Strange High Idle, some advice needed...

[DisturbedMXer8]

Okay guys, so my 88 2.9 decided to start idling high about a week or two ago (right around the same time that mornings started to hit the low 30's when I leave for work). As soon as it's fully warmed up, she normally idles perfect, dead at 800 RPM. Now, as soon as it's warmed up, she's idling at 900-950 in drive, 1k in park. What's weird is, if I snap the throttle, it comes back down to 800, then adjusts itself back up to the 900-950, as if it has no idea that it's idling too high. What's even weirder is that if I go on a longer than normal drive (say 10 miles/30 minutes instead of 5 miles/15 minutes) it will eventually correct itself and the idle will fall back down to 800... WTF.

So today, while trying to look for vacuum leaks, I spent a full bottle of carb/throttle body cleaner on it and didn't find anything. However, I did notice that when I stepped on the brakes, it will idle up a bit (50 to 100 rpm), and come back down when I let off. So, I then decided to pump the brakes for 10 seconds or so, and the idle came all the way up to 1100 or so, then back down to about 850 (had been idling in the driveway for close to half an hour at this point, after a short drive around town to warm up)... is this normal? I can hear a slight hissing sound when I have my pedal depressed that slowly goes away, could this be the source of my high idle? Or is it normal for idle to fluctuate when the brake pedal is depressed?

Any help is much appreciated. I am trying to install my center console soon, but I really can't justify spending the time on it when I know there is something else wrong. Thanks guys, 'preciate it ahead of time

 

[adsm08]

Booster might be on it's way out.

Higher idle is normal during cold weather operation though.

 

[DisturbedMXer8]

Yeah, but at 65 degrees or so on my way home, 950 rpm is a bit high... it's never done this before, at least. I've had the truck a little over 4 years now, and it's always had a solid 800 rpm idle, which is why this is driving me crazy. I'm just not sure if it's normal for idle to go up a bit when the brake is pressed. I'm just really anal when it comes to this truck, I can't think straight if there is something wrong with it that I can't figure out, hehe...

 

[adsm08]

Like I said, booster is probably at fault. Check it for fuel.

Mine has always idled around 950 or so.

 

[Tedybear]

Check the valve and the rubber grommet. I've seen this one personally. The rubber can get a split and cause some really oddball leaks. If the rubber grommet is nice and tight, and the valve is good? I'm with adsm....the booster might be on the way out.
(edit: this would be of course, the valve and grommet on the brake booster)


S-

 

[DisturbedMXer8]

How exactly would fuel get into the brake booster? I pulled the vacuum line off it earlier to see if it made a difference, but at that point it had been idling for a long time and had already come down quite a bit so it didn't do much.

 

[adsm08]

Failed fuel pressure regulator would allow fuel into the vacuum system, where it generally pools in the booster. It's the #1 cause of booster failure.

 

[DisturbedMXer8]

Okay, well the valve and grommet LOOK ok, I actually checked that earlier because I've had to replace the grommet before. I don't see anything bad there. So a bad/leaky brake booster can cause a high idle? That's what I'm most confused about, because I don't want to start looking into that if there's a chance that it's normal, ya know?

I know some cars idle at 900 or so normally, and I wouldn't be worried if mine did as well... but it never has, it's always been almost perfectly at 800. Now it's higher, all of the sudden. That's why I know there's something amiss, here.

 

[DisturbedMXer8]

Sorry, I keep missing your guys' posts here, haha (and thanks again for the advice). I had a fuel regulator go bad maybe 3 years ago or so... maybe it weakened it enough to cause it to fail in the cold weather this year? Is there a way to test this?

 

[adsm08]

Well the booster works on vacuum assist. If the rubber diaphragm is getting bad it can let air through, so when you push the brake is lets air through. Idle bumps because of the added air.

 

[adsm08]

Sorry, I keep missing your guys' posts here, haha (and thanks again for the advice). I had a fuel regulator go bad maybe 3 years ago or so... maybe it weakened it enough to cause it to fail in the cold weather this year? Is there a way to test this?

Pumping the brakes and watching the tach is the test.


If you can, run it for a few minutes, let it sit for a few days, then hit the brake pedal without starting it. If you have a rock-hard pedal you know there is vacuum bleed-off.

My dad's truck sat for 2 years with a blown engine and the booster held vacuum that whole time. I still got a few powered stops as we were pulling it on to the trailer.

 

[DisturbedMXer8]

Wow, okay... well there's an answer for me right there. It's NEVER been able to hold a vacuum assist. It will for a few minutes, but after a few hours (or a day at work, for example), it's rock hard when I get back into it. But it's always been like this, why the high idle now? Just finally got worse, maybe? The hissing while it's depressed definitely lasts longer than it used to. It used to go away after a couple minutes. Now sometimes it doesn't go away at all.