What is normal idle

i have a 1998 chevy blazer about 134000 miles v6 4.3 4wd…this last two weeks my blazer been running rough.i like to know what is normal idle for that blazer.when at idle its 500- 550 rpm when in drive its 500rpm.also i just like to thank you all for your help.it did have a new tune-up to it

With the engine warmed up, the transmission in park/neutral, the idle speed should be about 650 RPM’s.


+1 to Tester’s post.

Normal idle? TV on football game, beer in hand, feet on coffee table and a bowl of nacho chips nearby!

so my idle is little low.is that why my car is running rough.i still don’t know why my check engine light not coming on.that’s why i come to you guy’s.you guy’s know alot about car then i do.i just like to thank TESTER for your help

On a vehicle this age there are a number of possibilities. The first is the Idle Air Controller (IAC). That’s a valve operated by a motor commanded by the ECU that allows a specific airflow at idle, effectively controlling the idle speed. This can become gummed up over time and affect the idle. They can be removed and cleaned.

Another possibility is the Exhaust Gas Recirculating (EGR) system. This system is a solenoid operated valve that allows a bit of inert exhaust gas to be allowed to be drawn in with the fuel mix to prevent overheating of the cylinders when the engine is under load. It passes exhaust gas, and can become carboned up. If it’s not closing properly, it’ll allow exhaust gas to be drawn in during idle and cause idle problems.

And then there’s old fashioned wear. Have you checked the compression? If your compression is low, your engine may be having trouble staying running at idle.

Lastly, there are the sensors. The upstream oxygen sensor, the mass airflow sensor (which can become coated with stuff or whose heating element can become weak), and a few others. Normally I’d expect codes with these, but if you have no code and you’re still having trouble, don’t leave any possibilities out.

Start with the compression test and clean the IAC, and post back.

i did have a compression test done when i got my fuel pump replace and compression test came back good.i’m going to clean IAC and egr and ill let you guy’s know what i find and thanks for your help

ok guy’s i clean the IAC whitch it was really not that bad at all.i’m going to take it in to pepboys and have them give it a look to see what really going on.it’s going to cost me over 100.00 bucks for them to look at it

i forgot to tell you guys this.when i turn the a/c on and turn the wheels my rpm drop down to like 400 rmp thats is in drive and park

Everything you describe points to the IAC valve requiring replacement.

The idle is too low to begin with. Then if you turn on the AC or turn the steering wheel the idle drops even lower.

The AC system has a pressure switch that informs the computer when the AC is turned on so the computer can bump the idle up for the load the compressor imposes on the engine via the IAC valve

The power steering system has a switch mounted on the high pressure side of the system which informs the computer when the wheel is being turned, so the computer can bump the idle up for the load the power steering pump imposes on the engine via the IAC valve.


Tester, I agree. It sounds like the IAC isn’t responding to the ECU command.
To the OP; the valve is operated by a 'motor" that’s part of the IAC assembly. The whole thing needs to be changed as a unit.

As a quick verification, unplug the plug from the IAC assembly. If the symptoms remain the same, it’s the IAC “motor”.

so i unplug IAC when the car was running and there was no change in idle

That would suggest a bad IAC.

i unplug IAC when car was off.and i started it up RPM when up to about 2000 rpm as soon i plug it back in rpm went back down to like 600 rpm

Sounds like it needs a good cleaning.

This is a great video on cleaning the IAC. It’s not your vehicle, but they all work the same.

“ok guy’s i clean the IAC whitch it was really not that bad at all.”

Apparently he’s already cleaned the IAC

Time to change it out himself, for less than the cost of a diagnosis