[DayOff]

Okay. Yesterday, on my way to work, I was sitting at a stoplight when the car dipped below 500 RPMs then back to 1000 and down and up

(you get the picture). It was very rhythmic. Then last night, when I got home, I tried to reproduce the problem by sitting in my driveway with the car in gear and my foot on the brake. It dipped below 500 and stayed there. Then, when I put it in park, the idle started to climb. It was almost 3000 when I shut it off.

What's going on to my poor car?

 

[silvapain]

what year car? I am having some of the same symptoms, but my proble is because of the engine runing loo lean, and the 02 sensors trying to compensate. that's why it's cycling (at least, for me it is).

 

[DayOff]

Forgot to mention...98 Sable GS, 3.0 Vulcan

 

[silvapain]

IAC (Idle Air Control). it's on the TB.


to check, disconnect the IAC. turn the key on, but do not start the car. USing a voltimeter, place the red lead on the red wire at the connector, and the black lead on the white/blue wire. the meter should read between 10.5 and 12.5 volts DC.

 

[TheGreatZambini]

COULD be something sticking in the throttle linkage, or a bad throttle position sensor.

 

[DayOff]

Well, if I tested the right thing (top of the engine near the firewall and the window)...I got 11.68 for the IAT.

 

[silvapain]

The IAT and the IAC are different things. The IAT is the thermistor in the intake between the MAF and the TB. The IAC is a solenoid that is on the top rear corner of the TB itself. Just to clarify, which did you test?

 

[DayOff]

Okay, this is not my engine, so bear with me. The red arrow(a black plug) is what I pulled off, it has 2 wires going into it, a red and a white with blue stripe. Did I test the right thing?

Thanks

 

[silvapain]

yeah, that's the IAC.

 

[Intelliforce]

I doubt it's the IAC. I have the same problem on my Vulcan. This problem has stumped me forever. I've changed everything there is to change and I still have the problem. One thing I figured out is. The only time it does it is just before the thermostat opens. Once the thermostat opens it goes away. I've noticed with the Vulcans when you start them up cold it only takes 30 to 60 seconds and you have a rock hard rad hose already. I think it has something to do with that. What I'd like to find out is is it the water pump building up all that pressure or is it actually the coolant expanding cause it's warming up quickly around the cylinders.

 

[silvapain]

there are a great many things that are different when the car is cold, versus hot. Before the 02 sensors can get up to 600F, the MAF, IAT, and fuel tables determine the A/F ratio. If there is a vacuum leak, or the IAC is stuck, the car has no way of checking to make sure that the A/F ratio is where it should be, and cannot adjust for it.

IIRC, the water pump also has a bypass on it that will close when the proper operating temperature is reached.

 

[RobW]

My 95 GS also has this type of problem. So far, I've replaced the IAC and the TPS. I was really thinking it would be one or the other, and they each helped a bit. Now it only revs to 2,000rpm for a moment, then will settle to about 1100. Problem is, it's supposed to idle about 700. One of my brothers had a similar issue just this week with his Ranger, so he called to ask my opinion and it dawned on my to pull the EGR valve. He did this(I had no time yet) and called me back to say it was sticking about midway. I told him clean it and see what changes. Later, his truck is running normally again. So I guess that's what I'll do today.

 

[silvapain]

What did he use to clean it? just regular carb cleaner?

 

[Matthew Ebel]

I replaced my EGR valve less than a year ago, and I'm having similar idle troubles too. Haven't touched any of the sensors, though.

Pax,
Matthew

 

[RobW]

I didn't ask what he used, but told him I'd use carb cleaner, and then brake clean, since it dries fast and leaves little residue.

Yesterday afternoon, I took my EGR off and checked it over. It really didn't need cleaning and, unlike his, wasn't sticking at all. It easily plunged properly. I decided to do a quick cleanup with nothing but light air pressure and re-installed it. Thankfully, the gasket stayed in place and nothing leaked. My car runs just slightly smoother, but still idles at about 2,000 momentarily, then drops to what looks like about 1150. So I listened closely and traced things out in regards to vacuum lines. I found a leak, but couldn't repair it or replace the line just yet(no time). Anyway, I'm fairly confident this is the root of my problem and will likely replace the hose tomorrow or Sunday.

What really bugs me is, the car will coast in 1st gear and just run right up to 2,000rpm while doing it. This makes it move, what, 15mph? That's faster than I like to travel on my unpaved drive, which is some 950 feet long. Therefore, I simply must trace this problem to it's end and UTTERLY DEFEAT IT!

 

[speakerboy5]

I had that same idling problem. However, coupled with it, when the engine got hot, the transmission would stay in first gear. My local dealership checked it out, said it was the speed sensor. It was replaced, and with only a few occassions of the high revving (but no transmission problems), it's ran fairly smoothly.

1998 Ford Taurus sedan
3.0L Vulcan