Q: Why would my truck run better without the coolant temp sensor plugged in? 1990 Chevrolet K1500

asked by on January 13, 2017

I just replaced my stock 5.7L 350 with a 383 Stroker from Blueprint motors. It came complete with the throttle body intake and so on. I swapped water pump and all electronics over to the new motor, replacing only the coolant temp sensor, which the plastic was broke, so the clip wasn't present.

The motor runs very rich when cold and floods immedialty. This immediately stops if I unplug the new coolant temp sensor. I will leave it unplugged for about 10 min, or until the motor is at normal operating temp, and re plug the senor back, and it runs like a new motor. I took out the new coolant temp sensor and put the original back, and the symptoms are the same. Could I have 2 bad coolant temp sensors, or is something else I should be troubleshooting?
My car has an automatic transmission.

Car is getting worse gas mileage Inspection $79.99 - $89.99 Get a Quote

Hello - coolant temperature sensors are variable resistor devices - the colder the coolant temperature, the greater the electrical resistance value reported to the engine computer. (The warmer the temperature, the less resistance.) An "open" condition, as in an incomplete circuit, or not plugged in for example, would indicate infinite resistance. This would represent the coldest possible condition to the computer, and it would give you a very rich fuel mixture. Some engine management systems are sophisticated enough to recognize a wiring or sensor failure that shows an "open" condition like this, and it plugs in a default value (say 50 degrees Farenheit) arbitrarily. It sounds like in your case, the coolant sensor is either incorrect, or you have two failed sensors - it happens, although not all that often. Plugged in, the sensor is sending a signal for an extremely cold setting, with a very rich mixture as the result. Disconnecting the sensor may be getting you a default setting, and fuel mixture, that is much better for the engine’s performance. I would do some checking with the wiring, and sensor value at various temperature to determine where the fault lies. I would recommend an engine sensor inspection by a mobile, professional mechanic, such as one from YourMechanic, who will come to your location, diagnose this problem, give you an accurate assessment of damage and cost estimate for repairs.

The statements expressed above are only for informational purposes and should be independently verified. Please see our terms of service for more details

Get an instant quote for your car

Our certified mechanics come to you ・Backed by 12-month, 12,000-mile guarantee・Fair and transparent pricing

What others are asking

What does it mean when my headlights won't come on?

Headlights are relatively simple circuits. You will want to check all components from the switch to the bulb. To perform these tests, you will need to have a voltmeter and understand how to use it. You will test for available...

Car runs roughs and overheats

Hi there. This is a problem that comes up often on this car. The most common problem is a leaking intake manifold gasket. This gasket fails and causes a vacuum leak which will make it run rough. Not replacing the...

Replacing driver's seat

Hi there, thanks for writing in. You can use the same bucket seat out of any GM truck whether it is a GMC or Chevrolet from the years of late 1989 all of the way up to 1998. This can...

Temperature gauge fluctuation

There are two possible scenarios to check out. First, if the cooling system was not bled properly to get all the air out, then the cooling gauge can fluctuate, as the sensor can't measure the temperature of air/steam in the...

My 2000 Chevy Prizm is idleing hard and will not exlerate

You should have the computer scanned for misfire codes to see if it identifies the cylinder that is misfiring and you may then need a spark plug and coil replacement or plug wires replaced. If only does this when cold...

Why won't my truck start?

Hi there. Check the battery cables and see if they are tight and clean. If the battery cables are tight and clean, then check the ground wire to the fuse box and see if its loose or corroded. If the...

Steering wheel making noise

The power steering fluid on these vehicles gets too thick in cold temperatures and may blow any of the components or hoses in the system. The fix is to fix any broken component and flush and fill with new lower...

I have a coolant leak at the bypass pipe, where the hose from throttle body to the bypass pipe. 2000 Chevrolet Venture

Hi there - I suspect there is a tiny crack in the coolant bypass tube that is, against all odds, sealing itself as coolant temperature comes up. I recommend a coolant bypass tube replacement by a mobile, professional mechanic, such...

Overheating problem. Solution ?

Hello, thanks for writing in about your Nissan 350Z. This may be related to a leak somewhere in your coolant system or potentially a bad thermostat. As you may know when the thermostat is stuck closed this prevents the coolant...

Related articles

How Long Does a Speed Timing Sensor Last?
Your Your car’s engine needs a considerable number of sensors in order to operate correctly. Oxygen sensors, the vehicle speed sensor, the mass airflow sensor – these are just a few of them. Your engine also needs a speed timing...
Can a Bad Sensor Prevent a Car From Starting?
If your car isn't starting or has a hard time starting, a faulty sensor may be to blame. Car sensors monitor data to keep your car running smoothly.
How Are Car Wires Protected From Damage?
The highest tech cars can contain miles of electrical wiring. Wires must be secured and protected from heat and debris in order to work properly.