Slowly losing coolant, leaking inside or out?

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AbelTwo

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I have a 1994 GMC Sierra K1500 with the 4.3 l V6, TBI, and a 5-speed manual transmission. The truck has 180k miles, the last 15k of which are mine and I drive it gently. I bought the truck two years ago from a guy that did not offer that he had a shop put in a head gasket, water pump, and radiator before listing it for sale - until I noted fresh RTV on the water pump. He then produced a receipt for the job from a shop in northern Idaho. I presume that means that before he had the head gasket replaced, the truck overheated at some point. My first 11,000 miles in the truck included righting many of the wrongs dealt to the truck by neglect by its former owner and included a couple of ~1500 mile trips to Montana and Wyoming from eastern Washington. Last Fall I drove it again to eastern Montana, where it cracked a cylinder head, overheated, and spent the winter in a local shop over there getting a new-to-me cylinder head . I have driven it 3000 miles since March, when I got it back from Montana.

This morning I discovered that it was low on coolant, somewhere between a cup and pint low - on the eve of another trip to eastern Montana. I am starting to suspect that this rig has a long-standing leak that slowly reduces coolant levels until the engine overheats and that has so far avoided detection. I fear getting stranded again in eastern Montana.

I'd like some help determining where the coolant could be going. Is this amount of loss "normal" for these rigs? If so, that will take some adjustment in vigilance by me; I have been lulled by a 1995 Honda Civic with 400k miles that never needs topping up of oil, or coolant - ever. It really seems unlikely that this coolant loss would be considered normal, but this is my first truck and I am surprised that it requires a cup or so of oil per 1500 miles.

The truck doesn't leave puddles of coolant and the engine bay seems dry. I have felt around the rear of the intake and looked for leaks back there, but nothing is apparent - and the engine has a new intake gasket from the shop in Montana last winter. The heater does not leave puddles on the floormats. In short, I can't see any external leaks and that makes me suspect it is running coolant through the cylinders and out the tailpipe.

About 30 seconds after startup on a cold morning, the truck does produce some vapor from the tailpipe. The amount of that vapor increases considerably with RPMs until the temp gauge in the dash shows that the engine has reached about 160 degrees (on its way to 195 degrees) and then the vapor wholly disappears. The engine runs smoothly and pulls without complaint - to the limit of the 4.3, which I find a little too gutless for a full-size truck. There is no trace of coolant in the oil, nor is there oil in the radiator.

I am headed out to the shop to pull the spark plugs and to look for evidence of coolant fouling. Can anyone point me toward other ways to figure out where the coolant is going? Or maybe suggest chronic weak points that deserve special scrutiny? Thanks!
 

Erik the Awful

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Worst-case scenario? I had a '99 Suburban that got overheated to hell and back. It sipped coolant afterwards for a couple years, and we just kept it topped off. Then my son borrowed it. I told him to keep an eye on the coolant level, and he merely looked at the coolant overflow. He limped it into the driveway with nearly zero compression. When I tore the engine down, both heads were severely cracked. I should have taken the block to get magnafluxed, but I didn't. I reassembled the engine with fresh heads and a nice, balanced bottom end and a new spider assembly. It ran so smooth, but it kept burping exhaust gas into the coolant. The block was cracked.

Any coolant loss is abnormal, but it's driveable if you keep an eye on it. Best case scenario is that the head gasket you didn't have replaced is leaking, but if one head cracked, it's pretty likely the other is cracked as well.
 

PlayingWithTBI

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^^^This. Other suspect leaks can be your heater hose quick connect on the intake manifold, water pump bolts which go into the water jacket, fittings on the radiator where your trans cooling and/or oil cooling hoses go. I recently found all 3 of these with no drips on the floor. The intake manifold, oil pan lip, core support, and "K" frame (or whatever GM calls that thing where the motor mounts go) were all damp but the coolant would evaporate before dripping on the ground.

You can pressure check your cooling system to see if/where it's losing fluid.

This stuff works pretty good as a band-aid, you don't have to drain the system, just add it to your antifreeze and follow directions. If you put this in and it stops your loss of coolant, then you probably need to perform a head-ectomy.

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www.amazon.com/dp/B003RGIWO0?ref=ppx_yo2ov_dt_b_product_details&th=1
 
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Schurkey

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This morning I discovered that it was low on coolant, somewhere between a cup and pint low -
WHERE? Low in the overflow reservoir, or low in the radiator with an empty, or with a "full" or even "over-full" reservoir?

