Slightly rough idle?

**Aetos** · 07-27-2008, 02:42 PM

My car is a '99 M6 LS1 pretty much stock with 45k miles on the odometer.
I'm not sure what has been changed on the car since its purchase since I recently purchased the car.

Problem: The cars idle is a bit rough/shaky at idle, but the RPMs are always steady at I would say a 800 range or so. While looking in the engine bay with the car in idle you can see the engine pretty steady but every so often you will see it shake/tremor a bit then see it go back to a pretty much steady pace, but the idle will always stay static in this process.

What could be the cause of this(spark plugs, maybe? random misfire?) I don't have any codes popping up.

Could this also have something to do with my seemingly bad MPG? I was thinking that about 150miles on a tank was standard, but after reading some threads about people getting 200-300mpg on a tank I'm having my doubts.

**Fury_786** · 07-28-2008, 07:53 PM

mine does it sometimes...its an LS1 thing

changing the plugs/wires sometimes makes it smoother, sometimes not

**Aetos** · 07-29-2008, 02:52 PM

Originally Posted by Fury_786

mine does it sometimes...its an LS1 thing

changing the plugs/wires sometimes makes it smoother, sometimes not

how about the low MPG? lol

**KING Z28 HAWK** · 07-29-2008, 02:56 PM

Originally Posted by Aetos

My car is a '99 M6 LS1 pretty much stock with 45k miles on the odometer.
I'm not sure what has been changed on the car since its purchase since I recently purchased the car.

Problem: The cars idle is a bit rough/shaky at idle, but the RPMs are always steady at I would say a 800 range or so. While looking in the engine bay with the car in idle you can see the engine pretty steady but every so often you will see it shake/tremor a bit then see it go back to a pretty much steady pace, but the idle will always stay static in this process.

What could be the cause of this(spark plugs, maybe? random misfire?) I don't have any codes popping up.

Could this also have something to do with my seemingly bad MPG? I was thinking that about 150miles on a tank was standard, but after reading some threads about people getting 200-300mpg on a tank I'm having my doubts.

Sounds like a normal LS1 but MPG might want to change you fuel filter and air filter see if that hels any

**Aetos** · 07-29-2008, 03:03 PM

Originally Posted by KING Z28 HAWK

Sounds like a normal LS1 but MPG might want to change you fuel filter and air filter see if that hels any

The air filter seems fine, but haven't checked the fuel filter. The shake isn't only visual on the engine though. You can feel the car and the shifter shaking at idle also.

**Fury_786** · 07-29-2008, 03:42 PM

i get 28hwy, 18/city

with 3.90 gears

and yes, i drive it like i stole it

mine shakes like a epileptic on crack

**Aetos** · 07-29-2008, 04:00 PM

Originally Posted by Fury_786

i get 28hwy, 18/city

with 3.90 gears

and yes, i drive it like i stole it

mine shakes like a epileptic on crack

I get nowhere near that!!!! I'm lucky to get 150-160 a tank!!
I don't always push it either, a lot of times I'm shifting at like 2krpm

**Fury_786** · 07-29-2008, 04:01 PM

geeze

1. fuel filter
2. spark plugs
3. wires
4. 02 sensors

**Aetos** · 07-29-2008, 04:09 PM

Originally Posted by Fury_786

geeze

1. fuel filter
2. spark plugs
3. wires
4. 02 sensors

If either of these bad wouldn't it throw some kind of code? atleast the 02/plugs would?

**Fury_786** · 07-29-2008, 04:31 PM

not necessarily. they can be performing badly, and the PCM wont notice

alot of times they can mess up the AFR and have a code, but never a SES light.

**INMY01TA** · 07-29-2008, 04:59 PM

Clean the maf too.

**Aetos** · 07-29-2008, 04:59 PM

Originally Posted by Fury_786

not necessarily. they can be performing badly, and the PCM wont notice

alot of times they can mess up the AFR and have a code, but never a SES light.

Think it would be worth a shot to check for codes even if no dash light pops?

**Fury_786** · 07-29-2008, 05:01 PM

wouldnt hurt

any adv. auto could do it for free.

and as 01TA just said, check the MAF, sometimes they get a piece of fuzz or something in there and makes them go crazy

**Spaz** · 07-29-2008, 05:08 PM

Originally Posted by Fury_786

geeze

1. fuel filter
2. spark plugs
3. wires
4. 02 sensors

Originally Posted by INMY01TA

Clean the maf too.

check your air filter as well...

fuel filters should be changed with every 3rd oil change... easy to swap out and cheap...

**ws_sick** · 07-29-2008, 05:15 PM

I would start with the normal maintence mentioned above, if that doesnt work, you could have a leaking injector, that would explain the poor fuel mileage and rough running, also, does it shake when the a/c is on, off, or both? could be your compressor cycling

**Frost** · 07-29-2008, 05:45 PM

To maintain fuel economy, you need to replace your front O2s every 40K miles or 3 years if internally stock / still running manifolds or every 15-20K (with monitoring) if running headers, even if internally stock.

The problem with them is that they hardly ever fail outright (unless the heater fails), and over time they drift off. As they foul, the senor's exposure area is naturally fouled as well. This layer of "insulation" causes them to report leaner conditions, since more of the oxygen that they "see" is kept away from the sensor. As they report a little leaner, the engine responds by adding a little more fuel than it needs in closed loop. Basically, this is any time that the engine is over about 92 degrees and not at WOT. What I am saying is that they are basically in charge of your car's fueling all of the time except near WOT.

If mileage is really suffering, these guys are hands-down the most likely culprit.

**Fury_786** · 07-29-2008, 05:48 PM

sometimes you can temp. fix an 02 sensor by cleaning the carbon/buildup off od the tip

but its much better to just spring for some new ones at about $100/pr

**Aetos** · 07-31-2008, 02:58 PM

Scanned the codes and no codes were thrown

I think the first thing I'm gonna do is change out the 02s... now.. wheres the best price for 02s?

edit:

Without throwing any codes whats the thing that could affect the mileage the most? (shouldn't bad 02's throw codes?)

**Crimson Sin** · 07-31-2008, 04:51 PM

Read what Frost said about the sensor's, pretty spot on. I would just change them.. And it doesn't matter where ya get them.. a Bosch O2 is a Bosch O2..Just get the OEM style ( not the universal, wire the plug on yourself crap)..And yep, your mileage is in the toilet. I get 25 on the hwy. 3.73's with an stalled A4...Hell, do a tune up. Seafoam it, clean the maf and change your filters.

**Crimson Sin** · 07-31-2008, 04:57 PM

Most code *scanners* are crap, that's why they are 100 or so. Unless you have a MODIS or an OTC Genisys or similar, you can't read live data or freeze frame data. SO, unless they have failed, you can't tell dick. I have NO codes on my car, but lo and behold. I hooked up the Genisys and there's 125 or so items in stored memory. Most of it is normal stuff. But some is shit that should be looked into. A/F rates off etc..Hell, I had random misfires on all cylinders a few months back ( wires were WAY over on resistance) No SES light tho ( codes ). The local parts shops will pull codes for free, sure. You get what ya pay for tho. Most quality scanners are a few thousand dollars for a reason.. If you really wanna know, pay a reputable shop the 80-100 bucks to scan your PCM and show you the results. I do it all the time for guys that ask and aren't a dick about the charge.