I had a recent coolant smell coming from my car at startup, and got some help from a friendly local gearhead on trying to find the source. It was suggested to do a write up on what I did to try and find the leak (which is what this is). This is aimed more towards putting all the info in one place, which will help out any new guys with missing coolant, or coolant smells. If I miss anything, please be sure to add it. I put this in the 2000-2005 forum as this is what I worked on, but certainly most of this will apply to the older generations as well.


First things first would be to check for an UIM (upper intake manifold) problem or a LIM (lower intake manifold) gasket leak, as this is a big problem with our cars.

Pull out the dipstick to see what the oil looks like. It should look clean, and not like coffee with cream, or chocolate milk, and also should not be dark black. Also, you can wipe the dipstick off on a clean white rag, or paper towel. This will help to show off anything that shouldn't be there. It should look light brown with no other colors lingering. Next, pull the oil fill tube and the oil cap off the valve cover, and remove the engine cover. While the cap and fill tube are off, inspect the inside of it. There shouldn't be any milkiness in the tube or the cap. Also, look inside of the valve cover hole to see if any milky residue is there as well. If any of this residue is found it could be signs of problems with the UIM or a LIM gasket failing and leaking coolant into the lower part of the engine.

Next look at the top of the engine (with the engine cover still off) around the UIM and LIM gaskets. Look around the injectors for any puddling of coolant. The injectors look something like stubby sparkplugs, which will be right below some metal pipes (fuel rails). Also look inside of the valleys that the LIM makes. The coolant may also pool up inside these as well.

After this, start up the car and go around behind to the tailpipe. Look for any excessive steam coming out. Try to take a few good whiffs near the exhaust. If there are any sweet smells to it, or excessive steam, it could also mean UIM or LIM gasket failure. Another tip would be to hold your hand in front of the exhaust pipe and let some condensation build up on your hand. You can test this for any sweet smells as well.

If you have signs of UIM or LIM gasket failure, get it to a shop as soon as possible, and have them drain the oil. This is important, as if not done quickly, it could mean death to your engine. It is always good practice to watch the level in your reservoir tank and make sure no sudden changes in the level occurs.


Alright, if it appears that coolant is not leaking internally, it'* time to look to the outside.

Start with the radiator cap, make sure there is no leaks or seepage there. Then check the hoses leading to and from the radiator. Feel around where they link to the engine and the radiator for any moisture there. Also look around underneath where the thermostat housing is. This should be the metal elbow where the radiator hose attaches to the top, driver side of the motor. Make sure no coolant is escaping around it.

Go back to the passenger side of the engine again. Look right below the black plastic UIM for a 90 degree plastic elbow right below where the UIM atteches to the LIM. Check this elbow for cracks. Then if you go straight back from this elbow, you will see another one. It will be slightly lower, and a bit of a pain to find. It'll be hidden by pulleys and the serpentine belt a bit. Check this elbow as you did the first one.

Now look at the heater hoses as well. They will be rubber hoses that run from the firewall to the passenger side rear of the motor (near the alternator). Feel along the hoses and their connections on the firewall and the motor to make sure nothing is leaking there.

You can also look around the water pump, which will be on the passenger side near the bottom of the engine. This is kind of hard to check from the top due to it'* location, but you should be able to feel around it to feel for any fresh coolant.

The heater core itself could be another culprit of a leak. This will most likely cause a strong coolant smell inside the car. Fogging windows could be another sign of this being the problem.

Alright, this should be a really good running start as to what to check for when trying to find a coolant leak. Any input on anything else I may have missed, would be greatly appreciated.

a note to people, dexcool is UV reactive. so if it is really dark and you have a black light, dexcool is EASY to spot

ten_gig, that'* a pretty good write up that is written without assuming that people already know what and where things are. Good job!

I would add a couple items:

Note that after doing the visual checks on the radiator and heater hoses, you can start the car and let it run until it gets to temp (your fans will turn on as will the water pump and your radiator hoses will become hot). This will pressurize the system and if there is a pinhole hose leak, it will often become visible as it starts to leak. I've discovered Rad. hose pinhole leaks this way before.

Then turn your heat on and re-check the heater hoses for leaks while the heater core loop is circulating. This is also the best time to check for heater core leaks inside.

Another item that I did not see mentioned is the water pump. Often, if the water pump begins to fail, there will be a noise associated with it, but not always. Check for a "weep" hole at 12 or 6 o'clock (my '97 is a 6) on the pump casing nose. This is the part that surrounds the water pump pulley shaft. When the water pump bearings begin to fail, the weep hole will lose coolant.

The water pump is usually down low on the vehical and you will likely have to crawl under the car to get a good view of the weep hole. Again it is often easier to see after you have brought the engine to temp, but don't get under there while the car is on (safety first).

I'd recommend using a small mirrow and light to check up under the waterpump pulley to look for the brownish Dexcool stains.

The water is flowing through the heater hoses all the time whether the inside is calling for heat or for AC.

True in most cases, but when that heat blower is on, you will more readily smell that coolant in the cabin if there is a leak in the heater core.

There are a number of GM vehicles that used a heater bypass loop to ensure more rapid engine warm up. This meant that you did not get heater hose flow or heat until the engine reached a minimum running temp.

"Then turn your heat on and re-check the heater hoses for leaks while the heater core loop is circulating. " The statement was to recheck the hoses for leaks with water circulating through them, but it circulates all the time. The heater core also is getting hot water all the time, whether the unit is on vent, ac, heat. Seepage in the heater core often shows up at inital start after parking the car hot with pressure on the system, which causes seepage to maximize; then on restart the moisture has a higher concentration of water (and coolant smell).

"a number of GM vehicles that used a heater bypass loop"
I've not had a car with a bypass loop for the heater. How do they turn it off while the motor warms up?

Look, I'm not trying to get in an argument here, but you should know as well as I do that the best way to smell a leaking heater core is to bring the engine to temp and make sure you put the heater blower on. THAT is what gets the air into your cabin the quickest.

Secondly, your coolant does not begin to circulate until the engine reaches temp and the thermostat opens up.

I may have worded my statement wrongly above and accept that.

As far as a heater bypass loop, I'm surprised you've not run into them. We used to manufacture a number of GM heater hose assemblies with "H" valves in them that created a heater hose bypass loop that was heat activated (similar to a small thermostat) in an "H" shaped plastic assembly. This assembly was placed in both the heat and return hoses (they both went to it). This acted as a bypass loop to bring the engine to temp quicker. I honestly can't remember exactly which cars these went on, but they did exist in the '90s.

Great additions, guys. Keep them coming if anyone else has any more ideas/thoughts.

I always wanted to have a reason to buy a black light, now I got one. WooooHoooo!!!

The DexCool showing in black light is among the top 20 great things I've learned from this site --and I've learned a lot through the months.