4.0 comes up to temp and then stalls out-HELP

hey guys

im posting a question for a buddy of mine.

1990 XJ 4.0 auto

as he told me:

engine runs great, starts well, but when it warms up and comes up to temp, it begins to stall out. you can keep it running by giving it gas, but if u let off the throttle it will stall out.


any idea what could be causing this?

can this jeep be plugged in for diagonstic tests (like my 96 XJ with OBDII)?

im guessing it must be a bad sensor somewhere (IAC, MAP?)

please help!!!

thanks guys

CaptTrev

The '90 is a Renix system. It has diagnostics, but it doesn't store codes so you have to be scanning when the problem occurs. Also, most readers won't handle the Renix system.

Let's think about how the system operates. When you first start up, the system operates in "open loop" mode, which means it uses a pre-prgrammed fuel map to run a slightly rich mix for warming up. In open loop mode it ignores sensor inputs. When it reaches a specific temperature (which is somewhere around 170 or 180 degrees) it switches to "closed loop" mode, which means that it responds to sensor inputs and adjusts the fuel mix and timing accordingly.

It sounds like your friend's Jeep has some problem that doesn't let it run once it switches to closed loop mode. Could be one of the sensors, or it could be that the injectors are partially clogged and the open loop rich mix makes them squirt enough juice to run, but the sensors don't. That doesn't really work for me, though, because the O2 sensor should detect a lean mix and call for more fuel.

I'm thinking bad O2 sensor. Does he know how many miles are on it? The O2 sensor should be replaced every 80,000 miles, and when it starts to fail it can cause a number of erratic problems.

Eagle-

thanks for the quick replay! ( I actually read it last night shortly after you wrote it, but didnt have a chance to respond)

I passed the good info on to my buddy, and hes working on it today.

I think he's going to replace the O2 sensors and see how that does.

I'll let you know how it works out!

thanks again!!

CaptTrev

CaptTrev said:

I think he's going to replace the O2 sensors and see how that does.

Click to expand...

'90 only has one O2 sensor. It's in the exhaust manifold, just above the donut where the exhaust pipe attaches.

CaptTrev said:

Eagle-

...
I think he's going to replace the O2 sensors and see how that does.

I'll let you know how it works out!

thanks again!!

CaptTrev

Click to expand...

CaptTrev - so did replacing the oxygen sensor resolve the problem? Just curious. Thanks, Rick

If it's an early '90, there is a big plug in above the brake booster that causes problems ('87 to 10/'89) from corrosion. I unplug and spray all the connectors with contact cleaner. Even the one at the computer. That seems to have cleared up about 80% of problems like this I've run into on XJs.
If you unplug the O2 sensor, the motor will run better instantly if it's bad.