Light Throttle Hesitation

[carcrazyguy]

Hello, my car hesitates, or bucks somewhat at light throttle between say 2000-2700, or at least that is when it is obvious. This is most evident on a low speed highway of course when there is no load on the engine and the RPMS are low.

It only does this when the car is warm, or warming up, as it runs perfectly when cold and only starts this when the guage hits a certain point.
Also, the car always accelerates fine and idles perfectly (interestingly better than the previous 3 MKII's I have had).

So far I have checked all tune-up type stuff, bled the coolant, and looked for vacuum leaks. Any more ideas?

[kanga]

is it a stick or auto? and if its a stick could the clutch be going out? because thats how my mazda 3 was before the clutch went out and when i put the new clutch in all of that stopped

[carcrazyguy]

This is the engine cutting out, no question.

[TomsMR2]

unplug your o2 sensor and go for a drive. if it stops, you need to replace the o2. easiest, cheapest, fastest diagnosis for that problem.

when the car is cold, its in open loop.. which means its ignoring ALL o2 sensor input. once it hits a certain temperature, it starts reading o2 function.. if your o2 is bad, it'll cause a light throttle stutter.. high throttle angles = open loop again, and the stutter goes away.

[carcrazyguy]

Cool, that will be easy enought to try. I was also thinking about trying the same thing with the coolant sensor, as this is how I diagnosed an running too rich issue with a previous 240SX of mine.

[TomsMR2]

a bad coolant sensor will throw a code and CEL.. for some stupid reason a bad o2 wont.

[carcrazyguy]

Here's the update!

I unhooked the O2 as recomended, and it did seem to make the problem go away, or at least to a point where I was not sure if the problem still existed. Just as you said, the CEL didn't come on (as with most cars a bad or unhooked O2 will illuminate the CEL).

Then I hooked it back up, yet unhooked the coolant sensor. The CEL came on like clockwork and the car ran like mud once again, jerking and hesitating.

So from this, do you concur that my O2 is bad? Or is it possible that unhooking the O2 puts the ECU in a closed map so to speak? If money grew on trees, I would just run out and get a new O2 without question, but those are pretty pricey these days.
Also, if it is a bad sensor, what is the process to try to clean it to see if that helps?

BTW, thanks - so far I am further along that I have been for 3 weeks on another forum!

[baktasht]

You are better off replacing the O2 sensor rather than cleaning it.

[carcrazyguy]

Maybe so, but @$100, I would hate to do so and it not fix the problem.

Also, as I asked before, but probably not directly enough, does having the O2 sensor unhooked put the ECU in a closed fuel map, therefore potentially ignoring other issues?

[TomsMR2]

Spark plug, Spark plug wire, spark plug gap, spark plug cross reference and ignition wires at Sparkplugs.com

30 bucks for a new sensor. our old style o2 sensors only last about 30-40K miles.. sometimes less, sometimes more. especially with the well known oil burning problems with the 5s, id bet shorter than longer.

you can "clean" it with a propane torch.. burn it until it glows red, and its as clean as you can get it. dont use any chemicals or solvents.

i think an unhooked o2 just makes the ecu think your sensor is still cold and just ignores the signal.. calculates fuel off the other sensors. id assume thats right because it doesnt throw a CEL.. if it went into safe mode you'd have the cel on, like when the coolant sensor is unplugged.

id say you've got a safe bet on a bad o2 sensor. the bgb outlines a proceedure for checking with a multimeter, but it sounds pretty cut and dry, bad o2, to me.

[carcrazyguy]

Well, back to the drawing boards. I actually did some substancial driving today with the O2 unhooked. The problem is still there, only lesser so. It took a while for it to return but after say 15 minutes of driving it was there, and only, say 25 % less than before.
On top of that, after about 45 minutes of driving with the O2 sensor unhooked, the CEL did some on. After I stopped and plugged the O2 back up and restarted the car, the CEL went off and didn't come on again.
FWIW, I did notice a Bosch sensor at Autozone for $30, but at this point it appears that is not the issue.

[Gairloch]

If unplugging the sensor has an impact on what it's doing that indicates that it's at least related to the problem, don't you think?

Go with what Tom suggested. You don't want that Bosch sensor.

[carcrazyguy]

Originally Posted by Gairloch

If unplugging the sensor has an impact on what it's doing that indicates that it's at least related to the problem, don't you think?

No, because after much driving with the O2 unhooked I have found that it just makes the lurching / hesitating much less obvious because the car runs rather "flat" with the sensor unhooked. In other words, the car feels like it is running at 60% with the O2 unhooked, so when it hesitates, which it like short bursts of 35%, the difference is much less obvious. Did that make sense...as I know what I meant .

[Gairloch]

Yes, I get what you're saying now. Being full rich just makes the issue less apparent.

So it only does this when you're at light throttle, not at WOT?

[Mister2.2]

I still think you need to replace the sensor and then move onto the next step. Also check your engine grounds. Damn it, do I need to make a sticky on engine grounds?

[carcrazyguy]

Originally Posted by Gairloch

So it only does this when you're at light throttle, not at WOT?

Yes, only when the load and throttle are light. And as I said before, the problem starts when the temperature gets to a certain point. I mean it starts like a "on switch". In other words, the car will be running fine, and the exact second when the temperature gauge moves from the bottom to the incremented section, it will start jerking (simultaneously). This is why I still think that has to be a big clue.

I will go ahead and add a good additional engine ground tomorrow for good measure. However, this is a south car (non ocean area) so ground problems are never an issue on such vehicles, as engine bays always look a couple years old - even on stuff as old as this...but I will do it anyway.

As far as the O2 sensor, as I mentioned...after much time was spent going back and forth between no sensor and sensor hooked up, I am 99% sure it has no effect on the issue bad or good. I tried both ways back to back at least 5 times each as well as driving about 150 miles yesterday, so I feel reluctant to replace it, as it looks like a pretty newish one is already on there anyway. Toyota's sensor lists for $138 and everything else I found was $70 and up. Bosch is the only one I could find @ $30 - and I agree with what someone said a while back - to avoid Bosch (I have had poor luck with many of their parts in the past).

[baktasht]

ITB's? Ya, great suggestion...