After pulling my hair for months, I am going to attempt to begin collecting data on this common problem. If we can compile this stuff and figure out what works, I think it will benefit many of us.

First glance, I see cars with this problem typically have:
*90-120 thousand miles
*Common bolt-on mods (long tubes, ported heads, 1.6 rr's, etc.)
*Automatic Transmission
*Spent a bunch of money replacing sensors and opti's

I don't know about you folks, but I am sick of throwing money at this problem.

My mods are listed in my signature. I have the idle problem, an occasion backfire at initial tip in, and significant knock at 90-100 kpa/1500-2300 rpm.

Let's figure this out.

 

There was a recall for exactly this problem in 1992.

For the later ones, I would bet money there is an exhaust leak causing a false lean condition. The computer adjusts, chaos ensues.

The opti is not always the solution... from my experience, the following should be checked:

1) O2 Sensor
2) Fuel Filter
3) Coil Wire
4) Condition of throttle body
5) Condition of harnesses to various sensors
6) If equipped, MAF and relays
7) Aforementioned exhaust leaks

I look forward to others adding their 2 cents in. If we can create a consensus on what to check/repair/replace/fix, it will be added to the C4 Tech FAQ.

 

I just posted in another thread that my rough idle was leaking injectors at 80k, this was after LTCC upgrade so I don't think a rough idle is necessarily opti related.

In my experience, rough idles are usually spark related, (as bogus pointed out in the other thread). I think my problem was too much fuel for the plugs to efficiently light off. The car ran terribly rich at idle and the exhaust testified as such. Thankfully I don't have to pass emissions.

I wonder if the batch fire nature of the early LT1's inherently delivers too much fuel at idle. I thought I read GM went sequential injection specifically for idle conditions as at higher RPMs SFI is essentially batch fire anyway. I also remember seeing something in some tuning software about idle spark control... something about varying spark to help smooth idle. Anyone else see this?

I look forward to reading others experiences as our C4's are probably all in about this mileage range. Hopefully we can come up with some trends. I'm also curious, what the 92 recall was all about?

 

Without any codes being thrown your totally in the dark. Myself, Ill never again just start replacing parts or sensors to try to save a few bucks. Even with a new opti your symptoms point towards it. God forbid. Hopefully its a plug or even a bad wire. I would pay a Corvette shop or Gm only to put there scopes and scanners on it. Youll pay 100 to 200 bucks to find the problem but then you can repair it and save money there. Good luck

 

I agree with you on the not throwing parts at a problem and you will save money with proper diagnosis. But not all problems will throw a code. I've seen members post no start conditions while seeing no codes.

 

This list from bogus is an excellent start for a checklist.

1) O2 Sensor-Replaced with new
2) Fuel Filter-Recently replaced, plus we checked fuel pressure
3) Coil Wire-Tested with known good unit
4) Condition of throttle body-It looks okay (clean), are you thinking vacuum lead?
5) Condition of harnesses to various sensors-Checked/megged all that I could get at
6) If equipped, MAF and relays-Tested with known good. How would you know if you had a bad relay? Would this show up in datamaster?
7) Aforementioned exhaust leaks-I know I have a small leak aft of the O2 sensors, but I have plugged it temporarily and still got the same condition

I will add to the list:
8) Coolant temp sensor-Tested with known good
9) Plugs and Wires-Replaced with new
10) Fuel Injectors-Replace with new
11) TPS-Tested with known good
12) EGR-Removed and Plugged
13) ECM-Tested with known good
14) IAC-Tested with known good
15) New Opti- Replaced 3 times within this year. The first time was a definate opti failure (low res code), but the other 2 were us chasing the problem
16) Timing Chain-This is related to the cam change. I pulled the cover to double check that I had the timing marks right. They were.
17) Camshaft dowel pin-Turns out that the cam's dowel pin is longer then the original spec. TJWong suspected that this could be causing eccentricity in the rotation of the opti. We shaved it down to the correct spec.
18) Cooled down engine-This is more related to the knock situations, but we added a 160deg thermostat and replaced the coolant crossover tube on the back of the heads

Other crazy stuff on this condition-The car has been on some very sophisticated scopes at TJWong's shop, including the inductive oscilloscope and Tech 2 reader. On the ignition side, everything showed normal (even while the car was exhibiting the behaviors!!)

There has to be something going on here that I’m missing. Is it related to the mods? I noticed most of this start after the long tubes. I have also noticed that a few folks talking about this problem have long tubes. Backpressure issue?

Please folks, tell us to what level you’ve modified, the circumstances where this behavior began to get noticed (did you change something or did it just start on its own?), and if you somehow got it to go away!

