Rpm inconsistent at idle - 6 Cylinder Gasoline - Ranger Engine Tech

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Michael71009
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Well I'm here yet again with another problem. Hopefully I can get this one figured out a little easier than my frame issue.
I had my truck tinted about an hour ago here at home. Before the tint it ran fine, but anyways... I'm having 3 vehicles tinted and 3 doors on my house tinted. So I got my garage heated and pulled ole red in the garage. After he finished it the battery was dead so I had to jump it to get it started. Was thinking maybe the key was left on, so when I pulled it out I left it running to let the battery charge. Pulled my next vehicle in and went about my business. I went out to shut the ranger off and the rpms were dropping so low the headlights were dimming. If I remember right it usually sits around 500. It was jumping from around 300 or so to over 1000. When I would try to pull off the truck would die unless I run the rpms up real high. Has anybody ever experienced this on the 3.0 v6?

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Posted: Mon Jan 1st, 2018 10:14 pm

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JAMMAN

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Sounds like IAC is bad or you have a really bad vacuum leak. Check for rotted hoses especially going into the brake booster.

Pull the hose barbed fitting out of the brake booster if the hose looks good and put your finger over the opening. If it instantly starts idling better then your brake booster might have a hole in it.

Chances are it is your IAC but look for other sensors that have become unplugged and listen for vacuum lines.

NOTE the IAC is not a sensor but I just made it sound like it is LOL. The IAC is actually a little motor.

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Posted: Mon Jan 1st, 2018 10:24 pm

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CBB9M
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If you hook up a voltmeter to the battery and when the engine is running ~1,000 rpm, what's the voltage?

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Posted: Mon Jan 1st, 2018 10:37 pm

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CBB9M
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And, what caused the battery to drain?

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Posted: Tue Jan 2nd, 2018 12:02 am

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Michael71009
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I'll check for vacuum leaks tomorrow, and how do you tell if the IAC is bad?

The key was left on while the guy was tinting the truck. I'll check the voltage tomorrow as well.

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Posted: Tue Jan 2nd, 2018 01:05 am

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Hails from the land of rust!

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Hmm, ok so honestly, whilst your battery is probably not as strong as it once was, i think your only problem here is that you actually need to charge your battery.

If it was totally dead from the doors being open all that time, just idling the truck after a bump won't even be close to enough for it. Connect a battery charger (you have one, right?) over night and you'll be good.

Your only other alternative is a nice long run, 30+ minutes which will give it more chance for a baseline charge to get back into it. Also, lights on?? You are barely charging if you left the lights on.

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Posted: Wed Jan 3rd, 2018 09:10 pm

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ShovelheadKevin
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Kinda sounds like a bad alternator. Even with a low battery once started it should still run off the alternator w/all turned off but the ignition.

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Posted: Wed Jan 3rd, 2018 09:19 pm

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Right, but it was run dead and jumped, and at least the lights were still turned on.

Seriously, that isn't enough, you need about ten minutes driving just to replace the charge that starting the engine took. Sitting there idling won't do it in that time.

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Posted: Wed Jan 3rd, 2018 09:20 pm

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JAMMAN

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I thought the main problem was the erratic idle, I have never seen a bad battery or alternator cause erratic idle.

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Posted: Wed Jan 3rd, 2018 09:22 pm

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I once helped a guy at work, who had left his lights on the whole day (and his radio apparently)

We bumped the car, the battery was so dead that though we eventually managed to fire it up, it really struggled and wouldn't idle properly for the longest time.

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Posted: Wed Jan 3rd, 2018 11:03 pm

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ShovelheadKevin
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JAMMAN wrote: I thought the main problem was the erratic idle, I have never seen a bad battery or alternator cause erratic idle.

The wife had a Rodeo that became possessed when her alternator took a dump.
Wipers, locks & a couple of other things grew evil minds of their own and then it mercifully just quit......til we replaced the alt and then back right again.

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Posted: Wed Jan 3rd, 2018 11:55 pm

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Michael71009
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I'e never heard of a bad battery causing it either. Growing up we had an 80s model ranger just to get around on the farm with, I remember my dad would start it with a battery charger hooked straight to the leads on the truck and drive it all day without a battery. No issues with idle as far as I can remember.
I took mine and had it tested, it' a 7 year old Wal-Mart batter, tested bad so I just went ahead and bought a new one for piece of mind. I haven' put it in yet. My alt and starter tested fine. Not sure what the readings were off the top of my head. The little slip they gave me is at the house.

Last edited on Wed Jan 3rd, 2018 11:56 pm by Michael71009

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Posted: Thu Jan 4th, 2018 12:00 am

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Older stuff didn't have electronic running gear ;)

But yeah, a fully drained battery won't assist the alternator especially at idle, where it doesn't output much, that's why in the winter, using more electrical equipment in cars (lights, fans etc) is harder on the battery.

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Posted: Thu Jan 4th, 2018 08:44 am

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mhoward
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I would think that if the battery was completely flat, the alternator may not be able to supply enough power to the computer, injectors, etc. Low voltage on any of those components could theoretically cause them to malfunction (or not function at all). I've had various consumer electronics act erratically when the batteries got too low.

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Posted: Thu Jan 4th, 2018 12:41 pm

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That. ^^

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Posted: Thu Jan 4th, 2018 08:13 pm

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CBB9M
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Michael71009 wrote: I'e never heard of a bad battery causing it either. .....

Vehicle electronics are designed to operate within a defined voltage range, and when they are operated outside of thisdesign range, unexpected things can happen (been there, seen it, done that). Since your vehicle ran fine immediately prior to this situation, then the battery was drained wayyyy down, then it was jumped to start, and now the idle is f*cked, it's only a logical starting point to step back and test/verify/and rule out an unexpected vehicle voltage. To do test is also pretty simple. Alos, in light of what has led to this point/problem, it's logical to do a good visual under the hood to verify that no wiring or vac hoses were either disconnected by accident, or as a remote possibility, were aged, brittle, etc and got pushed and developed a leak.

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Posted: Thu Jan 4th, 2018 09:08 pm

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JAMMAN

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I didn't say it couldn't, I just said I have never seen it. I've never seen a ghost or a real tornado or hurricane either.

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Posted: Sat Jan 6th, 2018 09:07 am

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Michael71009
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I finally found some time to put my new battery on. So far the problem is gone. I let the truck sit and idle for an hour this morning with the headlights on and heat on high before I left. It didn't seem as weak trying to pull of in 1st gear like it previously had.
Thanks for the input everybody. I'm still impressed that the battery alone was the only problem but I'm glad it's running better.

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