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2003 gmc envoy_sle
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Discussion Starter #1
I'm new to this so bare with me. I have a 2003 GMC Envoy sitting at approx. 96,000 miles, when I bought it in August last year it had 92,000. When we got it, it had a slight knocking in the motor, mainly when it was idling. Almost sounded more like a tap so I decided it needed better oil and lucas and it'd be fine since dealerships don't put great oil in them. Over the past 8 months the knocking has become worse and more frequent. It knocks while idling, not loud but audible. It knocks when it is shifting between 1st-2nd and 2nd-3rd the worst. When the RPMS reach about 2 grand and up you'll start to hear it, and heaven forbid you let off the gas while it's making the noise it gets extremely loud. It almost has a diesel sound. I took it to one shop that said Spark knock, I asked about timing, nothing they could do if it gets worse bring it back, a.k.a when your motor blows we'll be glad to rebuild it. Next shop has it a day and says, probably a piston, you need a new motor. Now my boyfriend is a military mechanic and like he said, if it was a piston I'd blown the motor by now b/c I've towed our boat and stuff with it. Someone suggested spark plugs and coil packs. I bought spark plugs, waiting to put them in till I get a second opinion. Another friend mentioned it could be a collapsed lifter. I'm not sure b/c when I sit idle sometimes the truck will actually act like it's going to die, the RPMs will drop below a grand the rocket up to 2200 like someone is revving it and no one is even in the truck. Is anyone else having these kinds of issues? :bonk:
 

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2002 olds bravada
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It could be both. But you would have more of an effect (like your engine already blown up just like your boyfriend said) if it was rod knock. i dont' know..thats just my opinion. I say go ahead and do the spark plugs since you're close to the point where you have to change them anyways.

The idle issue is most likely a dirty throttle body though. It'll only happen when the A/C compressor is running. thats what you're basically describing. It's a real simple job, only should cost you the cost of the cleaner. Look it up in the articles section. hope this helps!
 

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2003 chevy trailblazer_ls
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5,268 Posts
like he said---
try cleaning the throttle body to see if it gets rids of that funny idle.
(usually happens when the AC is on)....

But the knock-- occur ALL time time- or more when cold 1st thing in the morning...

The I6 engines are prone to Cold Carbon Knock.. they knock like crazy when the engine is cold-- and then goes away once warm...

Chevron techron concentrate (fuel system cleaner) is good for this.
It can help quiet this down.

try both of the above.
 

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2003 gmc envoy_sle
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Discussion Starter #4
Thanks guys! I'm going to go ahead and have him help me with the spark plugs, I'm in line after the boat motor haha go figure. I had considered Cold Carbon Knock but it doesn't just do it when it's cold, it can actually get worse after it's run for a bit. Even worse if he drives it. I regularly put Lucas Fuel Treatment in it, every few tanks, and I fill it up with Shell Premium gas ONLY. Very religious on that. After seeing how well my Monte Carlo did with that, I'm stuck now. We do actually put 10W-30 in the truck now along with the Lucas Oil Treatment with every oil change. So hopefully the spark plugs help.

As for the idle problem, it does it with the A/C compressor off. The first time it did it to me in traffic my auto reaction was to turn everything off and then it still did it. We will try the throttle body though just to see.
 

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2005 chevy trailblazer_ls_ext
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Thanks guys! I'm going to go ahead and have him help me with the spark plugs, I'm in line after the boat motor haha go figure. I had considered Cold Carbon Knock but it doesn't just do it when it's cold, it can actually get worse after it's run for a bit. Even worse if he drives it. I regularly put Lucas Fuel Treatment in it, every few tanks, and I fill it up with Shell Premium gas ONLY. Very religious on that. After seeing how well my Monte Carlo did with that, I'm stuck now. We do actually put 10W-30 in the truck now along with the Lucas Oil Treatment with every oil change. So hopefully the spark plugs help.

As for the idle problem, it does it with the A/C compressor off. The first time it did it to me in traffic my auto reaction was to turn everything off and then it still did it. We will try the throttle body though just to see.
Personally if it were my vehicle, I would do a compression check when you have the spark plugs out!
 

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2003 gmc envoy_sle
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Discussion Starter #6
Personally if it were my vehicle, I would do a compression check when you have the spark plugs out!
That's a good idea, thanks! That might give us a big tip on something else being wrong. We've got a compression tester so we'll try that.
 

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2005 chevy trailblazer_lt
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The Lucas oil treatment is not a cure all. It could actually cause the oil to foam. I would clean the throttle body, change the oil to a Pennzoil yellow bottle or Valvoline 5W-30, with either an AC Delco or Purolator oil filter, then add a can of Seafoam(I would actually use AutoRx) but the Seafoam is a good quick clean. Run for 1000 miles, then drain and replace the oil and filter with the same stuff.. Replace the plugs with AC Delco/NGK Platinum or Iridiums. These steps will help the vehicle regardless, but may also help to expose the real issue.

OH, and I would also replace the front O2 sensor.
 

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Interesting.
so if it does it when warm- then cold carbo knock is out of the question.
let us know how the compression check goes.
 

