overheating?

[joshkbomb]

We need more info to help.

> What year is your truck?
> Is it TBI or carb'd?
>When did you last replace the hoses?
>Have you done a compression test to confirm you don't have a blown head gasket?

I would like to ask something very important: How much have you spent (time & money) trying to fix this by replacing parts and not actually testing them first? You know, pop the thermostat out and drop it in a pot of boiling water to see if it opens? If it did (you can't tell now, of course) then that was not the problem. Then you would know to move on to the next possible problem.

I suspect you can buy the GM shop manual and learn the actual steps to properly diagnose this for less than what you have already spent.

This manual might be what you need, for a 1991, and can save you a lot of money, teach you some great repair techniques, and solve your problem quickly.

I originally just took it to the mechanic because I didn't want to mess with it. They did the thermostat (which was stuck closed). I did the fan clutch which they said was marginal. The only thing they did that was not needed was the temperature sending unit. I'm not sure if the gauge is bad yet.

I have all the GM service manuals and wiring diagrams for the truck. Some times it's helpful, and sometimes not. For example, the 90 ohm testing thing for the gauge is not in the service manual. It says to just use some specialized tool.

Like I said before it's an '87 (so TBI) and I've checked the hoses for getting sucked shut and didn't see it happening. I don't remember how old they are though. No compression test. But wouldn't a blown head gasket produce other signs like coolant in the oil or exhaust in the coolant (I don't have either)?

I'll agree that my next step (after verifying that the gauge is good, which it sounds like my new one isn't) is to pull the new thermostat and verify it.

I do think that the Blazer's days of being my only car and putting 15k miles a year on it are drawing to a close though (it just rolled 200k miles with 50k more on the engine). I guess I can't complain about 9 years without a car payment though...

 

[joshkbomb]

The 90 ohm jumper should make the gauge read full scale. That is "accurately".

I used the jumper from the back of the temp gauge on the post that goes to the sending unit while it was plugged in and it still only went to half. I think this means there's nothing wrong with the wiring between the sending unit and the gauge.

So it sounds like both gauges are bad. Wonderful.

 

[Jessie James]

Like I said before it's an '87 (so TBI)

Ah, I must have missed that.

and I've checked the hoses for getting sucked shut and didn't see it happening.

I may be wrong, but I was under the impression that this was hard to test, other than being under load and at speed, at which point you can't really see what is going on.

I don't remember how old they are though.

If you can easily compress the hose with your hand they may be in need of replacement. Especially if you cannot remember when they were replaced, or if there are soft spots. New hoses are quite firm.

No compression test. But wouldn't a blown head gasket produce other signs like coolant in the oil or exhaust in the coolant (I don't have either)?

Usually, yes, but not always. I've had problems with cars overheating with a blown headgasket and the only symptom was overheating. To compound the issue, when the coolant heated up beyond the usual range and started boiling, it would spill/leak out and I wouldn't be able to check what was in it very easily. Now, the cars I had problems with were very prone to bad head gaskets, and the Chevy 350's are not nearly as susceptible, so you may want to put this in the back of your mind for a while.

I'll agree that my next step (after verifying that the gauge is good, which it sounds like my new one isn't) is to pull the new thermostat and verify it.

It sounds like the gauge is bad, based on your other post, but that just surprises me.

I do think that the Blazer's days of being my only car and putting 15k miles a year on it are drawing to a close though (it just rolled 200k miles with 50k more on the engine). I guess I can't complain about 9 years without a car payment though...

Meh, I have 235k on my Blazer, and it still goes just fine. Don't fool yourself into a new car payment, unless you really want to!

 

[joshkbomb]

So I go to start it up this morning and now its throwing a code 42. Looks like I did something I shouldn't have trouble shooting the gauge last night. I can't get a break!

I unplugged the battery waited a minute and plugged it back in, and the code came back.

 

[British Slave]

Check the timing disconnect wire. Next to the brake booster, Tan and black single wire.

 

[joshkbomb]

Check the timing disconnect wire. Next to the brake booster, Tan and black single wire.

It was disconnected. I'm at work so I don't have time to clear the code, but I'll do it tonight. Thanks!

Now back to your regularly scheduled overheating/gauge problems...

 

[British Slave]

Re-reading all the posts, something just doesn't add up. It kinda sounds like low coolant or a restricted coolant flow. When you say at higher RPM's it gets hot, then cools off in a short time when RPM's reduced, that makes me think it is something causing poor coolant flow. If you have an AC thermometer, stick it in the radiator fins and see if it really is getting that hot.

 

[joshkbomb]

Re-reading all the posts, something just doesn't add up. It kinda sounds like low coolant or a restricted coolant flow. When you say at higher RPM's it gets hot, then cools off in a short time when RPM's reduced, that makes me think it is something causing poor coolant flow. If you have an AC thermometer, stick it in the radiator fins and see if it really is getting that hot.

