94 SR5 V6 4x4 starts heating up after driving for 20-30min

[marcusSRG]

Well that's my current problem. After fixing a series of problems I've had this happen a few times... After running for about 20 to 25 minutes, after the engine has warmed up to normal temperature (right below the middle), the temp gauge starts creeping up past the middle. It wants to keep going but Ive prevented it from overheating each time by either cutting it off or running the heat until I got home.

I think the thermostat is getting stuck open or there is some debris occasionally clogging the cooling system. So I think I need to either replace the thermostat or do a system flush and possibly radiator replacement

However, this is the most recent problem with this vehicle, and I haven't had it long. FYI I'm not a gear head but I'm learning! Here's the history:

I'm a Tacoma/4runner guy and I have been driving a 2000 pre-runner but I was wanting to get an older 4wd so I sold my 2000 Prerunner and picked up this black 94 SR5 4x4. It drove great for two days, beasting it in the snow, and then the engine overheated, steaming from the back of the engine on the passenger's side. I took it to the guy I had been using, he said it was a heater core hose and replaced it. I got him to do the oil as well as check the fluids. A day or two later, after a 30mile or so trip, she starts overheating and smoking from the same area and was leaking some fluid, something it had been doing earlier that day as well. Not wanting to take it to the same place, I go somewhere else on the recommendation of a buddy and they change some more hoses, replace a couple of oil gaskets, I come to get it, it overheats right in front of them and they say they will do more work without any labor cost. Suspicious at this point, I said I wouldn't pay labor or parts so they agreed to not charge me anymore and kept it another day, supposedly changing the thermostat, which I don't believe they did because it looks untouched (both appear old and untouched, sealant around the casing looks old and untouched). Despite keeping it, they call me saying that they can't figure it out and it still overheats.

Pissed off, and now about $400 poorer, I picked it up and it seemed to oddly drive fine, and it wasn't leaking anymore but they said it was heating up for them so I needed another opinion. I found a mobile mechanic who was supposedly very reputable and had him come by. He found an **unplugged sensor wire** (???!!!!!), which he plugged back in, and said that my upper radiator hose needed to be replaced soon. Otherwise, he said he couldn't see any issues. Very strange...obviously the last place was tying to screw me and/or didn't know what the hell they were talking about.

So, assuming it's ok other than the upper rad hose, I drive it around a day and the rad hose finally blows, as expected. That I can replace with out question so I fix it, drive it, it heats up again due to low coolant, I add coolant and then it appears to drive fine to and from work, a 10 minute trip. But when I start to drive it for longer than 20 minutes or so it is now doing what I am describing above. As I said, each time I cut it off or ran the heat to cool it down and didn't allow it to fully overheat but I don't know what's causing it. The first time I thought it was an air pocket and let it sit for a couple days, ran it a few mins with the rad cap off and topped off the radiator completely, although it was almost completely full, but it would still start warming up after 20–30 minutes of driving.

All I can think of is that it must be the thermostat getting stuck open after engine is fully warm or a small blockage in my coolant system.

I realize that was a lot but I wanted to fully explain my situation. It's been a long journey with this vehicle in a very short period of time and it is starting to wear on me. I haven't even had a chance to enjoy it.

Thanks,
Marcus

 

[giro]

Why would the thermostat being stuck open cause this? I believe that would cause better flow of the coolant, thus cooling the fluid in the engine. It seems more likely that it's stuck closed or isn't the right thermostat. Mine had part of gasket stuck in the spring, so when I bench tested it and the 10 year old new one, while it opened at the right temp it was a little slower and not as complete.

I fiddled and replaced my thermostat about 2 months ago. Both times I had to refill the coolant, I got an air bubble back in the heater core area. This included going up and down a decently steep road, having the temp get high right before turning around, and not having heat from the heater any of that time. But at the end, 2 turns before our house, that air bubble went away and the heater started blowing hot air.

I don't see the info about the heater core in the AC or cooling sections of the FSM. My experience made it seem almost like there's a second thermostat back there that doesn't let in coolant until ready, but I don't think that's the case. Either way, it seems like many of your issues have been back in the heater core area (maybe something is plugged?), so I'd look there first. And the thermostat is easy enough and cheap enough to replace, so you might want to just do it if you aren't certain that they replaced it.

