ULTM8Z

So I'm driving to Donut Derelicts in Huntington Beach this morning...

On the freeway I look over at my fuel pressure gauge and it's reading at ~80 psi!! Thought the gauge was going haywire. I had my laptop doing some datalogging and see my BLM at 108, so I know it's real.

But after about 30 seconds or so, it drops back down to 40. Then after about 5-10 seconds, back up to 70-80. It's doing this oscillation continuously. If I let off the throttle, it goes back to 40 and stays there. When I get back on the throttle the oscillations start up again.

Once I get off the freeway and down to speeds below 30 mph, it stays at ~40 psi. If I start getting to ~40 mph, it starts going up to around 60 psi and oscillating again.

How can fuel pressure possibly be related to vehicle speed?

Before I go off and buy another regulator, wanted to get people's thoughts on this.

Fuel pump is not making any weird noises, no misfires, no other symptoms that I can see besides the bottomed out BLM's.

Vader

The only external influence on the FPR is the vacuum supply. Removing the vacuum hose from the FPR should only cause the rail pressure to raise about 7-10 PSI on an idling engine. Pump voltage can vary a bit depending on engine speed, but it should not create that kind of increase.

Another obscure possibility is that at road speed and/or under load, something is happening to the fuel return line. It may be getting kinked or pinched by something due to driveline movement or airflow under the vehicle.

ULTM8Z

So turned out the seal inside the TPIS AN adapter somehow dislodged and was partially plugging the output of the regulator. It's an adapter fitting that screws onto the output side of the regulator body, and allows an AN line to hook up to it.

Seems like that would explain the symptoms... build of pressure because the regulator couldn't return enough fuel to the tank.

Another Roadkill Classics moment...

I never liked TPIS's regulator setup and how that AN adapter sealed to regulator body.

I'm shocked I didn't have a fuel leak. Oh well, at least was an easy fix. Replaced the seal and back in business.

Vader

Bear in mind that even the army and navy gave up on the WWII-era "AN" fittings several decades ago. It seems that only the automotive aftermarket still clings to the less than ideal design.

ULTM8Z

So an update on this...

It started acting up again, so I swapped to my old TPIS adjustable regulator. Which seemed like it solved it. But it's acting up yet again.

I'm going to disconnect the return line and blow it out probably tomorrow.

But could this be an issue with the fuel pump?

Vader

Of course it's possible the pump is affecting it, but a functioning regulator should not be overwhelmed by a stock-ish pump.

ULTM8Z

Hmmm.... If the return line is clear, I'm not sure what else is left except the pump. There are only so many things that can cause an over pressure condition.

ULTM8Z

I'm thinking it has to be the pump. When I let off the gas, regulator is allowing more fuel to be diverted back to the tank. If the return line was blocked the pressure would be going up not down.

When the regulator closes under higher load the pressure goes up. The only way that would happen is if the pump was generating that higher pressure.

Vader

Keep in mind that any properly designed and operating pump always rides on its pressure/flow curve, and that there are only a few variables:

Flow (volume);
Head pressure;
Viscosity;
Pump speed.

Also understand that the pump in these systems should always produce more flow than is required by the injectors, otherwise it could not produce a head pressure. Thus, the regulator should always be passing some volume back to the tank depending on the spring pressure of the regulator, and the vacuum applied to the regulator canister only works to decrease that spring pressure. At no time should the pump produce more than the absolute spring pressure (without vacuum applied) unless there is some hydraulic restriction within the regulator or return lines.

This all presumes that the pump is running at a constant flow and pressure, as it is designed to do. The one variable that upsets this is pump speed. If the pump voltage is inconsistent, predicting the flow is about impossible. However, pump voltage would have to be considerably higher than its design voltage in order to exceed the return flow capacity of the regulator. My wild guess is that the pump would have to be operating at 18V or higher to potentially overwhelm the return line flow capacity.

I suspect you are well aware of all of this, but I'm listing it in the event someone with a similar issue sees this and not familiar with all of the factors involved.

