Sudden high temp on gauge.

[Timebender]

I had the oddest thing happen today while out. I had taken the car to go get some groceries, and the engine heated up to it's normal 220 on the gauge (which in reality is more around 185 according to a laser reading). For those that don't know already, I have a mid-80's 350 Chevy engine from a Vette.
So, like normal, while driving it ran just a tad below that mark, and at any stoplights it runs at or just a bit above it - and of course the fans, well, one big fan comes on - the smaller one had been out a while and I disconnected it so it wouldn't keep tripping the fan breaker. But even on hot days, the big fan always does a decent job of keeping things in check.

So after hitting Target, I run over to a Carl's Jr and after ordering, I'm waiting in line - and the temp looks fine. As I pull up to the window, after being in line about 10 minutes, I notice the gauge has jumped to just below the red. It seemed like it just climbed rapidly.

So I shut off the engine while waiting at the window for my order, about a couple minutes, then fire it up, and now the temp is down around the 200 mark. As I started driving, it went back to 220, and while at a light for a few minutes, it stayed there.

When I got home I let the car idle in the driveway for another 5 minutes and it didn't happen.

Any ideas?

Bad otterstat not turning on the fan and being intermittent? Or? I'm pretty sure i heard the fan on when it started to climb on the gauge as I had my door open when in the line. Maybe just a wonky gauge anomaly?

Maybe it was from being behind another car pretty close in line and the heat from their exhaust was causing an issue?

[DMC5180]

If the gauge sensor wire were to Go intermittent open the needle would point straight up 12:00.

I'm sure your fan is loud enough that you would hear it. If you say you could hear it run. Then I would lean towards a stuck thermostat or an air pocket in the system that found its way to the sender. I don't know where the sender is usually located on a 350.

I assume you've checked your coolant level.

[Timebender]

Possibly an air pocket - but I'm sure I bled it a while ago after putting in a new thermostat, but one can never be too sure. The sensor is on the side of the block above close to the spark plugs.

And I checked the fluid when I got home to make sure. But I'm thinking, and it might be slightly possible (?) that when in line, less than a foot away from the cat in front of me, the fan was pulling in hot air.. ?

As it seemed after that and when I left, all was back to normal. For now.

Meanwhile I'll be keeping an even closer eye on it.

[Timebender]

So I tried several things - put more coolant in, no luck Replaced the thermostat, and at first it did get liquid to the otterstat, but the fans still wouldn't come on until 220 - and so I let the engine cool off for a while, removed the thermostat housing plug to see if the thermostat was working, and it was, but not a lot of liquid would come up from the engine. So I put it back on, let the engine cool down again, put more coolant and water in the radiator, tried again, released the plug a little to let any air out- it did for a while, engine got really hot, fans didn't come on till 230 now.
So I left the engine off to let out any more air from the plug.
Replaced the otterstat, tried again, and noticed the hose above the thermostat was getting hot, but it was cool in the otterstat pipe. So I took off the hose and checked for blockage - none. Took out the thermostat and then took the plug out to watch, and this time the water just stayed in the engine - never came up and out.

I'm guessing it's a bad pump - it does spin on the outside just fine, but typically a bad water pump would leak before it just stops.. unless the impeller inside just came loose.

Thoughts?

[opethmike]

Shot in the dark - Take the cap off the coolant bottle and watch for if the coolant circulates?

[DMCMW Dave]

Shot in the dark - Take the cap off the coolant bottle and watch for if the coolant circulates?

That's exactly the test. Take the cap off (cold), start the engine, and look inside the bottle where the bleed hose attaches. You should be able to see water spraying out of the hose. With some design metal bottles you can't see it, but you can stick your finger in there and feel it (not to hot when you first start the car, but DO NOT do this if it's been running a while.

[Ron]

+1 on thermostat

The sensor is on the side of the block above close to the spark plugs.

It's probably not the problem, but the fan switch should be in the head, not on the side of the block. The correct port is between cylinders 6 & 8, a little higher than the spark plugs. (If the heads have been swapped, it's between 1 & 3 and OK.)
Check that a heavy duty connector (like OEM, usually has dark brown holder) is being used in any case, especially if it has headers!

[Bitsyncmaster]

If the gauge sensor wire were to Go intermittent open the needle would point straight up 12:00.

My testing shows an open the gauge would not move off the normal cold engine position. If the sensor or wire shorts to ground the gauge would point almost strait up. The cold sensor is normally about 2.5 Kohm and normal engine temp about 100 ohms.

[Jonathan]

I would run some tests under conditions you choose to figure it out. Or eliminate things anyway. Intermittent problems aren't always easy to find.

First test I would do would be to run the engine and let it get up to temperature. Do this parked in your driveway, somewhere flat and with room around the car for air to flow. Confirm the cooling fans come on as they should and watch to see the temperature come back down and the fans shut off. Let it idle some more, fans kick on, engine cools down, repeat. That would test the cooling system for normal conditions.

Next test I would do would be to try the same thing but this time try and limit the source of fresh air. Maybe do it while you're parked in the garage and the front of the car is up in there without a lot of fresh air circulating. The car should still be able to cool itself down like this.

After that, you might go looking to try the same drive thru test you did inadvertently the first time. Or you could rig something up at home to test it like that while you control the situation and won't fear a big mess (or circus). You could use another vehicle you have to be running right in front of your D and simulate the drive thru overheating or lack of fresh, cool air. See what happens and shut it off whenever you get past a point that makes you nervous.

The nice thing about running the first two tests is that you'll have seen what the cooling system does under normal conditions and will know exactly where on the gauge the needle points when the fans kick on or off. I don't doubt that trying to cool the car down using hot air from the car in front of you caused the problem. The rad is just a heat exchanger in the simplest terms and if the air on the side that is supposed to be cooler isn't, the hot coolant on the car side doesn't have anywhere to give off that heat.

Check the coolant levels of course beforehand, like mentioned. An air pocket is a possibility too and those first tests will help rule that out for you. I know your systems are modified slightly with the engine swap, but testing it without jumpers and as close to being in real conditions is the best way to go IMO. It at least lets you see exactly what happens with your car in the conditions you'll be using it. Jumpers can help in pin pointing the issue to an otterstat for example, but seeing the car perform (or fail) "as is" is the best test to put the issue to rest.

[DMC-81]

If you find that you are not able to repeat the issue, I bet the hot car in front of you overwhelmed the cooling capacity.
I'd replace /fix the smaller fan as perhaps that one helps the system perform during situations like the one you had.

I remember my V8 Fiero had a similar issue in drive thru lanes and long traffic lights. I think I rigged up a switch to turn on the stock Fiero fans manually for those occasions as I was nervous about overheating the car. Of course, a better solution would have been to get a bigger radiator/ fans. But it worked and I never overheated it.