ULTM8Z

I'm pretty sure it's the pump, but wanted to get other people's thoughts... (I know my car isn't a Corvette, but I come to this forum for info when it relates to similarities between my set up and a C4 Vette)

About a week ago I was noticing my fuel pressure climbing to 60-70 psi while driving along at low-medium throttle.

Symptoms and observations:

1.) I know it's not a faulty fuel pressure gauge issue since I can see the fuel trims in Tunerpro going way rich when it happens

2.) If I let off the throttle completely, it drops back down to the correct 40 psi.

3.) If I really get into the throttle, it'll drop down to around 50-ish.

4.) Doesn't do it at idle speeds.

5.) Key on, engine off, I get ~40 psi at the rail


At first I thought the regulator was going bad, so I swapped it with another one I had. But it's doing the same thing.

Then I started thinking the return line may be blocked. But then I reasoned that would actually have the opposite effect. When I'm off the throttle, the regulator is diverting more fuel back to the tank. So if the return line was blocked in some way, I'd see the higher pressure at idle or at no-throttle conditions, which would contradict observations 2 and 4.

During heavy throttle, the regulator keeps more fuel in the rails to support the increased fuel flow needed. But at the same time, the injector pulse widths are very high, so it relieves some of the higher pressure in the rails, thereby allowing the pressure to drop to some extent, which is consistent with observation 3. So this seems to be telling me the pump is (for whatever reason) putting out much higher pressure than its supposed to.

Any thoughts before I go out and buy another pump?

ULTM8Z

Other diagnostics I did...

I pulled the vacuum line off the manifold.

Hooked up a vacuum pump to it

Pulled the hose off the regulator and put my finger on it.

When I run the vacuum pump, I can feel the pull on my finger at the regulator end of the vacuum line. So that at least tells me the regulator is seeing the manifold vacuum.

The only things on that vacuum line are the regulator and my Autometer vacuum gauge. I know the gauge is ok since it's reading matches the Tunerpro data when I'm driving.

System voltage is rock solid at 14V at any rpm.

I blew some compresed air through the return line (injecting it at the fuel rail connection). Even with very little air, you can hear it easily coming through at the fuel tank.

At which point I blasted the air through. I'll have to take it around for a spin later and see if that solved it. Doubtful though... I wouldn't see what could cause a blockage... I have a filter on the supply side, so what kind of debris could accumulate in the return line?

Granted air will flow a little easier than a fluid, but at least I know there isn't a complete blockage.

Whaleman

Very informative post. Dan

2011KLR

Mayne edit it and use a diferent font color.

ULTM8Z

Oh... oops.

I didn't get it until I saw my post on a PC. On my phone it shows up in black text. Just fixed it.

I'm thinking, what the hell are these guys talking about? lol...

Any rate, yeah, I'm at a loss as to what else on the fuel system could be causing this.

EDIT: Looking around online... these electric pumps have some sort of built in pressure relief valve? If, so I wonder if mine is malfunctioning in some way, thereby overwhelming the fuel pressure regulator?

htrdbmr

Broken motor mount, engine movement collapsing rubber return line?

ULTM8Z

Everything is in good shape. I examined the return line front to rear yesterday.

Though I still think if I had return line blockage, the pressure observations would be the opposite.

ULTM8Z

Changed the pump and the problem is still there.

Clogged fuel filter? I woudn't expect higher pressure at the rails from that.

Bad fuel pump relay? Sending higher voltage to the pump could cause an overpressure, but if the relay only ever has 14V to work with, there's no way it could generate higher voltage on its own.

Not sure what to look at next... fuel lines? I've been running Aeroquip braided teflon lines since 2007... wonder if they have some sort of limited life span. I have them heat shielded where ever they come anywhere near exhaust.

Anyone else have any thoughts? Because I'm at a loss...

Vets-Vet

Not sure what car you have but what about a blocked vent, does your tank vent to a charcoal cannister or through the gas cap ? blocked vents cause high tank pressures. My C4 has a one way gas cap, it lets air in, the vent occurs via the charcoal cannister. Large buildups in pressure occur when the vent lines rust shut.

ULTM8Z

Interesting... I hadn't considered the venting.

It's a 1971 Camaro. Again, I come to this forum given I have some similarities to C4 TPI fuel and ignition setups...

I have a Tanks Inc fuel injection tank. However, since my basic EFI system is off a third gen Camaro (minus the different intake manifold), I adapted the third gen charcoal canister system onto my car. The third gen Camaro's make use of a one-way vent valve for allowing fresh air into the tank, with a vacuum line leading back to the charcoal canister. Gas cap is not vented. I've had this setup for the last ~15 years or so.

Hmm... so if the canister somehow isn't drawing a vacuum I could see the tank building up pressure. I'll have to check the canister solenoid operation when I get a chance. I looked in my ECM calibration and everything matches the factory settings (just in case I accidentally changed something w/o realizing it).

If the vent valve was plugged, then I'd expect the canister to be pulling a vacuum in the tank.

It was still worth it drop the tank though... I was able to fix the quirky behavior of the fuel gauge. So I got that years long annoyance ironed out finally. Plus my old pump was older than the tank, so I'm glad that's freshened up.

ULTM8Z

Dude, at the risk of jynxing myself here.... you're a genius.

I was poking around at the venting system and was noticing the vacuum line going from the manifold to the CCP canister was awfully soft. It runs from the passenger side of the throttle body, under the Miniram plenum and then over to the driver side and out to the canister.

I thought, what if this line is collapsing under vacuum? The engine wouldn't be able to pull through the gas fumes line and you'd get pressure build up in the tank. The problem never occurred when the engine was cold... the car needed to get warmed up first. And as the rubber heats up its gets softer...

So I grabbed some stiffer vacuum line and swapped over to it.... and I'll be damned....

After driving it around for a while, the pressure stayed rock solid at the normal 40 psi. I couldn't get it to repeat the problem. I'm just hoping its permanent now (not like the false positive when I swapped the regulator earlier). For now anyway, need to hunker down for Hurricane Hillary. Probably drive it some more next week and verify its fixed once and for all.

Vets-Vet

""Dude, at the risk of jynxing myself here.... you're a genius."" OH NO, Thanks for the compliment but I am not one of the sharper tools in the corvette forum shed, I am the guy that has a masters degree in the school of hard knocks with the cracked skull award .....

AZSP33D

No, it would raise the fuel rail pressure just as I have read you describe. Troubleshooting would entail a pressure gauge on the return line near the regulator, it should be less than 10 psi an more ideally less than 5 even.

Also if you have too much fuel pump, the orifice inside the OEM regulator would be insufficient.

ULTM8Z

It's fixed for sure. I've had the car out five times now and the problem has not repeated.

Crazy... collapsing vacuum line... lol...