98 S40 2.0 litre (non turbo) - behaves funny when hot

Flymeister · Post by **Flymeister** » 08 Dec 2009, 10:00

Hi There - New Here. DESPERATELY in need of some advice.

I drive the 98 S40 2.0 Litre non turbo, THe car drives fine when she's in free traffic(except maybe for minor power loss), After being in stop-go (bumper to bumper) traffic, like today, she starts idling uneven, I put my foot on the accelerator pedal and keep the revs at 1000, she seems fine, as soon As I take my foot off, the uneven idling starts and progressively gets worse. I had to pull off, when I tried to restart her she wouldn't budge. After waiting for about 30 mins I again tried to restart and she worked just fine and I drove her home.

here 's some notes on the situation:

* I replaced ignition coils earlier this year.
* The car's temperature gauge doesn't move so she's not overheating, no smoke from the exhaust.
* The times when it did happen, I had the aircon on full blast (it's summer in S.A)
* I took the car to the dealership, and the only fault was the (heated)rear oxygen sensor - This has been faulty for a while, the volvo techs informed me it wouldn't cause any major problems so I'm driving the car like that until I have the money to repair the sensor (4000 south african rands/ 700 american dollars)
* WHat I also noticed is that the car will do this when I do lots of short trips in a day

The one volvo tech i spoke to seems to think It's the ignition coils again, the other seems to think it's the oxygen sensor.....

PLEASE HELP?????I am at wit's end

Post by **billofdurham** » 08 Dec 2009, 11:50

Welcome to Matthew's Volvo Site.

The attached gives a list of possible causes:

S40 poor driveability.pdf: (670.76 KiB) Downloaded 366 times

Apart from checking the ignition leads the easiest is the Mass AirFlow (MAF) meter. These are self cleaning but do sometimes don't get rid of all the grunge. Buy an aerosol of MAF cleaner, at any parts shop, and give the MAF a spray.

Bill.

Flymeister · Post by **Flymeister** » 08 Dec 2009, 12:43

WOW Thanks Bill, I was told by the dealer this car doesn't have a mass airflow sensor.... could this be correct? The dealer pointed to some "other" sensor on the pipe from the airbox.... think he's smoking his socks?>

To test his theory I called another dealership and asked for a "quote" on the car's Mass Airflow Sensor (I even supplied them with the VIN)...they checked..and guess what...he gave me a quote....

See what I mean when I say i'm at wit's end?

Post by **billofdurham** » 08 Dec 2009, 15:49

Actually both dealers are correct - sort of.

The list I gave you is a generic list and does say to check the MAF. When I read it I thought it was like other models. Your car does not have a MAF it has a Manifold Absolute Pressure (MAP) sensor which does the same job as the MAF in a slightly different way.

A basic comparison:
The MAF measures the airflow using a hot wire which cools the more air passes over it. As it cools more current is passed through the wire to maintain its temperature. This change in current is then converted by the Engine Control Unit (ECU) into the information it needs for the amount of fuel the engine needs.
The MAP provides instantaneous manifold pressure information to the ECU. The data is used to calculate air density and determine the engine's air mass flow rate and thus the amount of fuel needed. The MAP also requires an Intake Air Temperature (IAT) sensor to feed temperatures to the ECU. That would be the other sensor on the pipe to the airbox.

A lot of mechanics and parts people use the term MAF for both.

You can't clean a MAP sensor like a MAF.

One thing we haven't established, have any fault codes been found apart from the one for the oxygen sensor? A faulty MAP or IAT would set a code.

Bill.

Flymeister · Post by **Flymeister** » 08 Dec 2009, 16:10

Bill... THANK you so much for your continued help with this!!!

Nope, no other fault codes except for memory loss.... i replaced the car battery a few months ago...

I'm thinking it has to be something electrical along the fuel/idle system that overheats as it only happens when the car's hot, after short stop start trips and or heavy traffic....and what makes the problem more complex is that the car doesn't actually cut out, which means it couldn't be fuel pressure or fuel pump related.

I started her up again after she got cold in the garage, no sign of trouble - I did notice a minor change in revs (estimate between 800 and 900 rpm) when idling though, but nothing seriously noticable....

Post by **billofdurham** » 09 Dec 2009, 15:26

It is strange that no other codes have been set as most of the fuel/idle setup should trigger one if it goes awry.

However, as one of the mechanics thought the IAT sensor may be the cause of your problems this is how you can test it:

S40 IATsensor test.pdf: (407.95 KiB) Downloaded 317 times

You need a multimeter and an ambient temperature of about 20C. The first you can buy quite cheaply if you haven't got one, the second should be no problem where you are.

Bill.

Flymeister · Post by **Flymeister** » 09 Dec 2009, 15:45

Thanks again Bill - I'll be sure to follow your advice

Yer a star!!!!!!!!