Hesitation when accelerating after gear change, goes away with battery disconnect
 

I have a slight hesitation when first accelerating after a gear change, it lasts about 1 second, then proceeds as normal. The interesting thing is that if I disconnect the battery overnight, the hesitation is gone and the car runs perfectly for a while ( maybe 200 miles? ), before the hesitation eventually comes back.

There are no codes and no check engine light before or after.

Any ideas would be gratefully recieved, it's kind of driving me nuts. :)

When you disconnect the battery the DME resets it's fueling maps to the default settings.
That's why the hesitation is gone for a while.

I am just making a.....very broad guess here..... but it could be the MAF sensor or your O2 sensor's
both sensors "age" and their signal slows down so the response from the DME to the fuel injectors will have a very slight lag.
It will not throw a code until "signal slow down" from the sensor exceeds a set limit in the DME.

So you need someone with the proper test equipment to test the sensors.
and the knowledge to run and interpret the tests correctly.

This is really helpful actually. I had cleaned the MAF sensor a while back, suspecting it might be that, didn't make any difference though. I'll take a look at the O2 sensors next, I hadn't considered those as possible culprits. :)

Thanks again!

Like I stated I am making very very broad guesses as I don't have any further info on your car.

One thing to try is disconnect the MAF sensor and go for a drive ...... see if there is any difference.
When you disconnect the MAF sensor.... the DME will revert to a default fueling MAP.


02 sensors that are starting to "age out" can be tricky to diagnose that is why I stated you need the proper test equipment and test knowledge.

Bosch: (They were the first to adapt the 02 sensor to automotive use)
States that an 02 sensors life is 100000 one hundred thousand miles.
So they are aging out or slowing down at that point.

I did actually try this at the time. There was no difference with the MAF sensor disconnected at all, so maybe I can rule that out?

My 2002 base model has 138000 miles on it, so not out of the realm of possibility that the O2 sensors are aging out at this point...

I would have the MAF and O2 sensors tested.
If you don't have the equipment or know how find a shop with a "scope" and Tech. with the test knowledge.

Another route is go to youtube look up "ScannerDanner" study all his videos on MAF sensor and O2 sensor testing.
He is very good at what he does.

Super helpful. Couple of updates...

I got a P1119 ( Oxygen Sensor Heating ) code, not sure if that helps point us in a certain direction here.

I also ran the live data function on my code reader and watched the voltages of the O2 sensors and recorded the lows and highs to see if they were in the expected ranges:

O2B1S1: Low = 0.080 / High = 0.720
O2B1S2: Low = 0.655 / High = 0.765

O2B2S1: Low = 0.175 / High = 0.730
O2B2S2: Low = 0.590 / High = 0.665

Not sure if any of this is helpful...