A Bad Coolant Sensor?

My '95 Saturn had its starter, elec. cables, battery, the whole starting-system-shabang tested. But occasionally it still won’t start…in the evening, after I get home and it sits for 2 hours. The mechanic says it could be the coolant sensor. Is this possible? Thanks!

(I failed to mention the word “passed” i.e., the car passed the electrical system test.)

Can it crank?
Your post is blank.
When it’s hot
Or when it’s not?

To elaborate on SteveF’s “Can it crank?” question, “won’t start” means lots of different things. You have to describe exactly what happens when it won’t start. You turn the key and…

Coolant sensor? Does he mean the coolant temp sensor or the coolant level sensor?

Of course it doesn’t matter. Neither will prevent the car from starting. Your mechanic is stumped and grasping at straws.

And, as cigroller mentions, please elaborate on ‘won’t start’.

It cranks like normal, i.e. GRRR-sounding, but doesn’t kick over to start. And, thus far, it only happens after I’ve driven to the gym from work in the afternoon where it sits for 2 hours. To summarize: morning-start is fine; afternoon-start from work to gym is fine; after 2 hours in the gym it’s occasionally not fine where it continues a normal-sounding GRRR without starting. I’ve recently started pumping premium gas (92 grade) and it might be helping - when the engine does its GRRRing without starting I put-the-pedal-to-the-metal and it starts, albeit barely.

You may need a coolant sensor, butiof that’s what were causing the no start condition you’d be experiencing it in the morning too. I suspect the coolant sensor he’s referring to is the one that provides the engine temp sensor to the ECU to get the proper metering for the engine’s temp condition.

I’d start by picking up a spark tester and checking for spark when it won’t start. What may be happening is that some component, perhaps the coil pack, is getting “heat soaked” and failing.

When you’ve been running the engine at full temp and you shut it off, the temperature under the hood rises. This happens because the temperatures of the engine’s internals (around the cylinders) and its exhaust manifold are far higher than the operating temperature of the engine. Cylinder temps can get over 2000F, and manifolds can get well into the hundreds. With the engine suddenly shut off, you no longer have the cooling fan(s) and the flowing coolant to carry the heat away. It slowly moves to the surfaces of the engine and manifold and radiates out into the underhood air. Some components, especially those with coiled fine wires in them (like ignition coils and igniters) can become sensitive to the elevated heat. The explanation is a bit technical and has to do with differences in thermal exppansion between the copper wires and the polyamide-imide insulation coatings and the coating breakdown, but the bottiom line is that they become heat sensitive.

That’s my guess. Of course, at 16 years old you might also need a temp sensor.

Could also be a crank position sensor getting heat soaked.
First thing to do is test for spark, then fuel pressure, fuel injector pulses.

In the past I’ve diagnosed a no-spark condition by sniffing near the exhaust pipe for the smell of raw gas while a helper cranked the engine.
Do it to the side in case there’s a backfire!

True. They’ve been known to become heat sensitive as well.

The first thing the OP needs to find out is if when the engine won;t start the spark is missing…or present. A bad position sensor of any kind will also manifest itself as a lost spark.

If flooring it will get it running then I might first suspect a flooded engine from something like a leaking fuel pressure regulator. CrummyVerses, holding the pedal to the floor is a flood clearing procedure. You can find out if the fuel pressure regulator is leaking by pulling off its vacuum line - it should be dry. If wet with fuel then you need a new regulator.

If flooring it normally gets it to run, and if the regulator is good, then you might ask a shop to check for leaking fuel injectors. Actually, a really good mechanic might be able to tell you if you have this kind of problem by pulling your oil cap and taking a whiff.

If flooring it is not a reliable method of getting it to start, then I’d revert to that basic question of whether its fuel or spark.

“…holding the pedal to the floor is a flood clearing procedure.”

This has always been the method with carburetted engines. Are you claiming it still works with electronic fuel injection?


SteveF, cigroller is correct. The computer is programmed to cut off the fuel when the throttle is wide open and the starter is engaged.

I love this site. There’s always something new to learn.

I semi-agree with cigroller too on the root of the problem, however I am leaning toward a leaking injector more than the regulator. But he may have mentioned the regulator first because its so easy to check.

LOL - actually Keith its partly true. If somebody asked, “how can I check to see if the FPR is leaking?” I’d say, “oh that’s easy…”

But if somebody asked “how do I tell if a fuel injector is leaking into the intake?” I’d say “uhhhhhhh…” Well, I guess you have to pull the fuel rail. Is there a simple way to figure that out? Pull the plugs & snort? I don’t know - not so easy to check.

Cigroller, it takes a small bit of effort, but fairly easy. Drive car, park car for 2 hours, try to start it once, pull plugs. The wet one is the leaky injector. If all are similarly wet, the computer is flooding the engine maybe due to a very bad CTS or IAT, although I’ve never seen this actually happen.

It could very well be a faulty sensor–the coolant temperature sensor on these cars tend to fail frequently, and they fail in a way that can make an engine get way more fuel than it can use. When these sensors fail “open circuit” they send a signal to the car’s computer indicating an engine temperature of something like -40 degrees F. So now the computer is operating the injectors as if it were 40 below zero–way more fuel than is needed for 60 or 100 or 150 degrees of engine temperature. The result–a flooded engine. Of course, it could also be a leaky fuel injector or regulator as indicated above.

This should be quite simple for your mechanic to figure out if he is connected to your car’s computer system and fuel injection system and watching sensor data when the problem is present. Or if you prefer to guess until you correct the problem try the coolant temp sensor first–they’re cheap.

Yep, I’m with asemaster. The computer should tell him exactly what the sensor thinks the temp is and how close that is to actual.

Since other possibilities have already been mentioned, maybe there’s a problem with the Idle Air Control valve and this is why the pedal must be depressed to start it. Just something for consideration anyway.