help with truck o2 sensor?

Question

i had an exhaust job done on my truck. the truck is a 1990 gmc 1500, 2wd, 350. i had the cat removed with a flowmaster muffler on it. now the truck runs horrible. after a while the check engine light came on and the code says o2 sensor. i only found one o2 sensor in the left manifold. i replaced it and it did nothing. i did find a wire though hanging free by the distributor. it is the same color, size, and same connector as the o2 sensor connector. but it's disconnected. i think there might have have been another o2 sensor on the truck and the guy just unplugged it under the hood when he removed the cat, while doing the exhaust. i dont know if unplugging an o2 sensor can make a vehicle run bad (miss, stumble, ect.) but ive done every thing else. ive even had the throttle body checked and distributor checked for damage and it all looks fine. i know newer trucks have 4 o2 sensors, but if anyone knows how many this truck should have or what i should do to fix it, i would appreciate it.

Answer 1

You are supposed to have one upstream sensor in the Y-pipe, and one downstream sensor, after the cat. If it is not there, the check engine light will come on and your vehicle will run horribly. I was just checking into having my 95 Chevy 2500 switched to a dual exhaust, but the 02 sensors are holding up my decision.

Answer 2

By replacing so much of your exhaust system, you got rid of the downstream O2 sensor. (downstream= after the cat.) The downstream cat monitors the efficiency of the catalytic convertor by telling the PCM that the cat is working properly or not. By having no sensor there, the PCM has thrown a code. The best thing you can do is put your cat back on and put in a downstream sensor.

A common misconception is that the catalytic convertor will decrease exhaust flow, increase back pressure, and therefore cause you to lose horsepower. This is not true. The way cats are made is such that they have a honeycomb type structure in the middle. If you were to unravel that entire honeycomb shape, it would be the size of a football field. So, there is plenty of space for your exhaust to flow.

Also keep in mind that GM designed that exhaust system on that truck to flow at optimum performance with the cat and muffler installed. If you want to do an exhaust performance upgrade, you have to do it from the headers back. It doesn't matter if the cat-back system is 6 inches, if your exhaust is only 2 inches up to the cat, then the extra volume of the cat back will do you no good.

Here's my suggestion if you really want to see some exhaust-related performance: Take the heads off and port them. Don't polish, you'll lost your bottom end power, but you can smooth out the flow a little with porting. Then, get an appropriate-sized exhaust system the entire length of the truck. Don't just mix and match parts, plan out your exhaust as units working together, not just parts thrown together. Make sure you have the cat and muffler installed, as well as both O2 sensors. You could also get a higher flow throttle body and a better intake to match your better performing exhaust. Hope this helps.

Answer 3

You should have at least 2. 1 b4 the cat and 1 after. if you had work done at a shop and they didn't install the second sensor then you should take the truck back 2 them. Without the 02 you will waste tons of gas, loose horsepower, and could even ruin your catalytic converter.

Answer 4

By Federal law you cannot remove the cat.