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  #1  
Old 12-08-2008, 11:18 AM
bordelond bordelond is offline
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Automobile engine revs up and down during parked idling -- what causes that?

Thanks in advance to all the mechanically-knowledgable on this board. You guys and gals do yeoman's work


Got a 2003 Ford Taurus SES, 6-cylinder, automatic transmission. There are three symptoms that are vexing me:

1) When idling, the engine will rev up and down all by itself, fluctuating between 600 and 1400 rpms or so. The effect is exactly like someone gently pressing down on the gas pedal and then letting up, over and over. This effect is most notable when the car is parked, but also occurs when stopped for red lights, or even during low-speed driving (under 35 mph or so).

2) The car struggles to go up bridges in the usual Drive "gear" now ... you really have to gun it or shift to D1 (?) to get the rpms up. Before, you could just gradually increase the gas while in the ordinary Drive "gear" and going up bridges was a piece of cake.

3) In concert with the above, the Check Engine light has been for about 6 months ( ), and it ain't the gas cap. I have avoided getting the code read because last time I had that done, it cost $100. My understanding is that I can buy a code-reader for myself for much less than that ... but I have put off buying the code-reader ( ). It is also said that I can go to Auto Zone and Pep Boys and get the code read on the spot for free ... but I have also put that off (I am not a car's best friend, I guess ).

The symptoms described in #1 have been on and off for since July ... when the Check Engine light first came on in May, nothing was apparently wrong. I originally blamed the problems on bad gasoline, and STP Gas Treatment (Engine Treatment?) improved the situation markedly for a few weeks, but the symptoms have since returned.
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  #2  
Old 12-08-2008, 11:24 AM
Pork Rind Pork Rind is offline
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My guess: one of your oxygen sensors or possibly your MAF or MAP sensor is bad or dirty. But you really need to get the code read for sure. Takes less than a minute...
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  #3  
Old 12-08-2008, 11:26 AM
bordelond bordelond is offline
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Originally Posted by Pork Rind View Post
My guess: one of your oxygen sensors or possibly your MAF or MAP sensor is bad or dirty. But you really need to get the code read for sure. Takes less than a minute...
I know -- code-reading should be gratis on a $600 repair ... but Firestone was happy to ding me for $100 all the same
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  #4  
Old 12-08-2008, 11:27 AM
Cerowyn Cerowyn is offline
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As a complete layman, and until Rick or another knowledgeable Doper comes along, I'd suggest taking the car in and getting it repaired (i.e. not just having the service code read). It sounds like the oxygen sensor, or perhaps fuel management system, is suffering a fault that causes a loss of power and random changes in engine speed. Not a good thing.
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  #5  
Old 12-08-2008, 11:30 AM
Pork Rind Pork Rind is offline
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Originally Posted by bordelond View Post
I know -- code-reading should be gratis on a $600 repair ... but Firestone was happy to ding me for $100 all the same
And that's why you go to Pep Boys or Autozone where they'll do it free!
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  #6  
Old 12-08-2008, 11:30 AM
Cerowyn Cerowyn is offline
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Originally Posted by bordelond View Post
I know -- code-reading should be gratis on a $600 repair ... but Firestone was happy to ding me for $100 all the same
That'll teach me to answer a phone call while writing a response.

Are you saying that you had the car repaired and that it still happened afterwards? And, I have never in my life heard of a shop that would dare to charge for getting the code read as part of a repair job. How the heck else are they going to be sure what's wrong?
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  #7  
Old 12-08-2008, 11:32 AM
beowulff beowulff is online now
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This is symptomatic of an air leak. Can you hear a hissing noise in the engine compartment?
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  #8  
Old 12-08-2008, 11:34 AM
bordelond bordelond is offline
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Originally Posted by Cerowyn View Post
Are you saying that you had the car repaired and that it still happened afterwards? And, I have never in my life heard of a shop that would dare to charge for getting the code read as part of a repair job. How the heck else are they going to be sure what's wrong?
Sorry to be unclear -- it was a repair on another vehicle. I like going to this particular Firestone, but was not pleased to be charged $100 for having the code read. It was itemized and everything.

