acting weird

[Andy D]

Alternators do get noisy when they charge, I haven't had an overcharging alternator, but once. It was quiet until the battery blew up.

 

[swynx]

Ill have to get a reading on it.

 

[V8RangerBoy]

Need to backtrack some before getting into charging system checks. I'm not saying a faulty charging system won't cause strange problems, as it certainly can. If you really suspect the alternator is your problem, unplug the small connectors to it or just take the belt off and see how it idles. My bet is that it will idle just the same. Ron had posted some very valuable checks that didn't seem to get much attention -

There are only two ways for air to be entering the intake on a healthy engine: one, past the throttle plates, and two, through the IAC motor. Remember that the IAC is simply an electronically controlled bypass of the throttle blades. First, you need to verify that the throttle plates are completely closed. Remove the large intake hose that comes from the air filter and make sure that the blades are completely blocking the throat of the plenum. If they are partially opened, back down the small screw under the throttle linkage until it is closed completely. Second, you need to verify that the IAC is not stuck open. Remove it, and you will see two holes on the backside of the IAC. Verify that the little plunger is not allowing air to flow between the two holes. Next, if the throttle plates are truly closed, you should be able to plug the holes on the intake manifold where the IAC goes (just use your fingers) and the engine should barely run, if run at all. It shouldn't be able to run without air. If you still have a high idle, or really ANY idle with your fingers over the IAC ports, you need to find out where the air is entering the engine.

 

[swynx]

My room mate has been moving out and its been raining so i just get random days I can park it in the garage. Hence the reason I haven't messed with it too much. He's supposed to be out Monday. Ill take the belt off then.

 

[swynx]

from what I can tell its closed. There is play in the linkage once the plate is all the wayvdown and I can't push the plate in further. I can blow into each hole on the iac and feel/hear air come out the other whole. very little tho. Won't the engine die without the air hose on because maf is disconnected?

 

[V8RangerBoy]

Yes, reattach the big hose but leave the IAC disconnected and plug the holes and see if it will run or not. Again, it shouldn't really be able to run. Also, just because the cables have slack doesn't mean the throttle is actually fully closed. As Ron mentioned, people will sometimes adjust the stop screw for the linkage thinking it's like a base idle adjustment, so you need to verify that the screw is not holding the throttle partially open.

 

[swynx]

Unplug tthe iac and it makes no difference. I put masking tape over the iac holes and put it back on. It ran smoother and doesnt hold a high rpm after I touch the gas. If I undo the bolts and pull it away it won't hardly run at all. The only difference the tape made was that it doesn't hold the high rpm after touching the gas.

Backed that screw out far enough that it wasn't touching. Truck idled at like 800 after that. Hit the gas and it smoothly went back down to 800 like it should. Drove it down the road and its idling above a grand again.

Also after I got it back to my house and it was idling high I decided to turn it off and start it it still idled high. Let it sit a few minutes. Actually long enough to right the first 2 paragraphs started it and it idled at 800 again.

 

[RonD]

Pull off 1 vacuum line at a time and cap it's intake outlet.
Check idle by reving engine and let it idle again.

Check every vacuum line.

On some TPS units the set screws were in oval holes so you could adjust TPS voltage, but if mis-adjusted it could hold throttle plate open a bit.

Without IAC hooked up, and with "idle screw" not touching throttle linkage you should be at 500 RPMs or stalled out.
So you are leaking air, either through the throttle plate itself or another hole.

 

[Andy D]

I lost track, have you replaced the throttle body ? One of my cars had a high idle. the butter fly linkage bushings were boogered up from age/heat/dirt. I have something going on with the Rat. It has a high idle that needs about 20 minutes to drop from 1500 to a still pretty high 1000 RPM. I haven't bothered to do anything about it. Except think about it. If I were to start looking for the trouble, I would start at the linkage and then on toward the gas pedal looking for wonky movement. Would a saggy body/ motor mount, put a bind on on the cable housing?

 

[swynx]

I have not replaced the throttle body. I did buy a new one and swap springs over because one of mine was broken. I've sprayed the crap out of the linkages there.

Oh and I noticed that the idle will stay below a grand if my girl friend drives. If I drive it idles high :/

Let you guys know what happens

 

[swynx]

I used map gas and unplugged lots of stuff. Idle never rose with the gas. Everything made a sound like an air leak and made the idle go up. I unplugged the pcv hose and it wanted to die. Till I hit the gas. I got another engine coming. So ill probably just rebuild it and drop it in. I can't find the air leak but there is one.

 

[RonD]

Yes, open PCV line would stall engine, that is too big of an air leak, so mixture gets over lean and starts to stall or lope, a small air leak raises idle because fuel mix at idle is slightly rich so adding a little air raises idle.

An engine is an air pump, when you restrict the air flow on the intake side(throttle plate closed) it creates a low pressure area(vacuum) this slows engine down because it can't suck in enough air for a high RPM.
Minimum air pump speed(idle) is maintained by fuel/air mixture combustion.

So when there is an air leak, two things are happening, extra air lowers vacuum so engine can spin faster, and it will create a leaner fuel mix which has more power so increases engine speed.

The perfect fuel/air mix is too lean to run in an engine, oodles of power but valves and pistons will melt from the heat, so all engines are run slightly rich to keep cylinder temps down, this allows an air leak to raise idle vs stalling out the idle.

 

[swynx]

How common is it for the intake Plenum gasket to leak? Engine has seen a lot of miles. Also what about furl injectors. I haven't changed either of them. This high idle has stumped even the ase mechanic I've taken it to.

 

[V8RangerBoy]

How common is it for the intake Plenum gasket to leak? Engine has seen a lot of miles. Also what about furl injectors. I haven't changed either of them. This high idle has stumped even the ase mechanic I've taken it to.

Then I wouldn't be going to that guy again. Honestly, ASE is a bit of a joke. I could train our secretary to pass them. Take it to the busiest place you can find.

As Ron said, an engine is just an air pump. It should only be able to pull air through the throttle body (which includes the IAC ports). If you block the IAC ports and the throttle plates are fully closed, the engine should stall. Plenum gaskets are common leaks on alot of engines, as are lots of other vacuum leaks. The PCV hose could be a problem - unhook it and plug the vacuum port with your finger or a vacuum cap.

Installing a completely different engine for a vacuum leak is a pretty extreme step. I would encourage you to stick with diagnosing this - you learn a lot more from the problems that fight you than the ones that you know right away!

Edit - I went back and looked for a specific note on what engine this is. I couldn't find one, so I will go with the truck in your signature - a '94 4.0 OHV. Obviously it has EGR because you were asking about the DPFS. Vacuum leaks are common around the big green o-ring between the EGR tube and upper manifold. Also common for the lower plenum gaskets to leak - these are the gaskets between the lower manifold and the fuel rail assembly, or the gaskets between the fuel rail and the plenum. Common vacuum lines are over near the A/C and heater box, but usually won't cause a high idle like this.