I will be brief. ***POST 11 is newest one with some more issues***

Car: 2002 Z28, M6, 63xxx miles.
Mods: Flowmaster exhaust (whoopee woo), Frost Tune (great stuff)
Recent maintenance: TR55s stock gap, Fram Drop in Air Filter (to replace K&N that was installed at purchase), fuel filter
Issue(s): Dreadful MPG (<200 miles a tank), vibration at idle under rev (feels like whole car is shaking up to 2k RPM), bucking while cruising along in 2nd (similar to a cammed car), while flooring the car it feels as though it's pulsing (like if your foot isn't pressing the gas evenly).

Recent developments:
- I unplugged the MAF today and drove around a bit on lunch. I wasn't entirely sure if I should keep it unplugged and drive around for an extended period of time considering all I have is a mail order tune, but the vibration was still there. All other issues I would call insufficient data, as I didn't floor it nor drive for an extended period of time.
- After the mail order tune it took a while for the idle to not flip out when coming to a stop, and the car seemed to pick up some power (especially in 3rd).

Parts ordered:
- New Denso 02 sensors, IAT sensor, Coolant Temp sensor (not expensive on Rockauto and I've read that all of them can go bonkers and not necessarily ALWAYS give a code... so I figured what the hell).

Admitedly, I can't completely remember if all of these symptoms (save for MPG, that was always **** poor) were prevalent from day 1 of ownership.

Any ideas are greatly appreciated. It's been 4 years since I had an LSx vehicle, so maybe I just forgot how it is. Thanks.

-J

 

Have you scanned the PCM for codes?

 

Scanned and found nothing. Cleaned my MAF (one of the sensors was CAKED in crap), put in a new Delphi IAT, changed my PCV valve (all was original), and put a new belt on. Did it help anything? Well it's too early to tell on MPG, so I will know by the weekend.

As for the vibration I get when revving it up? I can describe it as, it sounds almost like the car is running DOWN a cylinder and up to 2k RPM it feels like it's vibrating everything. When I changed the plugs none of them looked out of the ordinary (neither super white to indicate a dropped injector or black as night to indicate overfueling). As for power? As best as I can remember, the car seems to have all the power a stock 02 car has. Maybe the exhaust is banging or the stock mounts are shot. Or maybe it's normal and I just could never remember what the car felt like from 4 years ago (that's a possibility... should the car feel silky smooth when revving? Or no, considering it is a decently sized OHV V8).

I'll scan again tonight and check for anything in the history and see what's up. I would imagine if the cats were blown/clogged it would trip a rear 02 code, and if a coil was acting up it would give some type of code. The prior owner DID say he put on corvette coils (for what the hell reason I have no clue, I thought all LS1 coils were the same...), for what it's worth.

Thanks again for looking.

-J

 

I would climb under and check your engine and trans mounts. Prob doneski

 

Correct. Maybe its the mounts or K member or something like that. Does the vibration still happen when you are in neutral?

If your O2 sensors are old, changing the front ones may help you out with the mileage.

 

Just an update, I filled up with fuel yesterday and between the MAF cleaning, new IAT, new PCV, and whatever else, my mileage is around 18mpg. I am driving almost 100% city with hills as well on an M6 car. That sound about right? Have a mail order tune from Frost. Either way it's about as good as my prior car which was 2 cylinders less, automatic, lighter, and about 100% less fun. Will be doing the 02 sensors this weekend and also hooking up to HPT and seeing what each cylinder is doing. I have a feeling when I changed my plugs I might have fcocked up the plug wires (not seated all the way). I assumed that would throw a code, but I'll pull them all and reinstall listening for all the clicks.

-J

EDIT: one odd thing that the car did recently was yesterday. I went out on lunch to grab a jug of oil, and when I started the car, BOOM there was a backfire, then standard idle... never happened to any of my LTx/LSx vehicles. Struck me as odd, maybe others wouldn't find it as odd. Just saying.

 

Would you say your cars tune is rich?

I think that 18 city is pretty decent. He'll I average 20 ish.

 

Sounds good. Sometimes in the big city, I get 12 mpg. Because city driving mileage can fluctuate so much, I always compare/test my mileage figures on the highway.


Its happened to me before. Nothing to worry about if its every once and a while.

 

Thanks. Yea checked the misfires with HPT and nothing coming up, changed the O2 sensors and that was that. Car seems to be driving just fine and I figure I just forgot the cars behavior from not owning one for years. Oh well, now to just adjust my windows and replace my HORRIFYING stock shocks and good to go. Thanks for the input from everyone, I am planning a trip to Maine in the fall so that will be a good highway MPG test.

