94Z28, 135k miles. runs perfect until you run it hard (like 0-60 blast), then it will miss badly under part throttle (like 1/3rd throttle) from approx 2500-3500 rpm. I kind of goes away if you give it more throttle like half or wide open. Once it cools down it is fine until you get on it hard again. I suspect the o2 sensors need changing. I doubt they've ever been changed and the car has always smelled a bit "rich" and the exhaust is a bit sooty (not burning oil though). It is not just a heat related issue b/c it seems to run fine even when very warm (stop and go traffic), it's just when you run it hard it will miss for a while afterwards.

DOes this sound like bad o2 sensors? and/or bad plugs? I suspect both need to be changed. No idea if the plugs have been changed. Gas mileage hasn't went down a noticeable amount but it was never great since i owned it. I always figured the o2s were going bad.

thoughts?

 

Take a look at the EGR while you're at it......

 

If it is doing it at WOT I do not think it is o2 related. I will bet it is an ignition problem. Check the wires, plugs and if not then I would bet opti.

 

it's NOT doing it at wide open throttle. only AFTER a wide open run, not during. it isn't acting like an opti problem at all. i've had two of them go out on previous cars, doesn't seem like an opti at all.

 

Ya, an opti would do it at wot. Is this car all stock? And is your SES light on? I Would pull a couple plugs out and see what they look like; maybe the number 1 and 3 plugs since they are easy to get from the bottom.

 

it's essentially all stock. SES light is NOT on. going to pull some plugs tomorrow.

 

Plugs are black. running in WOT goes open loop and richens the mixture even more. Causing already crappy plugs to get worse until some of the fouled coating burns off and fire better.

As far as your diagnosis of the OPTI, you are dead wrong. While it's not running bad all the time, it's defective enough to lead to poor combustion. Just dare to remove the cap and look inside at the contacts of the opti.

The first place you should look is the air filter. The most often cause of fouled plugs.

I suggest a thorough tune up.

 

not sure on the opti, it ain't new, but runs fine otherwise, prob ok for a while, isn't acting up like my previous ones have when they went out was what i meant. we'll do plugs and O2s and see what happens. i believe that'll fix her.

speedy, what do you mean on the air filter comment? are you saying when they get clogged up or are you saying when aftermarket ones get dried out and start passing dirt?

thanks all

 

It's only my opinion, but, I don't think the air cleaner is the source of the problem. Sure, back in the day, clogged air filters could cause plug fouling but with FI and mass air flow that isn't much of a problem anymore. If the air filter was clogged you'd have trouble generating high rpm's...

The problem only occurs at the middle rpm range after a high rpm run. That would indicate that something isn't recovering...perhaps the O2 sensors or maybe the EGR. See if you can hook up with someone that can do a scan to see what's going on as you drive....

 

that's my thinking, bw_hunter. thx. the filter is a K&N but needs to be cleaned and oiled i think. i think i can scan the thing myself it's just pain. i have all the software i think, but need to figure it out. i haven't had the car in a week, as i left it at my dad's when i picked up a BMW last week. hopefully will get it tonight and start checking some stuff out.