BVSV Coolant Leak?

This site may earn a commission from merchant affiliate
links, including eBay, Amazon, Skimlinks, and others.


Apr 3, 2017
Plano Texas
So in the last 4-5 months have completed some cooling system work to my '84 60 series. This included new WP, fan, fan clutch, all new front hoses, etc. As the weather has gotten warmer, I have noticed some coolant drops on the front timing cover periodically. I cannot really figure out where this is coming from exactly however as I never see anything dripping while the engine is idling in my garage. Even running the engine up to 3K RPM never seems to produce any leaks I can see. I only see these drips after a highway commute of some distance. I looked under the truck to try and spy where this leak is coming from and I think I may see it coming from one of the BVSV's in the thermostat housing. I took a shot of the cooling system parts looking up from the ground past the front of the motor such that you can see the underside of the WP and thermostat housing with the BVSVs in there. So the orientation is me on my back under the front of the truck, looking up into the sky. There is what appears to be some white dried coolant across the WP housing just underneath the BVSV's. This would line up where I have seen coolant droplets on the TC in the past. See this circled in the photo below:



Wanted to just see if anyone would agree that my suspicions are correct and the leak is coming from the lower BVSV above the WP in the thermostat housing or if they could suggest another possibility as to where this leak is coming from. I am desmogged so these valves are not in use and only serve as plugs for the thermostat housing. If folks agree that the photos show a leak at the BVSV I'll source some plugs for this to try and get this stopped. Apologies in advance for the poor photos. They were the best I could get. Hopefully they are good enough to try and see whats going on.

I would also like to know the size of the holes that the BVSV's fit into. I believe these are Japanese type thread and not NPT (I forget the European NPT equivalent name right now) so if anyone can confirm the size I would appreciate it. I think Jim C makes some plugs to delete BVSV's in the thermostat housing, but would be open to ideas on known plugs that fit these holes. As always, thanks in advance.
The BVSVs are solid plugs themselves. Their thermocouple is embedded inside them. If there's a leak, it's most likely the threaded fitting.

There's a risk in removing the BVSV sensors that have been in there for 30+ years. They're screwed into aluminum and there's likely corrosion on the aluminum female threads.

If you want to unscrew them and then reseal the threads, use something like Permatex High Temperature thread compound.

That leak looks so small (for a 34 year old truck) that I'd just leave it as it is. It may heal itself. It's very possible that messing with those BVSVs will open up a whole nuther can O worms (damaged thermostat housing).
Thanks here. I had considered doing something with the BVSVs when I was doing the cooling system work but got talked out of it with much the same dialog as above, so I left them. I am not sure how likely the possibility of these leaking are. Its a small leak and not really causing me issues other than just me feeling like I should take care of it. Truck runs cool and still plenty of coolant in the overflow so should be fine. But thought I would see if some photos shown amongst the group would yield a source or at least confirm I am on the right track.

I took another shot after a quick drive and I noticed some coolant stains on the top of the WP in a place that seems more like it could be coming from the thermostat housing itself, perhaps the housing to block seal. I don't see anything on the block itself between the WP and the thermostat housing so not sure where its coming from. I recently tightened the clamps for the bypass hose and heater hoses off the thermostat housing thinking there was a leak there but I think those are good. But maybe the threads in the BVSV are leaking like you said, and running along the housing into the WP below. See photo below:


This shot is looking across the front of the engine bay from drivers side to pass. side, sort of over the PS pump. Coolant stain on top of the WP which I have never noticed before. Engine block looks clean to me. If it bugs me enough I might get a pressure tester and see if I can create a leak to see where its coming from. Truck seems to run cool otherwise. Would appreciate any tips on getting these out if I decide to get some plugs, as well as good plug sources. I know I am prolly getting way detailed here, but stuff like this bugs me.
So you did not do the thermostat when you did the water pump and hoses?

I may be worth a new thermostat and all the associated gaskets. It's not that hard and you can buy new BVSV's for a reasonable price. The sensors I wouldn't touch though.

Here is my quick write up with all part numbers and pictures.
FJ62 thermostat replacement quick write up
Sorry, I should have included that I did do the thermostat and gaskets (thermostat gasket, upper thermostat housing gasket, and lower thermostat housing gasket to engine block, all Toyota OEM) when I did the cooling stuff. Basically replaced everything cooling system related on the front of the engine, as well all the oil cooler plumbing (pipes and hoses). Had the radiator boiled and dipped and did the TC seal and gasket while in there.

I thought about messing with the BVSVs when I did the thermostat stuff but opted out of it based on recommendations from other mud members. But all the thermostat stuff is new, except the housing itself and of course the BVSV's. If the BVSV's are not prone to leaking then I'll skip messing with them. It just looked like maybe they could have been the source of my leak based on where the stains are in the photos.
Last edited:
Sounds like you did it right but got unlucky with the accessories installed in your thermostat housing. I'd swap the BVSV. Buy a new one and put penetrating oil on the old one for a few days before swapping. This assumes you can verify it's the BVSV.

All the spare BVSVs I pull at the junk yard off 4runners are the same old cracked plastic as ours.
Any value in using some of that UV dye to chase this down? Any adverse side-effects with using that stuff in these trucks? Waste of time or worth trying to try and pinpoint this to the BVSV?
You need a UV light. This dye is designed for automotive use.

