I bought this cooling system combustion leak tester. I have a few questions regarding its usage:

  1. While the instructions say take off the radiator cap and insert tester, it appears that at least some cars like BMW X3 2008 and Chevrolet Aveo 2007 don't have radiator cap per-se. They only have coolant reservoir cap. Just to confirm - in this case should I put this tester in coolant reservoir neck instead of the non-existent radiator neck?

  2. Do I need to run the car without radiator (or reservoir) cap and wait until the engine gets hot enough so that the thermostat opens, because, otherwise, the thermostat may prevent combustion gases from entering the cooling system?

  3. Any other precautions I should take with this test besides the well-known one - don't take off the radiator cap if the engine is already hot?

UPDATE#1: I tried to do the test according to the instruction. However, I think I actually contaminated the test fluid because after thermostat opened there was quite strong upward movement of coolant. The coolant level was so low that "low coolant" light was on in the dashboard. Now I am wondering if such upward movement of coolant is expected for BMW X3 or if it is sure indication of a blown head gasket? If it is expected then would this car be rather tested with two-level tester where coolant would be less likely to enter the second stage?

enter image description here

  • 1
    I don't think the upward movement of coolant is neccesarily indiciative of a blown head gasket. I'm not saying it isn't either, that is definitly more updward pressure than I am used to seeing in a radiator. I just don't think I would see that and then decide that the headgasket was definitely blown. Do you have some more test fluid you can try again with? The amazon link seems to suggest you could do this multiple times with the kit.
    – Zshoulders
    May 31, 2017 at 19:36
  • @Zshoulders Yes, I have more test fluid. However, it is not clear to me how to prevent coolant from contaminating the test fluid. Perhaps I should upgrade to the two-stage test device? May 31, 2017 at 19:40

1 Answer 1


If the system you are working on has a sealed radiator, with an expansion tank instead of a surge tank, with the only working pressure cap on the expansion tank, then yes, insert the tester in that opening instead.

The way these testers work is they test for hydrocarbons in the coolant system. If the engine is sealed correctly, there WILL NOT BE hydrocarbons in there. Any indication of them (where the testing fluid turns from blue to green or even yellow), it means one or more cylinders is leaking into the cooling system and there is an issue. If you have an expansion tank, hydrocarbons in the cooling system will wind up here as well, so testing there will be as conclusive as if you were actually testing off of the radiator itself.

Yes, run the car without the radiator cap on. You need to get it up to temperature before using the tester. Since you don't want to open a hot radiator, it only makes sense to have the cap off. Just keep an eye on it and make sure it doesn't overflow.

Just as a note, the thermostat won't keep combustion gases from entering the cooling system, it just won't circulate the gases forward to the radiator, but rather just run them around inside the engine instead. I know this is semantics, but I just wanted to clarify the point.

One of the instructions wants you to ensure there tester is completely free of coolant. You should be testing the fumes coming off of the expansion tank/radiator, not the coolant itself. This will foul your testing, so keep the tester dry of anything but the testing fluid itself.

For reference, here is a link to the instructions of your tester.

For those who might not want to follow the link, here's a copy of the instructions:

  1. WARNING! Since engine will be running during test, care must be taken to assure vehicle cannot go into gear accidentally. Take necessary precautions such as securely setting hand brake, firmly blocking all four wheels, and positioning shift lever with tape or other restraints to assure it will not move. Also take care to assure your hands and arms stay well clear of engine fan, belts, and any other moving items.
  2. Engine should be warm, and allowed to idle during the test. Coolant MUST be warm and circulating while test is in process. WARNING! Hot coolant can cause severe burns. A coolant system can fail at any time from hose rupture, gasket failure, radiator failure, etc. Protect yourself from these hazards. Wear goggles, heat resistant gloves and outer clothing that will protect you from burns if coolant system fails during test.
  3. Remove radiator cap, and check to see that radiator coolant level is low enough to prevent plugging of unit. Either siphon off or drain coolant so that level is 2" to 3" below the neck of the radiator.Combustion Leak Check color of test fluid, If blue, O.K. to use. If green or yellow, discard and obtain fresh fluid.
  4. Remove bulb by gently twisting. Pour test fluid from bottle into test instrument, through either opening in top cap, or remove top cap if more convenient, to the "fill to here" level line. Reinstall top cap (it removed) and install bulb by gently twisting into top plug, metal valve end up.
  5. Insert test instrument firmly into neck of radiator, by gripping the bottom rubber plug, with a rotating motion, so that a seal is formed with the inner neck of the radiator. Force radiator gases through test fluid by squeezing bulb.
  6. Continue to force radiator gases through test fluid by squeezing bulb for about 1 minute. If fluid turns YELLOW (in diesel engines, fluid maybe turn green-yellow), a combustion leak is present. If fluid remains BLUE, a combustion leak is not occurring while test is in process.
  7. If radiator opening is greater than 1 3/4", insert combustion Block Tester™ into Large Engine Adaptor plate (which may be purchased as an accessory). Block radiator overflow outlet with a suitable rubber plug or piece of _ tape. (Be sure to remove after completion of test.) Place tester with Large Engine Adaptor plate, on top of radiator opening, press down firmly to seal, and perform test.
  • Thanks Paulster2. I think I still failed the test, because there was quiet strong coolant movement that still got into the test fluid. Would you mind checking Update#1 where I attached a GIF that shows this movement? May 31, 2017 at 18:22

You must log in to answer this question.

Not the answer you're looking for? Browse other questions tagged .