Coolant loss in reservoir on 3.4L...

Just bought this truck...Coolant was fine the day I purchased, drove it 4 hours and it was slightly lower. Topped it off. Took it on a short 20 minute drive yesterday and noticed about an inch disappeared from the reservoir. REALLY thinking it might be a blown head gasket as I don't have any signs of leaking on the ground, even after running in place for awhile. A little coolant loss is normal on an engine this old, but that much shouldn't disappear after a short drive.

No smoke in exhaust, no major smells of sweetness. No overheating. Radiator is relatively new and water/timing was just done. Oil looks fine, not milky and the radiator cap looks fine as well as the coolant within. Really hope I didn't just buy a lemon. Any ideas?

 

If you have a skid plate on the bottom pull it off. A lot of times fluids get caught on the skid plate and don’t drip all the way down to the ground.

check out the skid plate for fluids and maybe take a short drive without the skid plate and check for leaks again

 

Define recently. A few days ago, a month ago, last year?

Let’s hope the cooling system wasn’t bleed properly, causing air bubbles to escape and drawing in coolant during the drive. Other than that, might be a blown gasket or crack head with coolant getting into the cylinder and burning off. Also check with a light, below the intake manifold and valley of both cylinder heads. That’s a common leak that traps coolant.

 

Recently, as in in the past month. I do have a CE light for an 02 sensor (P1135). I wouldn't think that would have anything to do with it, but I'm no expert.

 

If it’s been a month, that shouldn’t been the cause of coolant loss due to air pockets. Honestly, you might need to look elsewhere.

P1135 has the potential of being defective if coolant is soaking or cooling the sensor from sufficient heat source, might explain where the coolant is going.

 

That's another thing, looks like that sensor has been replaced recently as well. It doesn't look old at all when I was under there unplugging it and checking the connectors. I also removed the skid plate and the underside was dry too. Really stumped here and beginning to think its in the HG. Taking it to the mechanic tomorrow.

 

Seller probably did everything to try and chase the issue and gave up. As much as I hate to say it, it’s pointing towards a crack head(s) or head gasket. That suck

 

and its probably not the gasket itself if it's a head gasket issue. Like 9 times out of 10 on 5vz's, its a crack in the head causing the leak.

 

yep. Same with my experience on servicing Toyota, especially the Tacoma. Heads are known for the 5vz like burnt valves for the Honda B-series engine.

 

That's what I've been reading too unfortunately. I'm assuming the cracked head job is more time intensive/costly than the gasket too. I normally would tackle these projects myself but I'm a bit short on time currently. Going to be an expensive fix.

 

Basically the same job, but with new heads going on instead of the old ones. Finding a good reman head is the tricky part, in my opinion.

 

Honestly your truck is probably fine. Almost everything else looks ok and it’s driving fine. I wouldn’t overreact and start ordering parts for a head gasket job. Most of the coolant losses on these engines are small and can be driven like that for a long time without significant issue. These engines typically blow gaskets around the 400,000-500,000 mile mark. Both my 2004 Tacoma and 2000 CRV appear to lose a small amount of coolant over time as measured by the reservoir and they both are super reliable vehicles. Highly unlikely you bought yourself a lemon.

 

Appreciate the reassurance but I'm always overly-precocious when it comes to these things, especially since I have a long trip coming very soon. I wouldn't be so worried but it has been losing a concerning amount each drive. Like 1-2 inches in reservoir for a 40 minute drive.

 

AI Analysis:

You're absolutely right to be cautious here — that amount of coolant loss over such short drives is not normal, even for an older engine like the 5VZ-FE (your 3.4L V6). Let's break down your symptoms and what’s likely going on:

Possible Explanations (Ranked from Most to Least Likely):
1. Internal Coolant Leak (Head Gasket or Cracked Head)

• Key Signs:
  
  • Coolant disappearing with no external leaks
    
    
  • No white smoke yet (it can be subtle or only under load)
    
    
  • No milky oil now, but small internal leaks may not contaminate oil immediately
    
• Head gaskets on the 5VZ-FE are a known weak spot, particularly around 180k+ miles.
  
  
• Cracked head is less common, but can occur, especially if it overheated at some point.
  

2. Heater Core Leak

• Might leak inside the cab (check for damp carpet on passenger side, sweet smell with heat on)
  
  
• Could be very slow or vaporizing before dripping out
  

3. Radiator Cap Not Holding Pressure

• Even a "good-looking" cap can fail to hold pressure, causing coolant to boil off internally
  
  
• Replace it with an OEM or high-quality 16 psi cap — it’s cheap and worth ruling out
  

4. Evaporative Loss from Reservoir Overflow

• Unlikely unless the coolant is truly boiling due to under-pressure or a bad thermostat
  

5. Air Pockets / Improper Bleeding

• After water pump/timing belt jobs, air can get trapped and take time to work itself out
  
  
• As bubbles move into the reservoir, it’ll seem like you’re losing coolant
  

✅ What You’ve Already Done Right

• Checked oil: no milkiness
  
  
• No overheating: great sign so far
  
  
• No external leaks or puddles
  
  
• Checked radiator cap, skid plate, and undercarriage
  
  
• Aware of and monitoring reservoir level precisely
  
  
• Scheduled a mechanic visit
  

What You Can Still Check Before the Mechanic

1. Combustion Leak Test (Block Tester / Blue Dye Kit)
   
   • $30 at parts store; will detect exhaust gases in the radiator
     
     
   • Instant indication of a head gasket failure
     
2. Pressure Test the Cooling System
   
   • Holds 13-16 psi; if it drops, there's a leak (internal or external)
     
3. Check Spark Plugs
   
   • A single clean/steam-washed plug can tell you which cylinder is getting coolant
     

‍ Mechanic's Likely Process

• Pressure test the cooling system
  
  
• Possibly perform a cylinder leak-down test or block test
  
  
• May inspect for early signs of head gasket failure even without overheating
  

Cost If It's a Head Gasket

• Head Gasket Job: $1,500–$2,200 (labor-intensive)
  
  
• Cracked Head: Add $500–$1,000 if machining or replacement is needed
  
  
• If they’re already in there, replace timing belt/water pump/guides/seals if not fresh
  

You’re not wrong to worry — it’s the rate of coolant loss combined with no visible leaks that makes this suspicious. You're doing the right thing by getting it looked at now, before you're stranded on your trip.

 

Bit of an update. It completely escaped me that it could potentially be the heater core! the day I picked it up, it was raining so i didn't think much about it. But the passenger floorboard is damp and slightly sweet smelling. I ran the heater and all works well, but this could potentially be the culprit.

Pulled back the flooring and it's definitely saturated! While this isn't an easy replacement, I'll sure take it over a failed HG! Still getting it looked at tomorrow. Cylinder tests wont hurt anything.

 

I was going to ask about the heater core. Sounds like you found your culprit. You can bypass it to confirm that stops the leak before you pull the dash.