Bad TPS causing this issue?
03-15-2016, 07:58 PM
#16
CF Veteran
I'm going to have to jump on the upstream o2 sensor bandwagon. Mine did the same when it was bad only more random and more frequent.
03-15-2016, 09:53 PM
#17
CF Veteran
Thread Starter
Join Date: Jan 2014
Location: District of Columbia
Posts: 1,301
Received 279 Likes
on
210 Posts
Year: 1998
Model: Cherokee (XJ)
Engine: 4.0
Nope, though I only bought the Jeep two years ago. I was just poking my head under there, and the connector looks like sort of a PITA to get to. I'll likely have to pull the rubber splash mat out of the way to get to it. That's my next diagnostic test.
03-16-2016, 12:57 PM
#20
CF Veteran
Thread Starter
Join Date: Jan 2014
Location: District of Columbia
Posts: 1,301
Received 279 Likes
on
210 Posts
Year: 1998
Model: Cherokee (XJ)
Engine: 4.0
Unplug the upsteam o2 sensor at the connector. Driver side under the motor mount. Without a signal the ecu will run rich in a preset open loop map. You should get a check engine light but if it's the sensor it will run without bucking. No need to mess with the downstream o2 sensor. Test your tps with an analog volt meter, avoid a digital meter if you can. Fuel pressure test will be after these 2 tests simply because it's more testing equipment to buy.
Does anyone know what size wrench I'll need to unscrew the sensor? I'm assuming it's metric. I sure hope it is.
The following users liked this post:
87laredo (06-28-2021)
03-16-2016, 02:29 PM
#21
CF Veteran
Join Date: May 2012
Location: Lost in the wilds of Virginia
Posts: 7,964
Received 954 Likes
on
769 Posts
Year: 1998 Classic (I'll get it running soon....) and 02 Grand
Model: Cherokee (XJ)
Engine: 4.0
I'm thinking it's 19mm, but don't count on my memmory on that.
Two things I do know for sure:
- Use a flare wrench or an 02 sensor wrench of the correct size.
- Do it while the exhaust is hot.
03-16-2016, 06:45 PM
#22
CF Veteran
Join Date: Aug 2010
Location: Canton, MI
Posts: 8,357
Likes: 0
Received 82 Likes
on
67 Posts
Year: 1999
Model: Cherokee
Engine: 4.0
7/8" (22mm). Parts stores, like O'Reilly, will rent/refund special O2S wrench sets.
I do recommend changing the downstream O2S while you're at it.
Do use heat on the bung. Propane or MAP will do.
03-16-2016, 09:30 PM
#25
03-16-2016, 10:26 PM
#26
CF Veteran
Install the new upstream. If you get a code for a downstream then replace it. Down stream will not effect running conditions, it only reads catalyst efficency.
03-17-2016, 07:11 AM
#28
03-17-2016, 08:48 AM
#30
CF Veteran
Thread Starter
Join Date: Jan 2014
Location: District of Columbia
Posts: 1,301
Received 279 Likes
on
210 Posts
Year: 1998
Model: Cherokee (XJ)
Engine: 4.0
Here's what the '98 FSM says about the O2 sensors, which seems to support my earlier understanding. The text in Group 25 essentially says the same thing, though in much greater detail.
UPSTREAM HEATED OXYGEN SENSOR
The upstream O2S sensor is located in the exhaust downpipe (Fig. 6). It provides an input voltage to the PCM. The input tells the PCM the oxygen content of the exhaust gas. The PCM uses this information to fine tune the air/fuel ratio by adjusting the injector pulse width.
DOWNSTREAM HEATED OXYGEN SENSOR
The downstream heated oxygen sensor is located near the outlet end of the catalytic converter (Fig. 6). The downstream heated oxygen sensor input is used to detect catalytic converter deterioration. As the converter deteriorates, the input from the downstream sensor begins to match the upstream sensor input except for a slight time delay. By comparing the downstream heated oxygen sensor input to the input from the upstream sensor, the PCM calculates catalytic converter efficiency.
When the catalytic converter efficiency drops below emission standards, the PCM stores a diagnostic trouble code and illuminates the Malfunction Indicator Lamp (MIL). For more information, refer to Group 25, Emission Control Systems.
Last edited by IJM; 03-17-2016 at 09:04 AM.
The following users liked this post:
87laredo (06-28-2021)