Slow To Drop Timing Advance

larrym1961

5 Year Member
Dec 18, 2010
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Idaho Falls Id
I have had an issue for a while of that annoying "hanging idle" after stabbing the throttle and letting off the gas. It hangs @ about 1500 RPM for about 5 to 7 seconds then drops back to normal idle.

Doing some experimenting, first I unplugged the IAC then stabbed throttle, the idle still hung so that eliminated the IAC. Then I unplugged the spout connector @ dizzy so I would get no timing advance from EEC, stabbed the throttle and let off, the idle returned to normal immediately. Exactly what I am looking for.

So I have come to the conclusion that for some reason the ECU is slow to drop the timing advance therefore the "hanging idle". Any one have an idea what is going on here? Is this normal or do I have a real problem?
 
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No codes. Base idle is set at 800 RPM. Base timing set @ 10 degrees. TPS @ .98.

The question I asked, is if you messed with the idle screw? some aftermarket cams will facilitate the need to adjust the idle screw (something you normally would never do) in order to keep the car from stalling out. If you raise the idle too high, will hang.
 
I have a E303 cam and I have adjusted the screw w/ IAC and spout connector unplugged to set idle to about 750 to 800 RPM. Idle will still hang after the throttle is stabbed with no IAC but will not with the spout unplugged.
 
Have you checked for air leaks? Throttle body, intake pipe, intake manifold, cracked vacuum line. If any of them are sucking outside air, it could be the cause of your hanging idle.

You could also try cleaning the 10-pin connectors (salt and pepper shakers). It's not uncommon for them to corrode overtime and make intermittent connectivity, causing all sorts of strange issues.

I don't think your issue is ignition related.
 
I could not find any vaccum leaks. (not to say I could be missing something) 10 pin connectors are clean and good. I installed a new Ron Francis Injector Wiring Harness not to long ago. It is just odd that with the IAC unplugged and also the spout disconnected the idle does not hang but hangs w/ spout connector plugged and no IAC.
 
Well I went out and ran codes again and I am getting the dreaded codes 41 and 91

Left and Right injectors showing lean.

Sure sounds like it could be an air leak? Do you have a vacuum gauge? You should be seeing at least 14-16i nHg with that cam.

@jrichker has a pretty good diagnostic method to check the O2's. He'll probably chime in shortly.
 
Left and Right injectors showing lean.

Sure sounds like it could be an air leak? Do you have a vacuum gauge? You should be seeing at least 14-16i nHg with that cam.

@jrichker has a pretty good diagnostic method to check the O2's. He'll probably chime in shortly.
I don't think I have ever had more than 13HG's of vacuum @ idle since I installed my new motor in 2005. Probably time for a smoke test to check for vacuum leaks. And also if Joe puts up the 02 sensor check I will go thru that also.

With the new injector harness, I grounded 02 ground wire to intake. The 02's themselves are fairly new Motorcraft ones and I have also recently inspected and cleaned 02 harness connections.
 
As suggested...

Code 41 or 91. Or 43 Three digit code 172 or 176 - O2 sensor indicates system lean. Look for a vacuum leak or failing O2 sensor.

Revised 11-Jan-2015 to add check for fuel pressure out of range

Code 41 is the passenger side sensor, as viewed from the driver's seat.
Code 91 is the driver side sensor, as viewed from the driver's seat.

Code 172 is the passenger side sensor as viewed from the driver's seat.
Code 176 is the driver side sensor, as viewed from the driver's seat.

Code 43 is not side specific according to the Probst Ford Fuel injection book.

The computer sees a lean mixture signal coming from the O2 sensors and tries to compensate by adding more fuel. Many times the end result is an engine that runs pig rich and stinks of unburned fuel.

The following is a Quote from Charles O. Probst, Ford fuel Injection & Electronic Engine control:
"When the mixture is lean, the exhaust gas has oxygen, about the same amount as the ambient air. So the sensor will generate less than 400 Millivolts. Remember lean = less voltage.

When the mixture is rich, there's less oxygen in the exhaust than in the ambient air , so voltage is generated between the two sides of the tip. The voltage is greater than 600 millivolts. Remember rich = more voltage.

Here's a tip: the newer the sensor, the more the voltage changes, swinging from as low as 0.1 volt to as much as 0.9 volt. As an oxygen sensor ages, the voltage changes get smaller and slower - the voltage change lags behind the change in exhaust gas oxygen.

Because the oxygen sensor generates its own voltage, never apply voltage and never measure resistance of the sensor circuit. To measure voltage signals, use an analog voltmeter with a high input impedance, at least 10 megohms. Remember, a digital voltmeter will average a changing voltage." End Quote

Testing the O2 sensors 87-93 5.0 Mustangs
Measuring the O2 sensor voltage at the computer will give you a good idea of how well they are working. You'll have to pull the passenger side kick panel off to gain access to the computer connector. Remove the plastic wiring cover to get to the back side of the wiring. Use a safety pin or paper clip to probe the connections from the rear.

Disconnect the O2 sensor from the harness and use the body side O2 sensor harness as the starting point for testing. Do not measure the resistance of the O2 sensor , you may damage it. Resistance measurements for the O2 sensor harness are made with one meter lead on the O2 sensor harness and the other meter lead on the computer wire or pin for the O2 sensor.

