2000 V40 Temp Gauge Erratic and Engine Surging

barndoctors · Post by **barndoctors** » 27 Aug 2009, 23:23

Thanks in advance for any thoughts.
This car has 150k miles and lives at 6000k feet.
Two problems that may be related:

1. Temp. gauge swings wildly on cold start. P0117 code. Suspect ECT sensor. Are there one or two ECT sensors on this car? I know the location of the first - by thermostat. If two, do you know location of the second sensor? I had to pull cooling fan fuse to stop cooling fan from running excessively since this problem appeared.

2. Once warm and on occasion, engine will repeatedly surge to 4000 rpms on start! Sounds like it will climb to 5000 if you don't shut it off. Temp. gauge swings quickly to "very hot" when this happens. P0118 code. Seems to be a potentially dangerous problem . . .

barndoctors · Post by **barndoctors** » 30 Aug 2009, 14:02

Here's an update. While I wait for a replacement ECT (coolant temp.) sensor to arrive by mail, I have pulled the cooling fan fuse (it was running constantly) and disconnected the forward ECT sensor. Result? It starts and runs great . . . the temp. gauge climbs steadily to the half-way point - this appears to be an accurate reflection of engine temp.

Anyone have an opinion about whether there are one or two ECT sensors on this car, and if so, the location of the second one?

Thanks

VFrank · Post by **VFrank** » 04 Nov 2009, 18:37

I'm having a similar issue with my S40. The erratic temp gauge started shortly after my A/C stopped cooling. A/C needs to be recharged.

Appears that the temp sensor or thermostat incorrectly signal an "overheat" condition through the computer and the car responds as if it were overheating. When the engine is warmed to normal, the erratic 'hot' reading won't allow the car to start. Eventhough, the car is not overheated.

I'm replacing the thremostat first. I also only see 1 temp sensor (near the thermostat). I, like you - have also pulled the Fan1 relay to keep it from constantly running. I've tested the fan connections and relays and they are OK... not the problem in this case.

I'm also thinking, after the A/C is recharged, the car might return to normal.

The replies from myvolvoparts.com to other posts were very helpful in helping me diagnose the issue.

barndoctors · Post by **barndoctors** » 05 Nov 2009, 06:12

The new temp. sensor (ECT) did the trick - cured all of the temp. gauge problems. My only current problem is a P0128 OBD check engine code. My research suggests that the problem is probably an aging thermostat. I'll ignore it for awhile and see what happens. I hope you're planning to recharge that A/C system yourself - it's an easy fix.

VFrank · Post by **VFrank** » 09 Nov 2009, 20:29

My issues have been resolved:

As I suspected (and as doc's has noted...) replacing the Temp. Sensor resolved the whacky/erratic gauge behavior.

- the (old) thermostat looked/was ok - but since I had the area open to replace the temp sensor, I put in a new thermostat anyway. Cleared up my A/C for the moment... it is cooling again. I know it will need to be re-charged before next summer.