Open Loop with no O2 Sensor = Maybe It's Not Pinging Afterall

[WallyP]

Neither the Hall sensor or the knock sensors are ignored in open loop operation.

"Open loop" operation means ONLY that the oxygen senor input is not present or is ignored by the LH ECU, and the fueling is determined by the programmed maps and all of the other normal sensors.

If your WOT switch is functioning, you are ALWAYS IN OPEN LOOP at WOT.

If you have pinging or detonation at WOT on an S4, GT or GTS, you have a problem that the knock sensors can't control. Some possibilities include:
- Poor quality fuel.
- High carbon build-up in the cylinders.
- Heavy oil ingestion.
- Wrong heat-range plugs.
- A "hot spot" in the cylinder - carbon, thin plug electrode, localized boiling of the coolant, etc.

In a street engine, it is possible, but unlikely, that a lean mixture will cause pinging or detonation. Modern engines are designed to run at stoichiometric mixture, which is the hottest possible burn.

[mark kibort]

YEP, and our engines have a capacity to produce a lot more HP then the do. if you are only at 270rwhp, that is roughly 70% of what a high performance version of our engine could make. AND, if that engine (385rwhp version) was on the freeway, running part throttle, it could run without pinging and at max fuel economy and low EGTs and CHTs at near stoich.

I agree that the problem is probably a hot spot, carbon, or something else. the power loss could be fuel pressure or fuel flow (i.e. mixture) as when you go lean, the power drops off a lot passed 14.7 (stoich). also, voltage can drop power quite a bit at the rpm, but usually that is when an alternator is going bad and the battery is ready to die. a fully charged batter will run the car at full power for a while until it dies. (roughly a 10amp draw to drive the engine circuits)

Originally Posted by WallyP

Neither the Hall sensor or the knock sensors are ignored in open loop operation.

"Open loop" operation means ONLY that the oxygen senor input is not present or is ignored by the LH ECU, and the fueling is determined by the programmed maps and all of the other normal sensors.

If your WOT switch is functioning, you are ALWAYS IN OPEN LOOP at WOT.

If you have pinging or detonation at WOT on an S4, GT or GTS, you have a problem that the knock sensors can't control. Some possibilities include:
- Poor quality fuel.
- High carbon build-up in the cylinders.
- Heavy oil ingestion.
- Wrong heat-range plugs.
- A "hot spot" in the cylinder - carbon, thin plug electrode, localized boiling of the coolant, etc.

In a street engine, it is possible, but unlikely, that a lean mixture will cause pinging or detonation. Modern engines are designed to run at stoichiometric mixture, which is the hottest possible burn.

[Mongo]

I did a video recently on a prior thread I reviewed and did not have the issue at the time the video was shot during that WOT run. That was before I put gas in it. I should probably drive it again and do some more wot runs to see what the car does again.

[jcorenman]

Andy, you either need to do some more serious diagnostics or take the shotgun approach and start changing things. You don't know if the problem is too-lean at WOT, that's just one theory. If it is too lean, then fuel pressure is a likely cause. But it could also be a MAF issue. Or a load of bad gas.

I don't see how it could be related to the missing NBO2 sensor and running open-loop. However, strange things do happen with complex systems. On the other hand, coincidences do occur.

I think an important next step is to get a new O2 sensor back in there. It's an obvious departure from stock, and you keep saying that the problem started when the O2 sensor got disconnected. So get a new one, and connect it. (Also, if you have cats, they are hating life without the O2 sensor).

If it's been 35K miles since you replaced the fuel-filter then I would do that also.

As for the possibility of bad gas, drain it and use it for the lawnmower. It is simple to do that while you are changing the filter.

Mark K's advice to check fuel pressure at WOT is good but I would not be comfortable doing it on jackstands as he describes-- that much energy trying to go somewhere else would scare the piss out of me. But maybe I'm just a coward

Once you get the O2 sensor back online then connecting a voltmeter is a good idea. But remember that a narrow-band O2 sensor is basically a binary device-- more than 0.7 volts (approx) for richer-than-stoich (AFR <14.7), and less than 0.15 volts (approx) for leaner-than-stoich. There is a relationship between voltage and AFR above and below stoich (AFR's between 14 and 12 map roughly into a voltage range of 0.75-0.90 volts), but it is highly temperature-sensitive (exhaust-gas temp), which can vary widely with load. It's a crude check at best, but if you see the O2-sensor voltage dropping below 0.6 volts at WOT then you are leaner-than-stoich which is definitely a problem.

