My Subaru Baja (2003) occasionally makes just one pop when I turn the key over. Any clue?

DINKY

MEMBER
2003 SUBARU BRAT
75,000 MILES

Doesn't seem to have other issues. No leaking, I checked. Battery good, some noise coming from lifters. The pop doesn't happen every time I turn the car over, just every once in a while. It's almost like it's got a back up of pressure that needs to be released every once in a while. One single pop/snap and it's fine. What's going on? Mechanics don't seem to be able to make the noise. Seems more likely if it's been sitting awhile rather than if I've been driving and go in and out of store.

1 Reply

DRCRANKNWRENCH

MECHANIC
3,380 POSTS

The nosie could be from a failed lifter. Generally they make the valve make a , "Tapping nosie". However, they are hydraulic and pump up with engine oil and pass it out, so it is feasible, although unlikely, that a failed lifter is making the moise.

The more likely reason for the noise is the thermostat. Especially since you mention the fact that if it sits after running. The reason I mention this is that the engine actually gets hotter when it is turned off. This is called, "Heat Soaking". If the thermostat is stuck closed, the flow of coolant being forced by it can cause a popping noise, especially if it is how enough to the point where it should be open and the pressure in the cooling system forces it open.
If you don't have a temperature gauge, you can get an infra-red gauge for less than $20 at most auto parts chains stores. Point it at the themostat housing and see if it is overheating. The other possibility is that the gauge, if you have one is failing and it is used as the fan switch snesor for the cooling system. Usually this is not the case as if you have a temperature gauge, the sensor for engine temperature which actuates the fan is a seperate sensor. If you do not have a temperature gauge, this sensor may not be turning the fan on. Another component that will keep the cooling fan from working is the fan relay.
To determine if the fan is working, turn on the car and let it run for 10 minutes which is usually enough to reach operating temperature, dependant on ambiant temperature, to turn the fan on. Also, after driving, before turning offf the engine check the fan for a few minutes and see if it turns on. It is normal for the fan to turn on and off as it only stays on when it is in a specified temperature range for the coolant.
Also, check the coolant level in the raiator and the reserviour. Be careful to allow time for the cooling system pressure to drop before opening the radiator as this will cause high pressure steam to eascape and is very dangerous.
Which leads to another cooling sytem test. After operating the car, turn it off and open the hood. Grab the upper radiator hose. You should notice resistance to your squeezing it or pressure in the hose. This is necassay for the coolant system to work. If there is no pressure, there is a leak in the system somewhere.

I am including a link to a diagnostic walk through that discusses some of what I mentioned as well as some options that you may wnat to look over as you eliminate the possiblities I mentioned. Diagnosis is a matter of process of elimination from most probable to less liekly. So, I think it is good to have this information in this type of situation as chasing down abnormal noises can be tough sometimes.

Here is the link:

https://www.2carpros.com/articles/popping-noise

I hope this information is helpful.