High Speed idle adjustment

[shlepadoodle]

1.8 L V6. On a cold start the engine idles at about 2500 rpm and then gradually drops down to normal idle speed when warmed up. Everything works fine except for the startup. What is the procedure for adjusting the cold high speed idle?

[IanL]

Sounds like it's behaving normally, but you only see 2,500 in really cold conditions. Don't touch anything. If your idle when warm is 640 to 700 rpm with electrical loads off, no adjustment is required. There isn't a separate adjustment for the cold idle; it's determined by the ECU, based on coolant temperature and intake air temperature.

[sideshow]

As IanL said, that's normal, ESPECIALLY for Winterpeg

[shlepadoodle]

Everything seems to be working normally and there aren't any error codes indicating any faulty sensors. For Winterpeg when it's -40 I could see that 2500 rpm is normal (although I am used to about 1500 for other vehicles- Camry and Honda civic). This speed occurs year round regardless of temp (even at 100 deg F). The only thing that varies is the length of time to reach normal idle which is of course dependant on temperature. It may be the ECU or the actuator that controls the throttle position which gets it's drive voltage from the ecu. Is this a solenoid or a drive motor? Perhaps it can be adjusted. Back in the old carburetor days the mechanical linkage was adjusted separately and did not kick in or interact until the temp was low which was controlled by the choke coil. Without a shop manual I am flying blind. Anyone know where to download one?

[sideshow]

You can get the Chilton one (although not a 'true' shop manual) at Canadian Tire (had to order mine for me). It's definitely worth the investment though.

[IanL]

The actuation by the ECU is to the Bypass Air Control (BAC) valve and/or the Idle Speed Control (ISC) Valve. There is a procedure in the Mazda Workshop Manual for "inspection" of the BAC valve, and electrical testing of the ISC valve.

Inspection of BAC valve: "Connect the diag box terminals TEN and GROUND with a jumper wire. Connect a tachometer (to the IG terminal). Start the engine. The idle speed should decrease gradually as the engine warms up"

ISC valve testing: "Disconnect the ISC connector. Measure the resistance of the ISC valve with an ohmmeter. 10.7 to 12.3 ohm at 20 degC (68 degF). If not as specified, replace the ISC valve."

You could use the tacho in the car, of course. The manual may be written for cars without one.

Chilton's for the MX-3 (46802) is more helpful! It tells you to detach the ISC valve connector when the engine is cold and idling, & note the rpm. Reconnect, warm the engine up, disconnect again, and verify that the revs are lower (with the plug detached) when warm. (Detaching the connector may have the same effect as connecting the TEN and GROUND!)

Another test is to detach the valve from the throttle body, blow air into port A (where the pipe attaches) and verify at normal temp. that no air comes out of port B (communicates with the throttle body). As the valve cools, watch it open. Blow air into A again, and it should come out of B. You are going to have to be quick to get the valve off before it cools! Chilton's has good illustrations. People usually criticise Chilton's, but this time it seems better than the Mazda book.

All this puzzles me a bit, as Chilton's doesn't mention the BAC, but combining the stuff from both books, and reading between the lines, it may be that there is a thermo-actuated valve (the BAC?) which closes as it warms up, and an electrically controlled valve (the ISC?) which the ECU uses for fine control, all mounted in the one assembly.

If your thermo-actuated valve is sticky, it might give the symptoms you've mentioned (rather like a sticky thermostat).

<small>[ May 14, 2004, 04:23 AM: Message edited by: IanL ]</small>

[IanL]

I forgot to say - the Mazda manual illustration points to the BAC valve, which is the same unit the Chilton's identifies as the ISC valve - that why I think both are in the one unit.

Just wondering if the coolant water supply that some people have bypassed is the one that operates the BAC valve? Could explain a few things.

[shlepadoodle]

Thanx IanL, you may be onto something here. Will dig into the chilton's manual to identify the location of all these components and get a better feel of the operation of that circuit. I'll post back my results on my progress.

[IanL]

Just checked my car this morning. Temp in the garage was 18.5 degC, 66degF. (Yes, it's warming up in the UK) The car idled initially at 2200 rpm, but dropped within 5 sec. to 1650 rpm.

Looks like its ok to start with a high idle, even if it's not very cold weather, but the feedback loop should bring it down pretty swiftly if the weather is warm.

[shlepadoodle]

Checked this morning at 13C (55F) and it basically performed similar to yours - 2200 at start, 5-8 secs to get to 1600. It maybe ok then if it's performing similarly to yours. I guess it is high for what appears as a long period of time due to our extreme winters here. Will talk to a Mazda mechanic for some typical time vs temp. I suspect they rev the engine to that speed initially to get oil up quickly.