Coolant loss, slow

2007 Chevy Silverado Classic 1500 LT, 5.3 Flex fuel, 4x4. 225k miles

I’ve noticed a slow coolant loss since I bought this truck 2 years ago, about a quart between oil changes at 3k intervals. About 6 months ago, one of the QDs (quick disconnects) broke off the heater core and I thought that must have been the problem. It appears it wasn’t.

When the QD broke, I replace all the hoses, QDs, Y’s, water pump and thermostat. Still losing coolant at the same or slightly higher rate. I was worried that it might be a head gasket, and since I don’t have a maintenance history on the vehicle before I bought it, I decided to go ahead and replace the spark plugs and see if there is any evidence of coolant in the cylinders. I don’t see any but looking for a second opinion.

Spark plugs are in order as viewed from the drivers side of the engine.

I get a LOT of condensate draining from the AC so the heater core is my next suspect. I don’t see any coolant in it but it would be very diluted. Can’t find evidence of leaks anywhere else.

I can’t tell anything definitive by the plugs, myself. Make sure the radiator isn’t leaking around the end tanks (where the plastic meets the alum). I’ve had those leak slightly for a pretty good while before I found the leak. GM did had some problems with certain head castings cracking. Would probably be evident by removing the valve cover.

You might consider bypassing the heater core for a while and watching the coolant level. If the level still drops with it bypassed, you don’t have to tear the dash apart to get to the heater core.

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Top row 3rd from left, looks a little cleaner/whiter, possibly being steam cleaned by small leak.


I saw that too. Looks cleaner, but I wouldn’t want to change the gasket just yet because of it.

I am going to assume you also changed the radiator cap?

I see the difference in the plugs, but not sure it would be from the coolant.

How about pressure testing it. You can borrow a tester from Autozone and probably other stores.

For as small of leak he has, Head gasket would be on bottom of list seeing it’s been going on for 2 years and not getting worse.

Radiator cap, or even small leak, hairline crack in overflow reservoir.


Plus some more characters.

A weep hole in my water pump broke, but it leaked so slowly and only when I was moving, so it didn’t leave a puddle when I parked and whatever leaked evaporated before I could see a trail on the pavement. I was moving cross-country at the time so just kept on putting in more coolant and stop-leak until I had time to look more closely. '87 Toyota pickup.

Water pumps on this model have a history of leaking starting at around 250k miles so that is why I did a preemptive replacement. It’s a new AC Delco pump that came with a new thermostat installed.

It doesn’t have a radiator cap, just a reservoir cap.

If I fill past the parting line, the reservoir quickly goes down to just below the parting line, then slowly drops after that. I don’t see any stains outside the reservoir. I use Dexcool and it is notorious for staining. When I bought my 97 Nissan truck new, it lost about the same amount of coolant from new. When the first coolant change was due I used Dexcool. Within a month, I found an orange trail from the thermostat cover (gooseneck). I pulled the radiator hose and found a cold shut in the casting. Got a new one and never lost a drop of coolant after that.

I checked the radiator end cap seals and so far haven’t found any evidence of a leak there either.

I think I’m going to get some coolant dye that is compatible with Dexcool, they sell it on Amazon.

Try replacing the pressure cap.


OK I’ll try that, can’t hoyt.

no coolant smell inside the cab . . . ?

There should be a coolant transfer pipe on the front of the engine, plus 2 similar “plugs” at the back of the engine. I’m mentioning the plugs because left and right heads are identical, and each end has the hole for the transfer pipe. One end has the transfer pipe, the other has plugs insead. The seals commonly leak after several years.

I’ve seen the coolant reservoir develop hairline cracks on the bottom . . . very hard to see without a mirror and a good light

But it’s usually the radiator end tank . . . as already mentioned. Can be darn hard to see, because the fan shroud could make it hard to see the leak

One final question . . . did you use a coolant pressure tester?

No I haven’t.

@Tester I did buy a new reservoir cap but it will take a month or so to see if it helps.

I’ll keep an eye out and if the level goes down again, I will use a pressure tester on it.

A pretty good check for a head gasket problem is to get a helper to follow behind you in another car as you accelerate robustly up a freeway on ramp. Any signs, even small ones, of visible white clouds or steam out the tailpipe suggests a head gasket leak. For larger head gasket leaks you can often see bubbles coming up in the coolant when you look into the radiator at idle. May have to rev the engine to see the bubbles, depends.

To disprove the heater core theory, just bypass it. Figure out a way to connect the two pipes together, the ones that normally pass through the firewall to the heater core. You don’t need heat during the summer anyway, right? If the coolant loss goes away you’ve discovered where. I’m thinking this is the most likely culprit.

One thing you got going for you, one quart of coolant loss every 3,000 miles is a pretty big leak, so it should be pretty easy to find.

Without coolant exhaust gas analyzer I would suspect leaking intake or head gasket. Heater cores will generally leave spot under carpet on passenger side or you will see a film on windshield after running defrost. Should be able to smell coolant thru vent.

Pretty sure these trucks have a dry intake manifold with no coolant passages, unless it’s a 4.3 v6.

It’s a 5.3. There is no crossover pipe between the heads at the front of the engine. There is a freeze plug at the front of each head. I’ll be doing an oil change later this week or early next and I will check the backside of the heads then.

Never seen steam out the tail pipe. I had the heater core bypassed earlier this year for a couple of days while I got the parts I needed when the QD broke. Not enough time to notice if there was a coolant loss.

If there were bubbles coming up in the coolant, you would never see them in a GM. There is no radiator cap, the upper radiator hose goes into the end cap of the radiator, there is a hose to the reservoir bottom to keep the radiator full and a bleed hose that goes from the top of the radiator end tank to the top of the reservoir, and all of that is hidden by internal baffles inside the reservoir.

I am considering just replacing the whole reservoir because the one in there now is so cloudy from age that it is difficult to see the coolant level without removing the cap. So that would not be a Hail Mary to find the problem, but if it solved it, it would be a plus.

If I had any evidence that it is a head gasket, I would put that Subaru conditioner in the coolant system. My Subaru used more coolant than the Chevy when it was new, but in about a year, it stopped using coolant altogether. Hasn’t used a drop in the last four years.

@Tester, I put a new cap on this morning. It will probably take a month or more to see if that works.

@db4690 I could, I say I could be wrong on that. Today I noticed a 1/4-3/8" OD pipe that ran from one head to the other. This looks like it would only be big enough for bleeding the heads. Is that what you were talking about? Don’t see any coolant but they do look like they could have had some weeping at one time. Looks more like oil though.

Yes, that’s what I was talking about . . .

I’ve occasionally seen the line rust out and leak

But it’s usually the seals that are the problem


I used the UV detector and found the leak. I haven’t fixed it yet. It is the hose that supplies coolant to the throttle body. I did not replace this hose yet as it is so difficult to find, seems to be dealer only, even RockAuto doesn’t have it. The problem is that the hose is about a 3/16" or 1/4" hose at one end but only a 1/8" at the other. It has some sort of a rubber reducer built into the hose so you can’t find it just anywhere. If I can find a barb adapter, I will get it and use two hoses with the barb adapter in between.

Just to be clear, it is a molded hose with the same OD along its length, but a different ID at each end.