By on September 22, 2017

2004 GMC Envoy Radiator Cap Leak, Image: OP

Dave writes:

Hello Sajeev,

I have a 2004 GMC Envoy, 4.2-liter six-cylinder, with about 123k miles that seems to be leaking coolant from somewhere around the radiator cap. Every time I come to a stop with the windows down or pull into the garage I can smell antifreeze. The radiator appears full and the reservoir seems to be maintaining a fairly consistent level of the pink stuff. It never leaks enough that I see wet spots except now and then around the cap, and even then it’s only a few drops at most. I’ve taken it to the mechanic; they double checked all the hoses and connections and even replaced the cap, but it didn’t help.

I replaced the cap a second time thinking I got a bad one, but the smell remains. By happenstance the cap I bought happened to be the exact same brand the mechanic used. I’ve attached a picture of the engine bay showing the areas where I can see dried coolant. The perplexing part to me is that, judging by the splatter pattern near the oil fill and the air filter box, some of the coolant seems to be making it to the fan. Still, I never see any wet spots leading in that direction. Do you have any suggestions?

As a side question, I have been getting an intermittent CEL with an associated P0526 code (fan speed sensor). It will come on for a trip or two and then go away on its own. Any chance the supposed coolant hitting the fan could be causing the CEL?

Sajeev answers:

After much googling, I’m at a loss for words.  But your boy Sanjeev gets paid to type them words on the autoblogosphere, so let’s consider possible scenarios:

  1. Three bad (original one worn, two defective) radiator caps: you didn’t use a factory part, judging by the photo. But the odds of both aftermarket ones failing so quickly?
  2. Bad threads on the radiator neck? Reason being is if the cap isn’t bad, perhaps it’s the radiator: seems like a stretch.
  3. Bad upper radiator hose: not likely from the splatter pattern and your mechanic’s diagnosis.
  4. Engine compression forcing itself into cooling system? To the point the cooling fan is always running, running at a higher then acceptable voltage, triggering P0526?

Oooooh damn, son. 

A friend reminded me of that problem when Baruth and I suited up for an endurance race: one of the racers (not me, not Jack) ran the motor hot, popping the head gasket. It wasn’t a colossal failure (yet) but when the team leader started up the motor with the radiator cap off, we knew what went south. Like a little slice of Old Faithful, the car’s radiator shot out water like a geyser.

So do the same: start the motor (WHEN COLD!) with the cap removed. Look for bubbles in the coolant, but watch out for geysers. Maybe wear goggles — better safe than sorry. 

[Image: OP]

Send your queries to sajeev@thetruthaboutcars.com. Spare no details and ask for a speedy resolution if you’re in a hurry…but be realistic, and use your make/model specific forums instead of TTAC for more timely advice.

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24 Comments on “Piston Slap: The Envoy’s High Pressure Mission?...”


  • avatar
    Scoutdude

    That spray on the valve cover and surround area is no coolant that is just some mud.

    If a pressure test didn’t reveal anything about the only thing you can do is drive on until it starts getting worse.

  • avatar
    bullnuke

    Check the seating surfaces for the cap gasket. My 7.3 F350 was doing the same thing – smell of coolant, wet spots in the area of the cap. I cleaned the radiator neck seating area really well, wiped off both sides of the cap gasket and around inside the cap to remove any bits of foreign material. Fixed my problem.

  • avatar
    1500cc

    You could get some coolant dye and try to spot the leak with a black light.

  • avatar
    Jean-Pierre Sarti

    i would do this:

    1. get an OEM cap
    2. install new hose (guessing it is the original?)
    3. install new clamps for hose (same as hose, are they original?)
    4. make sure you bleed system correctly.

    I had a similar situation happen to me. I never did figure it out how it was leaking but these things helped. My best guess is the old spring clamps were allowing minuscule amounts of coolant by.

  • avatar
    Chris Tonn

    Or you could do what I’d like to do with MY (wife’s) GMT360 – the float test:

    Drive into nearest body of water. If vehicle remains above the surface after an hour, burn it.

    Otherwise…

  • avatar
    incautious

    I believe these things have a fan Clutch and the sensor is built into it. Could be bad sensor, wiring, or fan clutch. Smelling antifreeze. Could be your heater core is starting to leak. Running the engine to check for bubbles, that’s OK as long as the motor cool before the thermostat opens. much easier to check the condition of coolant or oil for the presence of oil which would indicate a bad head gasket. Of course a compression and or leak down test it best.

