LT4 LT4 Tuneup Question

[Nick S]

Ok, I have a 1996 LT4 Corvette with 65,000 miles. I recently replaced the spark plugs and wires with the stock AC Delco replacements. Everything other than the SLP intake on the car is stock.

Now to the potential problem. The car seems to idle a little rough at any operating temperature. I hooked up a scanner to the car. It idles at 720 rpm and then every five to seven seconds it drops down to 690 rpm for a split second and then back to 720 rpm. Before I replaced the wires I could see some arcing. So, I figured that was what was causing the idle issue. The wires did not fix that problem. Although, I have noticed a little better pickup at low rpm after the plug wire change. It seems that some horsepower was freed up. Also, there are no stored codes in the computer. Other than the somewhat rough idle, the car runs great. Do I have a problem, or am I imagining things? If I have a problem, any suggestions for fixing it will be appreciated.

Thank you.

Nick

 

[6 Shooter]

How does the Opti look?

 

[Nick S]

While changing plug wires, all that I could see was the cap on the optispark, and it "looked" ok. There were no signs of any kind of carbon tracking or anything. Although, I would assume that, that is something you cannot see from the outside. The assembly was clean. Also, from what I understand, you can never really tell if the optispark is bad until it either a) throws a code or b) is removed and disassembled. And so far, no optispark related codes have been set (or any other codes for that matter). As of yet, this is not a violent kind of idle. It is just, not smooth. It kind of reminds me of the idle that I had from the LS1 in my old Camaro. A bit rough, but considered normal.

Nick

 

[6 Shooter]

WOW - My LS1 runs pretty darn smoooooth.

I sure loved my old LT4, quite a piece of machinery. The Opti-II were suppose to be quite a bit more robust anyway.

 

[Aurora40]

I hooked up a scanner to the car. It idles at 720 rpm and then every five to seven seconds it drops down to 690 rpm for a split second and then back to 720 rpm.

Does the scan tool show what the IAC is set at? Or what the "desired idle" is? See if the IAC steps down or up when the idle fluctuates. Just a thought...

 

[Nick S]

If I remember correctly, the IAC read steady at idle. I'll look at it again, get the reading, and report back.

Nick

 

[Louis Bartay]

Aurora40 is correct, sound advice. I do hope you did not use Delco OEM stock spark plugs, as this is not your idle problem but the platium disc will burn or fall off and your plug gap will be to wide. Look at all of your old plugs and check and see if the disc is on the ground strap as three out of eight on my LT4 were missing the disc's after 55,000 orginal miles. My wife's LT1 T.A. had the same problem when I changed plugs on it. I went with Bosh Platium 4's as they do not have a strap blocking the spark as in the old days we use to buy a shim kit to index the spark plugs so the ground was parallel with the valves so the spark was not blocked and a open flame was facing the towards the exhaust port.Also make sure your vacuum line from the opti spark is not leaking and is in good shape.