Fuel rail pressure dropping

[zquez]

Alright Ladies and Gents,

Here's the deal. My 2006 GT has a GT500 eaton on it with the DOB manifold. Im running FRPP 47 lb injectors and the Walbro 405 pump with Jeremy's upgraded wiring straight from the battery. Several months ago, I had the fuel line on the hat rupture, and the car hasn't been the same since.

Basically my issue is one of the engine starving for fuel. It will sometimes (usually in the morning) buck on throttle tip in trying to get on the road. Im talking civilized, not wot to merge. After the boost bypass closes, it runs pretty smooth. Try and get on it, and there is a noticeable delay in throttle response, but it eventually catches up. If you try to go wot, it will go (albeit feeling a bit underpowered) but up at the top of the rpm range it will again buck and seemingly want to die.

So my first inclination was the tune, even though it seemed strange because I remember the car running awesome right after I got it tuned. What would cause the tune to go bad? nada. So I began to look for mechanical issues. I checked the gap on the spark plugs (.32 IIRC) and tried cleaning the maf but nothing helped. The ony thing I had suspected but never changed were the 02 sensors. Still feeling like it was a fuel problem, I pulled the hat to inspect the hoses having had a failure before. They were fine, but I replaced the PPRV with some left over submersible line I had. Went for a drive and the car felt better, but not great. I felt like the delete may have lowered the duty cylse and thats why it felt better. I then contacted Lito and he sent me a diagnostic datalog. I put it on the car and came up with some interesting results.

Heres the log if you want to open it in livelink.
https://docs.google.com/file/d/0B9h1BYadL1rnTElJSFY5SGVFd2s/edit?usp=sharing

@ 2000 RPM I begin accelerating

Duty cycle is .20 or 40%
Fuel rail pressure is at 38 psi
Fuel pump volts tab is 5.54

@ 2300 rpm

Duty cycle is .34 or 68%
Fuel Rail Pressure is at 30 psi
Fuel Pump Volts tab is 7.8

@4182 rpm

Duty cycle is .48 or 96%
Fuel Rail Pressure is at 40 psi
Fuel Pump Volts tab is 9.46

@4646 rpm

Duty cycle is .50 or 100%
Fuel Rail Pressure is at 36 psi
Fuel Pump Volts tab is 9.76

@5634 rpm

Duty cycle is .50 or 100%
Fuel Rail Pressure is at 20 psi
Fuel Pump Volts tab is 9.76

Lift to shift, fuel rail pressure spikes to 68 psi

@ 4100 rpm

Duty cycle is .47 or 94%
Fuel Rail Pressure is at 23 psi
Fuel Pump Volts tab is 9.59

By the time I reach 5500 rpm next

Duty cycle is .50 or 100%
Fuel Rail Pressure is at 19 psi
Fuel Pump Volts tab is 9.76



So basically what I get from this log is that the pump is reaching 100% duty cycle fairly soon and the pressure drops off fast. It stays a steady 38-40 psi under normal driving.

I recently sent my hat to Jeremy to look over and see what he could find. The pump appeared to be fine. He replaced it and sent my hat back to me. I installed it yesterday and alas! I didn't fix it. So here I am wondering what could be causing this if the pump is brand new. There are people making more power than me on this pump just fine. I assume its a voltage issue, but where? Is my alternator bad? Is my battery bad?

Where do I go from here?

 

[klaw]

Did you replace the fuel filter?

 

[zquez]

Did you replace the fuel filter?

I have not, but it was replaced less than 10k miles ago.

 

[klaw]

I have not, but it was replaced less than 10k miles ago.

Start with the simple/cheap things - replace it.

 

[zquez]

I guess I'll pick one up from autozone tonight.

 

[Fullboogie]

It's an easy job, but you will get gas all over your arms. No way to avoid it

 

[Roadracer350]

Did you replace the fuel filter?

I bet its the fuel filter

 

[one eyed willy]

something else you may want to check is your fuel rail sensor. i havnt had one go bad on me yet, but from what i have read if you remove the vac line going to it...and if that vac line smells like gas then the sensor is busted....

