High Idle @ Cold Start (~3k)

First off I'd like to say hello. My name's Jake and this is my first post on the bScene Forum. I just got my 92 B2600 EFI 2.6L and I love the truck. I do have a small problem.

When the truck is started it idles around 3k RPM's. Adjusting the idle screw doesn't change a thing. It does tend to smoke a little during these cold starts. I'm completely stumped. I did search but I could only find threads dealing with the carb'd version of my truck.

I would be grateful for any help with this as I am new to the B Series pickups.

There is an idle air control valve on the top of the throttle that regulates idle air - as it warms up, it allows less air to "Bypass" the throttle. In fact, Mazda calls it a Bypass valve. Yours is probably stuck wide open. They get gummed up over the years and will tend to stick.

Also, a bad engine coolant temperature probe will also cause idle problems. Does the idle ever come down once the engine warms up?

You can test each of the those two parts, but even if each tests OK doesn't guarantee that they're are not faulty.


IAC Valve:


Disconnect ISC valve electrical connector. Connect ohmmeter to ISC valve 2-wire connector. Measure resistance. See ISC RESISTANCE SPECIFICATIONS table. If resistance is not within specification, replace ISC valve.






Here is the procedure for the coolant temp probe. If you don't want to pull the probe and do the heated water thing, you can probably get a descent idea if there are any obvious problems by checking it still in the thermostat when you know the temp is in the neighborhood of 68 or 176.

Remove engine coolant temperature sensor. Place sensor and thermometer in container of coolant. Connect ohmmeter to sensor terminals. Slowly heat coolant. Note resistance at specified temperatures. See appropriate ECT SENSOR RESISTANCE table. Replace sensor as necessary.

Temperature Ohms
-4 F (-20C) 14,500- 17,800
68 F (20 C) 2,200 - 2,700
176 F (80 C) 200 - 400

Before you start checking the things above, have you pulled the intake pipe off and made sure that the throttle is in fact closing all the way when at idle? That would probably be the first thing to check.

thank you both for the advise. Ill get back to you and let you know how it goes.

well i ended up changing the thermostat bc it was stuck closed and building pressure in the coolant system. I have two vacuum leaks i have to fix and i'll get back to you guys

Still have high idle. It's worse when it's cold. The truck will just idle around 3k whenever it feels like it. One day I'll go out and it will idle around 1k like it's supposed to, then the next day it stays at 3k. I'm about to just go find a new throttle body all together and just replace the whole damn thing

Did you ever fix the vacuum leaks you mentioned earlier? Where are/were they? You definitely need to get those fixed before you start replacing other parts if you haven't fixed them already.

Try disconnecting the IAC valve connecter while it's idling fast. Just follow the wires out of the IAC valve about 8 inches or so and unplug at the connecter to see if makes any difference.

You might try the same with the temperature sensor (the one that has two wires going to it - the single wire sensor is for the gauge.)

As I mentioned earlier, you also need to make sure that the throttle is closing all the way. In only takes a small crack open to rev the engine a fair amount while idling.

Ok I think (as far as I can tell) I have all the vacuum leaks sealed up. I haven't tried unplugging the temp sensor or the IAC yet. The temp sensor was just changed (out of another truck at the JY) and nothing different from the old one.

I don't know if it helps but if I go drive the truck and let it heat up with the idle at 3k, then park with it off for about 10 to 15 minutes when I start it back it will idle at 1k like it should. Sometimes it stays at a 1k idle once I start driving again, but sometimes it will go back up to 2-3k when I'm out of gear.

I'm about to go pull another throttle body from a JY donor if I have to and if I can find one, bc this is killing me in gas money!

anyone have a clue as to what this could be? 3k idle until I shut it off for about 15 minutes then down to 1k like normal and sometimes after driving it goes right back up to stuck at 3k....

bump... anyone?