I don't have an AC thermometer, but I was planning on getting one of those cool infrared thermometers tonight to do just this sort of testing (to find out if it's actually overheating too). Hopefully it will work through the spinning fan, the A/C condenser is in front of the radiator.

 

[British Slave]

Take the temp at the thermostat housing.

 

[British Slave]

Take the temp at the thermostat housing. You can actually see a difference across the thermostat withthe infrared.

 

[dyeager535]

So I go to start it up this morning and now its throwing a code 42. Looks like I did something I shouldn't have trouble shooting the gauge last night. I can't get a break!

I unplugged the battery waited a minute and plugged it back in, and the code came back.

Pulling the ECM fuse will clear the code BTW. In the time that it takes to pull the fuse (or batt cable, or ECM connector, etc) and reconnect it, the code will have cleared. Much less than 10 seconds in my experience. No need to be standing around waiting instead of working!

 

[joshkbomb]

So I got the infrared thermometer and was taking temps when the gauge was reading 260+ . The thermostat housing never got above 201. That was the hottest I could find except for down near the exhaust manifold. I can't figure out how to measure the radiator except for along the side. It starts at the top near the hose in the upper 190's and drops to 125 by the bottom.

Today after picking up the thermometer and starting the truck up (so it was hot a few minutes before) I noticed a sloshing sound coming from the vicinity of the heater core. I popped the hood and checked temps and saw that the thermostat housing was in the lower 190's, but the upper radiator hose dropped quickly along its length to 125 at the radiator. I squeezed the hose and felt the fluid going along in spurts and sloshes. I could have just caught it right when the thermostat was opening though since I didn't notice as much sloshing in the hose later on when I stopped to measure temps half way home.

The sloshing is making me think I have air in the system though.

 

[British Slave]

LOL That's why I kept asking if the radiator was full.

 

[clandr1]

Does anyone have the correct procedure/method for burping the coolant system? I'm at work and my service manual is at home.

 

[Jessie James]

Burping? I'm not the brightest guy but I've never heard of that.

I just fill up the radiator, and fill the overflow tank, and call it a day.

Now, of course, my question is why do you have air in your system? It makes me go back to my blown head gasket question... That is the only way I've ever had any appreciable amount of air in my cooling system.

 

[British Slave]

Basically what JJ said. Check it when it is stone cold, fill the radiator full to the top, add coolant to the overflow, and drive it till it's normal operating temp. Then check it again when it's stone cold. And you MUST keep the level in the overflow at the recommended mark, MUST!

 

[clandr1]

Basically what JJ said. Check it when it is stone cold, fill the radiator full to the top, add coolant to the overflow, and drive it till it's normal operating temp. Then check it again when it's stone cold. And you MUST keep the level in the overflow at the recommended mark, MUST!

Good deal, that's exactly what I did when I replaced my upper radiator hose this past weekend. I assumed the system would pressurize when hot and force the air out via the overflow tank and suck in the coolant when it cooled down.

Thanks for confirming.

 

[joshkbomb]

Long post, sorry.

This morning I took the radiator cap off and saw that the radiator was full and the overflow tank was at the cold level. I decided to purge the system anyway so started it up with the cap off and left it for a bit. While I was gone it warmed up to operating temp and proceded to puke out a third of the radiator worth of coolant. I filled it back up and started it again and it seemed to pulse the water out in spurts. I thought the water pump supplied a steady stream, why would it be pulsing?

The good news is that after cleaning up the mess, the truck didn't go above 210 (according to the gauge) the entire 20 minute drive to work in near 80 degree weather. So this brings up some questions/observations/lessons:

1. Why was there air in the system to begin with? And why didn't my mechanic notice it or purge it properly when he did the thermostat?

2. The only way I can think of air appearing is if there is a head gasket leak. I need to find this out or it's going to eat at my brain. So, I need to do a compression test? I've never done one, is it hard and is the tool expensive?

3. This means that my truck really was hitting 260+ every day for the past week? If that's the case, then why did the thermostat housing never go higher than 200 degrees? Shouldn't the truck have done bad things like puking steam or something if this was really happening? Where should I be measuring to get the real temperature?

4. I was doing all this with the old gauge, which seems to be working now. I hope LMC is going to take back the new gauge so I can get my $50 back. Unless anyone here wants it.

5. I need to start working on this truck myself instead of taking it to the mechanic so that I can fix it right. Which means I need another vehicle so that I don't have to stress out about getting it working to get to work the next day (the main reason I haven't done much work on it since college) and can enjoy myself.

6. Don't leave the truck running in the garage with the radiator cap off or you'll have to clean up a big mess.

 

[British Slave]

It was puking because of the thermostat opening , then closing when cooler water hit it. That's normal.

 

[British Slave]

I really don't think you have a head gasket issue, I'm betting you fixed the gauge without knowing it. Unplugging and plugging wires, unplugging the gauge, checking the nuts on the gauge I think probably fixed the gauge. If you were getting that hot, you would have had other systoms.