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Old 03-08-2013, 09:20 PM   #1
Aaron K.
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2001 SC1
Default Rough idle and random high idle

I have a 2001 SC1 with about 125K on the clock. I bought it a few months ago with 119K as a commuter car and have been fixing and fine tuning things a little at a time. Most stuff I've found searching the forum but I wanted to ask about this one.

First is the rough idle. I've changed plugs (Autolight copper) and wires with no improvement. The idle is a little worse when its warmed up, so I'm thinking intake manifold gasket. However whenever I try to spray carb/brake cleaner I don't ever get a change in idle. Can it still be bad with no change in idle when carb/brake cleaner is sprayed around it? I'm thinking maybe its cracked and broken on the bottom side where I cant really get to it to spray? Is there any other test I can do? All the little vacuum hoses to the throttle body and all look good as far as I can tell.

Another thing I noticed is in park or neutral warm or cold, I can make the idle go up or down about 50-100RPM by pressing and releasing the brake pedal. I know thats not normal, any ideas what that could be? What should these cars idle at? Mine is around 600-700RPM.

Now for the random high idle, it seems like it may be my TPS based on the searching I've done. Every now and then when I come to a stop my idle will jump to 1000RPM, when I put it P or N it jumps to 2000RPM. If I shut off the car and crank it back up it will go back to normal. Is there anything else that could cause this, maybe something related to the rough idle?

Thanks for any help!

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Old 03-08-2013, 09:37 PM   #2
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Default Re: Rough idle and random high idle

What parts were replaced?

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Old 03-08-2013, 10:13 PM   #3
minn505
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Default Re: Rough idle and random high idle

Are you getting any codes or do you have a check engine light on?

Although it probably won't fully solve your problem, you should clean your iac and TB if that's something that you haven't already done. It'll at least help reduce the roughness of your idle if they're dirty, and while you're doing that, test the TPS.

There are a lot of YouTube videos on how to test the tps.

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Old 03-08-2013, 10:39 PM   #4
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Default Re: Rough idle and random high idle

Quote:
Originally Posted by fdryer View Post
What parts were replaced?
^^^^ Have you changed the ects and closely inspected the ects connector and wiring ? That should be the first thing. Next try replacing the pcv valve with a fixed opening one. Many have found this to improve their idle.

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Old 03-10-2013, 12:29 PM   #5
Aaron K.
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Default Re: Rough idle and random high idle

No codes currently.

I've replaced/cleaned the following:
- Replaced plugs
- Replaced wires
- Replaced PCV w/ fixed opening
- Replaced front O2 sensor (was getting a code on that, its now gone)
- Cleaned TB
- Cleaned IAC
- Cleaned EGR
- Cleaned post secondary air induction ports in exhaust manifold and head (was getting a code on that, its now gone)

I checked the ECTS connector and wiring when I flushed the coolant when I got the car. I pulled it out and checked that it was brass and that the connectors and wiring look good. Everything checked out there and my coolant temp goes to 3/8 when warmed up like it should. I'm thinking I may just put a new one in since they are cheap and try cleaning the IAC and TB again just to be sure since I did that when I first got the car too.

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Old 03-10-2013, 03:25 PM   #6
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Default Re: Rough idle and random high idle

I'll cast a vote for the TPS.

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Old 03-10-2013, 08:40 PM   #7
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Default Re: Rough idle and random high idle

i cast a vote for the ects and the connector replace both with oem. and make sure you put tap on the threads watch rich pins video on you tube and do not crimp the wires use a sotaring gun and shrink tubes

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Old 03-10-2013, 08:53 PM   #8
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Default Re: Rough idle and random high idle

sounds like a vacuum leak to me. i'd check all the hoses and ends, paying a lot of attention to the big ones on the pcv system. mine had a huge crack on the one that hooks to the manifold on the manifold side. check the ones on the egr and egr solenoid too. whoever had the bright idea to put 3 piece pcv vinyl vacuum lines under the hood of a car should be taken out back and flogged soundly.

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Old 03-10-2013, 09:07 PM   #9
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Default Re: Rough idle and random high idle

the break thing theirs a check valve on the booster line take that off and drive safely and if there is a difference in rpm is that but every s series does that movement when hitting the breaks in drive because the engine is creating a vacuum so its helping you stop the car

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Old 03-13-2013, 01:42 PM   #10
Aaron K.
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Default Re: Rough idle and random high idle

I just went out and re-cleaned the TB and IAC and to double check all my vacuum lines and all. TB was pretty dirty, almost like I had never cleaned it before, but it cleaned up much easier than the first time. The IAC looked good but I cleaned it again anyway. I haven't put it all back together yet so I dunno if what I've done has fixed anything yet, I'm taking a lunch/ask a question break.

