Bizarre Idle Behavior

anupaum · Nov 28, 2007

Hello everyone!

Three days ago my truck started acting strangely. Despite setting the air solenoid to control fast idle speed at 1 500 rpm, it began fast idle at nearly 2 000 rpm. Once warmed up, the idle would drop a few hundred rpm, and when my foot was out of the throttle (like when sitting at a light), the fuel cut would come in (as it's supposed to) and the injectors would shut off until the rpms dropped to below 1 000. Then, of course, the computer would turn them on again.

This "hunting idle" behavior used to happen a lot with the factory computer when I had air leaks. My first thought was that I had a vacuum leak happening somewhere. I searched all over the engine, yet the only thing I found was that my PCV valve had blown off. (I noticed quite a bit of oil on the back side of the engine and thought: "Uh oh, what have I done now?") I cleaned the valve and reinstalled it, but the high idle persists.

Thinking that maybe my IAC valve had the bun, I pulled the wiring off. Nothing happened. It's as if the IAC valve doesn't exist!

Now that's REALLY strange, because I've got the throttle stop closed and the IAC is controlling my idle. I'm thinking that maybe my IAC valve has clogged and is stuck "open." I have to check that, but I don't have time right now.

So before I become completely frustrated with my truck, do any of you have any ideas for me? How do you track down an elusive vacuum leak?

TireIron · Nov 28, 2007

you might be right on with thinking the IAC based on the behavior you describe. If the IAC is stuck open too much then it will be almost the same as having the throttle slightly pressed allowing too much air to pass by and jump the RPMs back up. The fuel cut happens because the throttle position sensor says its closed, then the RPMs drop regardless of extra air, then when the injectors kick back on the extra air bypassing the TB plate allows the engine to jump back up again. Of course this could also be from an air leak somewhere else along your intake plubing but if you cant find anything specific and it drives fine (an air leak after the MAF would make it drive poorly) and only has issues with idle then start with the IAC and go from there.

anupaum · Nov 28, 2007

TireIron said:
you might be right on with thinking the IAC based on the behavior you describe. If the IAC is stuck open too much then it will be almost the same as having the throttle slightly pressed allowing too much air to pass by and jump the RPMs back up.

It makes sense, but when I went to pick up my boys from school, I hooked up the laptop and studied what was going on.

Firstly, the "throttle" value is showing 22% when my foot is completely out of the pedal. When I query the TPS sensor, I'm getting a value of 55, which should be COMPLETELY closed with the way I have this thing set up. So that confirms that my TPS is working.

However, sometimes fuel cut comes in, sometimes it doesn't. That means there IS something wonky with the TPS.

I'll have to take the IAC unit off and clean it up a bit. Those things are REALLY expensive to replace, so I'd rather not replace it unless I have to . . .

Also, my A/F ratio is running in the 12's and 13's most of the time. Why would it be so rich? Strange!!!

anupaum · Dec 1, 2007

I was talking to a mechanic friend at church today. He thinks my problem may be a PCV valve that isn't seating properly. Has anyone heard of this before? I've NEVER changed my PCV valve . . .

TireIron · Dec 2, 2007

well if the PCV is popping off and you dont even have the blower hooked up then something is having an issue. Was the PCV clogged or plugged up when you checked it? Are you useing the PCV from a turbo coupe? which tube did it pop out of, the one from the intake or from the engine block?

anupaum · Dec 2, 2007

TireIron said:
well if the PCV is popping off and you dont even have the blower hooked up then something is having an issue. Was the PCV clogged or plugged up when you checked it?

No. Those two issues have me scratching my head a bit. The PCV valve rattles around in there just fine, so it doesn't SEEM like there's anything wrong with it.

Are you useing the PCV from a turbo coupe? which tube did it pop out of, the one from the intake or from the engine block?

I've been using the stock Ranger PCV valve. It popped out of the tube that goes down to the engine block.

Strange, huh? What's the part number for the turbo PCV valve?

anupaum · Dec 2, 2007

Ok, I have "progress" to report . . .

I swapped out the PCV valve. That changed NOTHING. I found a vacuum leak underneath my stock throttle body and fixed it. That changed NOTHING. I took off my IAC valve and cleaned it out, and that changed something . . .

Now the engine won't idle below 3 500 rpm!

Ugh!

Sometimes I HATE this thing!

Dave R · Dec 2, 2007

anupaum said:
It popped out of the tube that goes down to the engine block.

Generally, that behaviour indicates excessive crankcase pressure.

anupaum said:
Now the engine won't idle below 3 500 rpm!!

Are you sure the throttle isn't sticking?

anupaum · Dec 2, 2007

Dave R said:
Generally, that behaviour indicates excessive crankcase pressure.

Which is generally cause by a plugged PCV valve, right?

Are you sure the throttle isn't sticking?

That was the very first thing I checked!

anupaum · Dec 7, 2007

I swapped out the IAC valve, fired the truck up and that seems to have solved the problem. It idles beautifully at about 700 rpm, though the vacuum reading seems a bit high.

I probably have to adjust the settings for IAC control, given that this is a new valve. That's one drawback to owning an aftermarket system. Once I start changing things, I have to re-tune!

Dave R · Dec 7, 2007

High vacuum is seldom something to complain about.

One IAC solenoid shouldn't act that significantly different than another unless it was designed for another application, i.e. has a significantly large capacity to pass air through it.

anupaum · Dec 7, 2007

Dave R said:
High vacuum is seldom something to complain about.

Ack! Sorry Dave, that was poor wording on my part. The vacuum signal was running at about 14 inches, rather than 18, as it had been.

One IAC solenoid shouldn't act that significantly different than another unless it was designed for another application, i.e. has a significantly large capacity to pass air through it.

You're right, but I've tuned my idle by using the IAC valve. Apparently, the old, leaky valve was letting more air through, so I'd tuned the idle A/F ratio to run in the low 14's. When I put the new valve on the mixture was sitting around 12:1. I leaned the idle bins down a few notches to put the mixture back to where it should be and the vacuum reading improved.

My theory in this was that the old IAC valve wasn't shutting completely, so the engine was ALWAYS getting air around the throttle plate. When the valve finally went south, it was stuck open--hence the high idle.

Replacing the valve shut off the additional air I'd compensated for in my tune. Does that make sense?

Dave R · Dec 7, 2007

anupaum said:
Replacing the valve shut off the additional air I'd compensated for in my tune. Does that make sense?

More or less, yes. Doesn't MS compensate for fuel ratio automatically?

anupaum · Dec 7, 2007

Dave R said:
More or less, yes. Doesn't MS compensate for fuel ratio automatically?

It can, if I set it up that way. Megasquirt enables me to determine to what extent EGO controls the A / F mixture, but it doesn't do that at idle.

Bizarre Idle Behavior

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