'95 850 T5:- Engine coolant temperature sensor

Volgrrr · Post by **Volgrrr** » 02 Jan 2010, 05:06

I'm trying to establish if the instrument panel temperature gauge will appear to work normally if the engine coolant temperature sensor (ECT) is faulty.

My temperature gauge seems to operate O.K, but I suspect the ECT is not sending a correct signal to the engine management system, which gives the engine an extremely rich mixture (initially about 6 m.p.g on computer read out) until the engine is well warmed up (then improves to 32+ m.p.g). The vehicle averages over 30 m.p.g over a full tank of mixed city/highway driving.

Or am I barking up the wrong tree?

What's the go - any ideas out there?

Post by **jblackburn** » 02 Jan 2010, 07:18

About how long does it take your car to warm up to "operating temperature"?

In my case, the ECT displayed temperature correctly (I think), but it would give it such a rich mixture when the car was trying to start when the engine was warm it would blow out black smoke, or crank a LOT before starting.

A rich mixture on a cold engine is normal - I have never watched the trip computer in the mornings, but if you are using so much gas that it smells like rotten eggs or you're blowing unburned fuel out the exhaust, you may as well change the sensor.

Post by **polskamafia mjl** » 02 Jan 2010, 12:04

Once the engine reaches operating temp. should it stay that way regardless of the outside conditions? Usually when I drive the car, it warms up to the 5 or 4 O'clock position. If I stop at a traffic light or something the temp gets up to around the 3 O'clock position. Once I start moving again the temp will drop again to a bit below the 3 O'clock position. Is that normal or does that sound like a bad T-stat and/or Engine Temp. Sensor.

Post by **jblackburn** » 02 Jan 2010, 12:24

You've got a bad t-stat that's stuck open. It should warm up to and remain at the 3:00 position.

Post by **polskamafia mjl** » 02 Jan 2010, 12:42

Ok, thanks a lot. One more question, can the engine be damaged if I let this go? I'm planning on doing it right away, but it's been like this all winter and I wanted to know if there could be any engine damage.

Post by **jblackburn** » 02 Jan 2010, 13:00

No damage - it just eats more gas trying to warm up, and I suspect it also kills the ECT sensor with it, which you should go ahead and replace anyway while you're under there. Get some new t-stat housing screws. Its a good time to do a coolant flush if it hasn't been done recently, and I replaced both my radiator hoses.

I understand not wanting to work on it in the cold, I tried to do my PCV system a few days ago, and it took me 2 days because my hands got so cold and I kept dropping things. But! You'll have heat in the mornings much quicker

Post by **polskamafia mjl** » 02 Jan 2010, 13:36

Haha, tell me about it. I got 4 new rotors, pads all around, and stainless steel brake hoses for Christmas. I put those on in the cold and it sucked. Took me 2 days and I only managed to get the front end done. haha Thanks for your help.

Volgrrr · Post by **Volgrrr** » 02 Jan 2010, 17:29

jablackburn wrote:About how long does it take your car to warm up to "operating temperature"?

I would guess about 4 to 5 minutes as I've never actually timed it but the temperature gauge needle steadily rises to the 3 o'clock position and remains there without any deviation.

In my case, the ECT displayed temperature correctly (I think), but it would give it such a rich mixture when the car was trying to start when the engine was warm it would blow out black smoke, or crank a LOT before starting.

I noticed black smoke in the rear vision mirror swirling out of the exhaust when I took it out for a test drive yesterday.

So, I put my foot flat down and, after some slight hesitation, the T5 took off with the rev counter quickly climbing to nearly 6000 r.p.m. (in either 2 or 3rd gear, I'm not sure; but when I took my foot off the accelerator she immediately changed up to 4th and the road speed at that time was about 120 k.p.h).

It seemed to clear something out because after the initial hesitation the wagon took off and after a short period the black smoke seemed to disappear (it might have disappeared due to the full-on acceleration) however, the black smoke appeared to have ceased by the time I got back home (test drive about 5 miles).

A rich mixture on a cold engine is normal - I have never watched the trip computer in the mornings, but if you are using so much gas that it smells like rotten eggs or you're blowing unburned fuel out the exhaust, you may as well change the sensor.

No smell, but as I've stated above there was black smoke (unburned fuel) coming out the exhaust for a while.

I think it would make sense to go ahead and change the ECT sensor because, if nothing else, it will eliminate a suspect area.

Post by **jblackburn** » 02 Jan 2010, 17:42

Sounds like a good plan to me!

May as well change out the thermostat while you're in there - they're cheap anyway!