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2004 gmc envoy_sle
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I'm assuming there are no SES lights on? Does it seem to have reduced power at all? The statement you made about it getting worse when you let off the gas is concerning. That sounds very indicative of a bearing going bad. When you were towing with it, did it seem worse under load? Unfortunately, our oil press. gauges are more of a dummy light and don't tell us our exact oil psi. I believe early versions of these trucks had a TSB about bad cylinder sleeves. Usually you'll get a piston slap noise with that, but typically that goes away or gets better as it warms up and the piston expands.

Also, the throttle body/idle thing. My truck did it frequently even w/o the A/C on. After I cleaned it, 99% better.
Keep us posted and WELCOME!:thumbsup:
 

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2004 gmc envoy_sle
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I know that member jsprewell had a cracked exhaust manafold on his envoy and it make a nocking sound throughout the entier RPM range
That's actually a good thought. And when you let off the gas, the back pressure would make it sound worse....very interesting....:undecided
 

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Discussion Starter #12
I'm assuming there are no SES lights on? Does it seem to have reduced power at all? The statement you made about it getting worse when you let off the gas is concerning. That sounds very indicative of a bearing going bad. When you were towing with it, did it seem worse under load? Unfortunately, our oil press. gauges are more of a dummy light and don't tell us our exact oil psi. I believe early versions of these trucks had a TSB about bad cylinder sleeves. Usually you'll get a piston slap noise with that, but typically that goes away or gets better as it warms up and the piston expands.

Also, the throttle body/idle thing. My truck did it frequently even w/o the A/C on. After I cleaned it, 99% better.
Keep us posted and WELCOME!:thumbsup:

No, no ses light which is what I found odd. It tests clear, no codes. Power is fine, it gets up and goes. I mentioned the bearing to my boyfriend and he said the sound is more like timing slack, which would obviously cause the valves to chatter. He's quite determined on the timing chain, but it only has 97,300 on it. Under load the truck is fine. I hauled a trailer full of wood and I towed our boat and it just took off and went. Never hestitated or acted like I even had anything back there. My truck does have the tow package. I check on the TSB for the piston sleeve and my truck didn't fall under it.
 

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2003 gmc envoy_sle
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Discussion Starter #13
That's actually a good thought. And when you let off the gas, the back pressure would make it sound worse....very interesting....:undecided
Hmmm, interesting thought. B/c really it does chatter while I'm driving, you just don't hear it cause it's ever-so-slight and the tires make so much road noise. I think we may have him look at that tonight while he's outside messing with the boat motor. :undecided
 

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Discussion Starter #14
Well he just got out there and checked the exhaust manifold.... no such luck. No cracks, no damage. :hopeless Back to searching.
 

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2003 chevy trailblazer_ls
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Thanks guys! I'm going to go ahead and have him help me with the spark plugs, I'm in line after the boat motor haha go figure. I had considered Cold Carbon Knock but it doesn't just do it when it's cold, it can actually get worse after it's run for a bit. Even worse if he drives it. I regularly put Lucas Fuel Treatment in it, every few tanks, and I fill it up with Shell Premium gas ONLY. Very religious on that. After seeing how well my Monte Carlo did with that, I'm stuck now. We do actually put 10W-30 in the truck now along with the Lucas Oil Treatment with every oil change. So hopefully the spark plugs help.

As for the idle problem, it does it with the A/C compressor off. The first time it did it to me in traffic my auto reaction was to turn everything off and then it still did it. We will try the throttle body though just to see.
It could be the Premium Gas. Unless you have a tune, I would return to regular 87 octane. I would also ditch the Lucas additives.
 

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2005 chevy trailblazer_lt
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Yes, I forgot to mention ditch the high octane also, lose the lucas, do a cleaning of the throttle body and inside the engine. I actually had a customer with a new Trans am. She was running high octane gas and the vehicle ran like crap, would even shut off. She switched to 87 and the car ran perfect. Really, it is a true story. :eek:
 

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Discussion Starter #17
Well it would figure I just filled the tank at lunch today...... :duh: I'll do that next tank though, only takes a few days with my distance from my job. The guys at both auto shops I took it too said to run the higher octane so I decided I'd stick with it. Which Lucas should I be getting rid of? The fuel treatment or the oil treatment?
 

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2003 chevy trailblazer_ls
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I would stop using both Lucas additives. Run Techron in the fuel every few months and as long as you are using SM rated oil no additves are needed in the crankcase.
 

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I would stop using both Lucas additives. Run Techron in the fuel every few months and as long as you are using SM rated oil no additves are needed in the crankcase.
:iagree:
 

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2006 chevy trailblazer_ls
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Do you notice any difference between 5W-30 and 10W-30 in the engine on a colder start? Theoretically the 10W-30 would be thicker and make less noise but once they warmed up they would be the same 30 grade. I know a lot of people put a "heavier" oil in older engines to reduce the noise from the valve train. I've put 10W-40 in an older truck engine ('88 4.3L K1500) that called for 5W-30 to quiet down the lifters and rods. Some new engines have all kinds of small oil pumps, tubes, squirter mechanisms to lubricate different parts of the engine that a 10W-40 may clog shut if it starts to sludge as a 10W-40 will break down quicker and flow slower than a 10W-30. I have no idea if our 4.2s have those parts or if a 10W-40 would be safe in our engines so be careful.

This might help you track down the issue if you can notice a difference with different weight oils at start or when hot.
 
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