Datsun Roadster Forum-311s.org

67.5 1600. Flushing system after noticing slight rust color in coolant. Went to refill system, but it will only take 1 gallon. Ran engine with radiator cap removed in hopes of removing air from system but coolant level at the radiator cap remained full. While searching previous posts, several suggested that this may indicate a head gasket issue. A quick inspection shows no obvious signs of a head gasket problem but I am not an experienced mechanic. Can I get suggestions on how to diagnose the problem? Im hoping its something simple such as burping the system but I am concerned that I will overheat the engine by continuing to try to do so.

Hey Ken,
When you drained it, how much did you get out? If you only got a gallon out, you’ll only get a gallon back in.

Also, did you flush your heater core? … assuming you have one. Were the valves open?

If, after adding the one gallon of new antifreeze, the car runs fine and doesn’t overheat, you don’t have a big problem. Just reflush it again using the ideas you already found here and it’ll be fine. If however it’s now overheating, I would suspect a stuck (closed) thermostat or a plugged radiator core (very common with original equipment).

Also:
You can do a compression test to quickly check the condition of the head gasket, but usually you can tell right away by checking for a brown foam on the oil dipstick, oil sheen visible in the radiator coolant, sweet smelling white smoke coming from the exhaust pipe when engine is warmed up, or a heavily fouled (black tar like) coating on a spark plug - Any of these signal a blown head gasket.

I didn't pay attention to how much was drained out. The problem surfaced when I was bringing the engine up to operating temp to check the temp gauge after replacing the temp sender. The engine overheated and began blowing coolant past the radiator cap and the overflow reservoir cap. Thats when I noticed that the coolant needed to be flushed. The heater core was bypassed with the under hood valve being closed when I purchased the car. Since I live in an area thats always warm, ive never addressed it. In fact, I removed the heater core to more easily access some other under dash projects. Its on a shelf in my shop. Also, the car did not have a thermostat installed when I purchased it.Ive never bothered to replace it. I have checked the dipstick. No indication of contamination. No indication of oil in the coolant and no white smoke from exhaust. Spark plugs look normal. The whole timing of this problem seems odd. the car was running well. Suddenly its overheating with no obvious signs that the head gasket has failed. Very frustrating. I'll see if I can find a compression gauge to borrow or purchase one if need be. Any other suggestions on possible diagnostic tips? Id like to be sure what i'm dealing with before I start ordering parts.

Opps. Double posted

Well that info was helpful in eliminating a few possibilities … and creating a couple new ones … lol

Do you have a spare radiator cap you can try? If the cap fails and can’t keep the proper pressure, it may boil over. Stock caps are rated at 13 psi I believe.

Since you didn’t mention any coolant dripping from the water pump housing, I’m assuming your water pump is fine and turning freely - Double check that.

Let us know the results of the compression test. I’m still leaning towards a plugged rad though, because in my world, it’s never as easy as replacing a radiator cap …

i ordered new caps for radiator and reservoir today. pulling radiator to have it checked. eliminating possibilities before proceeding with head gasket. I have a compression gauge on its way.

In the mean time, pull the plugs and compare them. If it's burning coolant the plug will look different (likely cleaner). I agree with your step by step!

It's best to fill with the car frt of it 3 ft higher than the rear and use a high sealing funnel

BWK2000, You were spot on in your advice to check the radiator. It was in terrible condition. Local radiator shop went through it and brought it back to life. After reinstalling it and filling it with the proper ratio of antifreeze, the troublesome bubbles that caused my initial concern about the head gasket quickly dissipated and then stopped as the system cleared the trapped air. Just for peace of mind I had a simple compression test done this morning and all cylinders were around 140 lbs. Still nothing on the dipstick or on the plugs so my panic was unwarranted. As for my initial concern that the system was only taking one gallon to fill it, I wasn't taking into consideration that the heater core valve was shut or that not all of the fluid is removed from the system by opening the petcock, even though I did jack up the rear of the car. As an added precaution, I purchased an infrared temp gun to check the engine temp as my temp gauge always pegs hot after about 5 minutes. running at around 180 (F). Now to figure out the innaccurate temp gauge. Replaced sending unit, checked and cleaned all contacts, and made sure gauge voltage regulator had a good ground. Still pegging hot. The search for a solution continues.

Good job finding the cooling problem... I'm in the same boat with the temp gauge running hot on my 69 U20. Changed to a brand new sender, still runs right up to the red during normal operation, IR gun says 180-190 around the temp sensor and water pump area... gauge ground *seems* ok, but would need to pull to check.

bajaroadster wrote: Tue Nov 12, 2024 2:51 pm … Just for peace of mind I had a simple compression test done this morning and all cylinders were around 140 lbs …

Ken;
Glad the cooling issue was a relatively easy fix … Makes for a happy day!

As for the compression, the ‘plus’ is that they’re all within 10% of each other (that’s very good), but, those number do seem a bit low.

Question: Did you remember to open the throttle all the way when taking the readings? I would expect the numbers to be about 10-20 psi higher. It also makes a difference (lower numbers) if the engine not warmed up before hand.

I did not do all the recommended pre check set ups before checking compression, thus my description of "simple compression test". I was not necessarily looking for a completely accurate compression reading, just some added assurance that I did not have a head gasket issue. The throttle was not all the way open and the engine had not made it all the way to operating temp.

Everything is likely fine then. I never chase a problem that isn’t there … for long

Good luck with the temp sensor/gauge gremlins … Don’t pull all your hair out!