Extremely high idle, very low range of RPM

[ravinsaend]

Our '92 shows 3k+ RPM on startup, at all times idle, and ranges from there to just under 4k RPM. Just changed plugs, wires, IAC, air filter, oil/filter. It SOUNDS a lot better as well as running better, but the tach still shows the same. I'm not one hundred percent that the tach works properly. Before startup, when I turn the key to the on position, it jumps up to where it sits at idle. Aside from that it seems to work properly, though. Anyone have any ideas regarding this that will not involve randomly changing parts and hoping?

 

[Iron Weasel]

If you think your tachometer isn't accurate and you're that concerned about it, you can buy handheld tachometers for around $50 all the way up to over $300.

The general cause of a high idle like that is a vacuum leak. Unfortunately, vacuum leaks can sometimes be difficult to track down since it doesn't take a very big leak to make the RPM go crazy. The fact that the engine is running so fast just complicates the matter since you might not be able to hear the telltale whistling sound a vacuum leak makes.

One of the quickest ways to track one down is the "spray carburetor cleaner around the engine" method. It's not exactly the safest method, but it's very effective. Take a couple cans of carburetor cleaner and spray all the vacuum hoses, gaskets, and intake components. If the RPM spike higher than normal at any point, then you've probably found a leak.

One fairly common area on the first gen OHV 4.0 that develops a vacuum leak is the lower intake manifold. The bolts / nuts that hold the manifold down tend to loosen up over time. Sometimes, the fix is as easy as just tightening the fasteners back down, but if the problem has existed for a while, then the lower intake manifold gasket can literally break apart which causes a huge vacuum leak.

 

[ravinsaend]

I was afraid I was gonna hear vacuum leak. Unfortunately we haven't had the truck long enough to know what all is what on it. Guess I'll look and check out that lower intake, this thing wasn't taken care of at all as far as I can tell. guy we got it from supposedly haqd it for four months and everything I've taken out right down to the fluids have looked like they've not been changed in years, if ever. I honestly don't know how the thing ran at all after seeing what I've taken out. Even with the idle staying as it is the thing runs like a different truck entirely, and the only thing aside from general maintenance I've done is the IAC.

Don't suppose anyone with one of those hand held tachs lives along the I70 corridor of Missouri? I can't afford anything not 100% necessary right now as after we get a trailer for our Eagle we're gonna be about a grand over budget on our moving fund.

 

[FIND]

does it sound like it is idling really high even, or are you just talking about a tachometer that is not functioning correctly? If it was idling at 3k rpms, it would be difficult to stop with the brakes and it would lurch REALLY hard when you shifted into gear. When you turn the key to the on position with the engine off, your tach should read 0. If it is reading a normal idle RPM, then the dampening in your tachometer is not functioning correctly and you need a new cluster or to replace the tach portion of your cluster.