Let's talk about fuel issues -- Fuel Pressure Drop under Acceleration

[86GT351]

Alrighty. Here's a fun data log for those of you who know what to read!

The car was cold upon start. The car was started at ~70sec, when you can see the spike in fuel rail pressure.

I'm sorry, I didn't realize I wasn't recording RPM... Remember, I'm new to this!!

Anyways, viewing fuel flow, you'll easily see the three trouble times. The one that's strangest to me is the one at about 910sec.

Long term fuel trim bank 1 looks like it's messing up to me, as is one of the O2 sensor voltages, and the short term trims too.

Rail pressure and duty cycle and pump act. voltage all look okay, but those O2 sensors and fuel trims looks screwey to my untrained eye.

So, mechanical problem, or tune problem?

Unless I can't read, I am not seeing anything changing. The )2 sensor is but that is supposed to change. And the Load is changing. On my download every other number is the same.

 

[petersonb]

Unless I can't read, I am not seeing anything changing. The )2 sensor is but that is supposed to change. And the Load is changing. On my download every other number is the same.

I'm looking at fuel flow, O2 voltage bank 1, long term fuel trim bank 1, and short term fuel trim bank 1.

At 910, There's a weird spike in those banks 1s, which seem to have caused a spike in fuel flow. Long term trim seems to stay at 1.000 after that spike.

All bank 2s look normal throughout.

Obviously, I don't know much about all this data logging stuff, so I'm curious as to why bank 1 is doing weird things and bank 2 is normal. Does this suggest I have a bad O2 sensor on one side?

Keep in mind, that data log was taken with the car ONLY idling... no input from the gas pedal.

 

[petersonb]

FROM LAST NIGHT:



This is the fuel pump hanger with the factory check valve (the white thing), which I suspected was the culprit.



This is the check valve removed. Almost no flow was able to pass through this thing..... That can't be good!




This is the TEMPORARY fix. I replaced the hoses with Gates submersible fuel line, with NO check valve.

The car runs fine once it's going, but without the check valve, it doesn't hold pressure, so it's kind of a bitch to start.

New fancy check valve comes in Monday! (it will go in line with the black hose)

 

[lito]

Brad, you are a very dangerous newbie, lol. Why you did that?

 

[petersonb]

Brad, you are a very dangerous newbie, lol. Why you did that?

C'mon Lito... I might be a newbie to this forum and all this tuning stuff, but I'm not a complete newb when it comes to understanding what the motor needs to work properly!

To all, my terminology has been improved. This white thing is apparently called a PPRV ... Well, bottom line, my was clogged (I couldn't pull or blow air through the hose on either side) and now the car runs pretty well....besides belt slipping.

WOT only puts me at about 35 on pump duty cycle now that that stupid thing is gone. I used to max it out at 75.

..A step in the right direction, for sure!

 

[BruceH]

C'mon Lito... I might be a newbie to this forum and all this tuning stuff, but I'm not a complete newb when it comes to understanding what the motor needs to work properly!

To all, my terminology has been improved. This white thing is apparently called a PPRV ... Well, bottom line, my was clogged (I couldn't pull or blow air through the hose on either side) and now the car runs pretty well....besides belt slipping.

WOT only puts me at about 35 on pump duty cycle now that that stupid thing is gone. I used to max it out at 75.

..A step in the right direction, for sure!

FYI .75 is a code for a problem with the pump. .5 is maxed on fuel pump duty cycle. 1.00 is maxed for returnless fuel system duty cycle.

 

[lito]

A .75 happens when the read pressure is way off on the plus side of the desired, it normally tells the pump to shut down. You may see it while lifting after a WOT.

 

[Paps]

Why removing pprv is dangerous? Because fuel could go back to the tank?

 

[petersonb]

Why removing pprv is dangerous? Because fuel could go back to the tank?

Maybe others will oppose this, but these systems run at about 40psi toward the motor. There definitely shouldn't be any fuel flowing into the tank while the cars running.

HOWEVER, when you're trying to start the car, that valve is supposed to keep the lines charged, and since its not there, fuel will flow back into the tank (pump primes for a second or two when you turn the key, then shuts off until you crank the motor). This leads to embarrassment because your "badass" car seems like its broken to others.

Therefore, I'm putting a new valve in tomorrow!

 

[michael.konor]

HOWEVER, when you're trying to start the car, that valve is supposed to keep the lines charged, and since its not there, fuel will flow back into the tank... This leads to embarrassment because your "badass" car seems like its broken to others.

Did the same thing when that happened to me. That same day I had a Earl's -8AN Check Valve on way with overnight delivery...