If you're draining your coolant to change the thermostat, and you have any doubts about it, it's easy enough at that point to take off the bottom hose too. I did that on mine to check that the hose wasn't plugged, and check that the radiator wasn't plugged (by pouring water through from both the hose and cap sides). Both were fine.

Good luck

 

[marcusSRG]

I should have said 'malfunctioning' thermostat. Like I said, I'm not that knowledgeable in with this stuff, but I am definitely learning!

Either way, thank you for the detailed response. I reckon I will go ahead and replace the thermostat and I might as well remove the bottom hose and check for blockages at that point like you suggested. I really don't think the thermostat has been touched.

Don't really know how or where to start addressing the heater core though.

-Marcus

 

[good_ol_boy]

I'd replace the thermostat AND heater core, buddy.

First off, drain the cooling system.

On 3F-E engines, disconnect the 2 vacuum hoses from the Bimetal Vacuum Switching Valve (BVSV), the cold start injector time switch wire and the water temperature switch connector. Unbolt the housing, then remove the water inlet with the hose attached. If there is not enough room to pull both housing and hose out, you may want to disconnect the hose first. On 3F-E engines, remove the clamp bolt for the ISC water bypass pipe. Make certain to remove the gasket with the water inlet. Some of it will probably stick to the housing and require scraping. Then remove the old thermostat.

INSTALLATION:

When installing a new thermostat always use a new gasket. Be sure that the thermostat is positioned with the spring down or into the housing. On 3VZ-E engines, the jiggle valve (looks like a small rivet in the edge of the thermostat) must be at the top or 12 o'clock position. Install the water inlet. Install the bolts finger-tight until the inlet is positioned exactly. Tighten the bolts to 10-14 ft. lbs. (13-19 Nm). Overtightening will break the ear off the housing and spoil your afternoon-don't do it. Install the hoses and wire connectors removed for access. Make certain the coolant hoses are not kinked or twisted after installation. Do not overtighten the hose clamps.

Don't forget to fill the cooling system with coolant. Start the engine and check for leaks. Top off any fluid levels.

The heater core is an S.O.B. to freak with… requires taking off the dash to access the heater box. The heater core is inside the heater box. AT LEAST a 3 hour job. The heater core is around $100, give or take at any parts house.

 

[giro]

Marcus, if you don't yet have a Factory Service Manual, go to the link at the top of the classics section and get one. You can get it off the webarchive page that has a link there.

In my 92 FSM, it doesn't go into all of those steps. For the 3.0 3VZ-E engine it really does just say Drain Coolant, Disconnect radiator outlet hose, remove thermostat (though here it says to "remove the three nuts, water inlet and thermostat with gasket from the water pump."). There's a couple more steps for the 4 cylinder 22R-E engine, so apparently it depends a little on your engine.

The first time I did this was after my fan exploded, so I had a little more room. I took the replacement fan off when I redid it, but it shouldn't be necessary. But without it there, I was able to leave the hose on the "water inlet", I think.

On the 3VZ-E 3.0 it was pretty simple. The second time I did it was with a snowstorm coming in, and I was pretty quick. Personally, I like to take a few pictures along the way if I can remember, so that I can refer to them if I can't remember the direction something goes. Then just remember to have enough new antifreeze, either premixed or mixed with good water, along with something to collect the old stuff in. I try to remember to put on a set of disposable gloves, but often start off thinking I won't get too dirty and then end up with grease on my hands for days even after multiple scrubs.

Good luck. It really is fairly easy, only a slight step up from changing your oil and filter. One nice thing is that the gasket fits onto the thermostat, which makes it a lot easier than the first thermostat I replaced years ago on a prelude. So you just have to remember that the spring end goes in, and the jiggle valve goes up.

 

[marcusSRG]

Thanks for all the great responses, guys. I will definitely go ahead with the thermostat replacement but I don't know if it's worth my time to get into that heater core. That sounds like a beast of a job.

 

[marcusSRG]

Well I got a response to the same post I put in another forum and they said I was headed towards a blown head gasket based on my symptoms and merely said to 'monitor the coolant and oil' but didn't go into detail....

And I have my FSM and it explains replacing the thermo the way you were saying Giro.

