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Low/rough idle when engine is warm.

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Offline KyBash

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Low/rough idle when engine is warm.
« on: October 16, 2017, 01:40:23 PM »
2002 Chevy Tracker, Suzuki I4 2.0L engine. 2WD, Automatic, 217,000 miles.

I've had this vehicle about 3 1/2 years, and it sometimes dropped to 500-600 RPMs at stop lights and was consequently a little rough. But that was very intermittent -- would do it three or four times out of ten or twelve stops one day and then not again for a month.

I replaced the valve cover gasket Friday and installed a new PCV valve.

She ran fine Saturday, but had the low idle at some stoplights (not all). Sunday she ran fine, but I only drove a dozen miles or so.

Today, she idles at a solid 1,000 RPM when cold. When warm, she drops to 600-700 RPM and is a little rough, but about one time out of four, it'll drop to 300-450 RPM and is obviously very, very rough, like scary shuddering rough.

I put a vacuum gauge on her. It's 21-23 mm Hg when cold.

What did I screw up?
How do I make it right?

Possibly (but hopefully not) related: when I had the cover off, I pulled the spark plugs. 1, 2, and 4 had only beautiful, thin tan deposits and no noticeable wear after 27,000 miles (I changed them the day I bought her).

I didn't pull #3 because as I started to back it out, it got harder and harder until, maybe halfway out, I would have had to use force. I ran it back in -- it seated fine, but again, it got tighter as I tried to remove it. Rather than risking ruining something, I just left it (it's currently my only vehicle, so she has to stay operational). 

The only thing I can think of is it's crossthreaded, but I don't see how that's possible. When installing plugs, I run them in by hand, putting on the ratchet only for final seating. If I had somehow screwed up, I'd remember (no matter how bad my memory might be, I always remember every mistake).

Anyone know why it's hard to get that one out?
2002 Tracker, 4x2, 4dr, 2.0L 4I DOHC, virtually stock at present.

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Offline BRD HNTR

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Re: Low/rough idle when engine is warm.
« Reply #1 on: October 18, 2017, 02:12:34 AM »
You put it in too tight?  It doesn't take much pressure to hold, but then usually you can't get it started out.  Or you didn't use any lubricant on the spark plugs.  That is steel into aluminum and without any lubricant it can fuse to the threads so then you are pulling the treads out, which is why it gets harder the further you go.
Try pulling the spark plug when the motor is warm, aluminum expands more than steel as it heats up.
93 Tracker,XL7 springs & 1" raised spring pads in front with YJ springs in back, home built bumpers rear & front (w/winch), 2" x 4" rock tubes,  ARB front & rear, converted Sami rear to IFS, 33x12.5x15  aluminum rims, roll cage, 2.7L w/5 speed auto.

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Offline KyBash

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Re: Low/rough idle when engine is warm.
« Reply #2 on: October 19, 2017, 09:47:33 AM »
Thanks for the reply!

I doubt I torqued it too much. I'm not saying I never make such mistakes, but I'm nearly OCD about spark plugs. My dad worked at a Chevrolet garage (remember when a dealership handled only one make?), and one of his favorite stories was about a guy who got furious when the price for a tune-up went up $2 when it had already risen that much only five or six years before. Determined to save money, he bought some cheap tools and did it himself, including putting in some supermarket spark plugs. End result -- both heads had to be replaced at a very serious percentage of the value of the car.

I probably heard that story every three months from the time I was old enough to hand him tools as he worked until he handed over the ratchet, and doing tune-ups on the family car became my responsibility. 

It's stuck with me to such an extent I always hesitate before touching the plugs. I only pulled them this time because I replaced the plugs when doing a once-over when I bought her, and the plugs that were in it were in terrible shape. I was curious whether there was an actual problem which needed to be addressed or if it was the result of a decade+ of virtually no maintenance.

Spark plugs, exhaust system bolts, and the idle screw on a 2-barrel carb are my banes.

Someone in another forum suggested it might be a little carbon build-up, being squeezed tighter and tighter, digging into the threads as I unscrewed it. I don't know how the plugs fit in the head of one of these, but if any of the thread projects, then I think it's probably the problem. I know what a pain it is to tear down the shield around an automated welder because a stray spark landing on the exposed end of a screw can make it really difficult to take out.
2002 Tracker, 4x2, 4dr, 2.0L 4I DOHC, virtually stock at present.

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Online fordem

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Re: Low/rough idle when engine is warm.
« Reply #3 on: October 19, 2017, 02:12:08 PM »
Let's get serious here - from the sound of it I'd say you have enough experience to know that the only way there is going to be any "projecting" thread for carbon to build up on is if the wrong plugs have been installed, or, perhaps if some sort of threaded insert repair has been done and the insert was too short.

The plugs on these engines are "specced" for replacement every 30,000 miles, since you really have no choice, leave it alone for now.

Your idle problem is more than likely an EGR related issue, the EGR will be open when the engine is on the "over run" as you're slowing to a stop, and should close as the engine rpms drop into the idle zone, if it's dirty, it may not always close, and you end up with more "incombustible gases" in the mixture than you should have, causing it to stumble and run rough.
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