I'd like some help determining where the coolant could be going. Is this amount of loss "normal" for these rigs?
Coolant loss is not "normal".

Get a pressure tester from a local parts-store loaner-tool program. Pull the spark plugs, slide a sheet of clean cardboard under the engine just big enough to fit between the tires, and long enough to go from radiator to transmission. Or verify that the concrete floor is CLEAN before you start. Verify the oil level after the engine has been shut off for half-an-hour, and be exact about it. Verify that the radiator cap holds pressure. Then pump up the cooling system pressure to whatever the radiator cap is rated for. Watch the pressure gauge over the course of several hours. If the engine is cool when you start, it should hold pressure. If the engine is hot when you start, pressure will gradually reduce as the engine cools. After several hours, pumping up the pressure as needed, look for drips on the cardboard or clean concrete; see if the oil level has increased--or pull the drain plug since coolant will tend to settle to the bottom of the oil pan. Then have a helper crank the engine with the plugs out while you watch for coolant spray. Re-check the carpet by the heater core for dampness.

I am surprised that it requires a cup or so of oil per 1500 miles.
A cup is 8 fluid ounces. A quart is 32 fluid ounces. There are four cups in a quart.

A cup in 1500 miles is the equivalent to a quart in 6000 miles. That is well beyond the point of TOTALLY ACCEPTABLE. Even if we consider that oil usage typically increases as the oil gets "older", that's still likely acceptable oil consumption for a used engine.

I can't see any external leaks and that makes me suspect it is running coolant through the cylinders and out the tailpipe.
The other possibilities are:
1. External leak(s) so small they don't leave much evidence,
2. Coolant leakage into the oil pan, with the moisture removed by the PCV system. The anti-freeze would then be contaminating the oil. An oil sample, analyzed by a lab would conclusively prove or disprove that the oil is contaminated with coolant residue.

About 30 seconds after startup on a cold morning, the truck does produce some vapor from the tailpipe. The amount of that vapor increases considerably with RPMs until the temp gauge in the dash shows that the engine has reached about 160 degrees (on its way to 195 degrees) and then the vapor wholly disappears.
The primary (largest) tailpipe emission of a gasoline engine is water vapor. It's steam when it comes out of the cylinder, condensing into visible "smoke" as it cools in the exhaust system. When the exhaust system warms up, the visible white "smoke" stops, the water vapor is still hot enough to be invisible.

VERY difficult to determine "excess" water vapor in the exhaust system. If the exhaust doesn't smell like anti-freeze, it's probably normal.

There is no trace of coolant in the oil, nor is there oil in the radiator.
How do you know there's no coolant in the oil? HOW have you verified this?

I am headed out to the shop to pull the spark plugs and to look for evidence of coolant fouling.
Wisdom.

The block was cracked.
Often in the lifter-valley area, but anything is possible.

Any coolant loss is abnormal, but it's driveable if you keep an eye on it. Best case scenario is that the head gasket you didn't have replaced is leaking, but if one head cracked, it's pretty likely the other is cracked as well.
Yup.

Other suspect leaks can be your heater hose quick connect on the intake manifold, water pump bolts which go into the water jacket, fittings on the radiator where your trans cooling and/or oil cooling hoses go. I recently found all 3 of these with no drips on the floor. The intake manifold, oil pan lip, core support, and "K" frame (or whatever GM calls that thing where the motor mounts go) were all damp but the coolant would evaporate before dripping on the ground.
Yup.

You can pressure check your cooling system to see if/where it's losing fluid.
Yup.
 
