Hard start/stumble on start in cold? Check this

[Ian_12]

So for a while I was trying to track down a hard start issue when it was cold. I tried all the common things and even replace entire intake manifold. Nothing worked, and went for HP Tuners to look at the tune. I found the ECU is locked, but I was able to data log it and noticed something odd. The coolant temp was reading 120 degrees when it was like 60 outside. I have TorquePro and Blue Driver but disregarded this as a calibration error since it read fine when it broke that temp.

I decided to look into the temp sensor, and found something I didn't expect to find any more. If you look at the attachments, you will see corrosion in the sensor and the connector. I've seen this with old plastic temp sensors since they would crack and let coolant into the connector. Simple little thing to check if you notice any issue with starting. Coolant seeps through the sensor and corrodes the terminal. In best interest, you should replace the connector if you notice corrosion on it. You don't know how far that corrosion has gone up the connector into the wires.

Check your coolant sensor people! I highly doubt this is an isolated issue.

 

[Papasonic]

Good find!

After cleaning or replacing connection, a dose of dielectric grease should assure a trouble-free long lasting connection; unless it was the sender itself leaking internally. This info will no doubt save others some grief trying to diagnose a similar issue.

 

[Ian_12]

The sensor had a very tiny, nearly impossible to see, crack. It basically looked like a superficial scratch.

Car still starts up just fine even during the super cold spell we had.

 

[Maddmatts]

I had this exact problem today (155,500 km/97,000 mi). My car took multiple attempts to start and wouldn't keep idling without extra throttle. Ultra gauge immediately screamed an overheat alert showing coolant at 125C on what I knew was a cold engine. Checked the resistance on the coolant sensor and it showed 63ohms when it should have been around ~9.5 K-ohms (ambient was ~0C) so definitely an internal short.

While driving to the parts store, the rad fan was running the whole way and OBD code P00B7 (low coolant flow/pump performance) triggered.

I also noticed that the old sensor felt unusually loose in the outlet housing and it basically fell out once I removed the wire clip. The old o-ring had completely lost it's pliability and was allowing a slow drip of coolant out. The new sensor with it's fresh o-ring was noticeably more secure.

Like Ian_12, my old sensor had a tiny crack on the brass probe right where the o-ring sits although the connector on mine was still dry and corrosion free.

 

[Rango9]

I changed a ton of things getting imprivements.....after its warmed up. Under 40F.....repeated stalls 5 or 6.....then it smooths out. No problems. Changed battery,plugs,coil pack,radiator and thermostat sensors, both vvt solenoids, and cam sensor.....valve cover w pcv is 2 yrs old.....map sensor new....all this summmer...except valve cover.....so now what? Cold start stall remains.