I really thought you had it when you discovered the partly restricted return line. I wonder if there might be another restriction somewhere.

One potentially messy option might be to connect a completely separate return line directly to the fuel rail(s) and route it to a separate container, then simulate the load/speed conditions to monitor fuel pressure for variations.

ULTM8Z

Thanks for the explanation Vader... understood. Everything you're saying makes perfect sense.

But this is where I'm struggling trying to reason through this in my head, reconciling that explanation vs my observations.

As I'm driving along normally, the pressure starts to slowly climb to 70-ish psi.

When I let off the throttle, the pressure returns to normal. I would think the regulator has to divert more of the fuel to the tank since the injector PW is very low. So if I had a plugged return line, wouldn't I see a spike in pressure rather than a drop since the return line has to now carry more fuel?

Also at idle speeds, with very low injector PW, pressure is normal. Again, being much more dependent on return line flow at idle, I'd expect a pressure spike.

Key on, engine off, it's also normal pressure.

Then when I get heavy on the throttle, the pressure drops down to about 50-55. With the higher injector PW's, it's effectively relieving some of the built up pressure in the rails.

This all seems to point to an overpressure by the pump. System voltage on the car is right at 13.8-14.0. So I don't know how the fuel pump would be seeing a higher voltage than what the alternator is producing.

Does the pump itself have any kind of built in pressure regulator?

ULTM8Z

I just blew some compressed 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.

Probably just going to buy a new pump. This one is ~20 years old now anyway...

Vader

Practically anything that might make it through the pickup sock in the tank, then the turbine pump, and then through a fuel filter (even a Fram example) shouldn't be able to clog the return line.

Also, it would seen that a 20-ish year old pump might tend to fail by losing flow and/or pressure capacity, not increasing them. If that were the case, everyone switching to high-flow fuel systems to support 700+ HP would be looking for those "awesome" 1990's Buick Skylark V-6 pumps for their builds.

ULTM8Z

I've been reading that these electric pumps have built in pressure relief valves? I'm wondering if there could be something going on with that? BTW, it doesn't do it immediately... I have to drive it for a bit before it starts acting up. Maybe the pump getting warm has something to do with it too.

THe fuel system isn't that complex... there are only so many things it could be.

I always get the crazy unusual problems... lol...

Any rate, I ordered a new Walbro GSS340 pump. Any luck it'll get here on Wednesday/Thursday and I can do the swap on Friday.

My fuel gauge always acted a bit quirky after about 1/2 tank, but I never had the incentive to drop the tank to diagnose it (I've already looked at everything external to the tank). So this will give me an excuse to drop the tank and check it out.

ULTM8Z

Ok it's hair pulling time... lol...

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...

ULTM8Z

At the risk of jynxing myself (again), I think I fixed it...

At the suggestion of someone on Covette forum, I was poking around at the tank venting system... and something told me to start at the front of the car.

I was noticing the vacuum line going from the intake 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 guess over time the rubber just aged and got easier to collapse.

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.

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...

Vader

Thank you for the update. Too often we never see the ultimate result once something is resolved.

I've pondered the ramifications of a collapsed purge line and wonder if the tank pressure became excessive why the tank pressure control did not vent off the excess.

ULTM8Z

Yep, that's why I did it. I hate when I'm searching and come across an otherwise helpful thread that has no conclusion to the issue/question I'm searching on.

Vixen92Z28

I’ll have my mechanic try this fix. My 92 Z28 5.7 (my baby since he was 4) chokes likes he’s getting no fuel for the first five minutes we’re on the road then takes off like a new Corvette with no more problems. Until I turn it off and restart.

ULTM8Z

Fyi.... The exact line I'm talkng about is from the manifold to the solenoid on the charcoal canister.

erik69&85

iTrader: (1)

Interesting!!... I looked at this line on my 92z... And it's a hard plastic pipe until short hose sections are either end for connection to the canister and the throttle body...

seems maybe GM knew of this issue....