DEBrahms

I spent a long time last night reading a ton of forum threads about overheating and the issue of accurate gauge readings vs. actual engine/coolant temp. I've been nervously watching my '58's temp gauge needle climb in slow traffic and make seemingly improbable shifts, so I decided to ask a C5 buddy of mine to help me verify what's happening.

He's got this expensive, highly accurate, digital temp probe that he uses to, among other things, check his tire temps. He's a racer. Anyway, I stopped at his house on my way back from a cruise and sat idling in his driveway while I went to find him. He got out his probe and told me some war stories about how accurate it is. Meanwhile, my '58 (270 Duntov) sat idling for about five minutes.

We popped the hood and I asked him to take readings directly on the thermostat housing, at the base of the temp sending unit, and on the top rad hose just above the thermostat housing. All three readings were 167 degrees. My temp gauge was halfway between 180 (12 o'clock) and 240 (about 2 o'clock). My ballpark guess is that this indicated ~210 degrees.

So, you can see the difference between the gauge reading and the actual temp readings. I had assumed that I had a 180-degree thermostat, but now I'm wondering if it's a 160. What do you guys think?

Obviously, the problem is either the gauge or (more likely) the sending unit. Should I fault-isolate this or just leave well enough alone?

I'm running a stock rad with a six-blade fan spacered out closer to the rad. I've driven this car over 6,000 miles in two years and have had no cooling issues other than this squirrely gauge.

Oh, and while I have you all on the line, here's a related question. I have no expansion tank (or "supply tank," as it's called in the repair manual). It appears as though the hose that would have connected to the expansion tank just runs down the side of the rad and ends just above the rad shroud mounting flange. I see no evidence of puking. Any negative consequences for the lack of an expansion tank?

Thanks very much, guys.

Dave

Frankie the Fink

The point of the expansion tank is so that on the occasion your radiator fluid does expand (e.g. puke) it goes into the tank and is sucked back into the radiator when things cool down. Otherwise, if your car spews some antifreeze at times you are constantly adding fluid to the radiator.

As to the C1 temp sending units - they are notoriously inaccurate...I have heard as much as 30* in some cases. A Wells TU-5 is supposed to be one of the better ones.

Lots of posts on here about testing (ohming out) the sending unit and comparing it to original unit specs to try to get one that's close...if you can't find the info I can dig it up.

NSC5

Assuming any piece of test gear is (and remains) accurate is a mistake. A thermometer is one of the easiest pieces of gear to test because you have the "standard" as close as your kitchen stove. Distilled water boils at 212 at sea level, you can easily calculate the difference for your altitude. For temperature measurement I use a Fluke twin thermocouple probe with a separate infrared "gun" but like my Tektronix scopes, meters, etc. it and its various thermocouple probes get an annual calibration check. Rant mode off about relying on a piece of test gear just because it is expensive or was properly calibrated when new

Frankie the Fink

I worked with and calibrated avionics test equipment for more years than I care to recount. All you say is true but these cars are not F-14s and a temp reading doesn't have to be accurate to 3 decimal places. My $23 Harbor Freight IR temp gun has served me well for quite a few years now.

To the OP - you could easily have a 170 degree thermostat...they do exist. I'd have to fix the problem personally or I would paranoid every time I drove in hot weather.

NSC5

Frankie, I think we are in agreement. You certainly don't need accuracy to three decimal places for this task but I have seen more than one case of someone relying upon a very expensive piece of test gear that had a failure causing the reading to be more than 25% away from reality. I didn't want the OP to head down the wrong road because his friend was in love with a particular thermometer.

Your $23 HF temp gun is no doubt more accurate than a defective high end unit and a lot of the HF stuff is surprisingly good. I have a couple of cheap HF DMMs that I use for outside work (and will loan to friends on occasion). Both track within 4% on all AC and DC ranges of my highly accurate lab grade Hewlett Packard meter. And if I drop a $19 HF DMM while working from an antenna tower or a friend lets it sit on a hot exhaust manifold it won't bother me much

There are times when very high accuracy is needed but not often. But it is a good idea to check the gross accuracy of any piece of test gear from time to time, especially if the reading seems counter-intuitive.

