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High Idle Speed

Tiny
MWMIT
  • MEMBER
  • 1995 BUICK CENTURY
  • 6 CYL
  • FWD
  • AUTOMATIC
I have a 1995 Buick Century with the 3.1 V6 engine. One morning I started it and the engine was racing over 2000 rpms (estimate), I could idle at over 50 mph (It makes all the shifts) I turned the car off and on a few times with no change. I noticed a crack in the pcv tubing to the pcv valve, I taped that up and the idle went down considerably but still way too high now I idle at 15-25 mph. I replaced the IAC valve and cleaned the whole throttle body. I tested for vacuum leaks by spraying braking cleaner and found nothing. I plugged in the IAC and watched the piston move when I turned on the ignition before I put it in. But she is still idling way too high. If I unplug the IAC while it is running it makes no difference in idle speed.

Thanks in advance for any suggestions!
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Wednesday, March 9th, 2011 AT 3:52 PM

20 Replies

Tiny
JDL
  • MECHANIC
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Block off the mouth of the throttle-body, use a flat piece of board or something, see if any effect on the idle. It should kill it. If it is still running, you got false air getting in from somewhere?

If your fully OBD1, you can check for mil codes.
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Wednesday, March 9th, 2011 AT 4:01 PM
Tiny
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Thanks JDL,
I covered the mouth of the throttle body and it did kill the engine. Is there any way to check the mil codes without a diagnostic device? The check engine light nor any other warning lights come on.
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Wednesday, March 9th, 2011 AT 6:14 PM
Tiny
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If you have the 12 pin datalink connector, you should be able to use a jumper wire between A & B terminals at the datalink connector. Then turn the key just till the dash lites come on, no further, no crank, the mil starts flashing, you count the flashes to get the codes. Your looking for two digit codes. Each code may flash three times before moving to the next code. A code 12 can just mean diagnostic mode, I don't pay much attention to that code. The datalink connector is below.

Tap on the maf sensor see if any idle effect? I used to have an instructor, talking about engine sensors, he said when in doubt, take it out, what he meant was, unplug the sensor, see if any effect on the idle. I've had people say that unplugging sensors to see the effect, is poor diagnostics, that may be, but, if you have nothing else? I don't plug/unplug anything unless the key is off.
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Wednesday, March 9th, 2011 AT 6:47 PM
Tiny
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I tapped on the maf sensor with no idle effect & turned the car off unplugged it and started it again and still idled the same. I will not be able to try to get the diagnostic codes until tomorrow.
Thanks.
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Wednesday, March 9th, 2011 AT 8:36 PM
Tiny
JDL
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Some of the national brand autostores used to check codes for free, OBD2 codes, don't know about OBD1 codes.

Take the throttle linkage loose, maybe it's in a bind? The quickest way to see what sensors are doing, is to use a scanner that shows live engine data. There are other ways, but it takes longer. The tps and map sensors, you can check? Any vacuum at the map sensor?
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Wednesday, March 9th, 2011 AT 8:50 PM
Tiny
JDL
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Without something to point us in the right direction, all I can do is hunt and peck. Use a gage and check fuel pressure. Can you tell if system is running rich or lean?
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Wednesday, March 9th, 2011 AT 8:56 PM
Tiny
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Thanks for all your help so far, sorry I haven't been able to respond sooner. My OBD connector has only 3 live pins can't check codes through the car. I guess some 1994-1995 GM vehicles used a uniques OBD system. Nobody I called wants to check it. When I originally changed the IAC valve and fixed the vacuum leak it was much better maybe 1200 rpm versus 2400 rpm at idle, it appears to be creeping higher and higher, now maybe up to 1800rpm. It also appears to run much faster with a lot of humidity in the air. I will check the throttle linkage. I would prefer to buy a scan tool than to pay 50% of the price for a one time read at a dealer. I can't find any info that states a specific scan tool is guaranteed to work for a 1995 buick century.

