My '94 is running rough again, idling up and down at stops and sounds louder when I accelerate during driving. Seems to run okay for about the first 5 minutes, but then starts acting up if I have to stop for more than about 10 seconds. Have replaced all of the obvious, including the spider assembly about 2 months ago (had been leaking for who knows how long). From other posts I see that sometimes the cat converter is to blame, and am wondering what to look for on my own to see if that may be my problem. Have checked for vacuum leaks, replaced plugs and wires, egr is clear and has a screen already in place, new distributor....Also have what looks like water dripping out of the tailpipe, and I'm sure that's not good! Can't see any obvious holes in the exhaust pipes, muffler, or cat but all are rusty. Bought this one used so don't know if any of this has ever been replaced. Other than taking it to the shop, anything else I can do to check it?

 

If it were replaced at some point, you could probably tell because there would have be some welding involved, and you'd see that along the exhaust.

Is there any smell at all? Before I replaced my cat on the ol' Volvo I smelled like rotton eggs something fierce! If it were the cat you would likely have a bad sulpher smell as well.

You can't really see if it is clogged or not without removing it, to the best of my knowledge. You could remove the O2 sensor from the exhaust pipe and see if there's any change at all in how the truck is running.

How has your mileage been? You might see a drop in how great your mileage is if it's clogged. You might not accelerate so well either.

 

Actually the acceleration does seem kind of slow, at least compared to how it was when I first drove it. Mileage is hard to tell, because I only drive about a mile to work and a mile back, and that is the extent of most of my driving (plus with gas at $3 a gallon I am trying to conserve my driving!)...I can smell exhaust when the windows are down, but not the sulphur smell. Car sounds "throatier" when pushing on the accelerator, that's why I was wondering about the cat or muffler and if those would affect the idle.

 

I'm by no means an expert, but I would be lead to think it is probably the cat. The sad thing is you'll never know until it's off, and if it's not that, then you've wasted time and a lot of money replacing it. Perhaps some of the more experienced guys can step in and give an opinion.

 

Any experienced muffler shop should be able to run a backpressure test to see if the cat is restricting the exhaust flow too much. If you do not have an O2 sensor post cat, they will torch a hole and weld on a threaded nipple for their machine to use for a downstream reading. They will then tap off of one of the sensors pre-cat for an upstream reading. They should have a reference as to what a properly flowing cat should be and if it is significantly different, then you have a problem.

Good luck!

 

Here's a little more info I discovered today (don't know if this will help or only make it more confusing!)...While driving with the A/C on today (at 93 degrees I figured it was time to check and see if it worked) the car seemed to idle MUCH better at stops. It was a little weak on acceleration, which I would expect from the A/C pulling on the motor, but the idle seemed to not jump at all, or only a little bit. I pulled in the parking lot at work, let it idle in Park with A/C on and it seemed fine. As soon as I shut the A/C off it started to idle crazy again! Am I now looking for something different, or could this still relate to the cat? I love my Blazer if I could only get this figured out...one mechanic stumped and one to go!

Oh, not to sound too inexperienced (but I am)...to find an O2 sensor where exactly would I look and what would I look for? After the cat, but before the muffler? How do I remove it to check?

 

Can anybody help me with where to find the O2 sensor, and how to remove to check it? Would like to do it on Sunday when the weather's nice and I have time to tinker with it. Thanks!

 

Removing and looking at your O2 sensor will tell you very little. The outside conditions are very rarely indicators as to the health of the sensor.

I have an AC Delco pn of 25162600 for the O2 sensor for a CPI motor (not TBI). It should be located in the exhaust pipe before the catalytic converter but after the two banks come together (the y in the y-pipe). It will just screw out.

Be careful of which brand sensor you choose. I have had VERY bad luck with Bosch and Borg Warner sensors in any GM vehicle that I have put them in. I stick with genuine AC Delco O2 sensors for all GM vehicles I work on and have never had one come back with O2 troubles again. With my Autotap, I observed lazy responses from a brand new Bosch sensor and it wasn't until I installed a new AC Delco that I realized just how lazy the Bosch was.

The only true way to diagnose a lazy O2 sensor is with an oscilliscope viewing the voltage from the sensor that the PCM is using to determine the injector pulse width (or with a scan tool that can view the sensor data as well).

If you think that the sensor is original, then changing it may not be a bad idea. The condition of the plugs should be assessed at this time as well. Just pull the plugs, check the gaps and take a look at their color. If there is any carbon buildup on the plugs, replace them.

Also thought I would throw this in here too. Do not go with one of the Universal O2 sensors. GM O2 sensors breathe through the wires for the purpose of drawing in clean ambient air from the outside to compare oxygen content with that of the exhaust gases. If installed improperly, the Universal types can restrict the flow of air through the wires causing the O2 sensor to fail.

 

Thanks for the advice...Sounds like I should just go ahead and replace it and see if that helps any. The plugs and wires have been replaced less than a month ago, so that should not (I hope) be the problem. Once it quits raining I'll go out and check on the sensor. I'll let you know if that makes a difference or not!

 

It makes sense that your idle is better with the a/c on because the computer kicks up the idle to compensate for the extra load of the compressor and cooling fan.
You could(if you have one) connect a vacuum guage to a vacuum port on your intake. If it reads low , like 12" or less, you probably have a plugged exhaust. You can't get air in if you can't get it out.
Good luck