Amazon product
I’ve used UV dye to track down a power steering leak. I’d use it to find a water leak. Just make sure it’s a water soluble dye.
Last edited:
Dont worry. You do not appear to have significant leak. If you are uncomfortable with the situation i would do what @OSS advised. When i re did all my cooling system hoses I ended of breaking one BVSV. I was not happy. But you can replace if you break a BVSV
I sealed mine with Permatex, and now all is good.

I just replaced my radiator , water pump, fan clutch. Plus a whole bunch of cleaning and degreasing. I even used permatex type stuff on the WP bolts. I'm thinking of easing repairs down the road for me or another cruiser head.
Thanks all for the replies. I agree its not that big a deal and I am prolly just being paranoid. Truck runs cool so prolly won't press my luck with the BVSV. I might run the dye through just to see if I can figure out where its coming from. Whether I do anything about it is another story. If I could rule out the WP to block seal or the thermostat housing to block seal, I would feel better about things. WP looks pretty good and sealed so I think I am OK but would be nice to make sure.

I have never used the UV dye. Is it undetectable to the naked eye and only visible under UV light such that it won't turn my coolant purple or anything?
Can't see it without the UV flashlight.
It’s a lot easier to see the dye glow when it’s dark out. Add dye to cold radiator. Drive around. Wait until dark. Sweep engine with UV flashlight.

Make sure you use a water soluble dye.
Since you're running cool, I wouldn't worry about it.
The brass to aluminum thread combo is going to corrode and weep some, staining the thermo housing.
Both of my 2F engines have the whitish crud, but do not leak. On the 40, I have desmogged and left the bvsv's.

You might clean it up by using CLR and a brass wire brush, rinse clean, dry and monitor over time.
Thanks again for all the replies. I'll prolly just end up keeping an eye on it and see where it goes. A buddy of mine just informed me he got one of those diag cameras so I may try using that to just see under the thermos housing a little better and see if there is anything obvious going on. The dye idea is still there so if I get a wild hair I might try that. Already starting to get a bit warm down south so the cooling system will be tested pretty good here soon. If there is a change in the cooling situation as it gets warmer I might look into it then. With the hotter temps the leak may reveal itself in short order. Will see. Thanks all.
OK so thread revival warning here. I recently replaced my radiator because the original finally sprung a leak. The truck was not cooling as well as it had been with the original after the replacement so I borrowed a pressure tester and pressured up the system to see what the deal was. Immediately I found two leaks, one at the heater valve on the FW (tightened hose clamp seems to have handled that one) and one on the front of the engine somewhere. Dripping right down on the timing cover off the WP. Was a fairly steady stream. System held pressure pretty good but just had this stream of coolant leaking down everywhere. At first I thought it was coming from the WP itself. Looked like it was leaking from one of the WP mounting bolts. But got up and felt under the lower BVSV in the thermo housing and it was wet with coolant. I am fairly certain this is the source of the leak. I placed a rag under the BVSV to catch leaking coolant and the leak from the WP stopped and no more drips on the ground. I tried to get up under as far as I could on the thermo housing to feel for coolant and it felt dry there. So I think the engine block seals are OK. Just the BVSV. Now this was with the engine cold so maybe things seal up once the truck warms up some.

So just to clarify I should be able to remove the BVSV from the housing with it bolted to the truck right? I am still worried I am going to crack something doing this. I think somewhere I saw there were some plugs you could get to replace the BVSVs with. Since my truck is desmogged, I was considering just getting some of those instead of a replacement BVSV. I cannot recall where those are now. I think Jim C's desmog kit has them. Looking around at the proper thread pitch/size needed for this there seems to be some question as to what size is needed for a plug that fits in this. Have seen 16mm x 1.5 and NPT 3/8-18. Could anyone confirm for me what size I would need to cap this thermostat housing?

Also, would I need to drain the coolant from the system before removing this or can I just remove the BVSV catching the draining coolant once its removed? Seems to me this would be a small amount of coolant contained in this housing but am not sure.

As always thanks in advance here.
I do not recall the thread size. I got mine from JimC. Wait until you get the replacement in hand, make the swap. You will lose maybe a tablespoon of coolant.
As part of my '76 FJ40 oil cooler installation I used an FJ60 thermostat housing. I plugged the BVSV holes with metric oil drain plugs from the local Do-It-Best Hardware store. M16X1.5.
The BVSV(s) can serve as a plug. It basically IS a solid plug and no water passes thru it. Just temp. Just take it out and clean up the female threads in the housing an reinstall w/thread compound. I wouldn't bother draining the coolant from the radiator. It probably won't catch what coolant is in the bottom of the t-stat housing anyway. Just catch what comes out when you pull the BVSV.

Assume you're de-smogged but if you are still using your evap valve or your vtv for the dizzy like me (partially desmogd) you may want to leave BVSV 1 connected to the vacuum lines. BVSV 1 is the top one.

Also just FYI the t-stat housing bolts to the head and has a passage/gasket there too. Not to the block.

In this image you see BVSV 1 as part of the spark control system

Users who are viewing this thread

Top Bottom