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5bucks
April 3rd, 2015, 15:16
Anyone know how to force a '98 4.0 to stay in open loop after warmup? I've read putting in a 160F thermostat as well as disconnecting 02 sensors.

cal
April 3rd, 2015, 15:26
.....why?

87manche
April 3rd, 2015, 15:37
if you disconnect the o2 sensors it will run open loop with a "system malfunction" error

but yes, the question is why?

5bucks
April 3rd, 2015, 16:27
Having an issue where engine looses power after getting up to temp. Like 30+ miles at highway speeds. Thought to be a clogged catalytic convert. Getting more clogged when hot causing O2 sensor to give bad info for fuel air mix. Runs fine when in open loop. Need to drive the vehicle home from parking lot about 50 miles away

5bucks
April 3rd, 2015, 16:28
More details in earlier post

87manche
April 3rd, 2015, 17:39
then just disconnect the manifold o2 sensor and it will stay in open loop.

it's gonna run quite rich, and with a clogged cat that may be bad things.

5bucks
April 3rd, 2015, 19:00
I actually don't know if it is clogged. That a shops opinion. No CEL. Something causing a bad fuel ratio after extended driving at highway speeds. After things cool off it runs fine. Even when I have the problem it will restart and idle perfectly. Any chance this could be a bad O2 sensor that's not throwing a code?

carves
April 3rd, 2015, 20:08
Dont bother with changing the thermostat,
... as, Open to Closed Loop, occurs quite quickly after startup - when the coolant temp is still as low as 70*.

I havent tried it ... but disconnecting the O2 sensors may well cause the PCM to apply the Open Loop fuel presets in relation to whatever coolant temps are relevant.

Runnin'OnEmpty
April 3rd, 2015, 21:32
Cats get very hot when they're clogging up. If yours
is nearly cherry red, then it's indeed clogged. Running
in open loop still won't get you home; might be better
to loosen the cat flange so there's a gap for exhaust to
escape. Yes, it'll be loud, but you'll get home...

Digger87xj
April 4th, 2015, 07:57
MAP sensor failing?

old_man
April 4th, 2015, 08:17
Haven't you guys ever used a vacuum gauge to check for exhaust restrictions such as a clogged cat?