[Rydog2SS]

I have LT-4 HPFP and LT-4 Injectors, low side LT-4 and Aux low side pump.

Was at the track a couple weekends ago with the tuner, made a couple of run and a retune, then made a run, didn't get a scan, i am guessing this is when the issue took place, and the last time he flashed in a new tune, and when I went to start the car, it kept turning over but not starting and had low rail pressure, like under 300psi and then finally fired up after about 5 seconds or so of turning over. (note that I in no way think the tune or tuner did something wrong or messed it up, I am guessing whatever happened, it did it on that run that I didn't get a scan on)

After it fired up, the HPFP delivery angle at idle and at WOT is 130*. So, at WOT the injectors were running way high like 10ms and rail pressure is like 900-1200psi at WOT, and like 500-600psi at idle when the duration is at 130.

when I am doing normal acceleration, the duration goes down and rail pressure is fine, at times going back up into the 3000s and the injectors flow correctly.

Looking for input, is the pump bad or a sensor or something else that could be causing that. I don't know what that means exactly, but I went through several logs from the last year of scanning and never saw it run above 117 of delivery angle at WOT and never high at idle...

low side pressure has stayed good throughout the whole time.

Any input would be appreciated, sick week is only a couple weeks away!

Thanks

[bishopts]

Did it do it just that one time after the tune was flashed?

[Rydog2SS]

Originally Posted by bishopts

Did it do it just that one time after the tune was flashed?

Sorry I should have noted that, it has done this every time I started it up since that time, which was about a week and a half ago. I have run down the track 3 times, with different tunes, I even put in an old tune from about a year ago and it still does this. The only reason I kept running it, is because we thought it was a tune issue, which is why I went back and did a full write and unplugged the battery for a while. We also didn't notice the 130* delivery angle duration until after that...

[bishopts]

Have you checked pressure bleed down on key off? Maybe the valve in the high pressure pump is stuck slightly open and it's returning all the time. But you might see that in abnormally high low pressure numbers since it vents back to that.

[Megahurtz]

The angle shouldn't be 130 degrees at low RPM. That means the pump is using maximum effort to maintain pressure.

What cam lobe are you using? Is there a lash cap? Was preload checked when installing the LT4 HPFP? Do you have the stock check valve installed on the low side line still? I've seen where the check valve fails a certain way and starves the HPFP, but I have only seen it at high RPM...not at idle....but perhaps its possible?

130 degrees at WOT means the car is out of fuel, but it shouldn't need 130 degrees at idle to maintain pressure.

I would remove the HPFP and inspect for damage and see if you can borrow another LT4 pump to test with.

[Rydog2SS]

Originally Posted by bishopts

Have you checked pressure bleed down on key off? Maybe the valve in the high pressure pump is stuck slightly open and it's returning all the time. But you might see that in abnormally high low pressure numbers since it vents back to that.

No but I will look into that.

[Rydog2SS]

Originally Posted by Megahurtz

The angle shouldn't be 130 degrees at low RPM. That means the pump is using maximum effort to maintain pressure.

What cam lobe are you using? Is there a lash cap? Was preload checked when installing the LT4 HPFP? Do you have the stock check valve installed on the low side line still? I've seen where the check valve fails a certain way and starves the HPFP, but I have only seen it at high RPM...not at idle....but perhaps its possible?

130 degrees at WOT means the car is out of fuel, but it shouldn't need 130 degrees at idle to maintain pressure.

I would remove the HPFP and inspect for damage and see if you can borrow another LT4 pump to test with.

it's a BTR STG 3 with a 32% lobe, no lash cap, should I have one? hasn't been one on there since 2021 but that doesn't mean it doesn't need one...

Preload is good, I pulled the pump last night and put it back together, everything on the pump looked good, I pulled the sensor/valve piece on the back and inspected it, it looks good.

It is the stock check valve, I am not sure exactly where it is located to be honest, so I will have to still investigate that and will see if it has failed possibly.

The struggling at idle is what I thought was so weird too and then fluctuates under minor loads and under moderate load, the duration will return to what i would expect is normal and the injectors and rail pressure look good, then as soon as i let off and it is headed back to idle or if I make it WOT or close to it, it goes back to 130*, rail pressure drops off and injectors start working harder. the injectors will be at 2 - 2.5ms at idle...

