Engine stalls at idle and other problems -- need help

I have a '94 Thunderbird, with 3.8L V6.
A minute or two after starting, the engine will not idle properly -- revs go up and down and sometimes it will stall. It does not do this
immediately after starting, or when at normal temperature. Also, if in neutral, or if the A/C is on, it will usually idle OK.
I have also noticed that if I hit the gas pedal, the engine revs will dip slightly before picking up. It does this always (cold, warm or hot) as long as the engine speed is already low.
Fuel economy has dropped.
I noticed this after having a new MAF meter fitted. I have replaced the idle speed control valve and I tested the thottle position sensor (it seems to change resistance as the throttle is opened and there is a steady 5v supply in the connector).
I have replaced the plugs and wires.
I attempted to test the EGR valve and, when I applied a vacuum to the valve, the engine revs dropped (which is expected), so I don't think this is faulty.
I have listened for vacuum leaks and can hear nothing (even using some piping to help hear it). I did find one leak which I fixed, but it did not improve matters.
One final note: the clock spring has failed. There are some references on the web to this causing what looks like a TPS failure on some Chrysler vehicles, but the explanation is due to the clockspring problem affecting the 5v supply (which as I noted earlier, appears to be OK). I can't find any reference to clockspring failures affecting the TPS on Fords (only Chrysler models).
The MAF sensor was swapped out by a local mechanic after the car failed on the way to work. The idling problems seems to start after this, although I cannot be sure, since it idles OK if the A/C is running and that day also marked the beginning of colder weather.
The CEL is NOT on.
Suggestions as to what might be wrong? What to try next?
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Whoever wrote:

I would take it autozone or another place that reads the OBD codes for free (I think it is OBD I). I would also try to make sure that the plug for MAF sensor is improperly in place.
Jeff
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On Thu, 17 Jan 2008, Jeff wrote:

Will there be codes to read even if the check engine light is not lit?
And, yes, the check engine light does work -- the light comes on when the engine has stopped.
I already checked the plug for the MAF sensor, but I could double-check this.
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What happens if you disconnected the MAF? Have you cleaned the throttle bore behind the throttle plate? Have you checked your PCV valve and hoses?
Ed
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Whoever wrote:
<...>

There are sometimes codes if the CEL is not on. Not always, though. I recently had my car info for some work, and there were codes, even though the light wasn't on.

Jeff
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wrote:

Have you ruled out the IAC valve?
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On Wed, 16 Jan 2008, Tim J. wrote:

Yes. I replaced this early on when trying to solve the problem, with a very slight improvement.
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you didnt say how many miles on it, but i keep thinking timing chain.
http://www.minibite.com/america/malone.htm
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On Wed, 16 Jan 2008 17:36:05 -0800, Whoever

I have seen a similar problem in my old CV a couple of years back. Even after cleaning and replacing the IAC, it would have problems idling just after it went into closed loop operation. This usually occurs after about a minute and a half after startup. The only thing found was the base idle speed was not correct. Resetting the base idle speed to specs did the trick. Do not do this unless you have the equipment or scanner to correctly adjust it. You could end up worse off. The base idle speed needs to be checked after replacement of an IAC. It seldom needs to be reset. The fact that the problems is at leaset partly improved with the A/C operating is another suggestion that the idle speed setting may be involved. Could it be that the mechanic reset the idle speed unnecessarily and incorrectly or checked it at all? Keep in mind that base idle speed is greatly affected by a dirty gummed up throttle body. Make sure the TB is well cleaned before checking the base idle speed. It may have to be removed to do this if it is really bad.
Also, make sure your MAF is clean. You can use a non-residue electrical cleaner for this.
Another problem is vacuum leaks or plugged vacuum lines. Make sure you check the PCV system carefully for blockage. Even though the tubing may look good, it may be plugged.
None of these problems commonly set a fault code.
Lugnut
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On Thu, 17 Jan 2008, lugnut wrote:

Your explanation of the problem ocurring when it goes into closed loop mode sounds very plausible. How does one reset the base idle speed?
I have attempted to clean the throttle body, but perhaps I should remove it and clean it off the car before messing with the base idle speed.
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On Thu, 17 Jan 2008 11:32:19 -0800, Whoever

