I recently purchased my first Vette: 94' Black Rose Metallic coupe with 6sp. It has 86,000 miles and the only mods are the cat back Flowmasters and the skip shift disabled.

After I picked it up, I've noticed that under constant light throttle (about 2000 rpm), I notice a "miss", "hesitation", or "bucking". It doesn't happen all the time, but it is noticable. Under full throttle or in higher RPM's I don't notice it. I've not noticed any backfiring. The fuel filter has been replaced and the fuel injection cleaned. It does it with the fuel tank low and full. Any ideas? From reading previous listings (and its mileage), I'm beginning to suspect the Optispark. Again, any pointers would be helpful.

So far, this forum is great. Just searching through the messages, I've picked up a number of suggestions to look at other problems.

Dave

 

That is what it sounds like to me.

 

I tend to agree, the opti COULD be the culpret here.

First, tho, any codes? If not, check it. Get the codes pulled. Autozone will do this for free!

Check the coil and coil wire.

The problems you are describing do not sound opti-esque to me...

What is the condition of the plugs/wires? The factory ACDelco Platinum plugs are junk. They have these little platinum buttons on the ground strap that burn off, or fall off, I am not sure what. The end result? The gap grows from a stock .050 to a not-so-stock .065! Check'm!

I would start with the plugs... I recommend NGK or Denso platinums.

 

Also, 2000 rpm is EGR country. Might have an EGR related code stored. But that often triggers the Check Engine light. But too much EGR being let in could cause the symptom you describe.

If no EGR code is stored, you might try to still rule it out by pulling the vacuum line off the EGR valve, plugging it or cap it off, and driving it to see if the symptom happens. If it is EGR you're lucky, EGR solenoid is an $11 part.

 

I wouldn't bet on the opti just yet - if you're having troubles at 2000 rpm, you'd have big troubles at higher rpm.

I'm guessing the EGR thing too.

 

The NGK Iridium plugs are dope, too!

 

Thanks for the tips. The check engine light is not on. I'll try running the car by the local Auto Zone to run the codes Tuesday night.

Dave

 

Turns out the AutoZones in my area don't have the readers and my car doesn't have the 12 pin port. So, I guess I get to find another shop this weekend.

Also, I noticed that when cold, the car doesn't act up. Warm it does.

I'll check the other items this week.

Dave

Would a bad EGR affect idle? I noticed tonight that the idle is a little uneven.

 

Cold you're running in "closed loop", the ECU starts retrieving data from the various sensors in "open loop" after the engine warms up. This would make me think it's a sensor somewhere or maybe that EGR solenoid...

just my $.02

Joe

 

EGR typically shouldn't flow any exhaust gas back into the engine at idle, but if it's not working correctly and allowing exhaust back into the intake at idle, yeah, that could affect idle quality.

Ben

 

It wasnt my vette...
but I had the same situation in my daily driver cavalier...
New plugs fixed the problem...

just my $.02

 

A lot of '94's and 95's have a jsut a bit of chuggle here and there in the idle. MY '95 always has since I've had it. Looked at just about everything, can't pin it down. No big deal the moment it comes off idle its smooth and strong.

Don't buy a code reader yet!

If your '94 has the 16 pin ALDL port, you can still use a bent paper clip to get it to display error codes on the Speedometer LCD readout. Bend a paper clip into a tall "U" shape such that it will bridge the top and bottom pin outs on the port. Then bridge pin #4 to #12. Count goes from left to right begins with 1 not 0, so the top left most pin is #1 and the bottom right most is #16. Once its bridged turn the key to on but do not start, just to on where the fuel pump energizes etc. The codes will then display on the speedo. Here's more:

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How to View Stored Diagnostic Codes on 1994 – 1996 Corvettes

You display codes on the digital LCD speedometer readout, by using a re-shaped paper clip to short ALDL pin 4 to pin 12.

1994 – 1996 Corvette ALDL connector:



You short pin "12" to pin "4" and turn the ignition switch to "On" without starting the engine.

The speedometer will display any codes using the same protocol as the 1990-1993 model but there are some changes in the designation for the modules plus additional modules are added:

Module "1" is still the CCM module.

