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Old 10-26-2007, 15:37   #1 (permalink)
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Can anyone explain how the computer controls engine speed?

Mine is a 93 Taurus, 3.0L, 4spd automatic.

When I first start it up, it runs and idles okay, but after a five minute run down the road at 40-50 mph, the engine idles way too high. Don't know the exact speed. Its a lot more than the high idle I get when the engine is cold, its difficult to hold at a stop, and will take me to 25mph on a flat surface if I let off the brake.

When it does this high idle dance, if I turn off the car, and restart, the idle goes back to normal until the next time I speed on down the road. In fact, if I just click the key off and on, catching the motor before it even shuts off, the problem is "cleared"

If I drive the car around the neighborhood, never really getting past maybe maybe 30 mph, there is no issue. Its seems that if I get it up to 40-50 mph, then the problem of the high idle rears its ugly head.

I have verified that the accel cable is not hung up, the throttle body is clean, and the blades are closed tight when this increase idle speed is occuring. Pressing the throttle at the throttle body does not show any sign of the mechanical throttle being open at all.

I have replaced the TPS, and today swapped in a new IAC valve to no avail. No vacuum leak that I can find, and I tend to rule out anything mechanical since the problem can be 'fixed' by simply turning the key off and on.

There are no codes in the computer, but I would think that 2000 rpm standing still would cause the computer to light the check engine light, and attempt to 'fix' the problem.

Is there some form of a speed sensor that might be bad? The mechanical how-to of the throttle is simple, but I do not know all the details of what tells the computer the information it needs, to control the engine speed. It sure sounds like some form of sensor is giving bad data to the computer control.

I would sure like to hear from anyone with an idea or some conjecture as to where I need to be looking.

The transmission has an issue with 'disengaging' at the stop lights (done it three times in a few hundred miles) which mechanics tell me is a problem with the overdrive circuit, and I am not about to drop $2K to fix the trans, with the engine having this problem, so one thing at a time. :-)

Any chance the trans issue could mess up the computer control of the idle speed? Can't imagine it, but then I don't understand the system. I have a shop manual that touches on the IAC and TPS, but the computer controls are covered in another manual that I have no access to, and cannot find anywhere.

Much obliged for any help or speculation.

John
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Old 10-27-2007, 19:38   #2 (permalink)
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Re: Can anyone explain how the computer controls engine speed?

First off, did you replace the IAC valve with an aftermarket or OEM FORD? There have been many reported problems with aftermarket IACs.

Second, the most common cause of high idle is a vacuum leak. The MAF sensor measures the temperature and mass of the air and then the computer uses that information to calculate how much fuel to add. If there's a air leak anywhere AFTER the MAF, the extra air leans out the mixture. The computer sees that lean condition and adds more fuel to compensate. The extra fuel and the extra air now increase the idle speed. The increased idle speed with a closed throttle plate actually creates more vacuum and the engine sucks in MORE air, causing the computer to add even more fuel.

If you have access to a scan tool (not a plain code reader) you can look at short and long term "fuel trims." If either reads over 25%, that means the computer is battling with a vacuum leak. (Short term adds up to 25%. At that point, the computer considers it an long term problem and moves that amount of fuel into long term fuel calculations and zeros out the short term fuel trim. If it happens again, the long term amount starts to build. It max's out at 25%.)

Hope that helps you to understand how it all works.
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Old 10-29-2007, 08:15   #3 (permalink)
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Re: Can anyone explain how the computer controls engine speed?

Quote:
Originally Posted by RickMN
First off, did you replace the IAC valve with an aftermarket or OEM FORD? There have been many reported problems with aftermarket IACs.
I have read that, and I expect its probably true in most cases, but I lack the funds for a $150 part, so I had to buy an aftermarket one for a third of that. Since the symptoms are virtually identical, I feel like I have shown myself that its not the valve. I would have settled for something different to show me the new valve may be funky, but it behaves virtually the same way.

Quote:
Originally Posted by RickMN

Second, the most common cause of high idle is a vacuum leak.

If you have access to a scan tool (not a plain code reader) you can look at short and long term "fuel trims."
Rick, I really appreciate you taking the time to offer the detailed explanation. I helps me a lot, as there is precious little info that I can find to help me troubleshoot this issue.

Is there a speed sensor somewhere?

I have gone over and over the vacuum lines, and I keep coming back to the fact that the problem is not just a leaking hose or the like, as it doesn't occur until I drive it a few miles, at 40-50mph, and then it goes away when I cut power and turn it back on. That sounds like an electrical/computer issue, or perhaps some device that uses vacuum, which is failing.

I don't have any more ideas of what to test, as if I unhook the IAC under the high idle conditions, it stalls right out, as I guess it should <shrug>

I just hate to take it to Ford, as their charge for code reading is now approaching $100. I guess I could buy one for close to that :-)

Anyway, I might could use a kick in the head to get me past the fact that it only happens at speed, and goes away with nothing more than a restart, which keeps me from thinking vacuum leak.

If you or anyone has any other theories, I am most interested, and also very appreciative of you taking the time to help.

Thanks a lot,

John
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Old 11-01-2007, 01:36   #4 (permalink)
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Re: Can anyone explain how the computer controls engine speed?

