open loop, closed loop

[danthurs]

Working on a tune on a 98 grand prix. When the car is not moving, park, neutral or drive the O2 sensor will read about 100 or less and just hold there. As soon as the car starts to move the O2 readings will move up and down in closed loop like they should. Stop the car at a stop sign or light and the O2s will drop back down to 100 or less, start moving and O2s are back to normal. I can find nothing wrong with the tune, other then perhaps doing a real good VE tune. Was tuning another 98 GP, both tunes are set up the same and that one the O2s cycle normal. Am I missing something in the tune, or is there something else going on here. Ideas please.

[passingpower]

What are the idle LTFTs? You don't mention it so i'm guessing not but, does it stumble or miss at all. even going into or out of idle? If fine, start by looking for exhaust leaks.

[danthurs]

Here's a snap shot of the scan. You can see the LTFT is very high, +16 at idle, then as soon at the TPS moves above zero the O2 starts to move and the LTFT starts to move down. When cruising the LTFTs are around -9 to -4, need a little work, but that's a different issue. It's the O2 at idle that have me stumped.

[passingpower]

I've seen this twice before. one was an exhaust leak, the other was a bent exhaust valve. Seafoam makes a good smoke test for exhaust leaks, If that's not it, have a compression test done.

Something else. Monitor for misfires. If you have lots, that's a bent valve.
Do a cylinder balance check. If one chamber is more than 10% out of whack, bent valve.

One more thing. Cross ur fingers.

[TGP-37]

I've seen this twice before. one was an exhaust leak, the other was a bent exhaust valve. Seafoam makes a good smoke test for exhaust leaks, If that's not it, have a compression test done.

Something else. Monitor for misfires. If you have lots, that's a bent valve.
Do a cylinder balance check. If one chamber is more than 10% out of whack, bent valve.

One more thing. Cross ur fingers.

Low Fuel Pressures can do this too.

Failing fuel pump or clogged fuel filter. At idle the fuel pressure drops (FPR) due to vacuum. Part throttle increases fuel pressure. So the Fuel Pressure may be on the edge (low) and crosses into lower pressures at idle. The PCM will command more fuel but will not get it. So the LTFT keep rising.

Check the Fuel Pump is getting proper voltage, make sure Fuel Filter is clean/replaced, check Fuel pressures at idle and at key/on engine off.

[danthurs]

FPR is fine, there's a pressure gauge on the fuel rails. The only odd thing we found is the voltage drops. The seal on the supercharger snout leaked oil and it sprayed all over the alternator. So he's going to try swapping in a different alt and see if that helps.

[passingpower]

Are you monitoring rail pressure while moving? What circuit are you tapping while you monitor voltage drops? Whole system or just the fuel pump circuit? Do those same drops occur at the same time the O2s bottom out? Is the pump "rewired"? Is there a volt booster inline with the alt sense circuit?

Edit: Looking back at the scan I see what you mean by the system drops. 14v-15v and suddenly 12.5v-13v. While that looks hinky, and you need to get to the bottom of it, It doesn't look like there's a direct relationship: The O2s are FUBAR at 12.5v and 14.5v. The only time the O2s behave is when the airflow - and exhaust - is off idle. (bet you a dollar, when ur done, you'll find messed up field brushes in the alt. BTW)

[danthurs]

The pressure gauge is under the hood, but I convinced him to mount a gauge in the cowl on the hood. He just put a new fuel pump in because the last one wasn't holding pressure. I believe he's still stock wire but he was talking about doing a rewire. Voltage is the whole system, not just the fuel pump. The O2s seem to only bottom out when the TPS is at zero. Like when sitting still or stopping at a stop sign.

[passingpower]

try a cyl balance test. its free.

In VCM Controls>Special

1. Set the idle speed using the IAC Control

2. Perform a Cylinder Balance: The cylinder balance test calculates % of efficiency using current RPM. RPM is gauged with all injectors running normally. Injectors are then cut 1 cylinder at a time and the new RPM is compared to the pre-test RPM to determine % of efficiency. To use this test the main scanner must be stopped. You should also control the IAC steps as well for consistency if the vehicle supports it. (VCM help)

In other words, as the scanner kills the fuel in each cylinder, the power as indicated by idle speed (and NOT IAC compensated)should drop by 1/6. If idle does not drop when a particular cyl is disabled, its dead to begin with. Anything can cause that from a bad ignition module to leaking rings. But hey, at least you've isolated the fault to one cylinder. A dead cyl will allow unused oxygen into the exhaust stream and that will cause a false lean indication.

[danthurs]

I'll pass that on to him. The engine runs great and has a ton of power. It's only at idle that it's acting funny. He has a wide band O2, and it goes so lean it errors out.