My '93 has a very slight surge at idle. No hesitation and no problems when driving the car. Any ideas what could be causing this?

All plugs and wires have been changed recently.

Thanks,

Jrod

 

dripping or leaking injector would be my guess. Put a gauge on it at idle, shut it off and see how fast your fuel pressure goes away.

 

I would also check for vacuum leaks.

 

Mine was Idle Air Control Valve.

 

Could try cleaning your MAF and throttle body. I've seen people have had good results from this.

 

Injectors are not leaking (checked that pressure held with guage hooked up)

IAC tested and found to be operating correctly. Unplugged unit and checked resistance.

Checked for any vacuum leaks and none found.

Finally, cleaned MAF and throttle body.

The idle surges only when at operating temps (201 degrees and above).

Any other suggestions or what I should test next?

 

Have you checked for error codes by shorting pinA and pinB on your ALDL ?

Sounds like your problem occurs only in "closed loop" ...
O2 sensor(s) perhaps.....

http://www.troublecodes.net/GM/

 

I have checked error codes and none exist. I thought an O2 sensor error would give a lean error. I will scan the car and check the O2 sensors for the proper range. Thanks.

 

Ecm ??

 

What is the frequency of this surge ? Is it just a random "once in a awhile" thing, or is it periocic, like every 4 seconds ?

What is the rpm range of the surge? 700rpm normal to ???

In park, If you hold the throttle to increase to a steady 1K rpm, do you notice any surge ?

How many miles on your current Optispark ?

 

Once the car is at normal operating temperatures and the frequency of the surge is periodic like every few seconds. Range is 700 normal to 1000/1100 rpm.

58K miles on current (original) optispark

I haven't tried holding throttle in park yet

 

check throttle body shaft, for excessive play.

 

well I guess a lazy O2 sensor could cause this. If it is not cross-counting fast enough or reporting the fuel change fast enough it could cause a surge. I have never seen it, but that dosn't make it impossible for sure.

 

I just scanned my car and here is the data!

First Scan:
MAP Sensor 1.33v
Throttle sensor 0.60v
Battery 13.9v
Left O2 708 mV
Right O2 470 mV
Idle 625
Spark Advance 20 degree

Second Scan w/ idle bouncing a bit:
Everything nearly the same except O2 readings

Left O2 70mV
Right O2 88mV

Third Scan w/ idle bouncing a bit:

Left O2 572mV
Right O2 484mV

 

Unless the O2's are setting steady at that voltage it doesn't tell you much. They should continually sway from .2-.8 volts. These are called cross counts, or how many times the sensor voltage crosses .5 volts.

a general rule for fuel injected cars is that you want to see it switch about 8 times in 10 seconds. That would be .2-.8 (1) .8-.2 (2) .2-.8 (3). Yours may not see exactly but .1,.2, .3 or .7,.8, .9 is acceptable..

remember the car needs to be warmed to full temp(at least 170F) and should be held at about 1500-2000 rpm to check this.

 

Is there any way to test the O2s? If it's not throwing an error code are the O2s functioning properly?

 

Visual Inspection:

* Connectors and wires
* Housing and insulator for cracks/damage
* Sensor tip for silicone contamination (white powdery coating)

Performance Test: Checks working range and response (speed).

1. Warm engine
2. Install DVOM between sensor and ground (2 volt D.C. range)

NOTE: Leave the electrical connection between the Engine Computer and the Oxygen sensor intact. Use of a test lead at the oxygen sensor connector is recommended.

3. "RUN" engine at steady cruise speed (about 2500 rpm)

o Voltage should vary at least eight times in ten seconds between 0.2 and 0.8 volts (normal range).

4. Drive system rich: The use of a properly adjusted propane flow-meter (J-26911) is industry standard.

o Voltage should increase to at least 0.8 volts within two to three seconds.

5. Drive system lean: Create a vacuum leak from a source that is not an PCM input or output.

o Voltage should drop to at least 0.3 volts within two to three seconds.

NOTE: After each test the O2 sensor should return to normal operating range within two to three seconds.

6. O2 Sensor should be replaced for failure to pass any of the tests above.

NOTE: A Computerized Engine Control system that is operating excessively rich or lean will drive the 02 sensor to its maximum range and should be repaired first and not diagnosed as a 02 sensor failure.

7. Reconnect all electrical connectors. Clear all codes set and retrain idle.
8. When two oxygen sensors are utilized, test one and repeat procedure for the other oxygen sensor.

DIAGNOSTIC AID:
If voltage is fixed at greater than 0.6 volts, a rich condition or contaminated sensor is indicated. Make sure system is operating in "Closed Loop" and check for Diagnostic Trouble Codes. Codes 45 &/or 65 should be present. Check for excessive fuel system pressure and perform injector ballance test.

If voltage is fixed below 0.3 volts, a lean condition, grounded or open oxygen sensor circuit, exhaust manifold leak, or AIR system malfunction is indicated. Make sure system is operating in "Closed Loop" and check for Diagnostic Trouble Codes. Codes 44, 63, 64, &/or 13 should be present.

NOTE: Remember, the oxygen sensor only senses oxygen in the exhaust. If there is an exhaust leak at the manifold, or a cylinder misfiring, whether the misfire is due to lack of fuel, lack of spark, or excessive fuel, there will be additional oxygen in the exhaust. This may cause the O2 sensor signal voltage to be low, and all the cylinders except the one(s) misfiring may be operating excessively rich while the system tries to compensate for a "false" lean condition.

 

Dayum!! That sounds like a boat load of work !!

Whenever I suspect a faulty O2 sensor, I just throw in a new one to see if the "symptoms" change, or swap the left and right forward sensors.

 

Could use more scan data, but assuming this occurs after you've driven it, I'd be inclined to look at a couple of things that took place while you were driving; ie, EGR and the Fuel Evap system. For the latter, simply crimp off the throttle body hose and if the problem goes away, then something is letting the Charcoal Canister purge when it shouldn't. The EGR valve is a bit more problematic, but you might try disabling EGR altogether so that you can rule it out or confirm the problem.

Otherwise, take a look at: a/c signal and make sure it matches compressor engagement; Coolant Temp Signal to make sure it's steady; Park/Neutral to make sure it indicates one or the other; and then your MAP which is used to signal the Load, though I'd just wriggle the harness around and see what happens with the idle. You could also aim a hair dryer at the ECM when its cold and if it starts to fluctuate, then the board may have a weak solder joint though most of these problems lead to a stall and not a surge. Finally monitor the TPS signal and battery voltage - both should be steady.

If your scan data reveals a rich condition, then take another look at things that might be letting in extra air causing the O2(s) to sense a lean condition including you header checkvalves or exhaust leaks.

 

Tested the O2 sensors and both cross counts within spec.

Tested EGR on scanner and found the Duty cycle 99.4% under throttle and 0% @ idle. No vacuum leaks (hose in good shape) or no visable cracks in gasket.

Battery voltage registered 13.7v - 14.3v during cycle

When the surge in idle started - the IAC motor started bouncing all over the place. I unplugged the IAC and noticed the idle jumped up to 2K rpm and shut off the car and plugged back in and the idle went back to 700ish! I checked the resistance on the IAC and found it to be infinite.

I would expect the IAC to compensate for any other problems with idle - any ideas?