'92 Astro mysterious overheating

[m-theory]

This thing never used to overheat, then, immediately after replacing the alternator, it started to do so. I replaced the thermostat (which was bad), and it still over heated. I replaced the fan clutch, and it still overheats. Obviously, the water pump is the next in line, but I'm wondering if someone can shed light on a possible alternative culprit.

This isn't a consistent problem. If it were happening all the time, I'd be much better equipped to understand and respond appropriately to it, but as it is, I'm just shooting blind, using the expensive and time-consuming "shotgun method."

After replacing the fan clutch, I drove it around town for a full 30 minutes, with the gauge right at 200 for about half the time, and steady at 215 for the other half.

The next day, my son took it to work and said that it pegged out while on the freeway. As soon as he turned on the heater, it dropped back down to 215, where it seems to rest 95% of the time.

- 4.3l W
- 166k miles
- Antifreeze is clear, radiator clean and holding.
- With radiator cap open and thermostat open, revving the engine indicates flow in radiator.
- No bubbles appear in radiator while running
- Hoses seem fine, with no signs of collapse or excessive wear
- Front of radiator is clear
- Fan is in fine shape
- NEVER overheated before replacing alternator (belt alignment is correct)

Can someone point to a possible culprit here? Is it possible that something is causing the temp sending unit to freak out occassionally?

While replacing the thermostat, I found black and white wires that were bare, and twisted together. There was a crimp-style connector connected to one of the thermostat housing bolts. The wires come from the same conduit as the wires for the MAF sensor. I re-attached them to the connector. The overheating problem was present both before and after doing this.

I am also experiencing a fuel injection-related issue that is causing flooding at startup and idle, though this problem was present before my overheating issue arose.

Is this simply a case of a remarkable coincidence in having a water pump impeller fail immediately after the alternator was replaced? I'd really rather not have to go through the BS to replace that pump without exploring other possibilities first, but I'm frankly pretty stumped right now.

[old_master]

From your list, it sounds like you've covered all the bases. Time to go back to the drawing board for a minute. The two wires you found are for the computer temperature sending unit and computer ground, they need to be grounded to the engine block. Are you sure the belt is routed correctly? When you replaced the fan clutch, are you sure the fan was mounted correctly to the clutch? (It can be mounted backwards on the clutch.) Double check those items and post your results.

[m-theory]

//The two wires you found are for the computer temperature sending unit and computer ground, they need to be grounded to the engine block.//

Ok, those are good then. I hooked those back up to the connector that was bolted to one side of the thermostat housing. They were previously hanging loose.


//Are you sure the belt is routed correctly?//

That was my first thought. I checked, re-checked, and re-re-checked, and it appears correct. I'll check again tomorrow, though.



//When you replaced the fan clutch, are you sure the fan was mounted correctly to the clutch?//

Yes. I was careful to get that right, as I saw that it could go either way. The overheating was occuring both before and after I replaced that, however.

My kid's been driving this all week, to and from work. He said that the past few days, it's mostly hung around 215, only rarely popping up to the "warning" area (but never pegging out now), before falling back to 215. He said it just doesn't make sense to him, because it'll pop up just out of the blue, for no apparent reason, and then go back down on its own for no apparent reason.

I'll take a gander at the belt once more tomorrow. I wouldn't think it would even stay on if it weren't routed correctly, but it's definitely a spastic routing.

Thanks. I'll report back on the belt again tomorrow.

[brcidd]

does your bottom radiator hose have the spring inside-- that keeps it from collasping and shutting off the water flow while driving? they are designed that way for a reason......

[m-theory]

Ok, I've verified that the belt is routed correctly. The lower hose has no spring, but feels like a new hose, with no signs of weak spots anywhere. I can't see or feel any sign that it's been collapsing.

For the past week, this thing has been doing "ok." Not "great," but "ok." For about 95% of the time, it runs at a constant 215. Every now and then, it bumps up to 230 for about 20-30 seconds, then goes back down to 215 on its own.

Then, last night, my kid brought it home and said that it had overheated again. I checked things over again this morning and found nothing out of the ordinary. No leaks anywhere, oil level is fine and oil is normal, coolant level is good and coolant is clear.

Unless somebody can think of something else, I'm just going to go ahead and replace that water pump. I'll get a spring-loaded lower hose while I'm at it, just so I'm not kicking myself over that later.

[m-theory]

Somebody that my kid works with seems to think that this sounds like the radiator. Again, no leaks, coolant is clear, and I can see flow in the radiator with thermostat open and engine revving. To me, this simply cannot be the radiator, but I'm going to toss it out there nonetheless. Again, trying to avoid having to go the full monty on this shotgun repair.

[CD Smalley]

brcidd is dead on about the lower hose. It should have a spring it to keep it from collapsing. I'll bet if your watch you are only overheating when travelling at highway speeds. That's when it did it on your son correct? When you're doing that, the pump is pulling coolant under such pressure it is causing the hose to collapse....

[boatbuster]

When was the last time you had the rad and cooling system flushed, depending on the quality of coolant, you might have some build-up in the system. When you install the spring in the lower hose you might want to think about flushing the system at the same time.

[m-theory]

I appreciate the suggestions offered. I have to say though, that I don't believe the lower hose or radiator clogging is the issue.

The hose has no weak spots on it that would indicate collapsing, and the problem occurs and goes away completely randomly. If it were only happening at highway speeds, I could see that hose being a potential source of the problem, but that's just not the case.

The radiator appears clean and the coolant is clear, so I don't believe I've got a radiator blockage issue, either. Additionally, the problem is not consistent, as it would tend to be with a clogged radiator.

I've driven the van a few times myself over the past week, and, 95% of the time, it runs at either a steady 200 or 215, sometimes fluctuating between the two readings. Suddenly, with no apparent reason and no warning, it will climb to 260, and then, just as suddenly and also with no apparent reason, will drop back down again.

On rare occasions, it will jump to full peg out, again with no apparent reason or warning, and again, will drop back down just as suddenly. Nothing has to be done in order to get it to either peg out like this, or to come back down to normal. It pegs out from normal in mere seconds, stays there for 15-30 seconds, and drops back down to normal, without any changes in driving whatsoever.

I'm beginning to think that this problem is a bad sending unit, rather than a real overheating issue. I'm going to try replacing that first and see what happens. If that's not it, I'll dig back into these other possibilities.

Anybody happen to know where the temp sending unit is located on this vehicle?

[ericn1300]

Coolant Temperature Sensor location, testing procedure and R&R procedure here: http://www.autozone.com/az/cds/en_us...rInfoPages.htm