Annoying as hell...

Some IAC's will not work properly mounted upside down. I know that mine doesn't work correctly mounted upside down. Use the correct IAC and see if your problems go away.

The 91 code indicates a vacuum leak or bad O2 sensor.

Code 41 or 91 - O2 indicates system lean. Look for a vacuum leak or failing O2 sensor.

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
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 dark green/pink wire) and 43 (RH O2 with a dark blue/pink wire). Use the ground next to the computer to ground the voltmeter.

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.

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.

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

Jrichker is right-on. If the IAC was meant to be mounted upside down (a la the old Wells brand ones as I recall), there would have been a little slip of paper in the box it came in (unless some moron looked at the IAC in the store and tossed the slip of paper).

Also, I've never disconnected the SPOUT to set the idle. This would be inaccurrate since the timing advances as soon as you reinstall the SPOUT (increased timing = higher idle).

Be careful with the vacuum testing and sniffing around under the hood. As JR suggested, I'd be lookin real hard for a vac leak - it's kinda got that feel to it.

As always (and I'm sure you know), checking JRichker's own Idle surging sticky is a good idea. Many of the items and diagnostics would apply to your issue. And the background info is excellent.

Good luck.

More resource material...

See "Surging Idle Checklist for help with all your idle problems. The first two posts contain all the updates to the fixes. I continue to update it as more people post fixes or ask questions.

HISSIN50 said:

Also, I've never disconnected the SPOUT to set the idle. This would be inaccurrate since the timing advances as soon as you reinstall the SPOUT (increased timing = higher idle).

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When setting the base mechanical idle, the idea is to remove all possible variables, so you get the lowest idle speed. Therefore, removing the SPOUT to set the base mechanical idle is the best method to get the lowest idle speed. The lack of advance helps uncover any hidden mechanical problems that more spark advance might mask.

Jrichker, I can see removing the SPOUT connector for diagnostics (I actually never thought of doing it, but see your point).
However, as I read what was said (not knowing that was what you guys were going for), it almost comes across as removing the SPOUT is part of a normal, approved idle setting method. If this was done, people's dynamic idle setting would always be higher than what they had bargained for.

HISSIN50 said:

Also, I've never disconnected the SPOUT to set the idle. This would be inaccurrate since the timing advances as soon as you reinstall the SPOUT (increased timing = higher idle).



Good luck.

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It tells you to pull the pout in you guys's checklist on this website
Are you saying that its wrong? I need to know this so i can rest my base idle correctly and fix my idle problems

ShortThrow50 said:

It tells you to pull the pout in you guys's checklist on this website
Are you saying that its wrong? I need to know this so i can rest my base idle correctly and fix my idle problems

Click to expand...

I'm in a huge minority apparently and do it differently. You can never go wrong with Jrichker's wise words. Set your idle how he outlines it.

Jrichker,

does your surge check list have the TSB#91-25-7 in it(the kit covers more than just surging)?? it's for the Air-By-Pass service kit for the IAC.

if you don't have the TSB, let me know and i'll send it to you in PDF format.

take care,
cjones

HISSIN50 said:

Jrichker, I can see removing the SPOUT connector for diagnostics (I actually never thought of doing it, but see your point).
However, as I read what was said (not knowing that was what you guys were going for), it almost comes across as removing the SPOUT is part of a normal, approved idle setting method. If this was done, people's dynamic idle setting would always be higher than what they had bargained for.

Click to expand...

It might be a better plan on my part to move the base idle setting procedure to the later part of the checklist and changing the wording slightly. The emphasis should be on fixing all the problems before making any adjustments.

The idea is that the computer's ability to affect the idle speed is totally disabled. That lets you set the idle at its lowest stable point, and also helps to uncover any hidden mechanical problems. Once all problems have been resolved, the computer should be able to make the engine idle in the 650-725 RPM range. If it idles above that range, there are still problems like a faulty IAC that need to be fixed.

cjones: The Surging Idle Checklist is at: http://forums.stangnet.com/showthread.php?t=698148.It is not a standard Ford procedure, and makes no claim to being one. It is an attempt to compensate for what 12 years of wear and 12 different owners changed by doing things Ford never considered on a pollution controlled vehicle. It is an ongoing work, so your input is welcome. Please keep in mind that a microscopic number of people have access to all the Ford documentation and special tools. If you reference a Ford TSB or other document, please post it or the applicable sections of it so that we all can see what you are talking about.

jrichker said:

cjones: The Surging Idle Checklist is at: http://forums.stangnet.com/showthread.php?t=698148.It is not a standard Ford procedure, and makes no claim to being one. It is an attempt to compensate for what 12 years of wear and 12 different owners changed by doing things Ford never considered on a pollution controlled vehicle. It is an ongoing work, so your input is welcome. Please keep in mind that a microscopic number of people have access to all the Ford documentation and special tools. If you reference a Ford TSB or other document, please post it or the applicable sections of it so that we all can see what you are talking about.

Click to expand...

jrichker,
i see that you do have it, first paragraph #2 post. i understand that your checklist is not factory, it's real world(what actually works). i remembered the TSB last night and thought i'd ask if you needed it to add to your list or if you already had it. i'll go ahead and post the main part of the TSB the rest are proceedures you have already covered.

keep up the good work.

cjones

BigHairyMonkey said:

anyone care to let me in on where the ECT, ACT, and IAT sensors are in our cars?

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ACT = IAT. It seems to me that on cars where the sensor is in the lower intake, it's often called an ACT. On cars where it's in the rubber inlet ducting, it's called an IAT. Same part though.

Here's a neato diagram courtesy of Tim (Stang22) and Paul (Almost Stock):


View attachment 379561

EDIT: N/M I didn't read all the way thru