Jake braking noise on a 2003 GMC Envoy

[carpem1]

Hello Folks,

I might have a good one for you all. I have a 2003 GMC Envoy SLT (4.2L V6 4x4). I brought it into a dealer this weekend and amazingly they said they have never heard of this. Since they want to charge me about $500 just to look at it further I wanted to check the experts on here.

The problem only occurs when I am driving and its pretty straight forward. When I am driving the car and let off of the gas a 'glunk' noise happens. This noise is immediately followed by what can best be described as a semi truck jake braking. Even the dealership guys agreed with me it sounds 100% like a truck jake braking, but he had no idea of what it is. The noise is louder the faster I go, but only happens when you let go of the accelerator.

I took a look at it yesterday with a mechanic friend of mine. He also isn't sure what exactly it might be, however he thinks it might be the chain that powers the transmission. Since this would be a pretty extensive tear down, I wanted to see what others think first.

Thanks, Carp

 

[Molson02536]

It sound like your Secondary Air pump has issues. There is a check valve in on the exhaust manifold in the middle upper area. The valve is located on the top of the exhaust manifold just below the area where the air inlet hose is.

Start with that and keep me posted. here's a few more information on this issue.

There is a bulletin for this concern from GM. The bulletin says that if the air pump is working and the air check valve checks good, then replace the oxygen sensor on the exhaust manifold. I have seen may of these failures caused by the o2 sensor, The engine computer tests the air pump system by watching the o2 sensor readings, if the sensor is slow to warm up, the computer does not see the fresh air going in to the exhaust.

You can check the operation of the air pump system by removing the air valve on the top of the exhaust manifold. Feel for air coming out of the valve on the exhaust manifold side of the valve. The engine will have to be running, so there will be exhaust coming out of the manifold. You have to do this test on a cold engine start up, this is when the system is turned on, you only have about 30 seconds to do this test before the system turns off. If the air pump is running and there is air coming out of the valve, then the oxygen sensor is bad. If there is no air from the valve, then the valve is bad.

Good luck friend, surprise the dealer had no idea over this issue.

 

[carpem1]

I found out today at another GMC dealer that the issue is pertaining to the fan clutch. I guess it is a common problem for Envoy/Trail Blazers around the years 2002-05.

 

[Molson02536]

GMpartsdirect.com has it for 172.29. Definitely get a tool for the pulley. The fan clutch tool is used to hold the Fan Pulley still so you can back the Fan Clutch nut off of it. It's a PITA to even attempt it without one. Remove counterclockwise - standard thread. Good luck and glad to see your on your way to fixing the problem. Please keep us posted.

 

[carpem1]

Molson02536, thanks for the heads up. I will let you know how this ends up. I did a couple postings on some GMC owners chat logs and it seems like most people have had this problem ranging from around 70K to 110K miles. Amazing how many have and no recall.

 

[Molson02536]

Not a safety issue that's why there is no recall. You can always mode it out and install a electric radiator fan. this will he you an extra 3 to 6 HP from your engine. Keep me posted friend and can give you more hints to slight improvements and maintenance issue.

 

[carpem1]

Yeah I did tell the manager at the service center about the chain response. I was kind of clueless as well since I didn't think there were any chains in there but I am no expert.

Well I finally had time over the weekend to put in the new Fan Clutch which GMC told me after brining it in there they were 99% positive that was the problem. Hundreds of dollars later and the same noise. I haven't check to see if the new clutch was bad since it takes a while to jimmy it out there. I am going to bring it in to someone I believe since I am still clueless.

You are correct about the jet noise. When you are driving at about 35mph + and let off the gas the noise sounds like a jet engine powering down (just like in a plane when they are reaching their final approach) followed by the jake breaking noise (dada-dada-dada-dada-dada) which increases with intensity as the engine idols down.

Very Frustrating...