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I just had the mass airflow sensor replaced (AGAIN) at PepBoys, and noticed that occasionally the engine idles too high. Does anyone know off hand if the mass airflow sensor could cause an intermittent high idle?

The high idle issue seems to happen when the engine is initially started while cold; the idle doesn't come down (stays around 900 - 1000 rpm), even after driving for 20 or 30 min. Then, if you re-start the engine (while hot), the idle becomes normal (750-ish rpm).

I guess the air idle control valve could be acting up? But, it just seems "fishy" that this started right after the new mass airflow sensor.

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I also have had this same problem for last 3 months. Replaced the plenum gasket, checked all rubber hoses, cleaned throttle body same problem.

I can drive for 50 miles stop, and go rpm stays around 750.

Next stop for traffic light rpm ranges from 1075 to 1125. Shift to park, and re-start back to normal 750 rpm.

Seen several members with same question, but no answer.

So Mr. Padgett where are you? Larry

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Since the problem can be corrected by restarting the engine I would think it is a bad IAC or mass air flow, maybe even a TPS. Something electronic. If it was a vacuum leak restarting the engine would not clear the problem. Just my .02

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Have seen two things cause a high but stable idle. Most common is a vacuum leak. Less common is a faulty coolant temperature sensor. What is you IAC command dooing when it is high and when it is low ?

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Thanks for all input.

I finally got a service manual. Yay.

So, you recommend checking the IAC command when the high idle exists, and comparing that to when it isn't idling high? I assume that can be done through the CRT? I'm only familiar with checking fault codes; I haven't ventured further than that...yet.

I'm just annoyed, because the problem didn't exist until PepBoys replaced the fuel pump and the MAF sensor, and I think they supposedly cleaned the throttle body? I picked it up from them after they claimed they cleaned "carbon" from the IAC valve, but after driving 30 min to work today, idle was 1150 rpm.

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Found out how to get data from ECM on my 1989....way cool!

This morning, I started car and noted the rpm's were 1100rpm, and ED22 IAC signal was 60. I drove 30 min to work, and noted idle was 1000 rpm and IAC signal was still high (around 50). I shut and re-started engine, and IAC signal was then 24 with idle down to 700rpm. I also noted before and after shutting engine the ED4 (Coolant TP) was 100 degrees.

So, it doesn't seem like the IAC Valve is failing, because the ECM was commanding a high idle position, correct?

Any ideas on this?

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ED04 (coolant temp) is in degees C. 100 is 212F which to me is high but is considered normal by the factory and should not affect the idle command. What is the PROM ID (ED99) ?

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Well, it was 91, but went up to 99 C since I was idling there for a few minutes. I should have mentioned that. So, the temp seems OK.

I don't have the service manual handy, What is PROM ID? What's it do?

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ED99 is a four digit value that tells what PROM revision you have. Usually we refer to them by letter codes but the parts book indexes them.

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The PROM says it's 1094.

This issue seems to happen mostly when I start the car with a cold engine, the idle stays high even though I drive it long enough to thoroghly warm it up (30 min). But, if I shut the engine and start it right back up, the idle drops down to normal. Weird.

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Well something is making it think the idle is correct. Guess the next step is to take a full set of ECM readings (EDs) both before and after and look for outliers. Might also see if any odd arrows are lit around the service screen. Maybe it thinks it is in gear.

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After yesterday morning's high idle, it's been OK. I'll have to keep monitoring it, I guess.

I was thinking (Uh oh); the service manual mentions something about: if you disconnect the IAC valve w/ ingintion ON, then it needs to be reset by starting/stopping the engine, maybe the electrical connector is loose, or something, and then it needs to be reset. Or, would an open circuit of the IAC cause a SES light?

Also, I do think a little arrow was blinking over some Oxygen icon, in the ECM Data screen. Is that a problem?

I also plan to replace the thermostat, because I notice that I'm at 170-174F on the highway. I think it's been 10 years since the stat's been replaced. And, doesn't that seem low for a 195F thermostat?