-J

 

Car has been running well but still having small issues. When I start the car and run it before up to temp, occasionally I'll have an issue where (for example) this morning I was shifting 1, 2, to 3. And when I let off the clutch, I hit the gas, the car seemed to ignore my input (not try to die, just acted as though it was in neutral), and then all of a sudden came to life. I had the same thing happen in 5th the other day. When the car is up to temp I don't know if it's EVER happened. It feels like my old Formula when the fuel filter **** itself, but the one on my car has about 500 miles on it. Under WOT the car feels as though no power is being lost. NO SES codes, still. Still feel as though I have some exhaust leaks (can hear slight non rhythmic metallic tapping before the car is warmed up and sometimes when warm).

I have since the last post also put an SLP Lid on the car, about 200 miles ago (That install was... fun). Reset the TPS since I posted this about 5 minutes ago, and my MAF flow at idle is ~7.xx grams or whatever the hell the measurement was, can't remember the units. I will log on my way home with my handheld thing to see if I can catch it when it acts up and see what everything is reading at the time.

Thanks for the input.

-J

EDIT: forgot, that when coming to a stop, occasionally I'll push the clutch in, idle will drop to around 600RPM for a short bit, then eventually jump back up. I DROWNED my IAC in cleaner as I heard that that's basically all you can do, and there is seemingly no change. And I still get occasional bucking while cruising at RPMs that I didn't think would give me grief in this manner... I will be checking for vacuum leaks next weekend when I am at my parent's and have more of my tools/time.

 

Figure I will just update this versus starting more and more threads.

Recently car has been running well. Turned out I had a frost tune because I believed to have a flowmaster exhaust system. Turns out it was just a flowmaster drop in with some truck tips welded on. Yanked that off and put an actual 3" catback on, and right away the car seemed to run better (placebo? Who the hell knows... sure sounds better). After thousands of adjustments I got the exhaust nearly just right, but now it bangs at the highest point of the over-axle pipe (obviously I need to adjust this). Can this cause false knock, banging that far back?


- The other night I was driving home from work in the rain and had to use the defroster. I turned it on, the car acted like it wanted to DIE. Idle dropped to 500RPM, I gave some gas, moved along in the traffic (which was all it was for about 25 minutes), and when I clutched in, wanted to die again. After a few minutes it would snap out of it and idle normal. Mind you, when I first left work I had the defroster on with NO issue... no dropping idle or the like.

- Occasionally I will be sitting at a stop light and my idle will drop maybe 100RPM, then I can almost hear an audible change in the motor's tone, and the idle picks back up. Sometimes I'll hear what sounds like a fan kicking on or the AC Comp, when not running the AC.

- My car never seems to run the same way day to day. Some times the idle will rev up nice and smooth, and down nice and smooth... and other days it will drop so fast between shifts that it doesn't look like I know how the **** to drive a manual trans as when I hit the next gear, no matter how quick, the RPMs jump back up and jerk the car all over the place.

- Sometimes when I drive with the window down I can hear what sounds like very faint metallic clicking, almost like if you dropped a paperclip on a piece of metal. It's under light load, heavy load, doesn't matter. I suspect it's either my AIR gaskets and/or my exh. manifold gaskets, so I also have those to change if necessary.

Those are some new issues. This weekend I have a slew of maintenance I will be doing which includes a coolant flush (LS6427 write-up), power steering remote cooler, fuel filter, check my spark plugs, fix the banging exhaust, flushing my clutch fluid, flushing my brake fluid, and making sure all my plug wires are attached.

Side note: Anyone with Falken Ziex ZE-502 tires (245/55/16 or whatever stock size is, can't recall off hand)? I can't tell if they are just REAL noisy or some other issue...

Any thoughts you might have are welcome. Thanks for reading.

-J

 

Have you had your altinator check, ive read someplace these cars act funny when the altinator starts to go. Until you get all of these issues ironed out is it possible to get a stock tune rather then a mail order tune?

 

Funny you mention that, because this weekend I was planning on having my charging system checked... will see if kicking over that rock yields anything. Thanks.

I suppose I could have frost just retune the car for stock if I wanted to explore that route.

-J

 

Didn't get a chance to have my charging system checked. However, I find that my car actually drives and acts better with the AC running. When driving to work lately I have the defroster on because it's been cooler and I have no garage. As I'm driving, the idle is more stable, no stumbling, idle doesn't drop and catch itself... but when I switch off, it starts to occasionally act strange. I can't imagine why it would act better with the A/C on, any ideas? Thanks.

Unrelated: Also I discovered that my over-axle pipe is in contact with the body (upon first departure in the morning I can hear it and feel it vibrate). I am going to adjust this this weekend. Even though it's so far downstream, can this still affect the knock sensors and trigger false knock?

-J

 

You can go to advance auto and get your charging system checked in a few minutes. I would be more concerned about that then spending my time adjusting the exhaust. It will sound annoying vibrating on stuff but shouldnt cause any of the issues you are describing.

 

Well yea I was just without a car this past weekend so this coming one I'll just do both, as you are right it takes about 5 minutes to do either.

-J