Could a bad O2 sensor make the engine run lean?

LJameson · 08-26-2008, 11:25 PM

I'm pretty sure it's bad. I have no codes, but the engine has NO power. When I mean no power, I mean that my buddies 2.4 TJ is quicker then mine. Often times it gets warm (about 215-220) going up hills. My radiator looks cleaner then a hospital, and it's full of coolant with no leaks. The water pump only has 20k on it, so I know it's good. After thinking about it, i think the engines getting warm because it's running lean. Also causing it to have no power. I was thinking the cat, so i put a vacuum guage on it. Everything checks out good with vacuum. With a bad cat, when you blip the throttle, the vacuum will change slowly. However, with mine, it's very quick. Plus, it doesn't rattle. I had a thread earlier asking about the gutlessness, so i just wanted to ask if thats the problem. If its something else, i'd rather save the 60 bucks

dbell54 · 08-27-2008, 03:28 AM

Your 2000 is OBDII compliant, wherein the PCM (computer) checks virtually everything and issues specific codes. It is very unlikely to be the O2 sensor. It sounds like not enough airflow. Check the intake and air filter for restrictions.

LJameson · 08-27-2008, 11:24 AM

All intake and exhaust systems are good. As a quick little test for the cat, i disconnected the exhaust at the manifold to see if it'd have a little more oomph(MAN, it was loud). I drove it down the street and back, and even though it was fun to see some alarms go off, it still felt the same. So the cat is fine. I've also done a compression check, and all cyl. checked out good. So now I'm just kinda stumped. Also, as a little side note, I could not pull up codes on the odometer. I tried everything. On-off three times, holding down the odometer button....Anybody with a 2000 have this problem?

dbell54 · 08-27-2008, 04:14 PM

Check the fuel pressure. If the air flow is good, then there's not much else it could be. Be sure to verify that the f/p is up to spec WHILE the condition is occurring, i.e. under load. You have checked the codes with a code-reader or scan tool, right?

Pillswoj · 08-28-2008, 06:49 AM

I'll disagree with dbell (sorry) O2 sensors can be bad without throwing a code. You don't say how many miles are on the jeep but if the O2 sensors are original they might be due for replacing.

**THETROOPER** · 08-28-2008, 06:58 AM

pull the o2 sensors out of the exhaust,but keep them connected to the electronics, let them hang from something and see if it runs better. If it does replace the o2 sensors

Pillswoj · 08-28-2008, 07:10 AM

Quote:

Originally Posted by THETROOPER

pull the o2 sensors out of the exhaust,but keep them connected to the electronics, let them hang from something and see if it runs better. If it does replace the o2 sensors

Or disconnect the O2 sensor wires, it will throw codes but will also cause the jeep to stay in open loop mode which is slightly rich, if you jeep runs better again it is a sign your O2 need replacing.

LJameson · 08-28-2008, 07:49 AM

Well, I'm getting my paycheck this weekend so I'll go out and buy an O2 sensor. I'll let you guys know.

nolo263 · 08-28-2008, 08:52 AM

I have noticed mine does the same thing my tj has 94,000 miles on the odo. It runs very warm on the highway even though I have changed everything on the cooling system. I have noticed that sometimes it runs good and power is up and other times the jeep seems like it missing power. I was also thinking that it was my o2 sensor. Which one would you change? Both or just the front sensor? thanks

Pillswoj · 08-28-2008, 09:15 AM

Quote:

Originally Posted by nolo263

I have noticed mine does the same thing my tj has 94,000 miles on the odo. It runs very warm on the highway even though I have changed everything on the cooling system. I have noticed that sometimes it runs good and power is up and other times the jeep seems like it missing power. I was also thinking that it was my o2 sensor. Which one would you change? Both or just the front sensor? thanks

The front(Upper) is the only one that controls mixture. The rear (lower) is only there to check the Catalytic Converter efficency.

Slithering_Joe · 08-28-2008, 09:49 AM

How did you determine if it is running lean? Got any numbers to show this?
If the egine is running lean, the O2 sensors would be the last thing that would cause that. Don't throw parts at it. If you do, you will find the problem but at a high cost.

LJameson · 08-28-2008, 02:32 PM

Quote:

Originally Posted by Slithering_Joe

How did you determine if it is running lean? Got any numbers to show this?
If the egine is running lean, the O2 sensors would be the last thing that would cause that. Don't throw parts at it. If you do, you will find the problem but at a high cost.

We have a 5 gas analyzer at my school. We hooked it up and even under load, it's putting out 13 Hydrocarbons. Although it's good for the enviroment, it should have more then that. Also, the plugs looked liked they were "overheated." If it's not the O2 sensor, then what causes a lean mixture? MAP sensor?

Hardtop99 · 08-28-2008, 02:44 PM

Change the upper O2 sensor. My 99 did the same thing with the exception of a little kicking after the exhaust manifold heated up (O2 sensors dont start reading till theyre about 600 degrees). Best 65 bucks Ive spent performance wise on this thing.

Slithering_Joe · 08-28-2008, 03:06 PM

What do you mean by "13 Hydrocarbons?"

Lean condition: bad MAP sensor, vacuum leak (unmetered air), bad fuel pump, restricted fuel lines, bad/clogged fuel injectors, incorrect fuel injectors ....

burnt plugs: caused by pre-ignition, incorrect ignition timing, lean condition, wrong heat range installed, crossed ignition wires, incorrectly installed ignition wires

...to name a few

dbell54 · 08-28-2008, 03:44 PM

You still haven't checked the fuel pressure? Most like cause of poor acceleration with no codes: not enough air flow or not enough fuel. Low f/p can also lean it out some. Now I won't say that your O2 sensors couldn't stand changing, but I doubt it's the main problem. A scan tool will let you read the O2, MAP and so on to see itf they are normal. Not likely an ignition problem, your 2000 doesn't have a distributor or plug wires, timing controlled by PCM, and the PCM isn't setting a misfire code. A HC reading of 13 is not a sign of trouble in itself. Heck, my ancient Sub only put out 24HC under load last state emission inspection with the original converter, and it runs perfect. CO and NOX were very low, too. Were your CO and NOX also OK? If so, it just means your cat works good. Once again, check the f/p before buying parts.