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AbelTwo

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You guys are a godsend! Thank you for your replies and thorough suggestions for further diagnosis, which I plan to follow to a "t" although I expect it may take me a little while to do so. In the meantime, here are some replies to Shurkey's queries:
1) The coolant was low in the overflow reservoir but the radiator was full right up to the cap.
2) Yes, I meant that it only uses about a cup of oil about every 1500 miles. My personal rough guide for acceptable, which is really just my Uncle's rule from back on the farm, is a quart in 1000 miles. I just am so accustomed to using not one bit of oil between oil changes in the Honda (with 400k) that I hoped for the same with the truck - even though I read here that level of thrift would be unusual for one of these trucks.
3) I have not verified that there is no coolant in the oil with a diagnostic test. I have touched a drop of oil to the tip of my tongue (my Uncle's test) to test for sweetness (I know, foolishly old-school. But I do it very rarely. Nevertheless, I need a better way and you are pointing one out. Thanks.) It was not sweet. BTW, I checked for oil in the coolant by looking for a sheen on the coolant surface in the radiator neck.
4) Thanks for the note on water vapor in exhaust. I do sniff tests, but I have always had poor, intermittent capacity for olfaction. I don't trust my schnozz to detect that odor.
5) The spark plugs looked good to me last night, light brown to tan, pretty much identical color across all six plugs. Plugs were in really good shape, but I threw in some fancy new ones I had on standby anyway. Kinda frivolous.
 

Schurkey

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In the middle-1980s, GM considered a quart of oil to 700 miles to be "acceptable". I heard that directly from the lips of the Zone Rep for the Chevy dealership I was at.

Everyone on Earth knew this was a load of crap, just GM avoiding warranty repairs on oil-sucking junk.

At that time, a quart to 2000+ was pretty normal. Oil burning is much reduced now, I suspect better machining processes, leading to better ring seal are the reasons. A quart to 4000+ is pretty normal.

I wouldn't expect to taste "sweetness" in the oil by putting a drop from the dipstick tube on my tongue. But pulling the drain plug on an engine that hasn't run in awhile--so the water settles to the bottom of the pan--would be a quick test. Really, an oil analysis would be the most-helpful but also take the longest time to get results, and cost ~$30-ish.
 

AbelTwo

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Just got back to the house from the shop with an update.

I eased the oil drain plug out and allowed a few milliliters of liquid into a catch-cup. It was viscous, and oozed like oil. It smelled like oil. It left no odd colors after an hour on blotter paper. I haven't yet had it tested for presence of antifreeze, but I now have less cause to do so.

Once I pulled the plugs, registered the oil level on the dipstick and applied pressure to the cooling system I heard some gurgling but couldn't locate the source. No matter, in a few minutes I heard spattering on the cardboard under the truck near my feet. The hose clamp on the lower hose to the radiator was loose and after a minute or two of pressure, enough antifreeze had leaked into the catch tray under the radiator that the puddle finally found a drain hole tray and was dripping onto the cardboard on the floor. I tightened the errant clamp, and voila, no more leak.

This was a good lesson in seeking information and following principles to a diagnosis - calmly. I was fixated on the overheating history of this engine, and on the eve of a trip, was worried about getting stranded. So I was let myself focus on the worst possibilities. You all, and Shurkey in particular, got me back on track with a plan to get the information I needed to make a facts-based diagnosis. Thank you!

Of course, I am taking extra coolant with me and I will remain vigilant. And I may just get the oil test for antifreeze anyway, mostly for fun. The good news is that this IS still fun. Thanks again, all.
 

HotWheelsBurban

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Glad to hear you found the problem, and it was a simple fix! I'm in a similar situation with my crew cab; the shop it was at was trying to convince me it needs a $3500+ transmission job, and so it's at another (hopefully more honest) shop. I should have a diagnosis later today, but it seems more like a brake issue.
 

DerekTheGreat

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If I just changed my oil, it seems I can drive an unlimited amount before the engine (350 TBI) touches any of it. Once it ages by about six months or so, it starts to use a little, about a quart every 1500 to 2k miles. I am not sure why that is. Truck's engine has about 180k on it. Original engine was destroyed due to neglect by the PO. I'd say it's by people who never check the oil and/or neglect the cooling system. "If you french-fry when you should've pizza'd, you're going to have a bad time."
 

1998_K1500_Sub

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Just got back to the house from the shop with an update.

I eased the oil drain plug out and allowed a few milliliters of liquid into a catch-cup. It was viscous, and oozed like oil. It smelled like oil. It left no odd colors after an hour on blotter paper. I haven't yet had it tested for presence of antifreeze, but I now have less cause to do so.

Once I pulled the plugs, registered the oil level on the dipstick and applied pressure to the cooling system I heard some gurgling but couldn't locate the source. No matter, in a few minutes I heard spattering on the cardboard under the truck near my feet.

You’re methodical.

Good methods lead to effective outcomes.

Congratulations :waytogo:
 
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