Jeff

 

Anything you could think of that may be mechanical?

 

I had hooker shorties at the time. Interesting thought on the backpressure, I think only extreme backpressure would affect idle. I think more free flowing headers (including shorties) should have a negligible effect at idle.

 

[QUOTE=TankerVette]

Other crazy stuff on this condition-The car has been on some very sophisticated scopes at TJWong's shop, including the inductive oscilloscope and Tech 2 reader. On the ignition side, everything showed normal (even while the car was exhibiting the behaviors!!)

Will this equipment indicate a bad ground, or do you deduce that from the fact that the particular sensor is erratic? I thought these things could tell you everything. The fact that it shows "normal" when it is plainly not normal indicates some serious limitations.

Forgot to mention. I'll be replacing my opti and wires this weekend, and the above checklist is already setting on the tool box-thanks to Bogus & Tankervette

 

forgot to mention...

19) MAP sensor-tested with known good

 

I've been chasing this slight miss @ idle along with a popping in the exhaust for about 2 yrs now. I've replaced about everything and done most of what's on the check list(s). I have a K&N open lid .... But I really think that all this started when I installed my JBA shorities. I feel that these LT-1 like a little more back pressure. I know that it sounds counter-intutivite. I also think that the popping is from unburnt fuel igniting in the cats. Intergrators @ 126-128, But my BLM's long term trim have been split since I installed the headers. I have done the propane thing several times for vacuum leaks and have found none. JBA's are about as good of quality as they get, so I don't think there is a problem there other than the backpressure reduction previously mentioned..... in conjunction with the Borlas that also adds to less pressure.
I've had the car scoped with the tech II about 1 yr ago. Shows all is good and no problems but we (me and the tech) watched the engine miss while the tech II was hooked up. We both noticed that the "miss" went along with the "pop" in the exhaust. Go figure. On the positive side, the car runs like a scalled dog to red line.

 

I believe Mr. ittlfly my be on to something... Wasn't there recently a post about someone who put cats back on this car and it ran better?

 

Yes there was a recent post on someone putting their cats back on....and giving Bogus due credit for his suggestion.
I have replaced every thing on the list except the IAC, TPS and ECM. I did have the TB cleaned however. Since this all started around the addition of the headers, 1 and 1 is probably 2. So I 'm living with it for now.

 

ittlfly,

btw, how many miles?

 

So far,

k99ja04
80k miles
Shorty Headers
Proximate Event: Probably Worn Out Injectors
Probable cause: Leaky Injectors
Fix: Replace Injectors

ittlfly
? miles
Shorty Headers
Proximate Event: Install Shorty Headers
Probable cause: ?
Fix: ?

Tankervette
94k miles
Long Tubes, Ported Heads, Cam, 1.6rr
Proximate Event: Installed long tubes
Probable cause: ?
Fix: ?

Let me know if I'm getting boring

 

O.k. My brother found this problem an an Lt1 a couple of weeks ago.

Same symptoms.

Took apart opti- checked cap and rotor - looked good.
They were recently replaced before he had it in.

Just happen to notice evidence of a small burned hole where arcing thru the plastic shield plate to metal (ground).
If the rotor and/or cap were running worn out for any length of time, the spark will try to find another path to ground.

Even with a new cap and rotor it (the Ignition spark)continued to leak to this spot causing the miss.


Vettedave

 

It sounds like a way too lean condition to me.
Have you had a scanner on it ?

 

Just turned 40K

 

[QUOTE=DMGroh]
Just got inside my opti and found the source of my problem. 1 of those tiny torx screws holding the rotor backed out and was laying around inside there. With only 1 screw holding the rotor, it can flex quite a bit and caused the violent jerking/miss I had. The screw that fell out was not nicked or beat-up, so I believe the problem was from the rotor flexing. There were no carbon traces, but the contacts on the cap were screaming for replacement. Sure glad I went in there.

Question for the "Wiser/Better Informed/Experienced more problems than I have (yet)": Should I take out the rotor screws from the new GM opti and loc-tite them? Would they already have loc-tite? Any advice here?

Comment on a 10 year old rotor. I was surprised at how much the plastic had deteriorated..it was about to break into pieces and there were no signs of blunt force trauma. If you guys are upgrading to an aftermarket unit and decide to keep the rotor in there as a spacer, you might want to think twice about re-using the old rotor.

Still have a question about the sophisticated scopes and scan equipment. It is possible to have a problem that these scans might not be able to detect?

 

i have similar issues plus a high rpm miss (5250-6500).

i have done the hot cam kit along with ported polished heads, intake and tb and slp shorties.

those shorties seem to be causing problems for a lot of people....hmmmm