I do not have my scanning computer with me right now, when I get home, I will take some snapshots of the idle, moderate and WOT and post those.

yeah, the big thing is, I don't want to spend 1K on a HPFP and that not be it...

Thank you for the help!

[Megahurtz]

I seems like its starving for fuel. Bypass the factory check valve, which is in the hard line that comes from the fuel tank to the front of the car. You can build a replacement line with no check valve for much cheaper than replacing the HPFP.

[Rydog2SS]

Originally Posted by Megahurtz

I seems like its starving for fuel. Bypass the factory check valve, which is in the hard line that comes from the fuel tank to the front of the car. You can build a replacement line with no check valve for much cheaper than replacing the HPFP.

I am going to try that this weekend.

i attached the snap shots of the scans. not sure if there is anything helpful, but it's strange for sure.

[arpad_m]

To my amateur eyes your setup looks severely out of fuel supply capability.

High side fuel pressure < 1000 psi (instead of 2900 to 3100) and a crazy high 9.9 ms injector pulse width (instead of < 6 ms) even when the tune commands a lean 0.87 lambda at WOT is pretty bad. The low side should be above 70 psi, too, although 65.8 psi isn't that bad.

Also, you're running E44, which demands even more fuel than 93, and that 18 psi boost is way too much for a plain LT4 fueling setup with a +32% fuel lobe cam. Is the situation better if you run 93 octane?

That said, this still doesn't explain the high idle load you described... I'll wait for the pros to see what they think so I can learn, too

[bishopts]

Megahurtz has a good point about the low side check valve possibly being a choke point.
But... if it were, then you would see low side pressure being sucked down from the high pressure pump pulling from the line till it became a deficiency. It wouldn't stay even close to normal if it were losing enough volume to be 2000psi low.
And like i has said earlier. If the hpfp return valve is stuck open then you're going to see things like 200psi on the low side since it vents back into that line.

If the cam and follower aren't worn down then to me it seems like a high pressure pump.
But I've seen so many high pressure sensors fail on GM that i wouldn't rule that out. If you have the ability to check pressure with a gauge you would be able to determine that. If not maybe just throw one at it for good measure.

[Rydog2SS]

Originally Posted by arpad_m

To my amateur eyes your setup looks severely out of fuel supply capability.

High side fuel pressure < 1000 psi (instead of 2900 to 3100) and a crazy high 9.9 ms injector pulse width (instead of < 6 ms) even when the tune commands a lean 0.87 lambda at WOT is pretty bad. The low side should be above 70 psi, too, although 65.8 psi isn't that bad.

Also, you're running E44, which demands even more fuel than 93, and that 18 psi boost is way too much for a plain LT4 fueling setup with a +32% fuel lobe cam. Is the situation better if you run 93 octane?

That said, this still doesn't explain the high idle load you described... I'll wait for the pros to see what they think so I can learn, too

yeah, it wasn't like that before, like literally earlier in the same day prior to something happening, it is a mechanical issues that I am assuming is starving the HPFP during different times, everything is screwed up on the HPFP, Rail Pressure, and injector pulse. just trying to figure out what it is. The car usually runs about a .82ish Lambda and the only reason it stayed even close to the commanded amount is because of the methanol. I think if not for that it would have been close to .94 or .95... def not good.

just trying to avoid spending all kinds of money on another fuel pump and it be something else.

Going to try the low side check valve being stuck or failing in some sort of way.

[Rydog2SS]

Originally Posted by bishopts

Megahurtz has a good point about the low side check valve possibly being a choke point.
But... if it were, then you would see low side pressure being sucked down from the high pressure pump pulling from the line till it became a deficiency. It wouldn't stay even close to normal if it were losing enough volume to be 2000psi low.
And like i has said earlier. If the hpfp return valve is stuck open then you're going to see things like 200psi on the low side since it vents back into that line.

If the cam and follower aren't worn down then to me it seems like a high pressure pump.
But I've seen so many high pressure sensors fail on GM that i wouldn't rule that out. If you have the ability to check pressure with a gauge you would be able to determine that. If not maybe just throw one at it for good measure.

yeah, I have been debating throwing in my old LT1 high pressure fuel pump and see just want happens at idle to see if it goes back to max or if it stays low like would be expected at idle. may tell me right away if it's the pump or not. not sure if there is anything that would need to be adjusted in the pump setting or not just for a start up and idle.

[bishopts]

I would say this is a safe gamble.