07 Z06 long term fuel trim issues

[blindsquirrel]

It doesn't have to calculate anything, it's just a lookup in the flow rate Delta & Offset tables to figure out which cell applies. It knows what the rail pressure is supposed to be (either fixed or no FPCM, or variable via FPCM+sensor), and it of course always knows what the MAP is. I wouldn't call comparing those two to get the Delta a calculation, it's just rail pressure minus MAP, something you can do in your head (even for a number-dyslexic like me).

In my ECM I have specified the fuel pressure to be 55 psi at all times.

But, does that work? Or does it still stay at the mechanically regulated 58 and all your desired pressure change is doing is getting the ECM to look up a different (wrong) flow rate/offset cell?

(I am NOT an expert, at anything - other than maybe regular diagnostics. And maybe carburetor tuning. And cats.)

[blindsquirrel]

But, does that work? Or does it still stay at the mechanically regulated 58

To bring that back around to the theme of this post, have you verified with a mechanical gauge that commanding 55 PSI instead of 58 actually changes the rail pressure to the new number?

[92rsz]

To throw another curveball into the scenario, I run a fuel pressure gauge in my car full time (c6 corvette). It is almost never 58 psi. Cold start it is 61-62 psi. Hot soaked it gets down to about 59. Getting into the throttle hard it will touch 58 psi. Wot above 6500 it drops down to 55-56 psi. The pcm is wanting a static 58 psi at all times but that's not reality

[blindsquirrel]

Well the service manuals specify an acceptable range of "55 - 62 PSI", for the non-variable plain returnless models. Those have no way to get a real-time pressure reading so they assume "58 PSI" at all times for doing the lookups. Even though 55-62 is a range of 48kPa it doesn't seem to cause any unpredictable weirdness with injector data.

[obthomas]

To bring that back around to the theme of this post, have you verified with a mechanical gauge that commanding 55 PSI instead of 58 actually changes the rail pressure to the new number?

The reason I specified 55 psi is because my mechanical gauge on my injector rail is a solid 55 psi from idle to 5000 rpm. I have a single fuel line from the fuel pump that has a built in pressure reguator.

[blindsquirrel]

How are you seeing the rail-mounted gauge at 5K, camera? Just want to make sure you aren't mistaking fuel demand at 5K no-load with fuel demand at WOT 5K in gear moving the car/truck.

[blindsquirrel]

I'd also want to verify the accuracy of a rail-mounted gauge (I call things like that 'car jewelry'), checked against a real proper diagnostic style pressure gauge. Also make sure it's not a liquid filled gauge. Those have no place in an underhood environment with the wild temperature changes.

[Alvin]

To throw another curveball into the scenario, I run a fuel pressure gauge in my car full time (c6 corvette). It is almost never 58 psi. Cold start it is 61-62 psi. Hot soaked it gets down to about 59. Getting into the throttle hard it will touch 58 psi. Wot above 6500 it drops down to 55-56 psi. The pcm is wanting a static 58 psi at all times but that's not reality

This is completely normal.

[92rsz]

I know it's normal I guess I should've clarified my point that it doesn't matter if you change the value to 55 psi because there's a range that the vehicle will go through when driving

[obthomas]

I know it's normal I guess I should've clarified my point that it doesn't matter if you change the value to 55 psi because there's a range that the vehicle will go through when driving

I have no argument with the information being offered up in this discussion. Just adding to the data points my thinking is no matter the variations in fuel pressure if your car ECU has no fuel pressure sensor it has no way of following the fuel injector pressure curve except for a few offset tables most of which in my case are quite small or zero. BlindSquirrel introduced MAP as a pressure adjustment and has a point. Injector flow varies directly with the pressure from the inlet of the injector to the pressure at the outlet. In my case 55 psi to 14.7 atmospheric which is 0 psi gauge. MAP is likely used by the computer to adjust the outlet pressure affecting the look-up only slightly and more of an offset that anything else.

I do not see much to the fuel pressure argument. If the computer does not have a fuel pressure sensor my sense is that it will use what you specify and move along the pressure table using MAP and temperture or other things the computer can measure. If yu don't have the pressure you specify I can contemplate tuning issues.

My fuel system has a german design fuel pump with good capacity, and delivers a pressure regulated 55 psi into a good size 3/8 inch teflon lined fuel line to good size rails. The point is there is very little friction loss (pressure loss) to the injectors for the flow my engine needs at any speed. The gauge I am using is an oil filled gage with the span about double the pressure being measured. The graduations are small but I am mostly interested in changes in pressure and there are none detected.