Of the two vacuum tubes that mount to the top of the TB, the smaller one runs to the front of the car to the secondary air/smog pump. The larger one runs to the back of the engine to some sort vacuum junction type of thing underneath the intake manifold and above the started. It looks like there is at least one more line that runs to it (fuel line, maybe a return line or something). What is it and what does it do? I can barely see it from the top. It's a little loose, I can wiggle it a little with my finger.

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Old 03-13-2013, 02:11 PM   #11
Aaron K.
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Default Re: Rough idle and random high idle

After looking at it a little closer it looks like its just those two previously mentioned lines running to it and an electrical connector. I don't think the fact that I can wiggle it is an issue, its just a plastic little box hanging off a little mounting bracket
Still curious what it is though.

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Old 03-13-2013, 03:14 PM   #12
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Default Re: Rough idle and random high idle

Search rockauto.com for the evaporator purge solenoid. That may be what you're looking at behind and below the intake manifold.

For testing the intake manifold gasket leak, try using plain water in a spray bottle that you can adjust the spray pattern - a stream may work better to flood any part of the intake manifold with the engine idling without worrying about fluid run off.

The power brake vacuum boost unit uses a large diameter hose to supply vacuum to the boost unit behind the brake master cylinder. Find the hose and squeeze it while spraying water to soak the hose for dry rot cracking (engine idling) and allowing a vacuum leak. The grommet on the intake manifold for the hose may be worn out and leaking vacuum.

Your research on the tps is spot on. Eliminating all the above and anything else suggested may lead you to the tps. OEM seems to be the recommended replacement.

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Old 03-13-2013, 04:33 PM   #13
Aaron K.
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Default Re: Rough idle and random high idle

I put it all back together after cleaning the TB and IAC and all….no change in idle. I tested the TPS as described here:
Quote:
Originally Posted by OldNuc View Post
Go find 2 straight pins, stick them into the black and dark blue wire, turn key to ON but not crank, monitor the voltage 0.4 to 4.7v with a 0.2 tolerance from zero to WOT. No stripping or tape required. Do not pull on the wires either or you will be replacing the connector.
My result was 0.52 to 4.30. Since its more than the 0.2 out of the upper range I assume it needs replacement, but my question on that is this…Will a new TPS just fix my random high idle, or could it also fix my rough/low idle? The rough/low idle is what I’m more concerned with as I can live with the random high idle as infrequent as it is.

Also, it did the high idle thing again on my way home from work this morning. I plugged in my code scanner (OBDII Bluetooth reader with “Torque” Android app) to check the throttle percentage. It reads a 1.6% when the high idle thing happens in either P/N/D/or R and 0.0% after restarting and idling normally in either P/N/D/or R.

I checked the brake booster vacuum hose and it looks good, I’ll try the spray/squeeze technique you mentioned shortly. I pulled it off with the engine running and without covering either opening up it idled up just like it does with the random high idle, about 1000RMP in gear and about 2000RPM in P or R. when I put my finger over the hose it idles down almost to the point it wants to stall, and comes back up 50-100RPM and idles like it normally does plugged in. I checked my throttle percentage while messing with this and it stayed at 0.0%. Dunno what all that means, if anything!
Thanks again for all the help and suggestions!

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Old 03-13-2013, 04:56 PM   #14
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Default Re: Rough idle and random high idle

Although the temperature indications are ideal, the thermostat was never replaced(?). They wear out to lower operating temps. This may contribute to a lower idle as the coolant sensor tells the pcm temps that would make the pcm continue with slightly richer fuel mixtures than needed, lowering idle rpm. I would consider both, t-stat and tips as replacement items.

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Old 03-13-2013, 05:00 PM   #15
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Default Re: Rough idle and random high idle

I forgot to list it earlier, but the thermostat was replaced a few months back when I flushed the coolant after buying the car.

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Old 03-13-2013, 08:33 PM   #16
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Default Re: Rough idle and random high idle

Ordered a new TPS. Hopefully that does it.

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Old 03-22-2013, 06:16 PM   #17
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Default Re: Rough idle and random high idle

New TPS is in. Idle is exactly the same and the voltage is now 0.56 to 4.40.

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Old 03-22-2013, 06:27 PM   #18
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Default Re: Rough idle and random high idle

Quote:
Originally Posted by fdryer View Post

For testing the intake manifold gasket leak, try using plain water in a spray bottle that you can adjust the spray pattern - a stream may work better to flood any part of the intake manifold with the engine idling without worrying about fluid run off.
Did you ever check for an intake manifold leak ?

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Old 03-23-2013, 10:01 AM   #19
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Default Re: Rough idle and random high idle

Quote:
Originally Posted by el diablo viejo View Post
Did you ever check for an intake manifold leak ?
Yup, with carb cleaner and with water in a spray bottle as described above.

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Old 03-23-2013, 11:04 AM   #20
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Default Re: Rough idle and random high idle

This is an outside the box idea, but if you can view when the EVAP system is opening on your scanner, you might be able to see if the high idle is happening during an EVAP event, if it is you might have a cracked hose, fitting, etc.

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