Radiator cap on 1999 Jetta?



Hi there. I’ve got a 1999 Volkswagen Jetta and it’s got a slow coolant leak. To fix it, I bought some stuff called AlumaSeal Stop Leak (which Tom and Ray have mentioned on the show). The directions say to add it to the radiator via the radiator cap. Seems easy enough – except that I can’t find the radiator cap!

I looked all over the radiator for it, but I don’t see it. The Owner’s Manual makes no mention of a radiator cap (it only mentions the coolant overflow tank), and it is not shown in the diagram of the engine compartment. I’ve also looked online for schematics of the radiator and the engine compartment, but haven’t had any luck.

Could it be possible that my radiator doesn’t have a radiator cap?! Or is it just located in some impossibly out-of-the-way place that I can’t get to without extracting the radiator from the engine compartment?

Thanks in advance for any tips!


Aluma-seal may not “fix” the leak, even if you can get it into the radiator. I suggest you return it, get you money back, and have the leak diagnosed and repaired properly. Dumping this stuff in your engine may make the problem worse instead of better.


Thanks for the unsolicited advice. Any idea on the radiator cap?


No, sorry. Perhaps there isn’t one. In which case you’d have to disconnect the upper hose to pour in the Aluma-seal. Good luck.


Just some more unsolicited advice. I would not recommend using a stop leak of any sort. MC’s advice is correct. Also your vehicle does not have a radiator cap. It is a closed pressure system and water is to be added to the overflow tank ONLY!
~Michael (dartman69 cannot log in at all.)


Aha. So it doesn’t have a cap. Interesting.

The guy at the auto parts store did steer me away from the big bottle of AlumaSeal, saying that it would clog my heater core. So I just have a small tube. Is it really that dangerous? I’m surprised that the Car Talk guys would recommend it if it were. They indicated that they’d had some success with it in the past. Not your experience?


I’ve used Aluma-seal in the past, but only in beater cars that were on their last legs anyway. It’s a temporary solution AT BEST. I would not use it in a car as new as yours. As I said, I recommending have the leak diagnosed and properly repaired. You’ll be better off in the long run.


My experience is that fixing the source of the leak is the proper route. The potential exists that the sealer can plug the heater core, some of the tubes in the radiator, small coolant passages in the block and head etc. I recently repaired a 89 Taurus that the owner attempted to fix a mysterious water leak with a sealer. 1st it did not fix the small hole in the $8.00 bypass hose. 2nd it did plug his heater core which required complete removal of the dash and an AC discharge and recharge. I threw in replacing the hose because I felt bad about the labor costs on changing the heater core.
~Michael (Dartman69 still can not log in)


I see. Well, you two have given me something to think about. The situation isn’t too dire – I purchased a bottle of antifreeze about a year ago, and I still have enough for one more tank refill. So I have some time to address the situation. But maybe I’ll get an estimate on it after all.

Thanks for the advice.


You should get this problem checked out. A slow coolant leak could be as easy as a pin sized hole in a hose or flange, or as bad as a main engine gasket.

If the problem is the former, no biggie, just buy a new hose. If it’s the latter you need to repair the problem sooner rather than later or risk engine damage.

I’m not trying to scare you, but take your car to the shop soon.

BTW, the “radiator cap” is the cap on that ball-shaped reservoir with the pink fluid (no not the brake fluid reservoir.


You guys might be throwing the baby out with the bath water in the case of stop-leak remedies. I have been told that Ford products come from the factory with stop leak in them. It used to be that this is what caused the yellow and green coolant to be somewhat discolored, Now they have revised the formula so it does not do that. Formulation might be everything, however. I would not mix the Motorcraft pellets with pink coolant or anything else not used in Fords. I am thinking of getting some every time I do a coolant change. I doubt that I would use anything aftermarket.

From the Motorcraft chemical catalog:

Cooling System Stop Leak Pellets
Part Number VC-6 WSS-M99B37-B6
Specification Number WSS-M99B37-B6
Size 2 - 10 g pellets per blister card
Unit Package 24
MSDS / FIR Number 165817

? Seals and prevents minor leaks in the entire cooling system
? New formulation - does not produce coolant discoloration
? Safe for use with all automotive coolant types
? Use 1 - 2 pellets for cooling systems up to 8 L (8.45 qt) and 2 - 4 pellets for
larger cooling systems
? Add to the cooling system once a year and at every coolant change out

For the OP’s problem, I am betting on the water pump.


Everyone is trying to politely tell you that using stop leak instead of fixing the problem could adversely affect your radiator’s ability to disperse heat or even result in a massive hemmorage at an inconvenient time (like on the highway) that could even cost you an engine. And if Beads is right and the water pump is leaking (that was my first thought too) then it could take out your timing belt when it goes. If that’s an intereference engine it could then take out your valves.

It wasn’t unsolicited advice…it was kindness and decency. And I agree with them.


Baby? Bath water? I don’y know nothin’ 'bout no baby or bath water!

Coolants fall into three categories. One uses silicates to coat the inside of the system and prevent corrosion. The other encourages a coat of oxide inside the metallic components, that being an excellent corrosion inhibitor…the statue of liberty being the best illustration. The third category is allegedly compatible with both, but I’m uninitiated as to its “active ingredients”.

Stop leak as an OEM additive is a new one on me. But I know you to be as credible source, so I can only surmise that I’ve learned something new.


First I suggest returning that gunk. It seldom fixes a leak properly and can cause more serious problems. If you had a worthless 1989 truck, I might suggest giving it a try.

As for cap I seem to recall that there is not a conventional cap and that you loosen a radiator hose.


Green is/was silicates and phosphates. Phosphates are used in Diesels, but silicates are not, IIRC. Borate is also sometimes used. They all coat the metal. These are all inorganic oxides. Phosphates are a problem if there is a lot of calcium and magnesium in the water because scale forms.

The newer stuff uses organic oxides (OAT). Dexcool and similar are OAT. Europeans don’t like phosphates because they have lots of hard water. In general, they use a mix of carboxylates and silicates. This is hybrid OAT, but I think that the orgainc acids are different than in OAT (Dexcool). Japanese water pumps don’t like silicates because the seals are eroded by them. Hybrid OAT is supposed to be compatible with all, but I don’t buy it. My Ford specifies that it needs silicates and the so-called Universal coolants don’t have it. I think that I can use green or G-O5 which is an Hybrid OAT that does contain silicates, but it is not universal because it can not be used in Asian vehicles.

Confused? Me too! I think it is best to use whatever your vehicle came equipped with unless you understand why the engineers specified the coolant that they did.