Volvo 1993 940 faltering and stopping and idle speed

moussey · Post by **moussey** » 20 Jan 2011, 08:56

Hello. I have just joined the forum since I was impressed with the quality of the comments.

I have run a 93 940 for 10 years and its been pretty near perfect apart from a slightly variable idle speed, down to 600 and recently upto 1500. I have always suspected the idle valve but since its expensive I have lived with it.

All else is OK I believe and I do try and keep an eye on things.

The car is London based and used for short "station" trips mostly.

Recently is has started sometimes faltering within the first mile, when warm its fine. This very recently happened to my wife and reasonably un-nerved her. It also faltered and then stopped totally after 1/2 a mile with me. It then started and faltered but was driveable until it warmed up.

Fault codes were 232 and 231 after this I consulted the forum and my suspicions focused on the ECU temp water sensor as a possible culprit.

After locating the measured the resistance across the pins was 10.38k at 9 am after a -1 frosty night. So engine probably about 3-4 deg C. When warmed up, with the temp gauge nearly vertical, it was 680 ohms.

From the great stuff on the forum I see this is out of spec....to quote BillofD

At 14F (-10C) the reading should be 8,260 - 10,560 ohms.
At 68F (20C) the reading should be 2,280 - 2,720 ohms.
At 176F (80C) the reading should be 290 - 364 ohms.

After measuring the resistance I cleared all the codes and disconnected the battery to reset the fuel trim (great tip).

Since then the car has done about 10 miles perfectly and shows 111 on the codes

It would seem to me that the out of spec sensor causes cumulative problems for the ECU and after a while upsets the trim, I I'd be greatful for advice on how the fuel trim works and if thats possible/ likely.

I am waiting another day or 2 before I change the sensor. I have other reasons for thinking it runs on the rich side so hoping that will fix those as well.

Also, is there an easy way to check an idle air valve? should they make a noise?? click? The idle fluctuates quite a bit, unlike on my V70 where its much more stable and you can tell its working.

Thoughts on the above much appreciated.

Post by **billofdurham** » 20 Jan 2011, 11:29

Welcome to Matthew's Volvo Site.

On the readings from the temp sensor it looks like it needs to be replaced. However, as the codes haven't returned run the car and see what happens. This is always my approach to fault codes.

In very simple terms the fuel trim is determined by the Engine Control Unit (ECU) after gathering information from the various sensors on the engine. Depending on the information received it will either enrich or weaken the fuel mixture. If only one reading is wrong the ECU will make the wrong choice and the oxygen sensor will send a fault message indicating that the fuel trim is out.

Before you dive into the idle air valve, how long is it since the throttle body was cleaned, if ever? I could cheat and write a whole load of info but I shall direct you to another site which explains why I am asking this question. It is one of those jobs that only gets done when things go wrong and with your type of use it is a strong possibility.

Bill.

Post by **writer100** » 20 Jan 2011, 11:44

If interested, I did a writeup of the cleaning of the throttle body in this vehicle. Check the repair database. Good luck.

Post by **jimmy57** » 20 Jan 2011, 14:38

Does the car have Bendix fuel system (two wire inlet temp sensor in hose from air cleaner) or Bosch (MAF sensor with 5 wires at air cleaner hose)?

moussey · Post by **moussey** » 20 Jan 2011, 14:47

Many thanks all, i will look at the body tomorrow, from memory its ok but will make sure and come back.

And Jimmy, Its the Bosch setup

moussey · Post by **moussey** » 21 Jan 2011, 12:53

Wow, well I was wrong the throttle body was gummed up. Now clean, but maybe not the dazzle clean in the website referred to, , and all drillings blown out.

Car now drives better,idle more stable, not sure the idle air valve is totally free of suspicion but ok to live with.

Wife has driven car and pronounced good ;>)

I still have a feeling it runs a bit rich and maybe the thermostat isnt letting it get totally upto the right temp. MOT tomorrow so will look at the emissions test and see.

Thanks everyone, especially Bill.

Post by **billofdurham** » 21 Jan 2011, 15:17

Keep us updated on the MOT result, please.

Bill.

moussey · Post by **moussey** » 26 Jan 2011, 09:34

The car flew through the emissions and whole MOT test, so very happy and my paranoid thoughts about it being rich must be just that.

Post by **billofdurham** » 26 Jan 2011, 11:24

Welcome to the land of the paranoiacs. There's more of us than people realise. If we could get things straight we could rule the world, well I think we might if they stopped picking on us.

Thanks for the update.

Bill.

moussey · Post by **moussey** » 07 Feb 2011, 09:49

So, the saga continues, it did its again, faltering when cold, not badly but not right. Codes of 113.

After resetting it was back to perfect. So I either have a maddening intermittent fault fixed by opening the bonnet and poking about or this cumulative effect on the fuel trim.

This time my paranoia widened and I changed the thermostat (cheaper than the coolant sensor) since the gauge no longer got dead vertical and I still felt it was a bit rich.

Delighted to say that this seems to have cured it, and improved drivability.

Lesson for me is that ECUs of this age are good get confused if sensors dont all agree and the ECU has to make unexpected
changes to mixture.

Also, gradual degrades in drivability go un -noticed.