Not Running As Smooth As Others

[Rob1973]

This is kind of a tough one, but I thought I would post here.

I have a 2002 Z, 28,000 miles with a 6-speed. When I bought it, it ran noticably rough and wasn't as peppy as it should have been. We took it to a dealer, they determined the fuel filter was plugged, replaced it, ran the injector cleaner thru it, and it ran much better.

I have always thought it wasn't as smooth as others, so I test drove a couple other LS1 Camaros which confirmed this, both auto and 6-speeds. If you didn't know these cars, you wouldn't notice the roughness. It's not a misfire, more of a slight shudder that is more pronounced the harder you accelerate in lower RPM's. I was out cruising this weekend, and it is less noticable when you are really getting down on it, like full-throttle 4000+ RPM kind of runs.

The car was extensively checked over by a Chevrolet dealer to determine the cause of a misfire earlier this summer which turned out to be the heat shield arcing to the block. During this check, they drove themselves crazy looking at everything, ignition, emissions, fuel, and so on until one of them happened to be looking at the right spot when it arced.

I replaced the plugs, replaced the wires with MSD's, got rid of the heat shields, put in a new air filter, and yet this odd roughness is still there.

So, the car has been tuned up, scanned, and passed every test, but I have driven 4 other Z's and SS's and they ALL are super smooth while mine is not.

My "Non-SAE Certified" opinion is that I have an injector (or two) that aren't working 100% resulting in the slight roughness but not failing to the point of setting off the SES light.

So, my questions!

1. What are the opinions/thoughts out there? Can individual injectors be tested? Is it it worth the cost to test them?

2. Any other thoughts on the cause? The car was actually scanned at 2 different dealers, the first one who found the filter clogged ran a scan (found a lean code then found the fuel pressure was out of whack and replaced the filter) and then the 2nd dealer that I brought the car to with the intermittent misfire that turned out to be a heat shield arcing to the block.

I love the car, but I can't get over the thought that something just isn't right.

Thanks for your help in advance.

Rob

[Rob1973]

After driving it 300 miles this weekend, I refueled and noticed the car was running very smooth, almost perfect, as I drove away from the gas station.

This was a short lived moment, as the next time I took the car out, it was back to its "not quite right" state of running.

So, the car DOES have the ability to run smooth, it just chooses not to nearly 100% of the time.....

[occamaroz]

Does the car run any different when the motor is cold verses warm?

[Rob1973]

No, it seems to run the same. It fires up quickly, idles at 700 rpm. It hasn't been really cold up here in Minnesota yet though.....maybe once temps go to the 50's and 40's something will get more pronounced.

[jad628]

I recently had what I felt was a miss at lower rpm's while the car was under a load. The more throttle (but low rpm's) the more pronounced it got. As rpm's built, it was not noticeable. I was suspecting an injector at the time myself (and I changed the fuel filter too. Even created a thread here about it, but it turned out to be the plug wire).

My problem was a plug wire touching the exhaust. This seemed to "arc" the transfer of energy from the plug. I carefully repositioned the plug wire and the problem was solved. It was the plug boot that was touching. Clearance is close, but you should see about 1/4" (min) of clearance between the wire and the manifold/header. This can be done, but you need to ensure that the wire stays in the position you place it in. Use a little silicone spray on the inside of the wire boot to assure it slides on properly and push 'til it clicks. Drive for a while and make sure it doesn't shift back against the exhaust. I know you said you've replaced the plug wires, but that was something I did too. I replaced the plugs with Iridiums at the same time. My error was not ensuring that the wires stayed off the exhaust. The #6 plug ended up being against the exhaust because I did not take into account that once the wires (Accel) started breaking in, they "sagged" just a bit. It was enough to cause my problem

That may not be your problem, but I would at least start there to eliminate it from the possibilities. I have heard that if a plug wire is up against metal long enough, it can cause that particular coil pack to wear out prematurely. Just FYI. One last thing: unhook your battery for about a half hour and let the computer re-learn the car. I swear it helps.

I don't want to jinx you, but if that doesn't solve the problem I think your issue might be deeper (like in the valvetrain), but there are plenty of things to try before that bridge must be crossed. Right now, my money is on the plug wires positioning and contact with plugs/coils.

Good luck!! Let us know if (make that WHEN) you solve the problem.

[sic2001]

what this was with me and sounds strange but the front facia was not buttoned up into the radiator support and was kinda just dangeling there from some body work done at the dealership. I buttoned those into the holes that were there and no more shudder. I do have the air intake that draws air from the lower radiator via the aluminum scoop though but after plugs, wires,cleaning every sensor,inj cleaner and everything else I did that and purrs like a kitten now.

[Rob1973]

The only intake modification I have is an SLP lid, and the car ran the same before & after putting it on.

Since the car has had very short bursts of running smooth, it makes me think injector/s rather than a major problem....

Thanks for the replies, I know this is a tough one. Reminds me of a friend of mine in High School who was ultra obsessed with his 74 Nova. We would be driving around and he'd suddenly say, "Did you feel that?" "There. It did it again........THERE!" I couldn't feel anything....

Now I'm that obsessed person....."Hey, that's not smooth....It's pretty smooth, but not perfectly smooth.....now it's smooth....nope, not anymore...dang.."

[SeVeReDiStOrTiOn]

any exhaust mods?

[Rob1973]

Nope, no exhaust mods.....I'm torn between the GMMG and the Magnaflow though.

[02sscamaro]

i have a rough idle also it is tuned up perfect u can feel it in the car sometimes worse than others. 44,000 miles. i was thinking a injector also.

[Wheeler99WS6]

I had similar symptoms but it turned out to be a plug wire also. I hope you figure it out, that would drive me crazy too.

[1998Formula]

Any one know the rpm of a 1998 formula ls1 at 60 automatic? with 2.73 rear

[BowTied]

Have you tried adjusting the idle at the throttlebody?

Have you checked to make sure the intake doesn't have a mild leak?

28k is low mileage. Do you know if that car set a long time?

[ss~zoso~ss]

where do u get your fuel, change that up and eliminate one more variable, try some drygas in the tank, are you using premium 93 not 91, does a winter blend of gas have anything to do with it, higher ethanol percentage?

just some thoughts of mine

[ss~zoso~ss]

also since it was running smooth after you gassed up, maybe you could vent the fuel system, is there a air bubble trapped somewhere? breather valves / vents for the gas tank clogged or blocked, do a search for schrader valve / venting fuel system

[BowTied]

With that better after refueling...was it only once? No patterns?

[joedude02]

just a thought-- what if the fuel pump is over amping and short cycling?

[0rion]

thread is over a year old.....