    • 0 avatar
      Willyam

      I was wondering how Heater Core hadn’t come up, as it was always somehow involved back in the long, long ago time. Have they improved that much? My cars last longer mileage and I don’t see those wet-carpet problems anymore (knock on fake wood Brougham trim).

  • avatar
    MartyToo

    In the general time frame of 1997-2000 GM used a plastic plenum between the metal housing of the water pump and the iron block in 3.8 V6 engines. My ’97 Grand Prix met a wonderful dealer mechanic who properly fixed the seals such that we never had another problem with a leak on that car. My 2000 Bonneville was another story altogether. I was never convinced that the leaks were properly addressed by the two (or more) repairs that were done at the same location by different mechanics.

    (The dealer charged me an extra hour for labor over the book repair rate for the ’97 repair. I didn’t notice this extra hour charge until GM agreed to pay for half of the repair even though the car had about a thousand miles more than the 50,000 limit that was in effect for a repair campaign then in effect for these engines. That extra hour of labor was the best “investment” I have ever made in a dealer repair shop even though GM didn’t include that in my 50% repair allowance for the unexpected repair.)

    So if I remember correctly, when I was having the repeated issue with the 2000 leaking, I smelled antifreeze when the car was at stop lights and such.

    I don’t know if the crack engineers at GM solved this issue by 2004, but it is worth a look.

    I also think the neck at the top of the radiator could be at fault but the best way to check all of these issues would be with UV dye.

  • avatar
    CobraJet

    It is hard to get a line of sight on the water pump on most vehicles like this without removing stuff. There could be a small leak or weep of coolant from the water pump shaft seal. It doesn’t take much to detect an odor

    • 0 avatar
      bam210135

      I agree with cobra jet. its probably the water pump seeping caused by a faulty fan clutch that is locked up all the time putting stress on the bearing. replace the clutch and pump and possibly the fan blade too if its cracked. I’ve been through the same thing on my Saab 9-7x (trailblazer with a saab badge.)a few times…

    • 0 avatar
      Pip Diddler

      This. I had an 02 Olds Bravada. The weeping water pump is hard to diagnose because of the fan, it will scatter the coolant everywhere. The leaking coolant and the smell drove me crazy for a few months until i finally realized it was the pump.

    • 0 avatar
      Detroit-Iron

      Yeah it’s not a bad smell like transmission fluid, but it sure is persistent. Could try pressure washing and degreasing and see if that helps to either locate the leak or make the smell go away.

  • avatar
    nlinesk8s

    I had a Mini R56 that had a cracked head, with similar symptoms. Just sitting there running at idle, it was fine. But with highway driving, you’d get some lost coolant.

    The “smoking gun” was the strong smell of exhaust when you took off the coolant reservoir cap.

  • avatar
    APaGttH

    For number 4, a hydrocarbon test will show if you have exhaust gasses in your coolant and a failing head gasket. Almost any mechanical will do it, and they will typically do it for free. You can also buy a kit for $20 to $40 – but no sense if any shop will do it for you.

    If the test solution turns yellow, there is hydrocarbons in your coolant. The more yellow it turns the worse it is.

    With that said, I don’t think that is the issue, as you’d have coolant coming through your low pressure tank explosively first, and your temperature gauge would be going nuts. The exhaust gasses can’t pass the thermostat, so they build up behind it, creating the pressure problem, and the temperature sensor spikes, even though the coolant isn’t that hot.

    My vote is the unlikely multiple bad replacement caps, or something wrong with the top of the radiator (threads, hairline crack, etc.)

  • avatar
    Halftruth

    If the hoses have not been changed, I am banking on a hose. Here’s why. I once had a leak that I thought for sure was a head gasket/head leak. What had happened was the hose cracked in such a way that the leak started inside the hose about 3-4 inches in but traveled down and actually leaked out the tapered end of the hose on the thermostat housing. You could not see this. Picture a slice in the hose that is parallel with the hose instead of perpendicular. The leaking coolant then (quite sneakily) traversed the intake all the way to the back creating a leak that appeared to come from the back of the engine. I thought I was screwed (as I had just bought this car) but found it only when I went to pull the engine apart- first thing I did? Pulled the upper rad hose and that is when saw it.. Could not believe it. Hoses can be sneaky little bastards.. don’t trust ’em.

    • 0 avatar
      SaulTigh

      This, and at 13 years old with 123k on the clock, if the hoses have never been changed it’s dang well time. I’d replace every hose that’s easily accessible. I did this as a precautionary measure with my ’96 Grand Marquis when it hit 100k miles and never had any problems up to the point I sold it with 180k or so on the dial.

