I wondered why my gas mileage was horrible... well come to find out it IS stuck in open loop mode! HEre's some details:

1993 Z28

Replaced the 02s a year ago with Bosch's, but a few months after that I put in the hotcam kit, now for a little while afterwards my car was burning off some coolant ->(Had the heads done w/ hotcam kit). But the point I'm trying to make is that, Maybe I ruined the new 02s with the coolant that passed through the exhaust?

Also I think theres 3 keys that make the car go into "closed" loop, but can't remember them all, but I know it's Coolant Temp, 02 Temp 600 degrees(Single Wire) and possibly running time.

Anyhow After the car was running for 30 minutes with some excessive revving to speed up the 02 heating it still was in open loop! But the weird thing is that the 02s mV would not flucuate at all! They just stayed @ .454 - .460 the whole time, once in a while they might move a .001 or so, but nothing major when revved up to about 5000 rpm.!?.!?!

So anyone have any opinions? I'm thinking about changing the 02s again back to Delco, but wanted a few thoughts first...

Thanks!

 

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Come on....

 

Stupid question time what is closed loop and open loop is it when you start the car and the reves are higher for a while and then drop down to normal idle.

 

It's definately the o2's. Don't use anything but GM,s After market are never trustworthy since they make them to cover a wider range of use and may never have the tight operation tolerance requirements of OEM.

 

There's gotta be someone else out there with something else? I'm thinking it might be the 02's, but not completely sure yet.

Anyone else?

 

The three "gates" for closed loop are 1) coolant temp (120-140degF, variable), 2) O2 sensors active (as evidenced by the voltage dropping from 450mV to 00XmV as the engine/sensors warm up, and 3) timer, no more than 2-3 minutes.

To become "active", the O2 sensors have to reach approx 600degF. If you have LT's, and moved the O2 sensors, there's a good possibility that the single-wire sensors in the 93's might not heat up enough to RELIABLY keep it in closed loop, but for it to never enter closed loop, it would seem that there is a more significant problem.

Coolant is bad for O2 sensors. The "green stuff" containes silicates, and silicates destroy O2 sensors. That's one of the reasons they developed the "orange stuff" (Dex-Cool), to eliminate the silicates and phophorous.

For the sensors to stay steady at 450mV, they are either ice cold, or they are terminally deceased. 450mV is essentially the ECM seeing its own voltage. Check the wires, check the sensors, consider whether they are getting hot enough.

As far as the guy who want to know the difference between "open loop" and "closed loop".... it is related to a cold start condition, but has nothing to do with the idle speed. Normally the engine runs in "closed loop". The ECM calculates the amount of fuel required to attain the "target" A/F ratio (14.7:1) and the O2 sensors tell the ECM if it got the calculation right. If it didn't, the ECM adjusts the fuel calculations, first using short term (temporary) corrections, and then storing and using long term corrections, kknown as BLM's or Block Learn Multipliers. Closed Loop = the ECM is responding to feedback from the O2 sensors and using that feedback to keep the A/F ratio exactly where it needs to be for optimum balance of emissions and fuel mileage.

When the engine is first started, the O2 sensors are not operating (they have to heat up), so the ECM uses the stored fuel correction factors, and ignores the O2 sensors. That is "open loop" = no feedback control based on O2 sensors.

When you go to WOT, the ECM also ignores the O2 sensors, and uses the stored fuel corrections, but that mode of operation is called "power enrichment" (PE) mode.

 

Good post; Can you explain "power enrichment" mode to me?

 

then how do you know when it enters open loop or do you have to have it hooked up to a scan tool.

 

Sure... in many ways its like "open loop". The O2 sensors are ignored.

But there are some differences:

The ECM/PCM sets a new, richer "target" A/F ratio. Rather than the 14.7:1 stoichiometric A/F ratio it uses in closed loop, it calculates a "richer" mix for improved power. Unfortunately, the stock programming produces a typical target A/F ratio of 11.7:1, which is a little on the rich side.

The ECM/PCM may use the long term fuel corrections from the high load/high rpm "closed loop" Cell. If the long term fuel corrections were "adding" fuel to the standard calculations in closed loop, the PCM will continue to "add" fuel in PE mode, using the long term corrections from Cell 15. If the PCM was "cutting" fuel during closed loop operation, it will default to Cell 18, and lock the long term corrections at 128 (0%). This is why you can not alter A/F ratio in the long term by changing fuel pressure. If you decrease the pressure, the BLM"s will need to "add" fuel in closed loop, and they will continue to "add" fuel in PE mode. No change in fuel rate. (People claim this logic is not followed in the 93 ECM).

 

Thank You Injuneer, that's what I was looking for... where can I nominate you for a CZ28 award?!

Sometimes I wish I had 94+, 93s seem to be a real pain, especially since I'm getting into Tuning, I have to flash a chip everytime I want to change it

Ahwell, thanks everyone.

 

If you need to change your O2's, go ahead and upgrade to the heated 94-97 style. Get them from Jason Cromer (1-877-726-8295)....It sounds like your 02's are completely dead right now, if you replace them with O2 sensors for a 93, you will regret it later. They will not get hot enough, and they will constantly cycle you between open and closed loop. have fun

 

You need to have it on a scanner. When it is in open loop, it will only operate in Cells 16, 17 and 18, and the short terms will be locked at 128. When it goes into closed loop, it will operate in Cells 01-15 also, and the short terms will start to fluctuate. Some scanners have an indicator that will show "closed loop enabled".

If you have an A/F gauge, you can watch the mV's fall to "lean" as the sensors warm up, then start to jump all over the place when it goes into closed loop. Probably about the only thing those A/F gauges are good for .

 

Is there anywhere I can get more info on this. I'm not sure what exactly cells are, and "locked in 128" means?

 

Read the "Air/Fuel Management" section of my online ScanMaster writeup.

 

Thanks so much!