I'll await more reaponses here an elsewhere before I start drawing conclusions.

Wish I still had my Taco right about now!

 

[giro]

and merely said to 'monitor the coolant and oil' but didn't go into detail....

Basically make sure neither looks wrong.

The head gasket keeps the oil where it should be, and the coolant where it should be. You have 3 potential problems for a failure. Coolant or oil can get into the combustion chamber, where it will then muck with things, maybe put smoke out the tailpipe, and maybe foul things like your Oxygen sensor and catalytic converter. You'd probably fail a smog check. Or you could have combustion gases pushing through the antifreeze, which would tend to overheat it, make it spill, and also give the gas bubbles that cause the testing kits that you put onto the radiator (to show there is a leak) change color.

You could have coolant or oil escape the engine. This will cause drips, and likely also smoke on your engine when it gets hot enough.

Or you could have the gasket fail in a way that they mix with each other. This will cause oil to look milky, or antifreeze to look strange (milky too?). Basically they won't look like they should.

I don't know how 3.0 failures tend to happen, though mine went through the rebuild 12 years ago or so, before I owned it. I do know on the 3000gt that I had that I suspect had a head gasket failure, it had previously failed smog, it had some tailpipe smoke which got worse eventually, and at the end it started smoking under the hood, leaking oil onto the hot engine. But the oil and antifreeze still looked fine!

(there was also a loud knocking sometimes with one of the cylinders, which freaked me out. Even getting this free from a friend, I really considered not taking it because of this. Fun times. I really should have gone for it and replaced the head gasket just for the learning experience, but instead I sold it, fully disclosing everything, to a happy new owner who had 2 non-turbo 3000gt's)


Keep an eye on the oil and antifreeze levels and condition. Watch for drips. And watch for smoke. I think a major head gasket issue would present itself in one of those ways (likely in addition to a noticeable change in power, and possibly an overheat).

 

[marcusSRG]

Thanks. I will continue to monitor my fluids and look for smoke and drips, which have been absent. And of course it still starts getting warm/hot after 30 minutes of driving. I'm still hoping for a more straightforward answer, possibly from someone who had the same experience? From what I'm seeing my specific problem doesn't seem to be very prevalent. People have engine warming/overheating issues but not after 25-30 minutes of driving and no other symptoms.

I'm still going to proceed with the thermostat change and I will be able to check and flush the radiator out at that point.

Thanks

 

[2ndGen4runner4x4]

The 3.slo is notorious for blown head gaskets...
I believe that the reason that you are getting the run around from the repair shops is because they don't like to deal with this engine. It is a real pita to work on and will eat up a lot of a mechanics time. It is not easy money for them and your bill will get real expensive real fast.
Sounds to me like the previous owner was aware of a head gasket issue and unloaded the vehicle on you while it was still running good.
If you need to keep it on the road for a while, you can get away with removing the thermostat completely in order to keep it from overheating but overdrive will not function until normal operating temp is reached and gas mileage will suffer.
I unknowingly bought my 2nd gen with a blown HG a few years back. I ended up replacing the entire engine with a reman unit.

This is where this issue is headed (pun intended)...

 

[giro]

And I have my FSM and it explains replacing the thermo the way you were saying Giro.

Well yea, I wrote those down while looking at the manual. I'm not very good at finding things in the manual, so it actually helps to hunt it down for other people, and read it to understand.

Hear's a strange guess. Check that your fan is working, that air can actually flow through your radiator to cool it, and that there isn's some strange coating on the radiator or something.

I agree that it's strange that this is happening after 25-30 minutes. How cold is it there when you're testing this, and are you driving it really easy on flat roads, or on a freeway with plenty of airflow?

Keep an eye on those oil and coolant levels! Like has been said, it's really better to not damage things further while you're figuring it out.

 

[marcusSRG]

Fan is working as far as I know.

I'm driving on relatively easy roads, some slight hills and a freeway. Only driving it up the road (5-10 min round trip) or to work, which is only about 5-10min. Hasn't gotten warm on any of these trips.

Checked coolant yesterday before leaving work and it had lowered slightly, still covering the top of the vanes inside the rad but lower. It only took a little to top it off. Oil and trans fluid look fine...

 

[94_4runner757]

Any luck?