My 1989 IROC-Z 350 with rebuilt shortblock (w/ 30,000 miles) has recently started idling a little rough and stalls when the car has been driven for a little while and is hot. It seems to get worse and worse with the idle swining back and forth about 500 rpm or so. Does it just need a tuneup? Maybe some sensor is bad? Any help would be appreciated.

BTW, I also am starting to hear a pretty audible "ticking" sound coming somewhere around the passenger side head. Is this an exhaust leak, or something wrong with the head? The heads themselves are stock and have not been rebuilt. There are about 190,000 miles total on the heads.

 

One possible cause of an exhaust tick on a high miler could be a blown off oxygen sensor (but I believe thats on the drivers side though). Happened to my brother's '87 with 200k+. I saw the wire for the sensor laying in the engine bay. we followed it down and saw that the top half of the sensor blew apart from the bottom half (which was still in the pipe). It had an exhaust tick and ran a little rough, but wasn't stalling.
Another possible cause of bad idle is if the Idle Air Control Motor (IAC) is bad. Just put a new one on my parents '94 T-Bird (105k) because the engine would bog really bad at idle speed when you put the car in gear of turn on the AC. This cured it. On the IROC, I know its located right under the throttle body, but not sure how to determine if its good or not, maybe someone else can help???
One last thought would be the throttle position sensor (TPS). This happened to our '94 Bonneville @ 130k. The TPS was bad and the engine would idle at about 1,500 rpm instead of 800-1000. The problem was not consistent, but when it did idle high, it did so for several minutes. Hope this gets you started in the right direction.

 

Thanks for the great info. One other thing I forgot to mention is that it sounds like there is a big chunk of metal rattling around inside the catylitic converter. Maybe it's all related?? When you said the oxygen sensor might have blown off, I was thinking maybe it broke off inside the pipe and is now rattling inside my converter, which is causing the rough idle and the exhaust leak? But if you said it's on the driver's side, then, I'm not sure if that would be it. I'll check it out and let you know what I find.

 

Sounds like a vacuum leak to me.

 

How do I go about checking and locating the vacuum leak?

 

It may be hard to find the leak, check all the rubber hoses that come from the intake and carb. But first remove just the top of the air filter leaving everything connected. With it running, slowly start to cover the carb. opening with your hands. If everything is ok, it should want to stall. If the idle gets smoother and faster because you are restricting air, then you have a vacuum leak. I wouldnt worry about the noise in the cat., the honey comb inside comes loose and rattles around. If you want to take it off and check it, you should see light thru it. Mine in an 86 monte rattled around, but it was clear and clean even though it had 140,000 miles, and valve guides that puffed smoke every time I started it. Good luck!

 

And it might also be good to check the timing!

 

Actually, it has TPI, but I'm sure your instructions still apply.

The rattling in the cat is very loud. It's getting so loud it's embarrasing. The cat was replaced not 20,000 miles ago. It really does sound like something is lose in there. I wonder if something could've came from the engine and just settled in there??

I thought maybe the exhaust hangers came lose, but the noise is definetely coming from the cat. I wedged a big pry bar between the cat and the floor pan to prevent it from moving around, but it still rattles.

I'll check the timing and the vacuum sometime tomorrow. I just hope whatever it is is a cheap fix!!

Thanks again!