The adjustment is a stop screw on the throttle just like a carburetor had. From the factory, it has a sealer compound on the threads to indicate - if not prevent - tampering with the factory setting. As long as the throttle body and IAC are clean, the system has a lot of tolerence built in such that it would usually never need adjustment - usually. The adjustment should not be done without an accurate tachometer. The engine must be completely warm with all accessories "off". The IAC disconnected to read the base idle speed. It must be set for your vehicle setup. Sorry, I do not have access to the correct speed for your vehicle. The ones I have checked seem to set 100-150 rpm below the specified idle with the IAC connected. I would not claim that to be the spec for your car. Again, it is imperative that the PCV tubing be completely open because the air flowing into the intake from that system is accounted for in the system calibration. Likewise, a large vacum leak like an upper-to-lower intake gasket failure cannot be accounted for by the system. Other than the PCV and other vacuum operated losses, the MAF must be able to correctly measure the air intake of the engine. If it cannot do this, it will get a bit screwy trying to do it's job of controlling the engine.
You can hunt for vacuum leaks with something like an aerosol carburetor or TB cleaner sprayed at suspect points. A change in engine operation may indicate a leak. Carb cleaner is combustible but not like ether that some use. Ether is very dangerous to spray around a running engine because of the fire hazard and a large dose can damage the engine by blowing a head gasket or cracking a piston - can be dangerous in the hands of an experienced professional and bad news stuff in the hands of an amatuer. Use the carb cleaner. It does the same job quite a bit more safely.
Lugnut
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On Thu, 17 Jan 2008, lugnut wrote:

So the car should idle when warm with the IAC valve disconnected? I don't think my engine will do this -- I think it will very quickly stall.

I tried sticking a piece of tubing in my ear and listening for leaks (while moving the other end of the tubing around likely places on the engine). This seems to be quite sensitive but I'll try the carb cleaner trick. I was reluctant to spray carb cleaner around to search for leaks for fear of it igniting.
I haven't replaced the PCV valve. These are cheap enough that I should try replacing it.
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On Thu, 17 Jan 2008 13:40:06 -0800, Whoever

It should idle at some specified speed with the IAC disconnected. If you turn on anything or put it in gear, the engine should slow accordingly. All accessories plus putting it in gear may kill it. If all is well with the IAC operating as it should, you should be able to operate any accessory combination and place it in gear with no noticeable change in engine speed. Keep in mind that the transmission has a manual lever position sensor that tells the PCM what position you have selected. This info is use to adjust idle by the PCM. The reason I mention this is that the MLPS is a frequent failure in that vintage but, it usually first shows up as a problem with erratic transmission operation. Have you noticed any erratic or weird shifting? This sensor is relatively cheap and easy to change with a couple of ordinary metric hand tools. IT is mounted on the left side of the trans.

Carb cleaner should not be a problem. Just do not spray the exhaust manifolds or points where electrical sparks are usually generated like alternators or distributor caps and you don't need to drown the engine to get results.

PCV should be replaced about every 60K. Part of the service is making sure the tubing is in good condition and not plugged. The oil vapors seem to have a nasty habit of cooling in the tubing and solidifyng after a while. The rubber may have to be replaced. Any metal tubing can usually be cleaned pretty with a properly curved and bent coat hanger to break the gunk loose.
Lugnut
Lugnut
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On Thu, 17 Jan 2008, lugnut wrote:

No. It shifts rather roughly when accelerating mildly from rest, but it did this before the problem showed up.

The engine had a complete rebuild 30k miles ago. This was done before I bought it, so I don't know if the PCV valve was replaced, but, at $4, I can afford a new one.
One more thought -- I think there was some backfire when the MAF sensor went out, could this have damaged any of the emission control systems?
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On Thu, 17 Jan 2008 20:23:43 -0800, Whoever

Unless the trans has been built and modified, it should be hardly noticeable when shifting.

The MAF failure - if that is what actually failed - may cause any sort of fuel mixture problems. Many times, the MAF can be cleaned to return to proper operation. Also, since your engine has been rebuilt, does it have much observeable blowby. If so, a PCV malfunction may be the cause. If that is happening, the MAF sensor may be fouled with oil vapors. A strong backfire can also result in damage such as the DPFE sensor that controls your EGR system.
Keep in mind that the PCM may need to be reset after any changes so that it can or will relearn the changes and work correctly. This can be done by disconnecting the neg battery cable for 10 minutes or so. Turn off all accessories and start the engine without touching anything and let it idle 5-10 minutes to warm up. You can then move it into each gear selection without touching the throttle for a few seconds in each position to allow the idle to settle in. After that you should be able to drive the car around a few miles to allow it to learn the engines operation. It will continue to fine tune itself as it is driven. The learning it has done in the past includes all the adjustments it has made to compensate for anything that is out of whack. Resetting the PCM clears the slate and forces it to learn the setup as it is after resetting.
Lugnut
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