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AFAIR, you have to drive over (25|38) mph to reset the IAC. The lights sound right for being stationary and warmed up (in closed loop and O2 cycling).

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I also noticed that if I'm on the highway and in the IAC Data screen, the step (count) numbers increase, the more I step on the gas pedal.

Is that normal? I thought the IAC was only for idle (closed throttle position).

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I drove to work this morning, and the idle in Drive was 850rpm, and when I put it in Park it was 1150rpm. Then, I shut the engine, and started it right back up, and the idle in Park was 750rpm. I copied all the ECM data before and after the engine re-start. The main thing I noticed was that before re-start, the LOOP arrow was solid (operating in "closed-loop) and the OXY arrow was blinking. After the re-start, neither arrow was solid or blinking. Any ideas?

Note: the first column was before re-start with high idle.

ED01 .4 .4

ED04 81 93

ED06 3.8 4.4

ED07 .6 (changing) .82 (more steady)

ED08 28 20

ED10 13.9 V 13.6

ED11 1150 722

ED16 0 0

ED17 1567 625

ED18 5 (changing) 0

ED19 125 128

ED20 117 128

ED21 7.7 5.2

ED22 50 38

ED23 40 55

ED98 28 29

ED99 1094 1094

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I think both of your ED07 readings indicate a rich condition. Unless the first one is 0.06 not 0.6. On my car, which I think is normal, the O2 reading should be rapidly changing up and down in numbers so that the combined average of them is 0.45. Mine will stay at 0.70 or0.80 and then quickly drop to 0.06 or 0.20 so that if you were able to write them down fast enough and average them it would be an average of 0.45. They continuously run back and forth as the ECM manages the air fuel ratio. BTW for the readings to be flucuating rapidly, it must be in closed loop. Loop light flashing.

However, your ED19 and ED20 readings indicate that it's okay. I do not know what is correct on the IAC, but mine does change quite a bit when driving. I am just a back yarder. That means I only break my own car. Go more by what some of the experts here say. Otherwise, you will end up just throwing money at it piece by piece until you get it.

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Thanks for the resonse. The posting didn't put the data into columns. But the first data is with the high idle (ED07 is .6), and the next data was after re-start with normal idle (ED07 is .82).

After re-start, it wasn't in closed-loop, and OXY triangle was not lit. Although, I only ran it long enough to copy the data, after re-start. I don't think I can trouble-shoot this. It appears out of my league.

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There is about 25 seconds after a restart before the engine goes into closed loop. This allows time for the O2 sensor to light off.

The only possible outlier I see that is not explained by the higher rpm is the TPS. Try adjusting to abt .35v at idle and see if it makes a difference.

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Last Fri 17th drove from Louisville to Tenn. Avg mpg 28 to 29. Made first stop at 110 miles rpm was at 1100. Re-started rpm was at 650 to 700 rpm.

Before leaving on this trip cleaned the K&N filter, cleaned throttle body, checked for any loose connections.

Also replaced mass air flow sensor.

I think Padgett is on to something with the TPS. Auto zone cost is $44.99 part # TPS 119. Going today to my local pull-a part and pull one from a junk 3800

see if any thing changes.

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Thanks blazer! Keep me posted.

I just changed the thermostat today; but the high idle still happened when I started the car for the first time today. I let it warm up for 15 minutes to check for any coolant leaks from the t-stat change, and the idle was down to 800 rpm; then I drove it to a store and the idle was 1000. Stopped and re-started the engine, and idle was down to normal 650-ish. Strange.

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Auto Zone price for new tps around $41.00. If have a pull-a-part in your area cost me $7.00 for used tps.

Sun 07/26/09 drove around 50 miles, stop and go rpm stayed around 650 to 700rpm.

Let you know later this week if rpm staying same.