Backside view of the computer wiring connector:
a9x-series-computer-connector-wire-side-view-gif.71316


87-90 5.0 Mustangs:
Computer pin 43 Dark blue/Lt green – LH O2 sensor
Computer pin 29 Dark Green/Pink – RH O2 sensor
The computer pins are 29 (L\RH O2 with a dark green/pink wire) and 43 (LH O2 with a dark blue/pink wire). Use the ground next to the computer to ground the voltmeter. The O2 sensor voltage should switch between .2-.9 volt at idle.

91-93 5.0 Mustangs:
Computer pin 43 Red/Black – LH O2 sensor
Computer pin 29 Gray/Lt blue – RH O2 sensor
The computer pins are 29 (LH O2 with a Gray/Lt blue wire) and 43 (RH O2 with a Red/Black wire). Use the ground next to the computer to ground the voltmeter. The O2 sensor voltage should switch between .2-.9 volt at idle.


Testing the O2 sensors 94-95 5.0 Mustangs
Measuring the O2 sensor voltage at the computer will give you a good idea of how well they are working. You'll have to pull the passenger side kick panel off to gain access to the computer connector. Remove the plastic wiring cover to get to the back side of the wiring. Use a safety pin or paper clip to probe the connections from the rear. The computer pins are 29 (LH O2 with a red/black wire) and 27 (RH O2 with a gray/lt blue wire). Use pin 32 (gray/red wire) to ground the voltmeter. The O2 sensor voltage should switch between .2-.9 volt at idle.


Note that all resistance tests must be done with power off. Measuring resistance with a circuit powered on will give false readings and possibly damage the meter. Do not attempt to measure the resistance of the O2 sensors, it may damage them.

Testing the O2 sensor wiring harness
Most of the common multimeters have a resistance scale. Be sure the O2 sensors are disconnected and measure the resistance from the O2 sensor body harness to the pins on the computer. Using the Low Ohms range (usually 200 Ohms) you should see less than 1.5 Ohms.

87-90 5.0 Mustangs:
Computer pin 43 Dark blue/Lt green – LH O2 sensor
Computer pin 29 Dark Green/Pink – RH O2 sensor
Disconnect the connector from the O2 sensor and measure the resistance:
From the Dark blue/Lt green wire in the LH O2 sensor harness and the Dark blue/Lt green wire on the computer pin 43
From the Dark Green/Pink wire on the RH Os sensor harness and the Dark Green/Pink wire on the computer pin 29

91-93 5.0 Mustangs:
Computer pin 43 Red/Black – LH O2 sensor
Computer pin 29 Gray/Lt blue – RH O2 sensor
Disconnect the connector from the O2 sensor and measure the resistance:
From the Red/Black wire in the LH O2 sensor harness and the Red/Black wire on the computer pin 43
From the Dark Green/Pink Gray/Lt blue wire on the RH Os sensor harness and the Gray/Lt blue wire on the computer pin 29

94-95 5.0 Mustangs:
Computer pin 29 Red/Black – LH O2 sensor
Computer pin 27 Gray/Lt blue – RH O2 sensor
From the Red/Black wire in the LH O2 sensor harness and the Red/Black wire on the computer pin 29
From the Dark Green/Pink Gray/Lt blue wire on the RH Os sensor harness and the Gray/Lt blue wire on the computer pin 27

There is a connector between the body harness and the O2 sensor harness. Make sure the connectors are mated together, the contacts and wiring are not damaged and the contacts are clean and not coated with oil.

The O2 sensor ground (orange wire with a ring terminal on it) is in the wiring harness for the fuel injection wiring. I grounded mine to one of the intake manifold bolts

Check the fuel pressure – the fuel pressure is 37-41 PSI with the vacuum disconnected and the engine idling. Fuel pressure out of range can cause the 41 & 91 codes together. It will not cause a single code, only both codes together.

Make sure you have the proper 3 wire O2 sensors. Only the 4 cylinder cars used a 4 wire sensor, which is not compatible with the V8 wiring harness.

Replace the O2 sensors in pairs if replacement is indicated. If one is weak or bad, the other one probably isn't far behind.

Code 41 can also be due to carbon plugging the driver’s side Thermactor air crossover tube on the back of the engine. The tube fills up with carbon and does not pass air to the driver’s side head ports. This puts an excess amount of air in the passenger side exhaust and can set the code 41. Remove the tube and clean it out so that both sides get good airflow: this may be more difficult than it sounds. You need something like a mini rotor-rooter to do the job because of the curves in the tube. Something like the outer spiral jacket of a flexible push-pull cable may be the thing that does the trick.

If you get only code 41 and have changed the sensor, look for vacuum leaks. This is especially true if you are having idle problems. The small plastic tubing is very brittle after many years of the heating it receives. Replace the tubing and check the PVC and the hoses connected to it.
 
Did volt test on 02's. .2V @ idle on both. I did find a small vacuum leak. Vacuum hose from AM solenoid to smog pump was sitting against header and cooked. Removed the burned vacuum hose and capped it off @ AMS. Reset ECM and took for a drive. Ran another code test and now I get the desired code 11. 41 and 91 are now gone.
 
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