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...

That code is telling you that there is a problem with the O2 sensors heating circuit.
So it is either the sensor it's self, a resistance problem in the wiring harness, a bad connection,
or a problem in the DME.

An O2 sensor has to be at around 650 Degrees F. to function.
That's what the heating circuit is all about.
Older style O2 sensors were heated via the exhaust...so they took a while before they started to function and create feed back to the ECU.
Modern day sensors begin heating as soon as you turn the key on via the heating circuit.

P1119 Oxygen Sensor Heating 2 Ahead of Catalytic Converter - Below Lower Limit

Check resistance of H025 heating.

1. Remove connector of H02S 1/2 ahead of catalytic converter.

2. Connect ohmmeter on pin side to pins 1 and 2.

Display: 1.8 - 2.5 ohms at 20°C.

3. Connect ohmmeter on pin side to pin 1 and H02S housing.

Display: infinite ohms

Check wiring from DME control module to disconnection point of H025 1/2 for continuity, short to B+ and short to ground.

1. Remove connector of H02S 1 and 2 ahead of catalytic converter.

2. Connect special tool 9616 to wiring harness (DME control module connector). This special harness tool keeps you from shorting the wrong wires on the DME and blowing the DME up. If you don't feel comfortable doing this then please have a shop or dealer do it. Last time I checked DME's were about just over $1000.

3. Connect ohmmeter to special tool 9616, pin 30, and disconnection point of H02S 1/2 ahead of catalytic converter, pin 2.

Display: < 1 ohms

4. Connect ohmmeter at sleeve to connector, pin 2, and ground.

Display: infinite ohms

5. Remove ignition, injection and oxygen sensor heating relay and bridge terminals 30 and 87.

6. Connect voltmeter at sleeve to connector, pin 2, and ground.

Display: 0 (zero) volts

Check output stage of DME control module.

1. Remove connectors to oxygen sensors.

2. Start engine and run at high speed for 3 minutes.

3. Connect voltmeter at sleeve to pin 1 (positive) and pin 2.

Display: battery voltage

Brett posted on PPBB:

The O2 sensors do just that, sense the amount of O2 in the exhaust gas relative to the amount of O2 in ambient air. Perfect combustion of a perfect mixture of air and fuel (around 14.7/1 air/fuel ratio) leaves behind only CO2 and water as products of combustion. All the oxygen gets consumed in the combustion and combines with all the carbons and hydrogens. If there is not enough fuel (lean mixture), then all the fuel gets burned leaving some oxygen left over. Conversely, if there is too much fuel (rich mixture), then all the oxygen gets burned leaving behind extra hydrocarbons (fuel). Now an oxygen sensor outputs a voltage between 0 and about 1 V depending on the difference between the amount of oxygen in the exhaust and the amount of oxygen in normal air. If there is a lot of oxygen in the exhaust (lean mixture condition), the sensor outputs close to 0 volts. Conversely, if there is no oxygen in the mixture (rich condition), then the output is close to 1 V. These O2 sensor voltages are read by the computer. This is the feedback loop that tells the computer how the engine is performing with regard to air/fuel mixture. It's impossible for the computer to hold the exact perfect air/fuel mixture constantly, so the way mixture control is designed is for the computer to continually adjust the mixture from very slightly rich to very slightly lean and back again using feedback from the pre-cat O2 sensors. This means that the pre-cat O2 sensor signal will oscillate back and forth from high to low to high to low voltage as the computer adjusts the mixture. In a normal running engine at idle the signal goes from low to high voltage and vice versa about every 1 second, with a transit time from low to high (or vice versa) being about 200-300 milliseconds. This transit time is important because as an O2 sensor ages, the transit time gets longer, and eventually it can get too long such that the computer will call it a malfunction and signal a check engine light and fault code for a slow responding O2 sensor. O2 sensors need to respond to mixture changes quickly so that the computer can keep up with the proper mixture adjustments.

So the bottom line is that the pre-cat O2 sensors should oscillate between about 0.2 to 0.8 volts regularly (about every 1 second at idle) in a healthy engine.

The post-cat O2 sensors are identical to the pre-cat O2 sensors (same voltage outputs). They are there only to monitor the performance of the catalytic converters. So, as discussed, the pre-cat sensor signals are oscillating between 0.2-0.8 volts. Once the exhaust gasses pass through the catalytic converter, most (all, in theory) excess fuel (hydrocarbons) will be combusted thus reducing hydrocarbon emissions. The cat uses oxygen in the exhaust to combust the fuel. So what you end up with in the exhaust after passing through the cat is a gas mixture that is reduced in hydrocarbons and reduced in oxygen relative to the mixture entering the cat. The post-cat exhaust gas mixture should be CONSTANTLY low in oxygen if the cat is doing its job of burning excess fuel. Therefore, the post-cat O2 sensor signal should be a constant lower voltage reading (not oscillating). So, if the post-cat O2 sensor is seen to oscillate just like the pre-cat O2 sensor, that means that the post-cat sensor is seeing the same gas mixture as the pre-cat sensor meaning that the catalytic converter isn't doing its job of burning excess fuel. The computer monitors the post-cat sensor and compares it to the pre-cat sensor. If the signals are similar, it assumes the cat is bad.