 

[petersonb]

Did the same thing when that happened to me. That same day I had a Earl's -8AN Check Valve on way with overnight delivery...

That thing looks badass. Did you keep it in the tank, or cut your fuel line outside of the tank?

I ordered a simpler, cheaper one (about $45). I hope it does the trick!

 

[michael.konor]

I'm actually running the Fore Dual Hat with two -6AN's --> Y-Block --> -8AN line --> 10 micron filter --> Earl's Check Valve --> Engine.

 

[petersonb]

I'm actually running the Fore Dual Hat with two -6AN's --> Y-Block --> -8AN line --> 10 micron filter --> Earl's Check Valve --> Engine.

Sounds robust. I'm just putting a whimpy 465hp to the ground, so haven't replaced the lines on my car yet!

 

[petersonb]

The check valve came in yesterday. I know there's a lot of hose there, but it tucks into the tank just fine.

No pressure drops under WOT accel anymore, and system holds pressure!

Thanks S197

 

[muztangman93]

Most people delete it anyway. I know that mine no longer has it. As far as starting all you would need to do is turn key on when you hear pump finish priming turn it back off then turn it back on when you hear pump start to prime crank it and it will fire right up.

 

[petersonb]

Most people delete it anyway. I know that mine no longer has it. As far as starting all you would need to do is turn key on when you hear pump finish priming turn it back off then turn it back on when you hear pump start to prime crank it and it will fire right up.

Good info for the future. The car isn't to the point yet where I can do a WOT pull through the gears, so I don't know if this will restrict fuel at high RPM. But if it does, that sucker is comin off!

 

[petersonb]

Problem is present again.....

Removed the check valve again, problem still present. Guess I can check that off my list of possible culprits.

It has been fine the last few days, but I thought I was starting to see the pressure drop again. Today I went to visit some family about 1.5hr away, and about an hour into the drive, I drop to 4th gear to pass someone, push the accel pedal, and it's gutless. Sure enough around 15-20psi fuel rail pressure.

After this, I removed the check valve (after a nail-biting drive up the mountains) and do some pulls with data log. Lito sent me a config file, and I also made my own.

Attached are 4 data logs: 2 with low fuel (Manuel "Lito"'s file, and my file), then 2 with full tank (again, one with his config, one with mine).

Things I notice: fuel flow, pump voltage, and duty cycle all go up with throttle position. Fuel rail pressure goes down. O2 voltage goes down. (My wideband gauge shows 17.3, which is as "lean" as it goes). Fuel pump duty cycle is 50.00, which (as stated before) means "Maxed out."

It seems like this problem is worse when heat is involved. If flow is "normal" and pressure is dropping, that means leak, right?

I'm really losing heart on all this. Replacing everything and not making any progress.

 

[petersonb]

I spoke with Bobby at S&H Performance about maybe getting some new fuel lines, and he's going to let me try his old stock FPDM.

The list of possible culprits is getting smaller.....

Thanks for the input so far guys!

 

[petersonb]

Quick Question:

Is the fuel hat supposed to be "compressed" into the tank? This is where it sits with no pressure. To get the ring to lock down, I have to press it down and the ring holds it down.

Notice the ~1in. difference in tank depth and fuel hat length.

 

[petersonb]

I think I've got it (for real) this time!

I was talking to Bobby at S&H today, and we discussed the T-valve looking thing that connects to the line running to the passenger side. It has a cone shaped section that looks like it would be for allowing flow to only one direction.

I took that off, shook it, and it sounded just like I thought: a gunked up ball & socket setup.

I blew air in the direction fuel SHOULDN'T be flowing, yet I still could. PROBLEM!

So I took it out, connected the line like this:

Immediately I saw improvement, even before I started the car. Used to, I would "prime" the pump, but rail pressure (as read on my dash gauge) would go up, then fall very quickly. I thought it was because there isn't a check valve, but then, like someone said earlier, no check valve should be fine.

Now, pump primes, then holds pressure (still no check valve).

I go for a drive, do some data logging, and when I'm WOT, NO PRESSURE DROP AT ALLLL.... Last time I said I think I had fixed the problem, it wasn't a rock-solid 52psi. NOW it is!

I do several pulls, all the same! Attached is a snapshot of a data log, showing RPM, throttle position, and rail pressure (delta rail pressure should be 39.1 +/- 5psi, something like that) and you can see that it is! My dash gauge shows ~52psi because I'm running about 12psi boost.

Now I have to drive back home (115mi) tomorrow, and that will be the true test.

Are these common to fail?

-Brad