Invisible Coolant Leak

I have been observing the following for a year now and seem to have linked two issues together. The coolant reservoir is noticed to be low. So refill the reservoir to spec. Then within a couple of days driving the check engine light comes on (P0420) catalytic converter efficiency fault. Drive several more days and the check engine light goes out. Then the coolant level is very low again. The only other thing I notice is a faint sound similar to detonation in maybe a single cylinder.
Current plan: Top up coolant system. Remove spark plugs for inspection when the check engine light comes back. Hope to identify the effected cylinder. Pressure check the cylinder while bore scope at the same time.
Will update findings, but would like comments if the rest of you think I'm heading the right direction.

 

Head gasket leak? Are you noticing white smoke/steam from your tailpipe?

 

Sig says yours is an 06 which virtually garuntees bad HG.

you can use a block tester. Smell and observe exhaust for sweet coolant.
If the coolant is overly hot and pressurized that also indicates HG.

if it’s not leaking externally there’s only one place for it to go. Internal leak aka HG.
External leak can be checked with visual inspection and/or pressure test.

P0420 is typically cat worn out with age and miles. Never seen coolant being consumed cause this.

 

I was so fortunate when I bought my truck it was throwing a P0420 code and needed cats replaced. Fortunate thing was it had an outstanding recall for that very thing! Even at 125,000 miles Toyota took care of it. Even gave me a loaner to drive the day it was at the dealer getting the work done.

All it took was entering the VIN here and one call to the dealer to explain.

https://www.toyota.com/recall

 

I have had a fair amount of experience with head gaskets on Cosworth Vegas, Volkswagen GLI and 900 Ducati. The Tacoma just didn't act the same. No smoke or steam on start and no overheating yet. I'm avoiding the inevitable most likely. Next question, how much out of ture might the heads be? And should they be milled to bring them back in to spec? Thanks for the comments.

 

I was under the impression that it was only trucks manufactured in the first few months of 06?

 

That's correct.

 

Sometimes an oxygen sensor can get contaminated with coolant from a HG leak. It's rare but it does happen.

But at that age (2006) it could easily be a bad converter like you mentioned

 

OP I would check the FSM
I don’t know if the heads are serviceable or not. Like allowed by the manual and considered to have enough metal meat room on them for machining. I would hope so.
Then a shop can machine, hot tank clean, and replace valve stem seals.
You can paint valve covers while you’re at it. Maybe even do cam gears mod.

I’ll assume top deck of the block is still level and can simply be cleaned up with a carbide scraper. Do not use abrasive discs on rotary wheel tools as this is known to make the surface uneven and introduce a shit ton of grit into the engine that is too small for the filter to catch.

I don’t know what other kind of deals there may be. Like obtain used heads to refresh and swap in. Or if there’s any remans to put that in the end get their total price knocked down by mailing back old cores when done.

I would hope your current aluminum? Heads are not warped as generally on any vehicle (varies) that happens if it’s driven while overheating hot enough to warp them and cause a leak

as opposed to simply leaking from crappy HG design (known issue on early years), worn out HG, but still enough coolant in the engine and never actually overheated.
Some people claim it’s cheaper to buy a whole engine and drop it in.

if you do get a whole engine it may be convenient to take your time and refresh certain parts of it.
But it leaves unknowns like whether it had any major issues prior (junkyards lie, been there done that) or if the engine was well maintained or neglected (probably why it’s out of a car on a shelf)
that may also come with a core charge
I do see cheap engines for sale used and they can be taken apart to inspect
For example say you find a 4.0 on FB for sub-$500 considered by the seller to be done for due to a rod knock
That could be something to pull heads off and either sell the rest (unlikely to get anything for it) or scrap metal recycle it.

I believe the engine is aluminum. Aluminum wheels scrap for like $20-30 and they’re light. Engines are heavy.
I would hope you can get something for it in scrap.
You may have to separate dissimilar metals so it’s not all weighed as “dirty”.
As in giving them the block as aluminum (if it’s aluminum) and having the crank out separate that I’m sure is not aluminum, and would probably scrap as a lower price.
Given the weight of the engine it sounds like decent cash for scrapping the rest i would hope. Even recycling your old heads.

I would hope the Tacoma is magical with parts that never fail.
But having worked on other cars for work I just go by and assume regular timelines of when parts fail and apply it to the Tacoma, not trying to push my luck and limp extra last miles out of something and end up broken on the side of the road.

cats can also fail earlier if the engine burns oil, but it’s obvious when a car does and there’s known cars that do this. I’ve never seen a 4.0 burn oil.

good to replace O2’s at this mileage and cheaper to start with that than cats.
There’s also chemical cleaners that claim to restore cats that may or may not work.
Even removing the Y pipe to perk at the cat media to see if it has any obvious signs of being fucked up like clogged or shaking loose inside.

at least if replacing outside of CA and non CARB it’s cheaper

or park in wonderful riverside where they will shortly be stolen.

an exhaust leak, vacuum leak, or fuel pump dying can also cause cat related codes.