Fuel Pressure Drops When Revving

[twistedcm2]

Just to start off I have searched and searched but could not find anything related to this specific problem.

I'am working on a friends 90 Tsi, just got done installing a front mount kit (ETS race version), along with a MAFT and 3 inch mas, Walbro 255, and a new fuel pressure regulator (Fuel Lab). Just to let you guys know what I did.

Now the problem with the car is that it will start up and idle and at the time has a fuel pressure set around 38-40 just because the idle is fluctuating. When you rev the motor the fuel pressure drops to 10-15 psi and the motor starts to die and it sounds like a back fire nose from the turbo. I have spent hours checking everything and replaced the CAS and new oem injector seals (head and rail).

The only thing I can think of is that there is a leak at the pump and the line leading to the pump, but I have checked it twice and it looked fine and checked to tighten up the line at the top of the fuel sending unit. All the fittings on the fuel pressure regulator are teflon taped. I did notice a clicking sound when the car is running from the injectors and could feel a viberation with my hand when touching the clip.

This is the best I can discribe it and have been working on it for too long and need help from the wisemen. Any ideas?

 

[Calan]

You may have something clogged inside the regulator or supply line, especially if you used teflon tape. I would pull the lines back off and check everything internally for a blockage, and then re-assemble with some good thread sealer. (Permatex makes some that is sensor/fuel safe, but the number escapes me at the moment).

If you do this, be very careful to clean all the shredded pieces of teflon out of the fittings, and don't let any of it get down in the lines.

The first thing you may want to do is disconnect the return line and put the end of it in a big fuel-proof jar (the end leaving the regulator), and use the fuel pump test port (or logger if yours has the ability) to turn on the pump and see what kind of flow you're getting. Do this carefully and with the car off obviously. You may also want to check the voltage at the pump with everything together and the car running; rev the motor a bit and see what the pump voltage does.

You probably realize this, but it's worth mentioning that your pressure will drop if you pull more vaccum. You won't see a 1 to 1 fuel pressure increase until you build boost pressure.

EDIT:

If anyone has that thread sealer number handy, please let me know. Gonna need some more of it myself in a couple weeks

 

[twistedcm2]

I have checked the voltage at the pump but didnt know what wires to test. I did this by just unplugging the connector and I think one was around 10-11 volts and the other 6 volts and I don't think I got a reading from the other two just assuming there grounds. The battery was also low maybe just around 11.8 on the turbo timer volts.

I have also taken off the line coming from the fuel filter to the rail and gas was coming out. I was not sure how fast it should be coming up but it seemed fine. I'll have to make a run to the auto store and see if I can get that thread sealer. That sounds like a better Idea and I also got a new line for regulator (the rubber hose) So I can just slip that on also.

I know that the vacuum would cause the fuel pressure to drop down, but what I don't get is why it would be enough to kill the engine. I forgot to mention I can't rev past 2k and If you hold the throttle down then the engine will eventually die. The only think I can think of is that when you rev the motor is causing more fuel to be pumped out but if there is a leak then the fuel pressure would be lost through that leak causing the motor to not get any fuel.

 

[crimsondragon]

You can smell a fuel leak. If you don't smell anything, you're fine. I think it's a faulty FPR. You sure you connected the vacuum/boost source from the intake manifold to the AFPR? If you did, and it's still doing this. I would say either the pump or the AFPR. I had a guy come to me about a similar situation and it turned out to be the AFPR. Try to pressurize the AFPR seperately with the car running and see if you can manually raise the pressure that way.

 

[twistedcm2]

I have the vaccum line hooked up right off the intake manifold with the rest of the emission lines removed and that is also where the boost gauge line is hooked up. I had the same problem with the stock fuel pressure regulator also.

Tim, when you talk about pressuring the system how would I go about doing this?

 

[crimsondragon]

Well if you had the same problem with the stock regulator, don't bother pressurizing it then. It sounds like it might be the pump. Have you checked/changed your fuel filter recently?

 

[Calan]

I think it's a faulty FPR.

Another reason to check the flow after the AFPR, at the return line... not on the filter side.

Could also be a clogged return line....

I have checked the voltage at the pump but didnt know what wires to test. I did this by just unplugging the connector and I think one was around 10-11 volts and the other 6 volts and I don't think I got a reading from the other two just assuming there grounds.

Were you measuring from each wire to ground (which you should), or across them?

 

[twistedcm2]

I have not checked the fuel filter at all. I was hoping it was something else so I didn't have to take it off. I will check the return line tomorrow and let you guys know what happens. Should I just look to see if a steady flow of fuel is coming out?

 

[Calan]

Should I just look to see if a steady flow of fuel is coming out?

Yes... you can also squeeze it between your fingers with the car runing and see if you feel pressure, but actually seeing fuel come out with the pump test and the car off is a bit more positive result

When was the filter changed last? That would definitely be a suspect, especially if it's never been changed.

 

[twistedcm2]

When was the filter changed last? That would definitely be a suspect, especially if it's never been changed.

To tell you the truth I have no idea. My buddy bought thr car during the summer and at the time the car had a problem that if you ran it just like normally and shut it off, after about 15 minutes it would have trouble starting back up.

I might just end up changing the fuel filter It couldn't hurt anything and is never a bad idea.

 

[twistedcm2]

Were you measuring from each wire to ground (which you should), or across them?

Sorry I missed this part of your post. But yeah I measure it with a ground just in the back trunk area of the car.

 

[Calan]

I might just end up changing the fuel filter It couldn't hurt anything and is never a bad idea.

Yep.. I would definitely start there...may just fix your problem.

If you don't have any line wrenches (aka flare wrenches)... get some. They make working on fuel, clutch, and brake lines WAYYYYY easier, and you won't round off and ruin the fittings like you will with open-end box wrenches.

You can usually pick up a full metric set fairly cheap at autoparts stores, but most of our lines are gonna be either 10mm, 12mm, or 15mm. The brake and clutch hard lines are all 10mm, and I think the lower fuel filter line is 15mm.

 

[Car Cannibal]

Change the filter every 30k anyways but then hold it around 2k with the vacuum line off and see if it keeps your base pressure.

If you're reading the pressure off the regulator and it's still dropping, without any leaks, you could have a crappy Walbro. Walbro's been dying like madd for the GN crowd and Walbro has come out saying they found a production flaw on a part of the pump.

If the return was clogged up etc you'd have a pressure increase, not decrease.

 

[Calan]

If the return was clogged up etc you'd have a pressure increase, not decrease.

Ok... I'll give ya that one

I forgot about him saying the pressure dropped... I was thinking maybe he has restricted flow at the higher pressures, causing the car to die.

After the OP's last post, my money is on the fuel filter.

 

[twistedcm2]

Ok so I finally figured out what the problem was. I changed the fuel filter and it helped a little. I remember though that before the car would not start and I went to the the top of the fuel pump assembly and just turned the the line and it started. So to make this short and simple the hard line coming off the top of the assembly was bent, which was not allowing enough fuel to get to the engine but enough for the car to idle.

I feel dumb about not thinking this but that hard line did get bent when I was trying to break the fuel line apart from the assembly. So I bent it back as far as I could and worked fine but I'am still going to replace the whole thing with another one.

Thanks for all of your help.

 

[airmail96]

Nice congrats on fixing the problem and thanks for posting your fix.

 

[MaNGiNa24]

Grats on solving your problem

Great job on posting your fix for the problem as well.

For future reference you can rule out fuel leaks by simply smelling the possible areas. Fuel has a pretty strong odor