Can't adjust idle mixture - idling too rich

I am seeing several cars from the early 90's that are idling rich and we can't figure out what the problem is. After checking the simple things - plugs, vacuum hoses and connections, next idea was the computer was not getting the right signal from the mass air flow sensor. Swapping that out didn't improve anything.
All we really need to do is manually adjust the idle mixture, but as far as I can tell this is not possible due to all the controls coming from the computer.
Anyone run across this before and have an idea what to look at next? Or how to bypass the computer and adjust the idle mixture
Thanks in advance.
Bob
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On 2/10/2012 10:45 AM, Guv Bob wrote:

figure out what the problem is. After checking the simple things - plugs, vacuum hoses and connections, next idea was the computer was not getting the right signal from the mass air flow sensor. Swapping that out didn't improve anything.

Look for an IAC (Idle Air Controller) valve.
Lynn
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wrote:

can't figure out what the problem is. After checking the simple things - plugs, vacuum hoses and connections, next idea was the computer was not getting the right signal from the mass air flow sensor. Swapping that out didn't improve anything.

have it scanned by someone who knows how to interpret the readings. By "early" 90s I'm assuming you mean pre 1996, so not OBD2.
The Oxygen sensor has the greatest control over the mixture, with the possible exception - on some cars, of the intake air temperature or coolant temp sensors. Verify the operation and proper connection of 3 these 3 parts and you will most likely solve the rich running.
There is NO WAY to bypass the computer or adjust the idle mixture because the mixture is controlled by the pulse width of the injector - which is TOTALLY under the control of the computer.
If the computer is told the engine or intake air is cold bu bad sensors, the engine will run rich. If the computer is told by the O2 sensor that it is running lean, it will richen the mixture trying to make it right.
You need a good mechanic to check it out, or you need to read a few good technical books on EFI (if you understand electronics at all) and mabee take an indepth electronics course if you are totally unfamilliar with electronic controls and / or heat engine theory of operation. I suspect from your question you fall firmly into the latter category - and paying a knowlegeable mechanic to sort it out will be the fastest AND cheapest method for you.
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With these earlier cars, their computers always ignored the Oxygen sensor at idle, and until the engine was fully warm. Oxygen sensors don't output reliable information below a (Oxygen sensor) temperature of 600?F. Unheated Oxygen sensors are never that hot at idle.
It wasn't till much later that stricter emission regulations made electrically heated Oxygen sensors necessary... thus allowing computers to 'read' them most anytime, other than the first few seconds after starts. It was only then that they became part of the idle mixture equation.
A bum Oxygen sensor can make an engine run rich at times other than idle.
Erik
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Also, nobody said what KIND of engine. A unit with a L-Jetronic or K-Jetronic type EFI could have a leaky cold start injector. Any engine COULD have a leaky injector. Was REAL common on Chrysler K-Cars and their offspring. Actually pretty common on anything with throttle body injection - and not unheard of on port injected engines either. Another common problem is a leaky diaphragm on the fuel pressure regulator - the vacuum bias to the regulator sucks gas into the intake manifold.
A bum O2 sensor can make a HOT engine run rich at idle. Also at other speeds - rich at idle is a lot more noticeable than rich at speed - particularly if it is something like a leaky injector that adds a constant fixed amount of fuel per unit of time.
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is cold it will richen up mixture.
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On Sat, 11 Feb 2012 08:07:24 -0500, "John"

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