In summary, getting the injectors right is very important and always keep fuel pressure in mind. I think the ECM establishes a base flow pressure and without a sensor from then on pressure adjustments are minor and based on offsets. As always, I could be wrong.

[blindsquirrel]

A pressure gauge is the same as a differential gauge, just that it only has one port. It's comparing one pressure against another pressure and displaying the difference, hence, a differential gauge but one that can only use normal atmo as the second point of reference, and that's taken from inside the gauge case.

A liquid filled gauge is not good in a situation where its ambient temp spans a wide range. Liquid filled gauges have the reference chamber (the gauge case) sealed, otherwise the oil/silicone would leak out. When the gauge gets hot pressure inside the case increases, and the inside of the case is one of the two points being measured for comparison against each other. When the gauge cools, pressure inside the case drops.

A liquid filled gauge located in the engine compartment is not accurate.

[obthomas]

A pressure gauge is the same as a differential gauge, just that it only has one port. It's comparing one pressure against another pressure and displaying the difference, hence, a differential gauge but one that can only use normal atmo as the second point of reference, and that's taken from inside the gauge case.

A liquid filled gauge is not good in a situation where its ambient temp spans a wide range. Liquid filled gauges have the reference chamber (the gauge case) sealed, otherwise the oil/silicone would leak out. When the gauge gets hot pressure inside the case increases, and the inside of the case is one of the two points being measured for comparison against each other. When the gauge cools, pressure inside the case drops.

A liquid filled gauge located in the engine compartment is not accurate.

I use a liquid filled gauge because of its ability to dampen fluctuations which can be dramatic with pulsing discharges. I think my gauge is temperature compensated. If not, it should be for the price I paid.

[blindsquirrel]

True high quality diagnostic fuel pressure gauges are not liquid filled. 'Fancy' does not always mean 'better'.

[Guy With A Chevy]

I did do that didn't I. Sounded like another lecture maybe is why I got them confused?!

I had an understanding already that fuel pressure sensors are not found on most gen 4's so I knew the scanner fuel pressure data was theoretical. I just wanted it to be clear for obthomas that after mechanically checking you could always mount analog rail sensor.

Is the Map the only way a gen 4 senses barometric changes? I understand how the maf senses temp and humidity. The sensor on my 4th gen only has 3 wires and from my understanding is a linear voltage signal for just one sensor value. Everyone speaks of the computer knowing the ambient pressure as well as the manifold absolute, how so?

Left my computer on night before last just refreshed the pages yesterday. Got busy with life had no clue my response actually posted on the second page and went on...

[edcmat-l1]

I did do that didn't I. Sounded like another lecture maybe is why I got them confused?!

I had an understanding already that fuel pressure sensors are not found on most gen 4's so I knew the scanner fuel pressure data was theoretical. I just wanted it to be clear for obthomas that after mechanically checking you could always mount analog rail sensor.

Is the Map the only way a gen 4 senses barometric changes? I understand how the maf senses temp and humidity. The sensor on my 4th gen only has 3 wires and from my understanding is a linear voltage signal for just one sensor value. Everyone speaks of the computer knowing the ambient pressure as well as the manifold absolute, how so?

Left my computer on night before last just refreshed the pages yesterday. Got busy with life had no clue my response actually posted on the second page and went on...

GM since way back when has used key on engine off map reading as baro check. Very simple strategy. No need for an additional sensor. At least up until the LSA/LS9 days.

[Guy With A Chevy]

Thanks I was assuming that, even typed it in my last response but didn't want to continue being extra. Sig out.

[blindsquirrel]

BARO also updates at WOT on models that don't have a discrete BARO sensor.

[jtrakel]

OP here, just hooked up a fuel pressure gauge between fuel line from fire wall to fuel rail. At idle it sits around 56-57 PSI. A small blip of throttle will get it to drop a PSI at most for a moment.

If I really rev the motor it went to 42-38-36 PSI then picks back up, each of those numbers correlated to a throttle hit. If I sustain about 4k RMP it will drop but not far off idle PSI around 56-57PSI.

Working on posting a quick video

[blindsquirrel]

That is no good, straight up fail.

[jtrakel]

Could you expand on the failure part. Are we talking fuel pressure regulator, pump or another mechanical portion? I still need to figure out how to check the electrical draw on the fuel pump or if an electrical issue could be causing a mechanical failure, any ideas?