bdragonseven

I'm at my wits end. My Z has an issue thats been discussed over many of the different forums. It idles rough, almost like the car is misfiring and as well once the car warms up the issue isn't as noticeable but is still there.

Characteristics of the problem:
Runs really rich
Shoots smoke when first started
Sounds like the car is misfiring
Popping noise can be heard in the intake (like predetonation almost)
Kind of sounds like there's a vacuum leak somewhere although its subtle.
Surges at stop lights
Bucks in any gear and in any driving conditions.

I've replaced the spark plugs, cleaned the maf, cleaned the throttle body, installed new plugs, and cleaned the filter.

Issue is still the same. Car throws no codes at all.

I'd say the next step maybe a broken valve spring however the issue just didn't all the sudden happen, the car is H/C/I/E and in all honesty has always needed to be retuned but I've never had any issues with the current tune.

Any last suggestions before I take her back to a shop would be much appreciated.

AzDave47

You might check this although the affected MAF were mostly earlier than 09 (my 09 had the double digit code)

https://www.corvetteforum.com/forums...d-effects.html

It's always harder with a built engine to know whether the parts work together right, especially when you don't know what the parts in the build are.

Dan_the_C5_Man

Make sure primary O2's are not swapped (almost impossible on a C6), check for vacuum leaks anywhere after the MAF, especially the back of the intake / exhaust butterfly valve hose.

And don't be too quick to rule out valve springs, or a rocker arm with missing needle bearings, etc. Heck, one member had a lifter fail and depending on the position of the lifter wheel it would run OK one sec, bad the next, so pretty much anything is possible.

Have someone scan it wit HPT and see if one bank is running richer than the other, make sure it is going into close loop, etc.

outhouse

My 5.3 in a Sierra does just that if the TB butterfly is dirty. Spray a little brake fluid cleaner in small spurts or it will kill the engine until you see it cleaning up. You can also run it up to 3000rpm and give it a few squirts.


If dirty it will idle correctly afterwards.

Pb82 Ronin

Sounds like a tune from a reputable tuner is needed. Or at least some seat time from someone who knows these cars well.

Edit: Just re-read...if the car ran fine on the current tune and now it doesn't run right, it's NOT the tune. Something like a sensor going bad or vacuum leak is usually the culprit. Was ANY work recently done on the car?

Michael_D

Please clarify one point.... You mentioned it was running fine, and this is something new?? If it's new, probably not a tuning thing.

Also, does it start behaving when the engine is up to normal op temp? I think you said that, but please clarify.

Malfeitor

Might want to check the plug wires as well. My 05 vert was acting similar, albeit not very bad, and new plug wires (and plugs, which you've done) fixed it.

bdragonseven

I noticed the car wasn't really running smoothly one very hot day going out to a car show. The issue wasn't immediate it gradually got worse over time. To be honest the car has been sitting because I really don't feel running it to see if the issue worsens or not at normal operating temps is a good idea.

bdragonseven

A new starter was installed after the issues were already in place.

Michael_D

Well you should probably eliminate the simple stuff first. It really sounds like you have a plug wire that isn't seated. I had a wire pop off a coil pack one day and it behaved just as you describe. I'd verify that first. Broken valve spring is also a possibility. Sure wish you had a log to review.

Vac leaks can cause problems too. What you are describing though would have to be a pretty good sized leak... You can always make a smoke generator. Pretty simple to do. I built one using a 1 gallon paint can and some nichrome wire.

I'm really beginning to think that anyone with a modified car should invest in a scan tool.

C5 Hardtop

What color is the smoke? A lot of those symptoms make it sound like the timing is off. Definitely don't drive it because something broke. Hopefully its just a sensor that went bad.

AzDave47

I modded my car in May, bought a scan tool this week when check engine light came on, P0449 Canister solenoid, I guess I have a little work to do. I want to check any fuses that may be associated with that also.

outhouse

Common GM issue, my truck has flipped out intermittent codes for this for a decade. Sometimes it goes 2-3 years before flipping one. Ties into weather changes often.


The solenoid is cheap enough, but doesn't always address it.

bdragonseven

I will verify everything is seated properly but this issue started with the original plugs and wires in the first place. There's so many other possibilities to the cause that its honestly just more feasible to give the car to a shop just from a cost standpoint. I was thinking one of the cays might have gone bad since the issue slightly improves once the car is at normal operating temperature.

Tech

Where are you located?

bdragonseven

Virginia, South of DC

bdragonseven

So just an update, I was finally able to get the car to the shop this weekend. It appears as though the previous owner had a quarter sized hole in the bottom of the intake manifold, which they tried to patch with rtv. I was able to grab a used intake manifold off eBay so hopefully the issue can be resolved after this whole episode.

Dan_the_C5_Man

Wow. Great example of an issue that would have been impossible to fully diagnose over the Internet, and also yet another reason to stick with one-owner used / know the previous owner purchases.

Glad you are on the path to recovery.

redbird555

Why was there a hole in in the manifold? I would be much more intent on finding out why..sounds like it may have been ported and the went over the top with it

bdragonseven

The only thing that's ported is the throttle body, which I plan on retuning the car anyway once the new manifold is installed. There's always a risk of buying used but its worth it with a car like that.