I'm working on a friend's '88, his low coolant light was coming on at random. I replaced the sensor but it is still doing it. What is the next step? Coolant level is NOT low.

 

If it isn't the wiring to it, then you might still have air pockets in the system. This happened to me.

I had to "pack" my radiator a few times till all the air in the system was purged out, before it would stop.
I did this once a day on a cold engine over the course of about 2-3 days till I couldn't add anymore.

 

Yep, I suspect packing too. Even though the radiator may look full with the engine off or before the stat opens, it may not be.

Jake

 

Exactly. Mine looked full, but the light would intermittently come on while driving when the level in the radiator would drop. I knew it wasn't the sensor or the wiring by testing.

 

We can try packing but he lives 16 miles away so the car was completely warmed up when he got here and we only lost just a little coolant when I changed the sensor. When we started it back up I let the coolant flow for a bit then had him rev it up while I topped off the rad then also topped off his recovery bottle. So he should do the packing when the engine is cold then?

 

No the stat has to be open. Once it's open, hold the throttle to 2000 rpm, add coolant, replace rad cap, then release throttle.

 

It helps to park uphill or raise the front of the car some. This makes the radiator the hight point, since these early cars don't have purge valves. I was able to do it cold since my stat has a few holes drilled in it. You'll know when the stat is open by squeezing the upper hose with a rag around your hand. If it's hard, the stat is closed.

 

Upper hoses came with springs too? I thought only the lower hose needed it since that's the inlet to the motor. I'm missing both anyway. I could not fit the stock spring into a fresh new lower hose so I left it out. No issues.

 

Yes. I removed the factory hoses from my 89 and the upper hose had a spring. But for some reason, the lower hose didn't. I got a new GM upper hose and it had a spring. I got a new GM lower hose, and it didn't have a spring in it for some reason. Backwards. But I put the spring from the OLD upper GM hose into the NEW lower GM hose.

Can't believe those hoses lasted 20 years. Probably would have lasted a few more. But I had looked at Gates and Goodyear hoses. Gates looked horrid with white splotches all over it, and it was thin. Goodyear had bulges on it.

I think GM hoses look the best, and they are the thickest I've seen.

I also got specialty hose clamps. The kind that puts pressure evenly on the hose, and it doesn't have any slits in it to where the hose can sqeeze through. They also are stainless so they won't rust compared to the cheap Autozone type hose clamps I had pulled off.

 

I appreciate all the help but I can't help but think it may be something electrical. I don't want to sound like a smartazz but I'm no stranger to filling Vette radiators. I've had mine since '99 and have never had a problem. However, I am always open to suggestions so we will look at the packing process again. Thanks to all.

 

A member of my golf course has an 86 that had an intermittent light, for over a year. He took it to the previous owner, who has a repair shop. Each time he took it to him, the rad was full and the switch tested good, so after some research, he told him he needed to replace the low coolant module.
I told him that it needs packing, so a few days later, I stopped by his work with a jug of anti-freeze and demanded his keys. I packed it and returned his keys.
It's been a year and no low coolant light has appeared.

 

BTW, the low coolant sensor is just a thin blade of metal. Essentially, it provides a ground for the transistor that powers the light.
That's it, it's just a thin blade of tin.
If the radiator isn't sucking the coolant back in on the cool down, you have a leak between the radiator and the reservoir, a bad cap, or an intake manifold leak, hose leak, or some other way the 1-3 LBS of vacuum under the cap is escaping, and not drawing the coolant back into the radiator.
Be sure to check all over for heavy coolant residue somewhere, even under the car, say at the oil filter cooler, and other low lying places.
The clue that you have a cooling system vacuum leak is the reservoir fills up little bit at a time over a period of a week.
It won't spill it, the coolant is too dense to go through the vacuum leak, but when the pressure gets to zero and then turns negative in the cooling system, the slight vacuum leak prevents the coolant from returning back where it belongs.

 

Dan, you give some of the best advice available on this forum and I certainly don't plan on ignoring it. Coupeguy, I am positive there is no leaks, the engine underhood area is dry as a bone for which I am thankful since I just rebuilt his engine. Also there is no coolant loss whatsoever.
After I packed the rad the last time his light came on once right after he left my place but went off right away and never came back on and he drove it a considerable distance afterwards. If it does happen to come back on again I will try packing it again. How about this, could the vacuum be getting past the o-rings on the sensor without any coolant loss? We replaced his sensor with a good used one I had so maybe we need to put new o-rings on the sender??

 

Some guys like to drill a small hole in the stat to help with getting rid of air in the system. Some also say this helps to heat the coolant in the radiator, so when the stat opens up. Cold coolant doesn't hit the hot block.

 

It better be right?

 

If the "o" rings are old, and conformed to the old radiator, yea, they can weep a little air. There are no substitutes for fresh seals.
If the coolant light comes on just after the thermostat opens, it needs packing. You will know because you should know if it is a 170, 180, 0r 195 degree thermostat that was installed, and that's the temp that the light flickers on.
This indicates a settling of the coolant from the weight of the water, the water pump pressure, and how much coolant that is expelled from the radiator through the cap. which will create the vacuum. The void is large enough that some hot spots are boiling the coolant, and expelling steam to the coolant reservoir which will counteract the return of fluid due to excessive air volume in the radiator.
The other scenario is when cruising, deciding to accelerate moderately, and after you begin accelerating, the light comes on. This indicates a large air void distributed in the system, and higher RPM causes the water pump to pack the engine, and the air ends up in the radiator, and the light comes on.
The hose between the cap and the reservoir may be the original, and can be cracked. Mine was on the '86, and after watching the reservoir fill up after a few days, and seeing the low coolant light come on, I finally went and got a new hose, and when I removed the old one, the area under the radiator end clamp was cracked with a hairline crack, letting in air.
Usually, the succession of cooling and heating cycles over a short period, maybe 3 days will pull enough coolant in so the light stays off.
If it doesn't, pack it once more, and check hoses. Sometimes, heating and cooling cycles allow for the new hoses to conform to what they are clamped to, and the hose clamps can loosen from the rubber compacting around radiator outlets, thermostat housings, etc.

 

Here are some good Packing guidelines. When I last had this problem I had a very small pin hole way underneath of a remote heater hose. Took me a while to finally track it down, but the leak was there.

It also helps to have your Vette on an incline when you are packing the radiator.

 

I think we may have gotten it the last time we packed it, he drove it over 100 miles without so much as a flicker. We'll see what happens the next time he drives it which may be a while since we are to be blessed again with the white ****. Thanks to ALL who answered.

 

Great news. Hope you finally nailed it.

Jake

 

Sorry to be a novice but what is packing the radiator? Are you removing airpockets?