Last edited by bordelond; 12-08-2008 at 11:35 AM..
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Old 12-08-2008, 11:37 AM
bordelond bordelond is offline
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Originally Posted by beowulff View Post
This is symptomatic of an air leak. Can you hear a hissing noise in the engine compartment?
Yes, very much so after parking once we've arrived at our destination. I don't hear that while driving, of course.
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Old 12-08-2008, 11:39 AM
bordelond bordelond is offline
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Originally Posted by bordelond View Post
Sorry to be unclear -- it was a repair on another vehicle. I like going to this particular Firestone, but was not pleased to be charged $100 for having the code read. It was itemized and everything.
And for anyone that goes and reads that thread, the solution given in the last post ended up only being a temporary fix. About six months later, I needed a new distributor cap.
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  #11  
Old 12-08-2008, 11:58 AM
beowulff beowulff is online now
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Originally Posted by bordelond View Post
Yes, very much so after parking once we've arrived at our destination. I don't hear that while driving, of course.
If you want to try to diagnose this yourself, get a piece of flexible hose, and use it as a stethoscope to try to isolate where the noise is coming from. Look particularly closely at the brake booster, any vacuum manifolds, and vacuum hoses going through the firewall. Look for cracking hoses and broken connections.
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  #12  
Old 12-08-2008, 02:20 PM
Gary T Gary T is online now
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Idle surge and hissing noise certainly suggest a vacuum leak. Be sure to check all the PCV hoses. This may or may not be reflected in the code(s) related the Check Engine, and may or may not be related to the poor acceleration.

There's a good chance you can fix the idle surge by finding and fixing the vacuum leak. But if you want to be sure of getting everything fixed, it should be thoroughly checked by a competent repair facility.
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  #13  
Old 12-08-2008, 02:26 PM
Gary T Gary T is online now
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Originally Posted by Cerowyn View Post
...I have never in my life heard of a shop that would dare to charge for getting the code read as part of a repair job. How the heck else are they going to be sure what's wrong?
Wow. It's not 1965 anymore - the good shops invest a lot in equipment and training in order to perform effective diagnosis, and charge for doing it. I'm not familiar with any shops that don't charge for reading and clearing codes.

Of course, I've always wondered how physicians DARE to charge for examinations and diagnostic tests. How the heck else are they going to be sure what's wrong?
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  #14  
Old 12-08-2008, 02:30 PM
bordelond bordelond is offline
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Originally Posted by Gary T View Post
Idle surge and hissing noise certainly suggest a vacuum leak. Be sure to check all the PCV hoses. This may or may not be reflected in the code(s) related the Check Engine, and may or may not be related to the poor acceleration.

There's a good chance you can fix the idle surge by finding and fixing the vacuum leak. But if you want to be sure of getting everything fixed, it should be thoroughly checked by a competent repair facility.
Thanks, GaryT. Where are those PVC hoses?
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  #15  
Old 12-08-2008, 02:38 PM
bordelond bordelond is offline
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I'm not familiar with any shops that don't charge for reading and clearing codes.
People are telling me that Auto Zone will read the codes for free. Of course, they won't be clearing them.
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  #16  
Old 12-08-2008, 02:41 PM
Gary T Gary T is online now
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Originally Posted by bordelond View Post
Thanks, GaryT. Where are those PVC hoses?
The picture I looked at isn't crystal clear, but it appears that one goes from the air intake boot to the front valve cover, and another goes from the PCV valve (in the rear valve cover?) to somwhere near said boot. These are usually hose/pipe assemblies, so check their entire lengths for any deteriorated rubber parts that might be leaking.

Last edited by Gary T; 12-08-2008 at 02:42 PM.. Reason: clarification
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  #17  
Old 12-08-2008, 02:50 PM
Gary T Gary T is online now
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Originally Posted by bordelond View Post
People are telling me that Auto Zone will read the codes for free. Of course, they won't be clearing them.
Yes, parts stores often read them for free in the hope of selling you a part that logically relates to the code. And sometimes this works out. Often, though, just knowing the code isn't enough to really determine what's wrong. For example, a code described as "low EGR flow" could be caused by a faulty EGR valve, EGR valve position sensor, EGR solenoid, or EGR hose; or by clogged passages in the intake manifold. Paying to replace a good working EGR valve or sensor only to find out the problem lies elsewhere is not fun. A good shop is going to read the codes, research the codes, check and evaluate scan data, test as necessary to determine the root cause, then clear the codes and check for any signs of their recurring.
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