Hi , My 95' volvo 850 turbo used to be hesitation while accelerate from idle , and sometimes I stop from running the idle rpm run below 700~300 , then back to 700 about seconds.

I had replaced the Mass air flow sensor , it become much better , but the problem still happen sometime . Especially weather is hot ( > 30 oC , in Taiwan )

I had use a digital voltage meter to measure the signal ( black wire on my front oxygen sensor ) , while it heat , the voltage keeps high ( about 0.6 ~ 1.4 ) .

The error code from ecu is 2-3-1 . It happend error code , and always gone after 1~2 day(s) .

The fuel comsuption is keep reasonable , about 11.5~12 liter / 100km .

Should I replace the oxygen sensor ? I am planning to replace it with a bosch universal oxygen sensor .

I had the same problem and did research on the many posting from brickboard and found that the CAMSHAFT SENSOR is causing the hesitation.

ChrisQ 97 855 t5

Here's what the shop manual says: But check for any vacuum hose leaks first. All these problems can be caused by a vacuum leak in one of the hoses.

Code 2-3-1 (Long Term Fuel Trim Partial Load)

1) Check for other codes. If Code 3-1-5 is stored (EVAP system malfunction), perform testing on that code first. If Code 3-1-5 is not stored, go to next
step.

2) Check status message on Volvo Scan Tool (998-8686). If
status message reads LOWER LIMIT, go to step 8). If status message
reads UPPER LIMIT, go to next step.

3) Turn ignition on. Connect voltmeter between MAF sensor
connector terminals No. 2 and 4. If voltmeter reads .1-.2 volt, go to
next step. If voltmeter does not read .1-.2 volt, test using new MAF
sensor.

4) Check for air leakage in intake system. Repair as
necessary. If air leakage in intake system is not present, go to next
step.

5) Check for air leakage in exhaust system. If there is no
air leakage, go to next step. If there is air leakage, repair as
necessary.

6) Check fuel and residual pressure. If fuel and residual
pressure are okay, go to next step. If fuel and residual pressure are
not okay, repair as necessary.

7) Connect breakout box to ECM and check ground connections.
See COMPUTERIZED ENGINE CONTROLS in I - SYSTEM/COMPONENT TESTS
article. Reconnect ECM. Repair as necessary.

8) If status message reads LOWER LIMIT, turn ignition on.
Connect voltmeter between MAF sensor connectors terminals No. 2 and 4.
If voltmeter reads .1-.2 volt, go to next step. If voltmeter does not
read .1-.2 volt, test using new MAF sensor.

9) Check for air leakage in intake and exhaust system. Repair
as necessary. Check fuel and residual pressure. Repair as necessary.
If no air leakage and fuel and residual pressure are okay, go to next
step.

10) Remove hose leading to intake manifold from EVAP valve.
Connect vacuum pump to EVAP valve coupling. If vacuum gauge does not
show stable vacuum, repair as necessary. If vacuum gauge does show
stable vacuum, go to next step. Check engine oil and fill as
necessary.

11) Start engine and run at idle a few minutes. Using Volvo
Scan Tool (998-8686), read off value on SHORT TERM FUEL TRIM and note
value. Disconnect and plug crankcase ventilation. Read off value of
short term fuel trim once again. If value is more or less unchanged,
go to next step. If value rises, engine oil is diluted with fuel.
Repair as necessary.


Good luck......... Mike ........ Miami, FL

Hi:

Thanks your opionios .

Due to the hesitation is not always happen ( even I intend to reproduce it ) , it is real diffcult to find a rule to make it happen .

Mike ( Miami, FL ) had gave me direction to find root cause , thanks !
But I don't have volvo scan tool , as I know , it is a ODB-II tool , and I had found many people sell simple ODB-II scanner in Ebay.com . Maybe I should get to buy one to test it .

My oxygen sensor ( front ) voltage is rapidly change from 0.1~1.0 about every 1.5 seconds. (Last mail say to keep high is my fail , I don't connect signal- as ground properly )Seems it is ok ...

And chrisq said about camshift sensor , thanks , I will check it !

Warren Lee