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Runs good in open loop but pumping idle and with same throttle in closed loop

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Runs good in open loop but pumping idle and with same throttle in closed loop

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Old 07-10-2018, 04:54 PM
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Runs good in open loop but pumping idle and with same throttle in closed loop

Hi,I got a Z28 1991 manual with a 5.0 TPI.In cold start and open loop it runs fine but after a 10-15 min drive when she goes into closed loop, then the idle starts going between 500-800 RPM and also when driven with a little throttle the same result happens but at a higher RPM of course, hard to drive too.
If you give her more gas you can feel it's not the same performance as ion open loop.
Since she runs good in open loop so I get a little confused what's causing this problem which happens only in closed loop.
I did a test today when this happend, I stopped the engine, pulled away the temp sensor connector for the ECM and started her up. No problems since she was now running in open loop.
I checked for Error codes but there aren't any.
Like 400 miles ago every ignition part was replaced incl. injectors.
Need help!!
Thanks!
/Magnus.

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Old 07-10-2018, 08:55 PM
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Re: Runs good in open loop but pumping idle and with same throttle in closed loop

Has the O2 sensor ever been changed?
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Old 07-11-2018, 03:49 AM
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Re: Runs good in open loop but pumping idle and with same throttle in closed loop

Hi Fred and thanks for the reply!

I don't know if or when the O2 sensor has been changed.

It's a good idea to change it since the car have run for app.107000 miles.

Maybe I should replace the ICV too.

The parts that was replaced at a shop was 8 injectors, distrubutor cap, rotor, ignition module, pickup, coil, sparkplugs, sparkplug wires and fuel filter.
The reason for the replacement of those parts was that the car consumed lots of gas (app. 5 mpg) and had a to high co- value at emmision test.
After the replacement the car passed the emmison test but it was a close call on the O2 value. Now the car consumes 'only' 13 mpg... -
However, the problems I'm having started this spring so during the test the car was fine......

I read that you can replace the O2 with a 4-wire one with a heater instead of the a single wire one that I have. What do you think??

Thanks,
Magnus
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Old 07-11-2018, 09:26 AM
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Re: Runs good in open loop but pumping idle and with same throttle in closed loop

All your problems - low MPG, high CO, open loop/closed loop transition, etc. - point to the O2 sensor.

The heated O2 sensor is a good idea. If the O2 sensor isn’t hot enough (~600įF) it doesn’t work. If the sensor cools off, the ECM will drop out of closed loop. You also mention 10 to 15 minutes to enter closed loop. While I'm not deep into the ECM used with the 3rd Gens, most of what was there was rolled over to the 4th Gens, particularly the 93.

In the 4th Gens it typically takes about 3-1/2 minutes to enter closed loop. The things that have to happen the make the transition are 1) O2 sensors active (indicating they are warm enough to work); 2) coolant has to reach ~140įF; 3) timer in ECM has to time out (~206 seconds). Obviously a heated O2 sensor insures that will happen.

When you think about it, the only difference between open loop and closed loop is the fact that in open loop the ECM ignores the O2 sensor, and fuels the engine based on the learned (stored in memory) BLM's (aka “long term fuel trims”). In closed loop, the ECM reacts to the O2 sensor feedback, adjusting the fuel using first on the INT's (short term fuel trims) and then raising or lowering the BLM's to make a more permanent (learned = stored in memory) adjustments. In open loop, bad O2 sensor data doesn't affect anything. In closed loop, the ECM has a much broader capability of altering the amount of fuel, and bad O2 sensor data can really screw things up.

Shoebox has a guide to adding the heated O2 sensors to a 1993. While the 93 has an O2 sensor for each bank of the engine, the wiring for your single sensor should be similar.

http://shbox.com/1/heatedsensor2.jpg
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Old 07-12-2018, 08:34 AM
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Re: Runs good in open loop but pumping idle and with same throttle in closed loop

Thanks for info!

Well, I charged the o2 sensor today. Found a AC-delco with single wire. Took it for a ride but the same problem came back when the car got warm. I was running on a highway doing 45 miles and the car start shaking a little and the engine lost some power. i pulled of the highway and it was hard to keep it running on idle. On idle the rpm was moviing between 500-1000 rpm and it was running bad on higher rpm.
Maybe I will change the temp-sensor as well.
Does the engine go to Open loop if you disconnect the temp-sensor?
I did that and the engine was behaving the same.
Iím thinking it could have something to do with engine geting up in temperature or maybe even geting to warm.
I measured the ohm with warm engine on the temp-sensor and translated it and the temp is app 217F.
i also measured the tempsender for the gauge and that shows app 215F.
What more can I do??
Running out of ideas.....
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Old 07-12-2018, 09:49 AM
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Re: Runs good in open loop but pumping idle and with same throttle in closed loop

You can also check the reference voltage at the ECM coolant temp sensor harness connector. Key on, connector off the sensor, should get 5 volts between the two pins in the connector.

Just to repeat, I'm working on the assumption that the 3rd Gen ECM is similar to the 4th Gen LT1. Wish we had more active 3rd Gen owners on the site, but it seems like most of people still here are 4th Gen LT1 owners, and a few 4th Gen LS1 people. I do have the Chilton's engine control wiring diagram for the 91 5.0 TPI, but Chilton's is not 100% reliable.