      • 0 avatar
        golden2husky

        I agree to be suspect of the hoses as well. Most hoses deteriorate from the inside out – my MKVII had this problem and coolant was seeping out the end. The interior splits exposed the fabric like material in the core of the rubber and the coolant wicked its way to the end where it seeped out.

        Once you decide to keep the car, at this point change every single rubber hose with coolant inside of it, not just the easy ones or the upper radiator hose. The weak link is what will strand you, so make sure you change those little ones that go to the intake as well. I’ve kept old cars reliable for far longer than most ever consider doing and preventing an engine-killing overheating is a must. Do the belts and T-stat at the same time and you are good for another 100K. Keep an eye on any plastic fittings as they can split and leak.

  • avatar
    Pip Diddler

    Fan clutches on these are notorious for failing, they are electro-viscus clutches. They either can’t spin enough at idle, causing poor A/C performance, or they lock up and spin 100% duty cycle. The code is a good indication it is on its way out. I replaced the water pump, which I think is your coolant issue, and the fan clutch at the same time on our 2002 Bravada.

  • avatar
    StudeDude

    If you are seeing the coolant around the cap, I would say there may be a hairline crack somewhere around the filler neck. Plastic radiator tanks are only good for a certain number of thermal cycles before they become brittle and prone to cracks and the like.

  • avatar
    phxmotor

    Jesus – I thought this was a “car guy” site.
    Don’t all radiator issues start with checking
    clamps … the cap seat … peeking st the water pump
    ( if it’s visible) … and maybe glancing at the radiator
    itself
    (Ya think?!)
    But my god…. isn’t the real issue after a quick glance
    at the whole mess…. … to open the radiator cap and do the bubble test?
    God almighty already… …take it to the shop
    … do a fancy exhaust gas check
    …go to the web for advice
    Isn’t the first real test the 0bubnle test”
    Isn’t it always the first test a “car guy” does when looking
    over a used car?
    What am I missing here?

  • avatar
    Yankee

    I agree Phxmotor. Not sure why this even made it into the rotation of stories. Slow news day? lol. When I was a service manager I had a service advisor who was being pressed by a customer to diagnose his car over the phone so he would know exactly what the costs would be before a technician even looked at it. Finally, in frustration, the service advisor stopped the customer and said “First let me ask you a question: What do you think of the tie I’m wearing.”
    “I can’t see your tie!” the customer retorted.
    “Well I can’t see your car,” said the service advisor. The customer brought his car in.

    That being said, if the owner is too cheap to take it to a shop, many auto parts stores will loan you a complete coolant pressure testing kit (you pay something like $120 and then they give you it back when you bring the tool back). You find the correct adapter, screw it on to the radiator (or coolant bottle) filler neck, and pump up the tester and leave it on to observe where the leak is coming from. If non is found and the system holds pressure after a 1/2 hr or so, there are other adapters in the kit for testing the cap itself. If both of these tests don’t show anything, then the cooling system is likely over-pressurized from a head gasket leak that is causing the pressure to rise above the stated limit of the cap. A shop can put an emissions tester near the filler neck or use litmus-type test strips to detect combustion gasses in the coolant to be sure. Test, don’t throw parts at a car.

  • avatar
    pwrwrench

    Agree with Yankee, TEST, until you find something.
    Vehicles came in my shop with similar problems. Cracked plastic cooling system parts were often the cause.
    Those can be found by the, already mentioned, pressure and dye tests.
    I had some that took more time to figure out.
    Leak down and compression tests are excellent diagnostic methods. What I found on some was that the regulated pressure of the leak down tester (usually 40-60PSI) was not enough to find some head gasket failures.
    On a, “Let’s try this”, I put shop air (150PSI) into the cylinders. On one bubbles appeared at the cooling system pressure cap neck.
    Apparently some initial gasket leaks act like a relief/check valve. Only letting high pressure into the cooling system from the combustion chamber. This usually leads to coolant coming out when highway driving. Short trips there is little or no leakage.
    Standard cooling system pressure test was okay, but that is usually no more than 20PSI.
    Look at the plastic parts again. A neighbor removed the cylinder heads from a Buick V6 because coolant came out when the spark plugs were removed (engine would not turn/start). However the real cause was a cracked plastic intake manifold that had a coolant passage. This allowed coolant into the cylinders.
    Of course easiest would be to find a technician that sees that GM motor a lot. They might immediately point to something and say, “These crack/leak all the time.”


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