911fanboy

I would appreciate advice regarding a new issue for me:
997.2 gt3 cup
I recently got a red warning light due to low fuel pressure, at the end of a long straight at full throttle.
Started looking at FP, which runs around 70, but will dip to 40's under full throttle.
I checked the filters inline to the main pumps, found no debris.
Pulled all the foam in the cell, all appeared to be in good shape.
Replaced the 4 in tank pumps. One wasn't running, but replaced all.
On track, same results, dipping into 40's.
At this point, I thought maybe it could be the main pump. Since there is a reserve pump, I thought maybe I could use it? I have a red button on the dash that supposedly turns on reserve, but my button is always lit, and doesn't seem to depress or engage when I push it. Not sure how it SHOULD work. If you know, please tell me.
Anyway, thought I could force the reserve pump to work by switching the wires, between main and reserve pumps, so that main tells the reserve pump to work, forcing reserve function.
That didn't work! No change.
Could this be a defective sensor? I get pressures, but maybe inaccurate. I don't know where the sensor lives, how big a deal to replace.
I'd appreciate any thoughts. Thanks

Skibum1963

I would replace the filters regardless of how they look.

amso3

Just for reference, I thought I'd show fuel pressure for a full lap at Sebring in a 997.2 cup. It would be good to see your trace to see if the drop in pressure is gradual and if it stays lower lap after lap. If so you might have a vent issue in the tank or a clogged filter as Skibum suggests

911fanboy

Here's my FP tracing. FP down in 30's going into 7!
Further research: Both main fuel pumps work with 12v. Main fuel pump gets 12 v when car starts. Reserve does not, Button on dash always lit, but does not depress, or release.

amso3

911Fanboy

Can you do a screenshot with speed, throttle, brake and fuel pressure? It would be interesting to see what is going on with speed and throttle when the pressure drops

Dan Jacobs

Pressure regulator?

amso3

911fanboy

Did I miss the second graph earlier? Either way, the pressure drops when you are requiring high flow fuel so it would seem to me that you have a clogged filter or restriction somewhere on the input side of the fuel pump. You need to fix it before you drive much. I had a similar issue in an air cooled 911 and denotated new pistons and cylinders before I found that I had plugged my vent in my fuel tank. I don't think this is your issue but 30 ish PSI fuel pressure at full throttle is very bad for the engine for any extended amount of time. In the meantime, I think I'd set up an alarm to show when you drop below 45 psi fuel pressure until you solve your problem. Good luck!

Jerry

911fanboy

Thanks for your attention to my post. You didn't miss it. I added an additional screenshot as per your suggestion. As I mentioned, I looked at the prefilters in line with the pumps, which were clean. I'm thinking now that my reserve pump never comes on, which might be starving the system when my fuel level gets low. i'm afraid that I don't know how much fuel was in the tank, since I do not seem to have a gauge. I know I could just fill a bunch, then try again, but I didn't think of that when I was at the track. I plan to replace the reserve fuel switch, add lots of fuel and try again.

amso3

I'd start with a near full tank to see if it is an issue of starving for fuel. It is easy to set an alarm to come on under 45 psi fuel pressure. I would not run the engine if that condition persists. Keep us posted on your findings.

911fanboy

Thanks again. Could you tell me what's involved in setting alarm for low fuel pressure? Motec told me with red flashing lights, but I don't know where the trigger is set to come on.

amso3

To check or change your alarms, go to the Motec ADL 2 manager and "receive" your dash configuration. To open your file, launch ADL 2 and open your file which will turn the icons from grey to colors and your dash name will appear in the top center

Next, click the icon for alarm settings (circled below).



Look to see if you have an alarm in your settings like this: (Your fuel pressure may be named differently in your car....Germans!). If you do have a fuel pressure alarm already, check to see that it is set at 2.5 Bar (36 psi). If you have one like this, I'd change the 5 second delay to 2 seconds until you sort out your issue. If you don't have this alarm add a new alarm similar to this one then send the revised configuration back to the dash. Note that you want the condition of the alarm to include pressure and RPM >2000 so you don't have an alarm on at idle all the time.

911fanboy

Jerry;
I did as you suggested. Thanks for your help. Not sure when, but I'll try to give you followup when I get it.
Mark