Module "4" is now called the PCM module (Powertrain Control Module) because automatic transmission computer control was added to Engine Control Module in 1994.

Module 7 (on the 1996 model only) is the RTD module. (This is the Real Time Dampening module which replaced the Selective Ride Control module in 1996).

Module 9 is the ABS/ASR module number from 1994 through 1996.
Finally, Module "A", the DERM (Dynamic Energy Control Module --- (the air bag control module) will be requested to show any codes.

Just like the 1990-1993 display, you read the codes on the speedometer (big) and read the module number on the trip odometer (small).

A series of dashed lines for instance “ - - -“ indicates no more stored codes for that module.

A code showing as “C12” for the CCM module (module #1) means no codes are stored. So C12 is a good thing to see.

Again, you can cause the codes to repeat by turning the ignition off for five seconds and then back on.

Turn the ignition off and remove the short to restore normal operation.

To clear codes in the CCM proceed as follows:

1. With ignition "off" ground terminal #12 of the DLC (use bent paper clip to connect pin 4 to pin 12)

2. Turn ignition "on".

3. Press trip/odo button on the DIC until 1.7 appears in the trip
monitor area of the instrument cluster.

4. Press eng/met button on the DIC & hold until "---" appears in the
speedometer area of the cluster, this will clear CCM DTC'S.


To clear PCM/ECM codes do the following:


1. Press the trip reset button on the DIC until the desired system is displayed. In this case it will be 4.0.

2. Press trip/odo button on the DIC until the desired diagnostic mode(4.7) is displayed on the trip monitor area of the cluster.

3. Press & hold the eng/met button on the DIC until "---" is
displayed in the speedometer area of the cluster, this will clear PCM codes.


To clear ABS/ASR codes do the following:

Same as above but you’re looking for “9.7” on the trip odo



P.S. If the problem still exists the code(s) will return after engine operation

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Lone Ranger: Thanks. I just got my service manuals tonight, which went into detail about the 12 pin port, but mum about the 16 pin port.

The uneven idle didn't worry me as it wasn't racing or stumbling.

I guess I know what I'll be doing this weekend! Thanks everyone.

Dave

 

BTW, congrats on your new Corvette. That Black Rose Metalic is a killer paint scheme. Keep us advised on your progress.

 

Congrats on your new Vette. My '96 does the same - sometimes rough idle. Runs great off-idle. I've replaced just about everything except the injectors, and don't think it's worth that right now. No codes (since I replaced the #1 plug wire) so I put up with a slightly rough idle from time to time. My 2 cents.

 

This weekend I'll do some digging in to the car.

For sometime I've been looking for a sports car/hobby/toy. I kicked around several ideas. For the price of a used Miata, you could get a Vette and then I said why bother with the Miatas. After looking at several Vettes, I ran across this one. Althought the price was right and loved the color, I tried hard to find something to scare me off. I couldn't and ended up with it.

It's been driven, but doesn't appear to have been abused. Has the typical C4 stuff (weatherstripping cracking), but everything works (except for the horn which I found out the other day!). Rear tires are new, front tires worn (but even wear and no cupping). For having 86K miles, the manual is one of the most precise shifters I've ever used. That includes new Japanese cars such as Eclipses. Of course, I'm still getting use to the 300 hp!

Dave

 

Update:
I pulled the codes: none. As far as I could tell (with what I could do), the EGR appears to be fine. However, I did find the water pump leaking.

Due to what I had going on at work, I elected to have someone else work check it out and they found that the optispark was going bad along with a couple of iffy plug wires. I should get it back tomorrow.

Dave

 

Sounds like the start of Opti problems OR it could be just a spark plug. I would probably check and change the plugs and if it is not that, bite the bullit.

 

I got the car back and here's what they found (and fixed):

* Water pump was leaking through the rear bearing
* Opti was going bad. Rust was found in the cap.
* At least one spark plug wire was iffy.

While it was under the knife, I had them go ahead and replace the plugs, all the wires, the belt (it was worn and while they had it off ...), the pump, and the opti. The car runs great now.

Now it's already to put away for winter!

Dave