Unplugging the CLT sensor should set DTC 15 for low temperature (high/infinite resistance). Since a minimum coolant temp is required to put the ECM into closed loop, I don't see why it would stay in closed loop with an indicated -40įF coolant temp. The ECM also raises engine idle RPM based on low coolant temps. With manual trans, warmed-up idle is usually set at about 800 RPM. On cold start, idle can be elevated above 1,000 RPM in cold weather.

How fast does the idle surge occur? Can you relate that to the A/C cycling, for example? The idle air control valve should compensate for that.

In a speed-density based system, inlet air temp and manifold absolute pressure are critical inputs. The sensors are generally very reliable. But the harness connector, particularly the one for the MAP sensor, seems to dry out and crumble, causing intermittent readings. Worth checking the connector.

Do you have access to an OBD-1 scanner that shows real-time data for sensors and ECM parameters? 4th Gen owners are lucky to have scanning software that records all this data while driving. It's so much easier to diagnose problems when you can look at a spreadsheet and simultaneously watch what every sensor and parameter is doing. The first software, called DIACOM, was applicable to the 3rd Gen GM engines, but that was a DOS based program that, to my knowledge has never been updated.

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Old 07-12-2018, 12:11 PM
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Re: Runs good in open loop but pumping idle and with same throttle in closed loop

Thanks for the reply,

Well, I parked the car and took a swim sinne itís like 90 here. I prefer around 70-75!
Anyway, like one hour later I disconnected the o2 sensor and drive back home. The idle surge didnít occur at any time. It was surging a little around 1500rpm. I guess it was running in open loop all the time.
When the idle surge occur itís going around 500rpm-1000rpm in about 1 sec.
I have a non working A/C, maybe due to a empty cooling system. Everything seems to work with A/C except blowing cold air. Can the A/C affect the idle in any way? You mention pumping A/C but I donít know how that occur, never had the experience.
I havnít got any obd1 scanner but I thinking about getting one. Do you recommend any one?
I do have both the Orginal shop manual and the Chiltonís manual.
I will check the connections you mentioned
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Old 07-13-2018, 10:17 AM
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Re: Runs good in open loop but pumping idle and with same throttle in closed loop

Auto X-Ray made a decent hand-held OBD-1 scanner 20 years ago that would read some sensors. Only available used at this point. I'll see if Rob/Shoebox has a part #, I think he has one.

There are dual capability OBD-1/OBD-2 scanner’s available that will work, but you have to check compatibility. Just to be clear, we are looking for a true “scanner”, not a “code reader”. Some large, independent auto repair shops may have an OBD-1 scanner (e.g. Snap-On) but chains like Advanced, etc. typically only scan OBD-2.

Whet you need to see is the O2 sensor voltage. Cold, it should read about 0.450 volts. That would partially confirm the wiring is OK and the sensor has a good ground. When the engine starts, the voltage should start to change as the engine/sensor warms up. If your AIR pump system is functioning correctly, the voltage will start to drop. If the AIR pump is not working, the voltage will start to climb. If the voltage doesn’t move too far away from 0.450 volts, the sensor or sensor wiring is an open circuit.

Sounds like you may have a source of air entering the exhaust system before the O2 sensor. If the AIR pump system diverter valve is not working correctly, the system could be dumping air into the exhaust, throwing the O2 sensors off, indicating a false lean condition, and causing the ECM to add extra fuel the engine doesn’t need.
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Old 07-13-2018, 04:32 PM
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Re: Runs good in open loop but pumping idle and with same throttle in closed loop

Ok, thanks!

Do you have any advice how to measure the o2 sensor in its place with a voltmeter?
I just disconnect the 02 from the connector and measure the voltage on the wire and engine ground or?

Can you test or disconnect the AIR pump system diverter valve?
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Old 07-13-2018, 07:03 PM
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Re: Runs good in open loop but pumping idle and with same throttle in closed loop

Measuring the voltage at the wire can damage the ECM. The ECM is providing the 0.450 volt bias voltage. The O2 sensor adds or subtracts from that voltage to indicate lean (low volts) or rich (high volts).

Measuring the voltage with the wire connected to the sensor is less risky, but it requires a high impedance meter (~50 Kohms) to avoid damage to the sensor. Then, when you connect the meter, it isn't going to read 0.450 volts (450 milliVolts). Typically it will read around 360 mV.
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Old 07-13-2018, 07:19 PM
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Re: Runs good in open loop but pumping idle and with same throttle in closed loop

I know your engine has the AIR diverter valve. On cold start the air flow from the pump is directed into the exhaust manifolds, near the heads (before the O2 sensor). When the engine warms up, and the ECM goes into closed loop, the air flow is diverted to the pipe on the middle of the catalytic converter (after the O2 sensor).

I just don't know the details of the physical setup - the valve and the piping, hoses, etc.. If the valve is leaking or stuck, the air could continue flowing into the exhaust manifolds in closed loop. Or, there may be a melted hose or broken pipe that allows the flowing exhaust gas to educt (suck) air into the exhaust, throwing off the O2 sensor.
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