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Envoy XL Exasperation

Mechanics are stumped. 3 months ago, during a 4WD commute through the snow, I started noticing what at the time smelled like bus exhaust coming into the cabin. My SES lights comes on for 2-3 days, then off for 2-3 days. Codes say MAP sensor and low coolant coolant.level. When the SUV warms up fully, you can smell the odor strongly under the hood, inside the cabin while sitting at a stop light, and inside with the Heat or AC on. Mechanics had the car for days, replacing cam and Map sensors. There is no evidence of exhaust leaks or coolant on the engine. Level of coolant has been dropping slowly in the reservoir. Any help is greatly appreciated.

Since no one has responded and the year, engine, and mileage is unknown, I can throw a few wild guesses out.
Leaking heater core? (Maybe check the matting underneath the front carpet to see if it’s damp.)

Cooling system pressure cap not holding? This could allow coolant to puke over a bit at times, dependent upon engine temperature, and this would not be present on the engine itself.

Intake manifold leak, which may or may not be very visible?

What year, 98-2000 (Blazer clone) or 2002 and later (Trailblazer). If the year is 98-2000, I would suspect an intake manifold gasket leak. I had similar symptoms when my 2000 S10 Blazer’s intake gasket started leaking. My mechanic and local dealer had no problem diagnosing the leak. Is there any evidence of oil in the coolant?

A few years later the coolant level started dropping again. There was a crack near the top of one of the radiator’s side tank. I didn’t find it until the fan shroud was removed.

Ed B.

Sorry folks. 2003 Envoy I6 4WD 126,0000 miles. This started right around 110k miles.

Oil is clean, not milky. No white smoke from the exhaust. The odor smells like an old city bus that is running rich. Left hood open, ran from cold. No tapping around exhaust manifold. No coolant on block or visibly dripping. Heat and AC run well. Odor seems to eminate from right side of the engine compartment, but that could be simply the air being drawn into the intake neck.

Mat at front is dry, no fogging of windshield. The odor presents itself only when the engine is warm/hot. Ironically, it smells more like burnt fuel than coolant. This may sound silly, but I add 50/50 coolant ( yellow ) and the reservoir gives me orange.

Right side ( passenger side ) left side as you look into the engine. Exhaust manifold side.

This may sound silly, but I add 50/50 coolant ( yellow ) and the reservoir gives me orange.

The orginal long-life coolant is Dex-Cool which is orange. You better be sure the yellow coolant you’re adding is compatible with Dex-Cool or you could be making trouble for yourself.

The fact you’re using coolant is cause for concern. That is a problem you need to address sooner rather than later.

The rich exhaust smell- does your truck sound like there is a leaf blower running underneath the driver’s side seat when you first start the truck? If so, this is (IMO) a patched up A.I.R. system they found necessary to pass emissions and slapped it onto the trucks as a last minute add-on. That blower is piped up to a valve bolted to the right side of the engine. These valves are notorious for failing. The pintle gets carboned up and corroded due to moisture that accumulates on it. One scenario I can envision is this valve getting stuck open and exhaust gases being allowed into that valve and plumbing when the blower is not running. The valve should set an error code after a number of detections that it’s not moving but something I might at least check anyway. Feel around that valve and the rubber hose for a gas leak when the engine is running.

Twin Turbo, thank you. I was not aware of the coolant. Will either change it or drain and replace it. Mechanic recommended I grab traditional OTC coolant and occasionally run Sea Foam thru the fuel to keep the injectors clean. Of course, this is the same shop that could not find the issue.

The truck does in fact sound a bit noisy and rough on start up. The only codes I have received are the coolant level/temp and MAP/MAF sensor. Could the MAP sensor be the error. I will check the rubber hose.

I appreciate your help on this.

Look on the front of the coolant bottle you’re using to top it off. It will say something to the effect, “Compatible will all coolant types” if it is and then no worries. If it’s a more traditional formulation that is not compatible with Dex-Cool, it could gell up and clog your cooling system.

If the MAP sensor is faulty, it could cause rough running. But no matter how the engine is running, you shouldn’t be able to smell it in the cabin. You’ve got an exhaust leak somewhere or it’s burning off the manifold and getting sucked into the cabin intake like you suggested might be occurring.

A super make/model specific site is You might try going there for more vehicle specific advice.