Vacuum fluctuating, strange idle. - Ranger-Forums - The Ultimate Ford Ranger Resource


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Old 05-08-2015
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Vacuum fluctuating, strange idle.

Alright, so for a few years I've had an issue with my truck running rough. It feels as if it stumbles a little, and constantly catches itself. It never dies, but it tends to idle between 500 and 700. Trucks runs fine down the road, it just idles weird.

I hooked up my vacuum gauge to look for possible issues, and everything checks out. The truck idles at about 19in. I did all of the recommended tests, and the truck passes all of them. However, instead of the reading at idle staying steady, it seems to bounce between 18 and 19in. The bouncing tends to correlate with the audible sound of the engine rpm changing.

I've read that a slight change in the vacuum reading at idle would be an ignition issue, however my ignition coil, spark plugs, and spark plug wires are all new as of the last few months, and I still have the same issue.

So I'm just looking for suggestions on something I may have missed, or if there could be another cause for the fluctuating vacuum reading. I have a video of the gauge reading that I am uploading. I will add it as soon as it finishes uploading.

http://vid1308.photobucket.com/album...psdej67hmn.mp4

Last edited by Fordzilla80; 05-08-2015 at 12:35 PM.
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Old 05-12-2015
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uh, that's really nothing, the vacuum like that. It's actually good and steady.
What happens if you slowly increase RPMs to 2500? snap the throttle?
as far as the idle, that's the computer adjusting something(s).

First thing that pops to mind is fuel pressure - how's that?
spark plugs - are one or more out of gap? even .001" makes a difference. have you done a compression test lately? a couple of PSI on a cylinder would make a difference on idle and vacuum - and they all don't wear exactly the same.
fuel injectors/o-rings? a slight leak vacuum leak around one of the injectors would mess with spray pattern/idle.
sticky valve maybe? or a tired spring preventing full valve seal?
Clean the throttle body and IAC?
check the TPS? grounding?
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Old 05-12-2015
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Definitely some good points to try.

Slowly increasing the throttle gives me more vacuum. When I snap the throttle Vacuum drops way down and then comes back up.

I haven't check the fuel pressure, but It's on the list, as is the compression check.

Throttle body was cleaned a few weeks ago before I installed a new Motorcraft IAC. So both of those are fine.

Haven't checked the TPS in a long time, but I'll check it as well.

As for the injectors, I do have one injector that seems to be "louder" than the others. When I put my finger on the fuel rail above that injector, I can feel the injector pulse a lot harder than the rest of the injectors. Could that be an issue?
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Old 05-12-2015
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Fluctuating vacuum indicates misfire, a cylinder didn't do a complete burn so slowed down the crank, which of course changes RPM.

Most vacuum test results were for carbureted engines so assume fuel mix is equal for all cylinders, so for fluctuating idle vacuum it must be spark(or valve problem), fuel injection changed that.

Yes, fuel injector could be causing it, Ford injectors should read 9-16 ohms, test a few of them, if that one injector is off from the others change it.
Usually they test at 13-14ohms
OHMs test on injectors is like a compression test in one respect, it isn't all about the number, it is about the average number and the difference in one being off that average.
This is why you should test at least 3 injectors and the one you suspect, so you can get an average.

OHMs test doesn't mean injector is good mechanically, just electrically, only way to determine mechanical is to have it tested for flow.


Your 2003 3.0l is also in the timeline for recessed exhaust valve issue, this would lower compression in a cylinder enough to cause misfires at idle.
TSB was for '04-'06 Ranger 3.0l

Last edited by RonD; 05-12-2015 at 01:50 PM.
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Old 05-12-2015
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A "louder" or "harder" pulse is definitely indicative of SOMETHING. Could be it's just not seated as solidly as the others, and you're feeling the resultant movement of that pulse.
so if it's not seated properly, the o-ring seal might be letting some unmetered air in, making the ratio be off (lean), and by extension the pulse time would be off as well (short).
If it's an internal mechanics thing, if the pulse width is off, that cylinder would be either under or over firing...misfire.
Either way, by extension, rough idle. and "funky" vacuum reading.
As much science as it seems to be, there are tolerances...this is nit picking I'm sure. but in the best way.
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Old 05-12-2015
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I've got the recessed exhaust valve TSB in the back of my mind. Engine only has 91,000 miles, but I doubt that means anything when it comes to poor materials.

As for running lean, when I checked the gap on all of the plugs after I did the vacuum readings, all of the plugs were uniform in color and wear. None of them showed any sings of running lean, or running rich. That includes the plug from the cylinder with the injector.

Another tidbit of info. When it's hot, and I have the windows down, as I'm accelerating I can hear a noticeable fast ticking. Sounds exactly like the injectors, possibly from the passenger side, which is where the suspect injector is. I think at this point, investigating that injector is a top priority, along with the compression check.

Last week my coil went out on me. It was fine one night, and then the next morning it rained hard, and the truck ran like garbage. I knew it was misfiring the moment I started it up. Took it around the block, and was able to get a flashing CEL. Came up as a cylinder 1 misfire. Since the plugs and wires were new, I pulled the coil pack, and found a crack up the cylinder 1 post of the coil. I installed a brand new Motorcraft coil, and that took care of that.

Everything that has anything to do with spark has now been replaced (all Motorcraft) and/or checked, so I think I can cross spark off the list of possible culprits.
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Old 05-13-2015
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stone stuck in your tire's tread? hopefully not a nail or screw...
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