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Rough idle

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Old 25 Jun 2012, 10:25 pm
gvc2000's Avatar
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Join Date: May 2012
Posts: 28
Default Rough idle

hey all,

i've posted here in the general section about a blown head gasket, which i've since repaired. my concern now, and i've spent several hours researching this, is the rough idle i thought i didn't have once the head was replaced, but now it is very apparent, but really only at idle; like on the way to work in traffic at a dead stop. i feel it and i know it wasn't running this way prior (before i blew the head up). so i really would hate it if it were a tooth off, but, would it being one tooth off, show itself even at 65 miles/hour? it runs good. its this idle at dead stop i can't figure out. i replaced wires and put in some bosch plat 4's which i hear now are crap. i'll put my champs back if i have to. but i'm looking for the real pt performance experts to set me straight.........and on a path where i can begin to enjoy driving my rig again. and i've read all the possible things this could be; iac, emissions, fuel system etc.
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Old 25 Jun 2012, 10:30 pm
gvc2000's Avatar
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Join Date: May 2012
Posts: 28
Default Re: Rough idle

my bad


new tb, tensioner, pulley, wp, camshaft sensor, motor mounts.....
just put new plugs and wires this weekend
bosch plat 4+
oem wires though
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Old 25 Jun 2012, 10:47 pm
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Join Date: Nov 2007
Location: Mooresville, NC
Posts: 5,325
Default Re: Rough idle

There are just so many things that can cause a rough idle. More than likely you're getting a misfire on one cylinder. The best way to find out is to get you an OMBii reader. If you are located near enough to me I'd loan you mine.

There is just so many things that can affect your idle. Even crappy oil can affect it. Clogged oil filter, when the engine RPM's lower, the engine may have trouble pumping through it causing a rough idle, then when you accelerate you're pumping harder and faster and it could even out.

Could be a compression issue - that's not a good thing. I also have a cylinder leak test kit for compression testing. I try to keep just about every tool that I would use in nearly new shape in case I encounter one of these issues.

If you have an ammeter with an inductive coil pickup, you can clamp it around one of your spark plug wires near the plug and see if you are getting similar current to each spark plug. If you're running low on one, you could have a coupler issue, cable issue, coil issue, etc.

Heck, I think I could go on for hours on this subject. The main thing is getting someone to plug in an AutoReader to see what you've got going on. Most OBD tools are not true scanners, they are just to show you the DTC's and help you clear any error codes. I bought one of the high-dollar models just so that I can really dig into my engine. This way I also keep my wife's car running in tip-top shape too.

Whenever I have an idle issue, the first thing I do is pop the scan tool on and check out the engine. If I see a misfire (right now I'm having one on cyl 2, but it's pretty much intermittent), then I get my ammeter and see if I'm passing good current to each plug. If that reads good then I check my spark plugs for cleanliness and gap. Then I change my oil, making sure I'm using 5w30 synth. Most times my idle issues are gone by then. Actually, so far on my PT and my wife's T&C those have cleared all our idle issues...oh wait, but one. Both had AC issues, low pressure on both. Idle issue was during the summer and the AC was on all the time. Turned it off and the idle went smooth right away. Ended up pulling a 20 hour vacuum on both, then recharging both systems, and they've not had an issue since.

Last edited by Chromenut; 25 Jun 2012 at 10:52 pm.
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Old 26 Jun 2012, 12:06 am
NitroPT's Avatar
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Join Date: Jun 2012
Location: USA kalyfornia
Posts: 15,016
Default Re: Rough idle

"There are just so many things that can cause a rough idle"

Chromenut has touched on most coverage of this common complaint after a fairly straight foward service.

First I should recommend to the many reading... in any R & R job when a problem post appears, resist the temptation to start throwing more parts hoping to solve the first concern! Many times this makes it more difficult to slowly rule out the problem and more often times creates other issues besides the original problem.

One thing I might suggest in assisting to diagnosis this problem is to use a vacuum gauge. Because changing a head gasket, head, intake manifold all directly effect the vacuum response and how it does could effectively give a good clue to the problem.
If you get a chance to do this post up the readings and lets see if we can help you more?

This is a bit of a old school tutorial on how to read and use a vaccum gauge as a diagnostic tool. Hope it will help you?

How to Use and Interpret a Vacuum Gauge
How to Use and Interpret a Vacuum Gauge

The larger the face of a vacuum gauge the more sensitive the movments are in a helpful manor. Oh and for us old guys we don't have to run for the other seeing aids to read it!

Last edited by NitroPT; 26 Jun 2012 at 12:09 am. Reason: added web info
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