Strange idle, bad mileage help!!

**Shepp1691** · 02-05-2008, 04:32 PM

I have an 02 Z28 with 30k miles on it, lately it has been idling anywhere between 1000-3000 rpms and getting about 13 mpg. It is a six speed so the millage is way off. It does have an air box, flowmaster, and 4:11 gears. I just replaced the O2 sensors and it has changed nothing. I dont know much history on the car so it could have a tune in it or anything, but the dealer wants $80 to flash the ECM. I dont want to do that unless I am pretty sure it is the problem. Also no check engine codes, could be the MAF sensor. Any advice would be appreciated.

Thanks,
Dave

**Frost** · 02-05-2008, 07:55 PM

sure sounds like a vacuum leak after the TB...

**blackmagic00** · 02-05-2008, 07:59 PM

well i had the problem with weird idling but not at those rpms...anyways i got the MAF changed and my car runs perfect!

**M-Ten** · 02-05-2008, 10:15 PM

Originally Posted by Frost

sure sounds like a vacuum leak after the TB...

was gonna say the same thing... i had the same problem, turned out to be a hole in the pcv hose was sucking in a bunch of air.. taped it off temporarily and it's all better

**0rion** · 02-05-2008, 10:35 PM

yup, I'd check the pcv hose going to the intake first. Those have a history of either cracking or having a hole get rubbed in the backside of them.

**Shepp1691** · 02-06-2008, 07:36 PM

Checked the vacum leak with no success, getting the ecm re programmed by gm on friday. I unplugged all the emmission controls and never had a change idle or a code pop up, so something tells me the car has been tuned.

**Frost** · 02-07-2008, 06:08 AM

Originally Posted by Shepp1691

Checked the vacum leak with no success, getting the ecm re programmed by gm on friday. I unplugged all the emmission controls and never had a change idle or a code pop up, so something tells me the car has been tuned.

I bet money you are wasting your time blaming the PCM.

Why not look towards the most common and obvious of things; those devices and bits which are mechanical and/or exposed. I suppose an IAC coil driver could fail but it is unlikely and it will set a code right away. Some cars take a while to set codes after emissions items have been removed, so to say it has been tuned based on that is a bit of a stretch.

Do a search and find out how many bolt-ons guys have problems and how many of them blame the PCM... then look at how many say the PCM actually fixed their issue. Just saying...

**0rion** · 02-07-2008, 07:01 AM

Originally Posted by Frost

I bet money you are wasting your time blaming the PCM.

Why not look towards the most common and obvious of things; those devices and bits which are mechanical and/or exposed. I suppose an IAC coil driver could fail but it is unlikely and it will set a code right away. Some cars take a while to set codes after emissions items have been removed, so to say it has been tuned based on that is a bit of a stretch.

Do a search and find out how many bolt-ons guys have problems and how many of them blame the PCM... then look at how many say the PCM actually fixed their issue. Just saying...

yep, I agree. I doubt it's a pcm issue.

**Spaz** · 02-07-2008, 07:08 AM

have you checked your MAF sensors??? i know people who do the K&N recharge kit the wrong way and end up getting the recharge oil all over the sensors and it has the same symptoms that you have desribed...

**Shepp1691** · 02-08-2008, 10:31 AM

$45 to get the pcm flashed, ill let you know how it turns out, going today

**0rion** · 02-08-2008, 11:16 AM

please do. These threads always help other people down the road somewhere.

**Spaz** · 02-08-2008, 11:48 AM

Originally Posted by Shepp1691

I have an 02 Z28 with 30k miles on it, lately it has been idling anywhere between 1000-3000 rpms and getting about 13 mpg. It is a six speed so the millage is way off. It does have an air box, flowmaster, and 4:11 gears. I just replaced the O2 sensors and it has changed nothing. I dont know much history on the car so it could have a tune in it or anything, but the dealer wants $80 to flash the ECM. I dont want to do that unless I am pretty sure it is the problem. Also no check engine codes, could be the MAF sensor. Any advice would be appreciated.

Thanks,
Dave

also it could be a cat taking a shit on you... when mine went no SES... no codes... just reallly bad milage and until it lacked power and idles strange...

**Shepp1691** · 02-08-2008, 07:04 PM

Well after getting the ecm reprogrammed the idle is back to normal and seems to running great, as far as the millage I will have to see how many miles I get out of this tank, hopefully it will be anywhere from 250-300 miles will let ya know

**Frost** · 02-08-2008, 09:36 PM

Doesn't make much sense... If something were "bad" inside the PCM (minus a physical output driver or bad input) as far as programming, it would fail checksum test and the car wouldn't run.

Regardless, glad to hear it's better

**0rion** · 02-09-2008, 01:58 AM

yep, glad to hear it's better. I'd wonder what happens as it learns the trims again. How many miles have you put on it since the reflash?

**Shepp1691** · 02-09-2008, 05:38 AM

Only about 100, but the guy I got the car from had a cam and stuff in it, so im thinking he used an ls1 edit and then removed the cam and left the tune in.

**Frost** · 02-09-2008, 05:59 AM

Originally Posted by Shepp1691

Only about 100, but the guy I got the car from had a cam and stuff in it, so im thinking he used an ls1 edit and then removed the cam and left the tune in.

While on surface this would seem to be the case, it doesn't explain why when you first got the car it was fine, and idled and drove fine for awhile, and only later began having problems... regardless, glad to hear that it is sorted out.