Synthetic oil & 2000 GMC Sierra

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  #1  
Old 09-28-08, 05:07 AM
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Synthetic oil & 2000 GMC Sierra

2000 GMC Sierra 5.3 4x4 with 190K; solid truck; no oil burn; no leaks; tediously maintained. About 50K ago, tech talks me into full synthetic Mobil 1. I have synthetic in 1-ton diesel and 07 Tahoe, so I go for it.

All is well....I think....but of late, at 5-7K, I change oil and it's "muddy". Like dark milk chocolate, with some lumps at bottom of stream...just a few, gelatin-like. Oil is not true milky like water in it, though there is a small amount of condensation under oil cap; tech says that's normal. ?? Could it be that the synth oil is cleaning crap outta there? It's not overheating and there's no oil in coolant like a head issue. ?? It has always "used" about a pint of coolant every 3K.

Thoughts?

Thank you.

-Bryant
 
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  #2  
Old 09-28-08, 05:50 AM
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I believe the color of the oil is attributed to moisture in the oil. With heat as a catalyst, over a period of time (with the 5-7 oil change interval) your color would go from the milky to what you're describing as muddy.

Any cleaning of the engine, if any, by the synthetic would have occurred long ago. It will normally mix with petroleum based oiled, but either has detergent additives to help keep the engine cleaned. Some seem to perform better than others, while service intervals play a major role in keeping residue levels low. As the engine gets older/more miles, shortening service intervals helps a lot.

The moisture under the cap is normal condensation.
 
  #3  
Old 09-28-08, 03:43 PM
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coolant in the oil, most likely from the intake manifold , run a pressure test of the coolant system, but be prepared to remove the intake and either have it resurfaced or replaced with new. intake leaks are pretty common on gm products.

life begins when the kids leave home and the dog dies
 
  #4  
Old 09-29-08, 07:51 PM
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Coolant don't flow throught the intakes on these engines.

I'm suprised a mechanic recommended synthetic oil on a high mileage engine that used to run conventional oil. Every one I have seen switch to synthetic begins to eat the oil, more than a quart and a half to 2 quarts every 3,000 miles. Matter of fact, the neighbor has a 4.8 liter in his Sierra that eats it, and his ex wife just switched back to conventional for the same reason, on her Grand Prix GTP. I, myself, would never recommend synthetic on a high mileage vehicle that ran conventional up to that point.
 
  #5  
Old 10-01-08, 10:51 AM
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Originally Posted by ArmyAviator View Post
2000 GMC Sierra 5.3 4x4 with 190K; solid truck; no oil burn; no leaks; tediously maintained. About 50K ago, tech talks me into full synthetic Mobil 1. I have synthetic in 1-ton diesel and 07 Tahoe, so I go for it.

All is well....I think....but of late, at 5-7K, I change oil and it's "muddy". Like dark milk chocolate, with some lumps at bottom of stream...just a few, gelatin-like. Oil is not true milky like water in it, though there is a small amount of condensation under oil cap; tech says that's normal. ?? Could it be that the synth oil is cleaning crap outta there? It's not overheating and there's no oil in coolant like a head issue. ?? It has always "used" about a pint of coolant every 3K.

Thoughts?

Thank you.

-Bryant
Hi Bryant,

Let me make sure I'm clear. You swapped over to synthetic at 140K miles. At about 190k miles, you did an oil change and it comes out muddy.

The moisture under the cap might be from an incomplete warmup. I say might be as I'd make sure it's water. Two thoughts on that. 1) When the truck's cold, for the first mile you drive you may want to keep it under 40, then wind the truck up. Also, if it's the original oil cap, I think changing it out with a new one wouldn't cost that much.

Some questions:
1)Do you take a lot of short trips? If you're getting a lot of moisture buildup you might try to spend more time in prolonged driving (at one time) to drive the moisture from the oil and heat up the engine.
2)Do you spray water on or near the engine to clean it? Drove through high water? Both might get past the filler cap and into the engine.

As for the lumps, I can only guess. The one thought I had was maybe there was a substance in the pan before you drained the oil into it.

Hope this helps
 
  #6  
Old 10-01-08, 11:16 AM
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[QUOTE=Mr. Dummas;1438255]Coolant don't flow throught the intakes on these engines.

maybe not but the intake still seals a coolant passageway and still tends to be a leak looking for a place to happen.

murphy was an optimist.
 
  #7  
Old 10-01-08, 07:44 PM
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[QUOTE=Speedwrench;1439055]
Originally Posted by Mr. Dummas View Post
Coolant don't flow throught the intakes on these engines.

maybe not but the intake still seals a coolant passageway and still tends to be a leak looking for a place to happen.

murphy was an optimist.
No, it don't even block a coolant passageway. This is a 5.3L.
 
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