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Author Topic: Engine idle lunges and falls  (Read 5404 times)

AZRedhawk44

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Engine idle lunges and falls
« on: May 21, 2010, 06:54:23 PM »
2005 Dodge Ram 1500 SLT, 4.7L with auto transmission and about 87K miles on it.

I had engine idle problems, when I would turn the truck on and watch the tach, and it would go from 400 RPM to 1200 RPM, bouncing back and forth every few seconds, almost like my foot was gunning it.

Then, when I would put it into gear and drive off, I could hear engine pinging.

I had previously changed out the original copper plugs for platinums at about 60K miles, and I've noticed a drop in performance and power since then so I just changed the platinums out for new coppers this last week.  Pinging sound is gone, so I don't think I was getting good combustion or I was getting a late spark or something.

But, the revving problem is still there.

It tends to go away after driving for awhile.  But, if I come to an abrupt stop at a rapidly changing stoplight, it comes back.  The RPM's dip to near 400 from the rapid change in engine demand, then bounce back up.  Once the truck is warmed up, the lunge/fall is less dramatic than when it is cold, but I'd say 400-900 is the swing when it happens when the truck is hot.

I'm thinking of two things right now as tests...
-Replace the fuel filter.  Haven't done that to the truck yet.  Probably time.  Perhaps there's a restriction in it, and the fuel pump is getting wonky "hungry" signals from the engine ECM?
-Run some fuel injector cleaner through a tank.

But, I understand this can also be caused by a failed head cover gasket leak, a faulty PCV valve, or a vacuum leak elsewhere.  I don't see any oil leaking on either of the head covers, the PCV valve rattles when I pull it so it isn't stuck.  Vehicle vacuum systems have always plagued me.  Had a car with bad brakes for several years due to a leak in the master brake cylinder that I could never trace down.

I've also heard that the distributor cap can contribute to this.  A weak spark along the spark lines, or on the contact itself.  Given the fact that my pinging problem is gone, I think that unlikely, after the change in plugs.  Am I wrong?

Anyone got any ideas?
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charby

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Re: Engine idle lunges and falls
« Reply #1 on: May 21, 2010, 07:46:19 PM »
I would look first at cap and rotor, looks like you never changed them, probably be a good idea.. Fuel filter next, also replace that fuel pump if it is getting wonky.

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bedlamite

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Re: Engine idle lunges and falls
« Reply #2 on: May 21, 2010, 07:47:33 PM »
Found the problem:

Quote from: AZRedhawk44 on May 21, 2010, 06:54:23 PM
Dodge



Fuel filter wouldn't hurt, but they usually start causing problems when you need volume and pressure at full throttle, not idle.

Try cleaning out the EGR valve and idle air motor, otherwise I'd start looking for vacuum/evap system leaks. Have you got a Check engine light? If so see what the ECU says.
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Nick1911

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Re: Engine idle lunges and falls
« Reply #3 on: May 21, 2010, 08:03:05 PM »
Sounds like a vacuum leak to me.

Good luck.  =|
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dogmush

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Re: Engine idle lunges and falls
« Reply #4 on: May 21, 2010, 09:08:44 PM »
Idle air control.  They gum up and keep the engine hunting idle when the TB closes.
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Marnoot

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Re: Engine idle lunges and falls
« Reply #5 on: May 21, 2010, 09:41:23 PM »
Could also be the air flow sensor, bad MAFs often cause symptoms exactly as you describe.
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Perd Hapley

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Re: Engine idle lunges and falls
« Reply #6 on: May 22, 2010, 12:37:28 AM »
What are we going to do with these MacRamCultists? 
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Phyphor

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Re: Engine idle lunges and falls
« Reply #7 on: May 22, 2010, 01:44:05 AM »
It's a 2005.... does it show a Check Engine light?

Even if it doesn't, have you considered getting an OBD-2 scanner and hooking it up and seeing what codes your engine control comp throws?
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Brad Johnson

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Re: Engine idle lunges and falls
« Reply #8 on: May 22, 2010, 10:21:20 AM »
IAC's will cause that kind of problem.  Out of range O2 sensors will, too.  At 87k both could probably stand replacing anyway.  It's possible the TPS is giving trouble, but that usually results in a dead spot somewhere in the sweep.

As Phyphor said, pull the codes.  Most chain auto parts stores will do it at no charge.

If it shows no codes, try disconnecting the battery for 20-30 minutes to completel kill the computer's memory and force it to relearn all the operating parameters.  Do this AFTER you have the codes pulled or it will wipe any trouble codes.

Brad
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drewtam

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Re: Engine idle lunges and falls
« Reply #9 on: May 22, 2010, 10:42:11 AM »
Usually its the last thing touched, and sometimes its the simplest thing...

When you reconnected the plug wires, did you reconnect them to the correct cylinders?
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Brad Johnson

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Re: Engine idle lunges and falls
« Reply #10 on: May 22, 2010, 10:45:16 AM »
Quote from: drewtam on May 22, 2010, 10:42:11 AM
Usually its the last thing touched, and sometimes its the simplest thing...

When you reconnected the plug wires, did you reconnect them to the correct cylinders?

That would (or at least should) result in a dead miss and all sorts of complaining from the engine computer.

Brad
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Tallpine

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Re: Engine idle lunges and falls
« Reply #11 on: May 22, 2010, 11:26:35 AM »
Quote
Anyone got any ideas?

Well, I was going to suggest tapping gently on the side of the carburetor to unstick the float ...  :P
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Brad Johnson

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Re: Engine idle lunges and falls
« Reply #12 on: May 22, 2010, 01:20:20 PM »
Quote from: Tallpine on May 22, 2010, 11:26:35 AM
Well, I was going to suggest tapping gently on the side of the carburetor to unstick the float ...  :P

You sadistic SOB. Next you'll suggest he changes spark plugs on his diesel.  :laugh:

Brad
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Re: Engine idle lunges and falls
« Reply #13 on: May 22, 2010, 01:32:44 PM »
Quote from: Brad Johnson on May 22, 2010, 01:20:20 PM
You sadistic SOB. Next you'll suggest he changes spark plugs on his diesel.  :laugh:

Brad

If he's going to do that he better clean his plug wires first
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Tallpine

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Re: Engine idle lunges and falls
« Reply #14 on: May 22, 2010, 03:39:26 PM »
Quote from: Brad Johnson on May 22, 2010, 01:20:20 PM
You sadistic SOB. Next you'll suggest he changes spark plugs on his diesel.  :laugh:

Brad

Oh - if it's a diesel then the problem is the pony motor  =D
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Brad Johnson

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Re: Engine idle lunges and falls
« Reply #15 on: May 22, 2010, 04:44:14 PM »
Quote from: Tallpine on May 22, 2010, 03:39:26 PM
Oh - if it's a diesel then the problem is the pony motor  =D

*snicker*

Brad
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