62Jeff

Check your temp sending unit for the Ohms at indicated temps
http://www.lbfun.com/warehouse/tech_...g%20gauges.pdf

Review this because it's interesting and good info:
http://www.lbfun.com/warehouse/tech_...eCF-Thread.pdf

Chances are your issue is a temp sending unit.

Frankie the Fink

toms silver 60

I'd be somewhat concerned with your temp gauge because you don't have a lot of gauge room to play with. My 60 gauge routinely reads near the 120? mark, but if it climbs up towards the 180 point, I am definitely watching it very closely. A loose fan belt would typically be the cause if it got there at cruise. A stainless flex fan goes a long way towards cooling off in traffic.

I had my radiator frame rewelded due to corrosion, probably due to the overflow discharging onto it and not draining. I have since added a few more drain holes.

Overall, if your car is not puking, you're probably doing ok. Car will typically do some overflowing after you refill radiator until it equalizes, but after that there should be zip....

DEBrahms

Excellent advice guys, as always. I'm on the case!

Thanks again.

Dave

JohnZ

You have the classic/chronic problem of an inaccurate sending unit, especially if it's been replaced in the past; chances are your gauge is just fine.

Your '58 never had an expansion/supply tank, and doesn't need one - it's built into the radiator; the OEM stacked-plate aluminum radiators used after 1959 didn't have integral tanks like the previous copper/brass radiators had, so they had a separate expansion/supply tank.

You don't want the fan spacered-out close to the radiator - that does nothing for airflow, and in fact reduces it. The ends of the fan blades should be half-in/half-out of the ring formed by the rear edge of the fan shroud for maximum idle/low-speed fan effectiveness.

DEBrahms

John, thank you very much for that insight regarding spacers. A gentleman on the NCRS board just mentioned the same thing. I'll check my fan alignment.

Also, as I mentioned in response to him, the pictures and text in my '58 Corvette maintenance manual show and mention the tank, but the engine pictured is a 327, which is odd, I think, in a '58 manual.

Dave

JohnZ

That's because the ST-12 manual was printed in 1962, and covers all Corvettes from '53-'62, and the 62's were all 327's.

Pilot Dan

John hit the nail square on the head, but just to augment, I had a "temp issue" after replacing my intake with an identical stock one but date correct. Got out the IR gun and ACTUAL temps were still fine. Best I can say is sending unit (formerly calibrated) did not like being R&R'd and now reads a bit higher now than actual. Gonna have to replace it AGAIN to get accurate readings. I think that is also your problem. To confirm drive around a bit and idle and have an IR gun in hand as I did. Pilot Dan

dplotkin

I put a Wells TU-5 in my 65 396. It indicated 160 at a confirmed 180 running temp. At least its consistant.

Dan

Frankie the Fink

It's not just C1s....the crappy SunPro aftermarket gauge set in my '67 Chevelle showed temp readings near 210 when the I/R gun confirmed 180. Replaced the gauges with nice AutoMeter stuff and now they read quite close to the I/R gun temp.

You think a civilization with the technology to come up with smart phones and American Idol could make a device to read water temperature accurately for Pete's sake.

cbernhardt

I tried several TU5 sending units on my wife's '62 and none were very accurate. The car has a Ram Jet 350 engine and normally runs around 190°. I solved the problem by placing a variable resistor in line with the wire from the sender to the gauge and "tuning" it so the gauge read correctly when the engine temp was 190.

Charles

Frankie the Fink

That is often what folks do. My SeaRay BowRider boat temp gauge was way off and SeaRay's recommended fix was to do the same trick with the resistor. Of course the results are non-linear at any temp besides the one the resistor was selected for.

DEBrahms

Now my rad cap is stuck. I think the cap's rubber gasket is bonded at the sealing point. I can turn the cap and get the notches to align for release, but it won't pull off. I don't want to pry it for fear of damaging the filler neck. The rad is OEM copper.

Ever had this problem? Any tips on how to unstick that gasket without ruining the cap/rad?

Thanks.

Dave