Rokee99

I have a 99 xj with 125K miles


When I cold start it and put it into drive I have about 5-10 seconds of hesitation under acceleration. Then it gets better and drives perfect.


After letting it sit for hours and turning it on again I have the same issue.


If I go to the store take 20 min and come out I have no issues. Usually happens in the morning and at the end of my work day.


Any ideas?


Thanks in advance.

Gatchmo5710

Verify fuel pressure on cold start up vs warm.

tjwalker

Any check engine light? If so, pull codes for clues/guidance.

When was your last tuneup? Fresh Champion copper plugs, quality plug wires, distributor cap and rotor.

There certainly are a lot of possibilities including fuel pressure, but it's "simple stuff first" and if you need a tuneup, do it and then see where you are at.

Rokee99

No check engine..

new cap and rotor .I want to switch the plugs and wires next.

But its weird cuz it only does it upon cold start up and then gets better immediately

CCKen

I suspect the upstream Oxygen Sensor.


With that many miles on your Jeep I would consider replacing the upstream O2S. Actually, you should replace both O2S' at the same time.


I experienced this several years ago and after I replaced the upstream O2S the problem went away.


NTK Upstream O2S part number 23151. Downstream 23099.



.

jordan96xj

Definitely do the fuel pressure testing.

In addition to the oxygen sensors (although wouldn't the symptoms for these be a -good- start followed by rough running once things warm up and go closed loop?, CCKen is pretty knowledgeable so I will assume I'm wrong), I would next suspect that your are getting unmeasured air into the system when it is cold. As things warm up and expand, they seal up and the leak diminishes or goes away.

If it is resolving relatively quickly, the mixture might not be improper long enough for the computer to raise a code (which would probably be a -lean- mixture code P0171). If you had a scanner, you would be able to see the short/long fuel trims during the cold start up, and it would probably provide a clue as to what the air/fuel mixture is doing during that cold startup.

You or a mechanic can smoke test the system (when dead cold) to determine if air is finding a way in.

Fred/N0AZZ

If I may make a suggestion before starting the first turn the key to the on position leave it for 10-15 seconds then turn off then start foot off the throttle. It may do nothing but nothing to try either.

Rokee99

Both 02 sensors are new. Wouldn't this be an issue after it reached operating temp? not right at cold start up and then get better 10 seconds later?

Rokee99

My dad has an obd2 scanner.. is this what you mean by scanner?

CCKen

What brand are the O2S'?


I'm not saying for sure your problem is an O2S, only that my issue (like yours) was cured by replacing the upstream O2S.


You may have different issues, but yes, a symptom of a failing O2S heater could be a cold hesitation then cured when warmed up.


In Open Loop the PCM does not use O2S inputs but relies upon all sensor inputs and PCM presets. The PCM does look at every sensor during Open Loop, including the O2S', but not its inputs. That happens when Closed Loop occurs. Closed Loop is a timed in feature of the PCM, not a coolant temperature dependent feature. Going Closed Loop can happen pretty quick on some Jeeps (like mine). I can watch mine on my scanner and it doesn't take long at all. If, when going Closed Loop the O2S has not preheated the O2S up to around 600*F the O2S input will be flawed.


Just say'n.

jordan96xj

I figured you probably had a good reason for just say'n. Just Say'n.

Rokee99

o2 sensors are bosch, the most $$$ ones.

i think ifi let it heat up for a few min it doesn't do it. Tried it twice today.

I read on an old post some guy said it was the temperature sensor on the thermostat housing. does this sound logical?

CCKen

I don't want to spend your money but it is a widely accepted opinion that Bosch O2S' don't work very well in Jeep applications.


The NTK O2S' were original equipment, and still are, and seem to work well in Jeeps. See post #5.


The Bosch O2S' usually fail after some time in use, not right away so I'm not sure if they are your problem.


Edit: You may want to have a quality scanner that shows live data hooked up and see if you have any pending codes - and check the operation of the O2S' before start then immediately after start. The O2S's should show 1.00 volts before being activated then start trimming (toggling) at Closed Loop. If the upstream O2S stays at 1.00 volts for a long time that's bad.

Rokee99

So yesterday i tried waiting for a few minutes before taking off to let the engine generate some heat then took off . Both times i didn't have issues and it ran perfect.

Strange. Any ideas

CCKen

See posts 2 thru 13.