Hello forum,

I have a 94 LE AWD non-supercharged Prev, 256500 miles.

The head-gasket had been going since the vehicle had around 210000 miles. Also, At times bit sluggish to start when cold - ten to thirty seconds - until warm enough to support normal combustion and likely a bit of moisture in one cylinder - I presume.

The big disaster was a failed thermastat deep in remote Idaho mountains 236500 in summer 2011. Car got hot. I removed thermostat to get it out and the car ran fine. I put a new thermostat back in when back in civilization. But more head gasket deterioration led to the rest of the story.

The head gasket failed badly at around 237400 miles later in summer 2011, so badly that I hydrostatically locked it up three times. After the third time, I used Thermagasket, started the car dry, added water on the fly and then Thermagasket and followed the instructions. Needless to say, the product worked remarkably. Now the vehicle has 256600 miles.

Ran pretty well for about 14000 miles with care and not charging around too much.

About 5000 miles ago, the vehicle would have a hard time staying cool without running the interior heater and fan on high and especially with a grade.

After eight fresh doses of Thermagasket as preventative medicine since summer of 2011, my Previa now presents with trending toward over-heating after about thirty to forty minutes of driving, a little faster at highway speeds of course. Still, in cooler weather, with a headwind, I can drive at 60mph for four hours or more as long as there isn't much grade and I keep the engine temperature nominal while using the interior heater on full-blast. Last week, I drove from Grants Pass OR to Boise ID and only stopped once before Bend OR. On leg from Bend to Boise, I drove non-stop, mostly at 55-60mph. Hwy leg Ontario OR to Boise ID, had to slow down to 50mph as strong tailwind and still ran on warm side.

I've now seen a number of Prev motor images with cracked cylinders and blown gaskets. Most likely, if you took the head off, my motor would be a contestant in the preliminary image contest and would be a most blown head-gasket finalist.

To specifics.

Thermostat is in and new.
Overflow container and cap good.
Hoses original as far as I know.
Plugs and wires two years old.
Fan-clutch is eight months old.
Radiator flow tested last fall and nominal.
Water pump new.
Run thermagasket solution full-time.
Gas mileage good. 21 to 24 hwy.
Idles fine.
Does not lose fluid if operated in normal range.
Loses about a half a cup every hour if running on hot side.

Does not blow excessive moisture not visible as when gasket was head-gasket was really bad upon start, though holding a hand at tailpipe detects moisture. I know that water is a byproduct of normal combustion. Less moisture on hand after motor is warm.

Still slightly sluggish to start as one cylinder (a presumption) seems like it takes ten to thirty seconds to be warm enough for normal for normal combustion, but after that runs smoothly, as like a couple of years ago. When warm, starts right up.

After it gets to normal and trends towards hot, after thirty minutes or more of driving, a little quicker at hwy speeds, with very slight accelerations, it can knock once here and there, I just change the engine rpm and that alleviates the knocking. I'll even take off over-drive to increase rpms on slight declines using cruise to avoid the knocking. This is a relatively new symptom in last month.

The things I can do myself are:
1) Replace the main hoses, they might be a bit soft, especially lower but it's hard to say. They feel OK when cold and I notice no dis-figuration on forward hoses when warm or on warm side of normal. How likely is overheating from bad main hoses?

2) Replace the return hose from overflow container, as saw this fix mentioned in another forum.

3) Replace radiator (aftermarket) just because I don't know what else to do.

4) Add an additive like BG44 K if contaminants are messing with valve operations, the knocking, incomplete combustion - though I know occasion knock is most likely from heat in a cylinder (a presumption).


Most likely

1) Get a new house, as for seven months of the year, the Previa is my home.

Possible

2) Put in a used Japanese motor. How much might I expect to pay installed? Of course this solution inherently involves the overall value of a twenty year old vehicle.

Remote

1) Get a custom radiator. As I understand it, the Prev comes with one row. There might be enough room to add a second row if the fan cowling was modified.


Specific questions

1) Can a damaged, warn or sticky valve cause the overheating in conjunction with a presumed head-gasket issue as described? Hence the idea of using a treatment like BG44 K.

2) Does anyone have a referral in the Boise area.

3) Any other thoughts?


I do not have the experience to do engine work.


Thank you all in advance for your thoughtful responses.

Chuck