 

[klaw]

something else you may want to check is your fuel rail sensor. i havnt had one go bad on me yet, but from what i have read if you remove the vac line going to it...and if that vac line smells like gas then the sensor is busted....

I believe there was a revision to the FRPS during 2006. The earlier design had issues iirc.

 

[one eyed willy]

I believe there was a revision to the FRPS during 2006. The earlier design had issues iirc.

he has a 2006

 

[05yellowgt]

Check for a vacuum leak to the FRPS as well. If it isn't seeing accurate vacuum/boost in the manifold, it will send all kinds of crazy data to the ECU. It is strange that the duty cycle is showing pegged though. Are you running stock fuel lines to the rails? If not, check and see if there is a kink or pinch in the line somewhere while you are changing the filter. Did you ever find anything that might have caused the line to fail in the first place?

 

[zquez]

Check for a vacuum leak to the FRPS as well. If it isn't seeing accurate vacuum/boost in the manifold, it will send all kinds of crazy data to the ECU. It is strange that the duty cycle is showing pegged though. Are you running stock fuel lines to the rails? If not, check and see if there is a kink or pinch in the line somewhere while you are changing the filter. Did you ever find anything that might have caused the line to fail in the first place?

Stock lines and GT rails. I haven't found anything that would be a restriction in the system. Im going to the store to get a filter now and Im going to do some logging before and after. Hopefully it fixes it. If so, that'll be the easiest fix I've ever encountered on my car.

 

[zquez]

I didn't get around to putting the car on the lift and replacing the fuel filter today, but I did smell the vacuum line going to the FRPS and it smelled like good 'ol 93 octane. So I guess thats a pretty clear indication that the FRPS is bad. I'm going to pick one up tomorrow and we'll see what happens.

 

[one eyed willy]

Nice, hopefully that's the end of your problem!

 

[BruceH]

I didn't get around to putting the car on the lift and replacing the fuel filter today, but I did smell the vacuum line going to the FRPS and it smelled like good 'ol 93 octane. So I guess thats a pretty clear indication that the FRPS is bad. I'm going to pick one up tomorrow and we'll see what happens.

FYI new ones are reasonable on ebay. New from the dealer are over $100.

 

[klaw]

update?

 

[zquez]

FRPS is on order from ebay. (Thank you btw BruceH) It should be here hopefully on Wednesday. Fingers Crossed it helps.

 

[zquez]

I got the FRPS today and put it on. I was only able to drive it into town with rush hour traffic, but initial impression was that the FRPS was my needle in a haystack! The car seemed a lot more responsive and seemed to pull harder. I'll be doing some datalogging tomorrow to verify things are better.

My hypothesis is that because the FRPS was bad, the fuel pressure readings I was getting were erroneous. But those are the numbers the computer saw and so it told the pump to pump harder. This accounts for the 100% duty cycle. So really I wasn't leaning out, I was getting richer. (I need a wideband I know) This also accounts for my horrendous gas mileage (220 to a tank).

But what made it fail? Was it a bad design like Klaw said or did something happen to make it fail. I noticed the tip that was submerged in fuel was discolored. I don't know if its just from the gas or heat. Right now my fuel line is passing behind my engine and the blower. I have it wrapped in another larger ID hose but that may not be doing much. Is my fuel collecting ambient heat?

I am switching to GT500 rails which will keep it from going behind the engine and I'm also going to sheath it with DEI heat shield for kicks anyway.

It will be interesting to see what the data tells me tomorrow.

 

[klaw]

The sensor actually reads differential pressure (intake manifold vs fuel rail) across a membrane. I believe that the original sensors were only rated for a differential pressure of 70 psi or so. Apparently if you're WOT and flowing a lot of fuel and then suddenly snap the throttle shut, these fuel systems can see a significant pressure spike at that membrane and it can rupture. The newer sensors have a more robust membrane. The situation is exacerbated in FI applications due to the much larger swing in manifold pressure from full boost to full vacuum at throttle cut, higher fuel flow rate, and higher fuel rail pressure under boost.

 

[one eyed willy]

Glad you got it fixed!