error = 300kPa + [Manifold Pressure] - [Fuel Pressure]
Ignore the mess on the left side of the chart. I imagine a lot of the error there is just due to the fact that manifold pressure is sampled at 10Hz while fuel pressure was sampled at 5Hz, leading to regular errors through the range where there is a fast delta MAP.
On the right side though, you can see a clear linear increase between the error and the fuel flow. So yeah, probably is fuel pump limitations.
Here is another chart:
X axis - Calculated fuel flow = ([Injector Stage 1 Duty Cycle]/100 * 550 + [Injector Stage 2 Duty Cycle]/100 * 850)*sqrt([Fuel Pressure]/300kPa)
Y axis - Fuel Pressure error = 300kPa + [Manifold Pressure] - [Fuel Pressure]
Ignore the mess on the left side of the chart. I imagine a lot of the error there is just due to the fact that manifold pressure is sampled at 10Hz while fuel pressure was sampled at 5Hz, leading to regular errors through the range where there is a fast delta MAP.
On the right side though, you can see a clear linear increase between the error and the fuel flow. So yeah, probably is fuel pump limitations.