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Idle speed control


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6 replies to this topic

#1 mulcheese

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Posted 07 March 2006 - 07:21 PM

The CEL came on the other day when the car was going through one of it's bucking fits and the codes are these:
P0303 cyl 3 misfire, P0304 cyl 4 misfire, P0500 VSS A (i think this is from last week, the speedo cable broke but CEL went out after replaced), P1507 Idle speed control, P1540 idle speed control. Now the car started bucking last summer and has become much worse now that it is colder. It only bucks in the morning when the car is cold and stops after a few minutes when the car starts to warm up. I originally thaught it might be in need of a clutch replacement but now with the codes I dont know. I did mess with the IAC this last fall because the idle would not drop below 2500. A little gum-out and fiddling with the armature and the idle was back to normal.

Any suggestions?

97 Impreza OBS, manual, 140k

#2 blitz

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Posted 07 March 2006 - 08:49 PM

I get the feeling your more of a "fix it after it breaks" rather than a preventative maintenance type person.

The casual manner that you throw a "maybe this code is actually part of an unrelated fix" means that you complicate the diagnosis for everyone here the same as you do for yourself.

When was the ECU last reset? How many of those codes are freshly logged? How many have been accumulating? What exactly is "messing with the IAC"?

Finally; what is the general maintenance condition of the vehicle? How much crud is in the IAC, the PCV, the T/B? How are the underhood grounds? Are the battery terminals clean and tight?

Sometimes sensors go out of range (defective), but most of these things that "go bad" are related to dirt and corrosion.

**** ******!

#3 mulcheese

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Posted 08 March 2006 - 10:28 AM

I will admit that it probably does need a tune-up (pugs,wires,cap,.....) but other that that yes i am like most people and do not fix something until it breaks. On the other hand I have done some smart preventitive maintanence to the extent of T-belt, water pump,...... so I do understand the preventitive part. Now tell me what preventitive maintanence are you refering to?

The "maybe " I through in is because the CEL went on 2 weeks ago, speedo stoped, I fixed it and the light went out. When I had the codes read at that time the P0500 came up but the light went out and I have not been able to clear/read the codes until now. So other than that code (P0500) all others are new.

"Messing with the IAC" was taking of the magneto, disconnecting hose, spraying carb cleaner in there, and reassemble and adjust.

As far as for what condition are the others components well that is why I started this post to ask for suggestions as to what should I look for. Should I clean somthing? Should i disconnect/reconnect somthing? This way I can do some diagnostic work and come back with more info.

#4 mulcheese

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Posted 09 March 2006 - 10:06 AM

Another thing that I realized is that the codes I get ,P1540 P1507, both say idle speed control on my print out yet looking at a previous post http://www.ultimates...ght=P1540 P1507 they are show for different codes. The print out does say OBDII. Which are correct?

#5 blitz

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Posted 09 March 2006 - 10:45 AM

I will admit that it probably does need a tune-up (pugs,wires,cap,.....) but other that that yes i am like most people and do not fix something until it breaks. On the other hand I have done some smart preventitive maintanence to the extent of T-belt, water pump,...... so I do understand the preventitive part. Now tell me what preventitive maintanence are you refering to?

How much crud is in the IAC, the PCV, the T/B? (All air-flow at idle is through these three orifices ..two of which are fixed & one which is variable.) How are the underhood grounds? Are the battery terminals clean and tight? When was the fuel filter last replaced? When was the air filter last replaced? When was the last time you've looked at the plugs? Are they tightened properly? Are all the intake hoses tight? Are all the vaccum hoses secure and not cracked? Do any of the underhood connectors have green on the terminals?


The "maybe " I through in is because the CEL went on 2 weeks ago, speedo stoped, I fixed it and the light went out. When I had the codes read at that time the P0500 came up but the light went out and I have not been able to clear/read the codes until now. So other than that code (P0500) all others are new.

"Messing with the IAC" was taking of the magneto, disconnecting hose, spraying carb cleaner in there, and reassemble and adjust.

I just looked in my service manual and I don't see any adjustment specs for the IAC. What specs did you adjust to?

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#6 mulcheese

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Posted 10 March 2006 - 12:25 PM

This weekend is when I will have time to complete the " tune-up", plugs wires, pcv, ...... and fuel filter but where is the filter? One problem I have is that I have not been able to locate a manual for this specific vehicle so beyond a a tune-up I am at a loss. Can some one tell me where I can find info? I have been told to but legacy manual and Im sure that will work but I would like to find a OB sport manual.

The adj. to the IAC that I performed may not be an actual adj. Some people have said that it is adjustable and some say it isnt adjustable. What I did is on the top, the magneto has sloted holes so that you are able to turn it to some degree and this did seem to effect idle speed. I found a adjustment that returned the idle to 750 rpm's and its been fine all this time. It probably isnt an actual manufactures method but it seemed to make a difference for the better.

After this weekend with some cleaning up I will know more.

#7 blitz

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Posted 10 March 2006 - 02:04 PM

Any dealer will be able to order you a factory engine manual. I ordered a service manual for both of my subes and familiarized myself with all the sensors and their operation before I did any work under the hood.

The IAC & the TB both have adjustments, but they're factory set to a specific measured airflow. To the best of my knowledge, neither are user servicable items. If you changed any of these settings, I wouldn't be able to help to get them back to where they were.

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