Thanks!
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Thursday, March 24th, 2011 AT 3:31 PM
Tiny
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If that is a crossover connector between obd1 and obd2, not sure how to check codes? How many cavities at the datalink connector? Should be a decal under the hood, says if your obd1 or what.
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Thursday, March 24th, 2011 AT 5:01 PM
Tiny
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I checked all the decals none say anything about obd. It is a 12 pin connector like in your diagram but it only has 3 wires A, M and the blank one under C. I took it apart to make sure the linkage wasn't binding and started it with the throttle linkage disconnected and it idled beautifully. I was optimistic that something had now bee freed and reconnected everything and was back to a high idle. I wasn't too disappointed and disconnected everything again to find what was the cause and even with the linkage disconnected it was idling very high again. Could the binding be in the throttle body itself?
Attached is a picture of the connector.
Thanks.
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Thursday, March 24th, 2011 AT 7:37 PM
Tiny
JDL
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You may need to clean throttle-body and iac/idle air control. Take the throttle position sensor loose, start vehicle, any difference?

I don't recognise your data link connector, it may take a special scanner to check codes? Those cross-over years are a pain.
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Thursday, March 24th, 2011 AT 7:56 PM
Tiny
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Thanks for all your assistance JDL. She idling properly now. I completed removed the throttle body and took out all sensors and completed cleaned everything. TPS was very gunked up. When I removed the NEW IAC valve, THAT I REPLACED, I saw that the oring was pinched and crimped. I replaced the oring and reinstalled everything and she is running great. Whether it was the oring or the cleaning I am not sure but she is finally idling so low and smooth again that I can hardly tell if she is running or not.

Thanks Again!
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Thursday, March 31st, 2011 AT 3:39 PM
Tiny
JDL
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Good job, glad you got it going your way.
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Thursday, March 31st, 2011 AT 4:32 PM
Tiny
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I spoke too soon. Yesterday I started it and stopped it 5-6 times drove about 10 miles all good. Just got back from an errand and it has resumed it's high idle. I am going to rule out the IAC - I am sure the oring is seated properly. Could the TPS be working sporadically?

Thanks.
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Thursday, March 31st, 2011 AT 6:27 PM
Tiny
JDL
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You may have to use a scanner so you can look at live engine data. Unplug the tps, start vehicle, any difference? If you want to do some testing, you can check that sensor for ref voltage--ground--signal. You need a digital multimeter, at least.
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Thursday, March 31st, 2011 AT 6:59 PM
Tiny
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I took it to a mechanic and it has stumped him as well. He connected his diagnostic scanner and he said he was getting a value of 8 for the maf and he would expect to see a much higher number 500 or 1000 and that the ecm was causing the iac to run it so high. He replaced the maf sensor and no change in idle or diagnostic reading. He checked for vacuum leaks and tested the tps. He is thinking maybe it is the ECM.
Could it be the ECM?
Thanks.
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Friday, April 1st, 2011 AT 2:51 AM
Tiny
JDL
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I don't know, hard for me to tell anything from here. Those numbers you posted don't look right to me.
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Friday, April 1st, 2011 AT 3:07 PM
Tiny
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Could a carboned up EGR cause this issue? I have it off right now and it appears to be wide open and really doesn't move at all. Should it be closed or open with no power to it? If I plug the connector back in but leave it EGR unconnected should the piston move when I turn on the ignitiopn?
Thanks
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Friday, April 1st, 2011 AT 3:13 PM
Tiny
JDL
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If the egr opens up at idle, it will kill the engine.
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Friday, April 1st, 2011 AT 3:59 PM
Tiny
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I currently have a backwards resolution. With the mechanic stating that the ECM was telling the IAC to open (Could need new ECM) I took out the IAC and turned the piston until it was completely extended and then put it back but did NOT plug in the electrical connection. She has been idling perfectly for 2 days now. Does the IAC server any other purpose than providing a high idle in the cold or increased RPMs for AC/accessories (AC doesn't work needs new compressor and leaks)? Does it sound like it could use a new ECM?
Thanks
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Sunday, April 3rd, 2011 AT 2:53 PM
Tiny
JDL
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The computer uses the iac to control idle. If you use a scanner that shows live engine data, you could look at (idle) and (idle required). (Idle required) is what the computer is trying to get the idle. The (idle) is what the engine is actually doing. Something could be telling the computer to go to higher idle if that is the case, (idle) and (idle required), should be close to same. It could be an ecm issue?
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Sunday, April 3rd, 2011 AT 5:32 PM

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