Ethanol percentage flex fuel

[Fless]

That fuel pressure spec is for the Flex Fuel trucks??? I have always read that the FF trucks run 43.5 psi.

I know the FF psi spec for the NBS is a few psi lower than non-FF, but the FF pumps have more volume to feed the larger injectors. The specs are with KOEO (Key On, Engine Off).

Obviously there are further tests to determine if there is too much leak down, perhaps a leaky injector or the fuel pump leaks down when off.

 

[wsteele]

You say you want a pressure gauge?

Like always, lots of gauges found, not one fuel pressure gauge.



I am off to Napa to see if they have a rental.

 

[wsteele]

So I am thinking it probably isn't the fuel pump.

Key on engine not started.




Key on engine idling.

 

[swathdiver]

So I am thinking it probably isn't the fuel pump.

Key on engine not started.


View attachment 280365

Key on engine idling.

View attachment 280366

No take it out and womp on it and see what the pressure is.

As for the pressure, I don't claim to know everything, but I do know that the Gen IV 5.3s and 6.2s have a running pressure of 43.5 psi all the time. So being that yours is well over that idling, I would be suspicious that something might be wrong but would verify such with the shop manual specs first.

 

[wsteele]

No take it out and womp on it and see what the pressure is.

As for the pressure, I don't claim to know everything, but I do know that the Gen IV 5.3s and 6.2s have a running pressure of 43.5 psi all the time. So being that yours is well over that idling, I would be suspicious that something might be wrong but would verify such with the shop manual specs first.

I couldn’t find any reference to running pressures or testing pressures under load. Actually, the only reference I found was the key on engine off test, which lists 50-60 PSI as nominal, with leak down limits. Mine didn’t seem to drop more than about a 1 PSI in 5 minutes.

None of the above means it isn’t there, just that I couldn’t find it. Frankly the manual seems purposely written for someone to just follow rote without any judgement.

I did rev it up a few times but had to watch the gauge through the window. The pressure didn’t drop more than a few PSI at like 4K. But no load tests.

The hose wasn’t long enough to snake it up trough the hood opening, and the gauge was a free loaner with a $200 deposit, so no road tests were conducted.

 

[iamdub]

I couldn’t find any reference to running pressures or testing pressures under load. Actually, the only reference I found was the key on engine off test, which lists 50-60 PSI as nominal, with leak down limits. Mine didn’t seem to drop more than about a 1 PSI in 5 minutes.

None of the above means it isn’t there, just that I couldn’t find it. Frankly the manual seems purposely written for someone to just follow rote without any judgement.

I did rev it up a few times but had to watch the gauge through the window. The pressure didn’t drop more than a few PSI at like 4K. But no load tests.

The hose wasn’t long enough to snake it up trough the hood opening, and the gauge was a free loaner with a $200 deposit, so no road tests were conducted.

You can't verify the accuracy of the gauge, either.

 

[kbuskill]

I couldn’t find any reference to running pressures or testing pressures under load. Actually, the only reference I found was the key on engine off test, which lists 50-60 PSI as nominal, with leak down limits. Mine didn’t seem to drop more than about a 1 PSI in 5 minutes.

None of the above means it isn’t there, just that I couldn’t find it. Frankly the manual seems purposely written for someone to just follow rote without any judgement.

I did rev it up a few times but had to watch the gauge through the window. The pressure didn’t drop more than a few PSI at like 4K. But no load tests.

The hose wasn’t long enough to snake it up trough the hood opening, and the gauge was a free loaner with a $200 deposit, so no road tests were conducted.

Here are the pages from the factory service manual that I found to be relevant...



I hooked up the Tech2 tonight and I am more convinced that it is the fuel pump, at least in my case.

Following the service manual it states that KOEO should be between 50-60 psi and should remain steady for 5 mins. *Mine does not remain steady for 5 mins.

The service manual then states to check to see if it loses 5 psi in 1 minute. If it does, it says it is a leaking injector. If not replace fuel pump. *Mine does not lose 5 psi in 1 minute.

The manual then says to relieve the pressure down to 10 psi and verify that it doesn't lose more than 2 psi in 5 mins. *I did this and mine lost all 10 psi in 5 mins. It lost the first 2 psi within the first 1 1/2 minutes.

The only caveat I have in my testing is that I didn't verify these pressures with a physical pressure gauge hooked to the fuel rail. I just watched the pressures through the Tech2.

I know this may not be 100% accurate but I believe it is close enough for me.

As an FYI for @wsteele since you can't see your trucks FPCM with your Tech2.

The truck commands the desired fuel pressure to 58.5 psi with the key on and engine off.

The truck commands the desired fuel pressure to 43.5 psi when the truck is running. This includes while the truck is being driven, even under hard acceleration.

I watched mine today while driving and the fuel pump seems to be operating at the proper pressures while driving, only dropping a pound or two of pressure when first going WOT. The commanded fuel pump percentage of course varies with demand and during normal cruising my pump was sitting around 43% and upon hard acceleration would go up to around 70ish %. I don't know if that is considered normal or not since I don't have a baseline to compare it to.

Also my fuel pump short term trim is sitting at 1.00 and the fuel pump long term trim is sitting at 1.34... I don't have a clue what any of that means or how it would compare to a new pump but thought I would post the info in case someone else can interpret any of it.



Also @wsteele your engine idling PSI seems high as it should be 43.5 psi.

 

[wsteele]

You can't verify the accuracy of the gauge, either.

Right, but to assume the gauge is way off when it is displaying what the shop manual says is a nominal value for an engine with no faults, all the while the engine exhibits no problems that are listed as things that get you to check the fuel pump in the first place? Of course it might not be accurate, but Occam would tend to favor the opposite.

 

[iamdub]

Right, but to assume the gauge is way off when it is displaying what the shop manual says is a nominal value for an engine with no faults, all the while the engine exhibits no problems that are listed as things that get you to check the fuel pump in the first place? Of course it might not be accurate, but Occam would tend to favor the opposite.

I thought your engine was supposed to be running at 43.5psi. I can't keep track anymore. lol

 

[wsteele]

I thought your engine was supposed to be running at 43.5psi. I can't keep track anymore. lol

If you can’t keep track, I have no chance.

Everyone tells me my engine should be running at 43.5psi. I can’t find that in my shop manual (doesn’t mean it isn’t there), in fact, I can’t find it anywhere in GM documentation.

For whatever reason, the Tech 2 on my model year can’t see the Fuel Pump Control Module, so I can’t see values that module is determining is the desired pressure based on rpm, load, etc. , I can’t see even actual pressures at the rail with my Tech 2. For all I know, there might not be an intelligent fuel pump drive module in my model year and my pump might just be a single speed with a basic pressure relief valve on the return, if there even is a return, if not maybe just a relief valve at the pump, I have no idea.


All I know right now is the truck runs great and can’t figure out what percentage of Ethanol it has in the tank.