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Old 09-26-2013, 11:02 PM   #1
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Surging Idle, Poor Mileage

Car: 2003 ZX3
Zetec, 81K miles.

Issue: The engine surges a bit at idle (up to 950 or so from the regular 800RPM), then drops back down. Sort of like the surging idles I've seen on tuned SVTs. My car is stock, however.

I was originally thinking it's a vacuum leak, but the engine doesn't stumble when the idle drops back down. I'm not sure where to start.

Additionally, I've been getting pretty shit mileage. ~260 to a tank when I normally get 300 (80 city/20 highway). It's not winter fuel yet.

I'm wondering, based on my very limited knowledge...if the MAF is effed, and the computer's adding fuel to keep the A/F mixture stable? If I'm running rich, it'd explain the MPG loss as well.

The car pulls just fine.


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Old 09-26-2013, 11:35 PM   #2
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I dunno... can you get some scan data like long and short term fuel trims, MAF sensor flow in g/s at WOT. (or MAF counts I'm sure some of these guys will want) spark plug condition etc.
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Old 09-27-2013, 12:23 AM   #3
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I don't have a ScanGauge so it's tough to get anything precise or do any datalogging.

But I can tell you I changed the spark plugs when I changed my oil about 4K ago. Autolites.
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Old 09-27-2013, 10:26 AM   #4
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Idle air control valve or, IACV for short. Remove it and clean it.
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Old 09-27-2013, 11:07 AM   #5
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Quote:
Originally Posted by freemind View Post
Idle air control valve or, IACV for short. Remove it and clean it.
That was another suspicion. Found this video:

YouTube Video
ERROR: If you can see this, then YouTube is down or you don't have Flash installed.
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Old 09-27-2013, 12:44 PM   #6
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Update: I couldn't find the IAC where the video says it is. Occurs to me that when he says Zetec...he probably means the 1.6L model.

Disconnected the battery for a while, started the car and let it idle for 5 minutes. Problem didn't return, but we'll see what happens. May wind up getting a jack and replacing the IACV.
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Old 12-15-2013, 12:44 PM   #7
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Second update:

Changed the IACV. The idle still hunts. Also, whenever I depress the clutch while I'm in gear, the revs will shoot up to about 1800 RPM. As I come to a stop, the revs will stay high, but settle after a few seconds to the hunting range (600-1000RPM).

Notably, the revs will also shoot up a little bit between shifts.

The hell am I missing?
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Old 12-15-2013, 01:32 PM   #8
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having the same problem, need to figure it out. its not really affecting my mpg but the idle and cutting out gets old.

im thinking its my iacv or throttle positioning sensor. im gonna clean the iacv like it said in the video later today, ill post results to this thread when I get them.

have you checked the vacuum hose that has a 45 degree bend right before it goes into the intake manifold? they like to collapse on themselves at the bend with age.
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Old 12-15-2013, 07:12 PM   #9
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^ No, but I suppose it could be a vacuum leak. Our cars are at the age when rubber shit starts to crack and break.
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Old 05-02-2014, 08:53 AM   #10
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UPDATE:

A fellow fanatic and I did some exploring, and we discovered a hissing sound at the back of the engine bay. We're pretty sure we found the culprit of said hissing (see attachments).

The question is...what is it, exactly? We removed it, but there were no rips or cracks to be found. Are we on the right track?
Attached Thumbnails
Surging Idle, Poor Mileage-20140419_153600.jpg  

Surging Idle, Poor Mileage-20140419_153615.jpg  

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