billscamaros

iTrader: (1)

'99 Z28 M6, approx 160K miles, stock motor and trans. This is my daily driver and I consistently average 26 mpg on my normal commute, a combination of distance and city driving. My normal commute to work is 45 miles each way, mostly four lane highway,

When I first start the car up, the idle will sit at approx 650 rpm. But once it has warmed up to normal operating temp, the idle stays at around 950 rpm. I've noticed this happen over the last month. Gas mileage has remained the same.

I tried to pull the codes with my OBD II scanner, but it reports no codes.

This sounds like an O2 sensor issue to me, as the sensors are original to the car. But I want to check with the LS1 gurus. No other changes are noticed.

Any ideas? There would be four sensors, right? Two before the cats, two after the cats? Change in pairs, or change all at once?

Thanks for your thoughts.

Bill

NonGoggleGuy

Try cleaning your MAF & air filter.

And yes, there are 4 O2 sensors. If you have a scanner that can read them, a typical O2 sensor should change rather rapidly. If it's slow to change or seems to hover in a certain range, it should be changed. Otherwise, I think they'd throw a code if they weren't responding.

sroach20

iTrader: (31)

ignore the rear o2 sensors, they have no uses.

bballr4567

iTrader: (5)

Also on GM vehicles the O2 sensors can be VERY VERY VERY slow and it still wont throw a code. They literally have to not be working for it to give you an SES light.

So if they are the stock sensors might as well replace them. 160k miles is a lot for an O2 sensor.

billscamaros

iTrader: (1)

Replace the front sensors and leave the rears in place? Check for the change? I pulled the MAF and cleaned it. I'll change the air filter, although that is fairly new. I'm looking for 200K miles out of this motor and trans, so I keep the filters and lubricants changed regularly.

sroach20

iTrader: (31)

check your iac also and look for a small leak in one of your vacuum lines.

bballr4567

iTrader: (5)

The rears do zero for the performance of your car. They are there just to monitor the cats to see if they are doing their job.

billscamaros

iTrader: (1)

thanks for the insight. got to love this forum.

1 FMF

iTrader: (1)

My vote is for the IAC- idle air control valve. That is what controls your idle, because the main throttle valve is closed and it's the IAC that controls the amount of air into the engine at idle. It is common for them to stick when they've got some years on them, I've heard you can pull them off and service them, or you can just pick up a new one from the part store.
If you have no codes, and the car runs fine otherwise, and you still get good highway gas mileage, don't mess with the MAF or O2 sensors. And the air filter would have absolutely nothing to do with it!

NonGoggleGuy

But I thought the IAC usually shows a code.

sroach20

iTrader: (31)

nope, when i started tuning my idle i went from 0 all the way to 300 without a code. the valve could be sticking or the hole it goes into could be gummed up.

1 FMF

iTrader: (1)

I believe it will only show a code if the electrical circuit is faulty or if the IAC is so bad the idle is not within a set range for a certain amount of time. They can not function properly, the pintle valve in them can stick and not flow the air it should, so idle goes silly, but with the PCM in closed loop monitoring the O2's the air fuel ratio is maintained and the engine idles fairly well, just not at the desired 800 rpm.
per the manual, "if the engine idle speed is out of range for a calibrated period of time, an idle speed DTC may set". Key words = may set.

what may also affect the idle is the PCV valve. with 10 years and 160k miles i would not be surprised if the IAC is sticking, nor if you need a new PCV valve, a pcv valve ought to be a maintenance item anyway, given it's cost i would replace that first then drop bucks on an IAC. With no codes, the engine running strong, and good gas mileage, it's not going to be the MAF, O2, or restricted intake.

have you replaced spark plugs? I would do those before the IAC. I don't think it would be the main cause of the idle but it certainly isn't helping. Other than that, for the mileage if it isn't those things then the next step is a leak down test to check for valve seal and valve guide leakage.

intense3123

Normal idle for an M6 is about 950. It's really around 800 but shows up as 950-1000 on the stock tach. I don't think you have anything to worry about.

dtown_racer

is there a way to test the iac? can i take it to like autozone or a parts place and see if they can test it? i think im having the same problems except my idle goes up and down from 1000 to 2000rpms!!

Zerokube

iTrader: (4)

Dtown you probably have a vacuum leak. my dodge ram has a surging idle at cold start up and dealership found out it was a leak, might be the same for you idk.

dtown_racer

probably is just have no idea were its at ive checked everywere lol

billscamaros

iTrader: (1)

well, i pulled the IAC and checked it. the sensor is still working, but was pretty cruddy. so i cleaned it and then cleaned out the bung in the throttle body. the idle is back down to normal now.

i noticed a small hole in a rubber tube leading into the throttle body. this tube is on the passengers side of the motor, runs along to edge of the intake and connects into the throttle body just below where the EGR ties in. any ideas on what that tube does? i sealed the hole, but need to get a new one.

also, as i indicated on another thread, i did finally have a DTS code appear. it's P0171, indicating a lean condition on bank 1. i'm not sure if this is a result of the IAC being dirty and open? i disconnected the battery to clean the computer. will see if the engine code returns.

thanks for the tip on the IAC.

bballr4567

iTrader: (5)

That tube is just a vacuum tube IIRC. If you replace just get a complete rubber house as the stock ones are very prone to get that hole in the same exact location every time.