02 Sensor Data

tobyw · Jun 22, 2016

I'm chasing a low power and fuel economy issue on my '04 Wrangler with a 155,xxx mile 4.0L inline six. On hot startup, it will stumble and stutter for several seconds before smoothing out, but overall power is way way down and fuel economy has gone right with it. No CEL and no stored codes according to the cheap OBD-II scan tool I have. I replaced the air filter, cleaned the throttle body, and checked the plugs and found them all to be in extremely good shape.

I have ordered a wireless dongle and am currently downloading the Torque Pro app for my iPhone, so as soon as the dongle arrives I should be able to see live sensor data. Some interweb research seems to indicate this problem could be related to one or more malfunctioning oxygen sensors, but I'm not about to drop $80+/- each on guessing... So here's the question: if I can view live data for the 02 sensors, what exactly am I looking for in terms of values? Are there certain RPM or engine temp ranges I should be specifically looking at?

zukkev · Jun 23, 2016

You are more likely not dealing with an 02 issue on a hot restart stumble, unless of course the hot soak puts the 02 into a temp range that it is failing at, but I doubt it...most failed 02's I have seen happen while running.

I'd be looking at a possible injector hot soak issue, or the fuel boiling in the fuel rail, or the fuel pressure bleeding off when sitting for a bit. Does it have an extended cranking condition? Those phone vehicle data apps, although kinda neat, and can help, most times it's all French to most looking at it.

tobyw · Jun 24, 2016

Thanks for the reply and insight. No extended crank condition, it seems to fire up as quick as ever just stumbles and stutters for a bit. I did a little more research on 02 readings, and it seems they are supposed to bounce around between 0.1 and 0.9 volts if things are functioning properly, and mine are doing their thing within that range. Good and bad in that I suppose. However, the fuel bleeding off theory... Do you suppose a weak fuel pump could cause the low power issue as well? Just not getting enough fuel? Not sure how that contributes to poor economy if it can't get fuel to burn LOL

comanchekid · Jun 24, 2016

At 155k it wouldn't be a bad Idea to replace the up stream o2 sensors. But on most hot soak issues on the 4.0 liters its the injectors getting hot.

Do you still have the tinfoil fiber thing on your intake manifold?

And we have fixed a few Jeeps with your problem having the Injectors tested and cleaned by Doctor Injector.

GaryTJ · Jul 6, 2016

Hook up a pressure tester to the fuel rail. watch the pressure when you turn it off. If it bleeds down, it is either returning to the tank or your injectors are bleeding off. from your description, it sounds like injectors. try a can of seafoam, that should help with the injectors leaking.

tobyw · Jul 7, 2016

Thanks for the info, I'll give it a shot and see what the fuel pressure is doing.

TreeClimber · Jul 7, 2016

What's the temp sender doing on the live data, during hot re-starts? It's possible, that the temp is reading 'cold' during 'hot' starts, and flooding the engine. But once running, the O2 corrects that condition???? Just another idea.