Here's more info on O2 sensors than you ever wanted to know... scroll down and find the graph:
http://www.carlton24v.co.uk/datasheets/o2sensors.pdf

Taking diagnostics to the next level would be a WBO2 sensor (in addition to the NBO2, or instead of). A LC-1 with a laptop for readout works fine and is around $200, or you can get a separate readout for another $100 or so. Beyond that, if you know someone with an ST2 that you could borrow then you can fit PEM's to your boxes for another $200. Once that is done then it's a simple plug-in to the car's diagnostic connector to find out anything you could want to know. It's not cheap, but parts aren't cheap either. And if you can save replacing some parts that don't need replacing then maybe it is cheap.

[Mongo]

Thanks for the advice Jim. I'm going to do some more multimetering this weekend and start all over from Box A again just to make sure the sensors, WOT and idle switches are accurate and functioning. Next, I'm going to put the O2 sensor in drive it to see what happens. When adaption of the LH is complete and the problem still persists at high RPM, I will drain the gas and change the fuel filter.

I don't have much patience left for this thing and have decided to just keep it sitting around the house undercover lately hoping a mouse doesn't move into it.

[Mongo]

Don't knock me for saying this but I'm thinking of going back to Bosch Platinum +4s to solve a possible hot spot issue too if that is the case. I ran those plugs from 2001-2005 with nothing but happiness...


Flame Away

[Mongo]

I drove in today to work and I had no pinging issues. I hammered the gas at 55mph and went WOT to 80 without one hiccup or hesitation. The exhaust note at high RPMs just sounds like the car is running richer. Either Saturday was a fluke and the knock sensors picked up a hot spot, or in first gear at WOT what caused the car to shudder was the fact that my tires weren't biting on smooth pavement..

[86'928S MeteorGrey]

Here is a possible stupid question. Could an AFR problem be caused by an out of adjustment throttle body cable?

[dprantl]

Originally Posted by 86'928S MeteorGrey

Here is a possible stupid question. Could an AFR problem be caused by an out of adjustment throttle body cable?

The MAF measures the air coming in and the brain decides how much fuel to put in. The throttle is not electric so the brain only knows if it's idling, at cruise or WOT. Now, if you are not properly hitting the IDLE and/or WOT switches, that will definitely affect AFR at idle/WOT.

Dan
'91 928GT S/C 475hp/460lb.ft

[Mongo]

I had an overtight TB cable that wreaked havoc at idle when I first got the car. It was almost as if when I let off the gas the TB slammed shut hard and caused running issues. I reduced the tension on this cable and it went away. If the cable has too much slack, the idle switch cannot be activated and the car may stall out. On a side note, the cable mentioned is the cable going directly from the linkage to the throttlebody on S4 & Later cars, NOT the cable to the accelerator pedal.

Seems like cable slack is a science on these cars too.

[Mongo]

Well after some more WOT runs last night, the car feels like it has power (A LOT more power on open loop), but it's not dropping off or falling flat in high RPM. The exhaust note is completely different on the car though. It has this snarl that starts just before 5,000 RPM and continues to redline.

Is it normal for an exhaust note to change if the engine runs richer with this default fuel map?

[Xlot]

Originally Posted by Mongo

Well after some more WOT runs last night, the car feels like it has power (A LOT more power on open loop), but it's not dropping off or falling flat in high RPM. The exhaust note is completely different on the car though. It has this snarl that starts just before 5,000 RPM and continues to redline.

Is it normal for an exhaust note to change if the engine runs richer with this default fuel map?

Under WOT, 928's run open loop regardless of whether an O2 sensor is present or not.

[Mongo]

I was under the assumption that there are 3 fuel maps stored in the LH: Closed Loop for idle & cruise, WOT Open Loop & the Default Open Loop map in the absence of an oxygen sensor.

[dprantl]

Originally Posted by Mongo

I was under the assumption that there are 3 fuel maps stored in the LH: Closed Loop for idle & cruise, WOT Open Loop & the Default Open Loop map in the absence of an oxygen sensor.

Almost... there are a bunch of maps, but for this discussion there are two main maps, one for operation with an O2 sensor and one for cars without. Then there are two WOT maps; one for O2 and one for cars without. But... the WOT maps are not whole maps, they are just additive values that add to the main map at WOT. If you disconnect the O2 sensor, the LH will NOT switch to the non-O2 maps. What the brain will do is use a "middle" value in the O2's absence (it also triggers the check engine light on later model year 928s). The only way to use the other maps is to change the coding plug or to use a sharktuner to override the coding plug's value.

Dan
'91 928GT S/C 475hp/460lb.ft