-40F Coolant Temp?

[EdW]

The day I get my evap problem squared away, car ran great, passed inspection and I thought all was well. Later that night the check engine light comes back on with a P0115 code set. Coolant temp ckt. I reset the codes and was watching the coolant temp on the scan tool, all seemed ok but then it momentarily dropped to -40F and the engine rpm went down for a second or two. Car shook a bit, the check engine light came right back on with P0115 set and P0116 pending, then the temp switched back to 175 and the rpm's were fine. All this only took a second or two.

I'm wondering if something else may have caused this momentary blip, or if the CTS sensor could be at the root of all this? If it went intermittently bad for a second, I'm not sure what effect it would have on the RPM, etc. The temp gauge on the dash didn't move during this, so I assume there are 2 coolant temp sensors on the engine? Took a quick look and there appears to be one 3 wire coolant sensor next to where the upper radiator hose enters the head, and there's another single wire sensor in the block, on the front of the engine, just under the intake manifold (this one seems tough to get at). Which one does the ECM read from?

Anyone have any thoughts or experience with this?

[Milky]

Originally Posted by EdW

The day I get my evap problem squared away, car ran great, passed inspection and I thought all was well. Later that night the check engine light comes back on with a P0115 code set. Coolant temp ckt. I reset the codes and was watching the coolant temp on the scan tool, all seemed ok but then it momentarily dropped to -40F and the engine rpm went down for a second or two. Car shook a bit, the check engine light came right back on with P0115 set and P0116 pending, then the temp switched back to 175 and the rpm's were fine. All this only took a second or two.

I'm wondering if something else may have caused this momentary blip, or if the CTS sensor could be at the root of all this? If it went intermittently bad for a second, I'm not sure what effect it would have on the RPM, etc. The temp gauge on the dash didn't move during this, so I assume there are 2 coolant temp sensors on the engine? Took a quick look and there appears to be one 3 wire coolant sensor next to where the upper radiator hose enters the head, and there's another single wire sensor in the block, on the front of the engine, just under the intake manifold (this one seems tough to get at). Which one does the ECM read from?

Anyone have any thoughts or experience with this?

the ecm reads the coolant temp from the 3 wire on the head. the single wire one is for the gauge. the CTS is the reason for the dip in idle and shaking. the lower coolant temp cause the engine to dump more fuel, thinking it was in cold start. it sounds like the sensor is starting to go bad, or might have a bad ground. i would check the wiring first before buying a new coolant temp sensor.

[EdW]

Thanks for the reply Milky, appreciate the assistance. Haven't had time to pick a new one up yet, but will soon. Any thoughts on whether this could increase the engine idle rpm's? Sometimes they're normal, sometimes they're up at about 1100. I had a bad\leaking Evap VSV and it was alwasy high until I swapped it out for a new one. Now it's only at times, but the CTS went intermittent shortly after...

[Milky]

Originally Posted by EdW

Thanks for the reply Milky, appreciate the assistance. Haven't had time to pick a new one up yet, but will soon. Any thoughts on whether this could increase the engine idle rpm's? Sometimes they're normal, sometimes they're up at about 1100. I had a badleaking Evap VSV and it was alwasy high until I swapped it out for a new one. Now it's only at times, but the CTS went intermittent shortly after...

yes it could cause a high idle as the engine would not be getting the right temp for coolant and would try to high idle to warm it up. its part of the cold start stuff inside the programming of the ecu.

[EdW]

That seems to have been the case as I swapped the CTS out the other day and the idle has been normal ever since. Not a moment too soon as the old sensor was only registering between 125-160 when the car was fully warm, and the engine idle would go up as high as 1800 rpm when just sitting. Now the scan tool shows 190+ when fully warmed up and engine idles nicely around 700. I think my fuel economy will improve too now that the ECU thinks the engine is fully warm. Thanks again for the help, it's so nice to not see the CEL light lit up on the dash, it's been a while!