The TPS wiring may be bad. You would have to trace the output wires of the TPS back to the PCM connector, back probe them and see what the voltage is. Kind of hard to do without a scanner or data logger of some sort but it can be done with some small gauge solid wire like found in fluorescent lights and a volt meter.

When the idle is high you need to see if the output voltage of the TPS is the same as the voltage on the same wire at the PCM connector. The output wire of the TPS is the one with the variable voltage.
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Old 11-03-2007, 09:24   #5 (permalink)
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Re: Can anyone explain how the computer controls engine speed?

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The TPS wiring may be bad. You would have to trace the output wires of the TPS back to the PCM connector, back probe them and see what the voltage is. Kind of hard to do without a scanner or data logger of some sort but it can be done with some small gauge solid wire like found in fluorescent lights and a volt meter.

When the idle is high you need to see if the output voltage of the TPS is the same as the voltage on the same wire at the PCM connector. The output wire of the TPS is the one with the variable voltage.
If I had any idea what levels and variations I should have on the IAC and TPS, I would be so much better off :-) For my Chevy and VW, I can find this data, either in shop manuals, or online, but there is simply no apparent help for Fords across the internet beyond how to unstick the door, or adjust the headlight.

I can fix anything, if I can understand the way it is supposed to work. I have replaced the TPS twice, the IAC once.

If the engine is in its "high idle" mood, I pull off the IAC connector, and it stalls right out.

I am going to have to build a small harness and bring those signals into the interior where I can monitor them as I drive (and talk on my cell phone, eat my lunch, and shave <g>) but I am not sure what it is I am looking for.

I am considering, after verifying the wiring is okay, of replacing the computer. The scrap yards sell em for about $50. No way to know if they are all good, but if I replace it, and the problem goes away or changes, I will have learned something.

<shrug>

Thanks,

John
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Old 11-06-2007, 08:53   #6 (permalink)
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Re: Can anyone explain how the computer controls engine speed?

TPS voltage should read from ~0-5V, from closed throttle to WOT. There should be a smooth gradual increase in voltage. This can also be read with an Ohm meter. TPS base idle voltage is relative to the PCM and it will take whatever is there for a base. The key is a smooth range.

IAC is a bit trickier. Best way to test that is when it is idling high unplug the IAC and see if the motor stalls. If it does not there is either a vacuum leak or the IAC is sticking. If you suspect the IAC is sticking while it is idling high tap the IAC with a screw driver handle or something to see if it unsticks.
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Old 11-06-2007, 16:39   #7 (permalink)
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Re: Can anyone explain how the computer controls engine speed?

Quote:
Originally Posted by SHOZ123
TPS voltage should read from ~0-5V, from closed throttle to WOT. There should be a smooth gradual increase in voltage. This can also be read with an Ohm meter. TPS base idle voltage is relative to the PCM and it will take whatever is there for a base. The key is a smooth range.

IAC is a bit trickier. Best way to test that is when it is idling high unplug the IAC and see if the motor stalls. If it does not there is either a vacuum leak or the IAC is sticking. If you suspect the IAC is sticking while it is idling high tap the IAC with a screw driver handle or something to see if it unsticks.
Thanks. That is good information. I will be wiring up the IAC and the TPS so I can monitor them while I drive. The TPS is very linear and smooth on the ohms scale, but then the problem only happens after driving fast for a time, so being able to see and compare levels will help.

When it is idling really high after highway driving, I have banged on it with a screwdriver to no avail (its a new device, so it should at least be clean) Then, if I unplug the IAC, it stalls right out. That confuses me. It will also clear the problem if I just turn of the key, and turn it back on. As I noted, even if I do it quick enough to catch then engine before it shuts off, the idle comes right back to normal, so its likely being driven by the electronics. Just not sure what else there is besides the TPS and the IAC that can do that.

Very much obliged for your help and advice.

John
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Old 11-07-2007, 00:53   #8 (permalink)
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Re: Can anyone explain how the computer controls engine speed?

Well if it is any help to you I have exactly the same problem and do my own tuning. What is happening is the IAC adaptive values are being set too high for some reason. This is why when you clear the PCM the problem goes away. But after a while the PCM is learning a higher value for the IAC.
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Old 11-07-2007, 08:50   #9 (permalink)
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Re: Can anyone explain how the computer controls engine speed?

Quote:
Originally Posted by SHOZ123
Well if it is any help to you I have exactly the same problem and do my own tuning. What is happening is the IAC adaptive values are being set too high for some reason. This is why when you clear the PCM the problem goes away. But after a while the PCM is learning a higher value for the IAC.
So would you lay the blame on the PCM? The Taurus computer is abundant in the junk yards. Lots of different serial numbers, so if I could get a match, $50 isn't too bad to take a chance at it.

Very interesting observation. I will be disconnecting my battery and seeing if the computer will change the symptoms. Its pretty much repeatable as it is now, so a change would tell me a lot . Hate to loose those radio presets tho :-)

Thanks a lot,

John
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Old 11-07-2007, 11:42   #10 (permalink)
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Re: Can anyone explain how the computer controls engine speed?

No the PCM is being told to learn the new values for some reason.

'97s don't lose the radio presets. :)
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