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Good question Padgett - YES, when my car is idling high (1100-ish) in Park or Neutral, it is around 850-ish rpm when it is in Drive. Plus, the IAC Valve setting is usually around 50, and then if I re-start engine and idle is normal, the IAC is then around 30.

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My car does something that may or may not be normal concerning the idle. When I start the car from sitting overnight the idle will be fast as expected. If I immediately put the car in reverse and let off the brake, the car will start going out the driveway without touching the gas under fast idle condition. As it gains speed going backwards the idle will drop down to a normal idle and the car slows down as well. What is odd, is if I leave the car in the garage and let it idle, it will fast idle for quite a while until the engine gets warm.

Does anyone know if the computer senses the car is moving and drops the idle speed even though the engine is still cold? Has anyone else noticed this behavior in your car?

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This moring, the idle seemed ok, and was down to 800-ish when I got to work (although, that still may be a little high?).

In response to padgett; I don't know anything about the PROM or memory, but it seems like the ECM is "commanding" the IAC valve to be around 50 when the high idle exists, and maybe being in Drive slows the engine down a little, to around 900-ish? Possibly from the extra load from the torque converter? Don't know. But a IAC setting of 30-40 seems to coincide with normal idling.

I just replaced the thermostat with a 180F one (wasn't replaced for 10+ years, and running around 174F); and was wondering if maybe I should have just put another stock 195F back in, because now it runs around 160F-165F while moving. Could this slightly lower temp be causing the ECM to kick-up the IAC valve a little?

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Kick-up ends arounf 150F but sounds like new thermostat is not closing (I have had very good luck with STANT SuperStats).

Idle speed is not a matter of load with a computer car. There is a table of values that kicks out the rpm. it then uses the IAC to match that rpm regardless of load. Reason I asked it that there are two tabels, one for P-N and another for "in gear"

On mine, I increased the warmed up idle maps from 650 to 700 rpm to get a little better cooling from the a/c at idle.

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Starting my 5th day and have no problems with high idle after changing the tps.

When stopped in drive idle staying around 650 to 700rpm.

For $7.00 at pull-a- part try it. If doesn't work lost $7.00 and start looking for the cause.

The old tsp was purchased at auto zone and was adjustable. This time found the stock tsp that only has 2 hole where the screws go into with a metal retainer plate that goes on first.

Padgett was one that got me thinking about the tsp, and made a good point.

Spent 2 weeks looking for the problem cleaning, changing parts.

Not shure if inside it is spring loaded on the arm start to hange due to lost tension on the spring not letting the arm pull all the way back. Like you after restarting guess send a signal to the tsp and re-sets the spring to idle.

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So, I bought a new TPS at PepBoys for $35.99. But, I think I'm waisting my time, because the TPS seems to be working fine. It shows .4-ish (.38 this morning) whenever I'm off the gas; and .4 is within spec.

Plus, whenever the high idle is happening, the IAC is being commanded to be 50-ish (53 this morning). And then after a re-start, the IAC is down to mid 30's. That probably explains why even though the rpm's are lower in gear, they're still a little high (800-900-ish). Not sure if Padgett agrees; but my theory is that the extra load of being in gear slows down the rpm's from the 1000-1100-ish (in P or N) down to 800-900-ish (in gear), because the IAC is slightly high (50 in lieu of 30-40).

This new TPS has elongated mounting holes; I'm assuming that's the adjustability that's required.

This new thermostat I just put in was a Sant SuperStat, 180F. Seems to run 160-165F with significant air flow over the radiator. Was wondering if I should just go back to a stock 195F, or leave it as is.

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No, no codes. Car's been starting-up real quick, too....better than ever. Such a fun car to drive (just recently switched with my wife).

The ECM just seems to be giving the IAC Valve a slightly high command (50-ish), when the car is started for the first time of the day; until/unless I stop the engine and start it back up. Then, all seems fine.

It's not a major issue for me, but kinda annoying. Makes shifting into R or D kinda "clunky", too, since the rpm's could be around 1000/1100.

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