I finally got my new clutch put in and now it is on to the engine trouble.

It all started a few months ago when I got the SES light. The code said bad O2 sensors. The car seemed to be running rich, I could smell fuel, power was down, and the idle was rough. I replaced the sensors with bosch units and everything was fine for a few days. Then the same code came up and same symptoms of the engine.

When I read the codes now, it says a bunch of O2 codes and one for a multiple cylinder misfire. My gas mileage is terrible, in the 10mpg range, there is black soot on my rear bumper and the occasional puff of black smoke. The car is way down on power, idle is rough enough to shake the car,(especially with the ac running), and the engine just doesn't run as smooth as it should.

The shop that did my clutch (Corvette Masters of Orlando) suggested changing spark plugs, wires, and buying AC Delco O2 sensors. Should this do the trick? Also, could this be a sign of a failing opti.

Basically the engine runs but just not as good as it should. (No trouble with starting either.) A few of my questions are; how many O2 sensors are there (I thought there were a total of 4, 2 pre-cat and 2 after cat), and what plugs should I buy. I plan to run nitrous in the near future so should I take this into consideration?

Also what are some tips to changing the plugs and wires? I've heard its pretty difficult. Again, my car is a 96 LT4 with 75,000 miles.

 

First of all what codes are you getting? Just because it sets a code doesn't necessarily mean the device is bad. For instance if a code 44 was set, it means its a lean condition. Several things can cause a lean code such as a plugged fuel filter or faulty injectors.


It sound like you have a miss firing problem with the miss fire codes. You need to correct that before doing anything else. Spark plugs are not too bad to replace. Wires on the other hand will take a long afternoon for the average home DIY'er. The passenger side is relatively easy. The drivers side is a pain because the wires are routed behind the large AC and ALT bracket. You can remove the ALT, and AC compressor, just unbolt them and lay the compressor to one side. Loosen the PS pump and unbolt the large aluminum bracket. That will give you access to the wires on the drivers side. It isn't that bad really, and I can tell you its not even close to as difficult to replace plug wires on a LTx F body! Now that is a royal pain in the a** job.

As for the Opti, that is always a possibility with a reoccuring miss. The best thing to do to eliminate guess work is to have the engine scoped on a ignition analyzer. That would tell you if you have a wire or a plug bad. If these things look good and if the miss is persistent I would be looking at a Opti.

 

I have a LT4 and I replaced my wires by doing it from under the car. It can be done without taking all of that stuff off the front of the motor. I roughted my wires below the motor along the side of the oil pan and it looks alot better. It really cleaned it up

 

gee...

I don't think it's a failing opti, but I sure would check the coil and coil wire. The coil wire has a tendency to rub on the water pump. This is not good at all. Problems like this can ensue. Mainly, cause the spark is poor... and with an OBDII car, which is much more intellegent, I would be worried about it throwing a code.

OBDII has more sensors, and can make many more assumptions based upon the data being inputted into the ECU. Many times, those assumptions are correct.

For example, there isn't any one sensor that tests the cats, but, the post cat O2s do test the function of the cat, so when it doesn't get the level of improvement it is expecting, it will throw a code.

There are other data elements that are a result of the OBDII doing its analysis.

I did a search on Road & Track's website, and came up empty, however, Dennis Simanaitis, Engineering Editor for R&T did a tech article on OBDII a couple of years back. Most enlightening.

 

I wonder if they hit one of the wires to an O2 sensor when they changed the clutch? It could just be disconnected.

 

The problem was there well before i had my clutch done. I appreciate the help and hope more will chime in. Also any plug suggestions.

 

Use genuine GM O2 sensors. I tried bosche only to get a bad one out of the box.Lesson learned just like rebuilt Optis.

 

NGK Platinum or Iridium.

Stay clear of the AC Delco Platinums... they are poor. They run fine... for a while! They have this little platinum puck on the ground strap. For 30k miles or so, they are great, but over time, the puck either falls off or burns off. I don't know which. What that leads to is gap growth. From .050 (spec) to .065 (not spec).

 

Bosch are low on my list. OEM or Densos are best!