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black goop in intake manifold

Lv1G6He3V2

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Lv1G6He3V2 Enthusiast

Lv1G6He3V2

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I'm doing the intake manifold gaskets on my '99 5.3L (GMT-800). When the computer is reset, the truck idles very roughly and my LTFTs quickly climb to 25%, at which point the idle smooths out. This means unmetered air is getting into the engine. A smoke test revealed nothing, and I could see no loose/cracked tubes. A TSB said intake manifold gaskets could be the problem; given the truck's age and how cheap the gaskets are why not replace them?

 

So I ordered new gaskets from Felpro. When we took the manifold off, the old gaskets had the original orange seals, not the teal replacements. Also present near one port was something like orange insulation (almost like a chewed up shop rag had been stuffed in to mask the problem). Given my "factory" clear coat is bubbling and peeling off in 3" wide spots I'm not surprised by anything the previous owner might have done...

 

What I am concerned about is the inside of these parts is covered in tons of black goop. The backside of the throttle body, the intake manifold, even covers 1-2 inches into the aluminum (?) ports. The stuff looks like tar, or wet black paint. When wiping it off, it gets stringy and smears so I got a few spray cans of intake/throttle body cleaner to act as a solvent - works okay.

 

Is this old oil, possibly accumulating from the PCV oil consumption problem? I'm going to check the PCV tube later to see if it rattles when shaken. If it does I have the old style valve and will replace it.

 

Or is the black goop from some kit like stop-leak? I see nothing copper-colored, just black goop.

 

 

BTW I hate the injector wire connectors! Little things took me 30 minutes to remove, just from guessing and the tight spots GM stuffs them into. 'course once you know how, they can be popped off in a few seconds.

08lt Enthusiast

08lt

Posted
Posted

It seems like it is carbon buildup. (Photos would help....) I'd do my best to try and clean as much off as possible... (try soaking your intake in Purple power) It's probably from the PO not changing the oil regularly and using an inferior oil. Good luck and post up some pics....

Lv1G6He3V2 Enthusiast

Lv1G6He3V2

Posted
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Carbon buildup is like dust, right?

 

The stuff we cleaned off was a very thick, gummy, greasy sludge, e.g. the kind of stuff that builds up on the inside walls of my gas grill.

 

This is my first time pulling something off the engine so I can't say what's normal. My friend who used to be a professional mechanic said it was unusually sticky and felt like a mix of soot and coolant, as if the throttle body had a pinhole leak in the coolant channel. It was a huge mess that took several hours to clean.

 

Back and sides of the throttle plates was covered with it, to the point of being unsafe IMO. It ran throughout the intake but only on sides of walls the air hits as it bends, and the sludge was caked down the aluminum ports but stopped a few inches before the push rods (?). Left an orange film when thinned with a solvent spray.

 

Intake's back on with new gaskets, but LTFT hit 17% by the end of a 5 minute test drive. Better but still bad. No pictures - tried to get it back together before rain hit - but at least it's clean now. Later I can loosen the bolts on the throttle body to see if it reappears, maybe get a picture then.

 

EDIT: I suppose the next step is to get my hands on a vacuum gauge.

 

EDIT: Found another thing to test: the EGR valve. If it's stuck open then it will cause a vacuum leak and rough idle but not necessarily set a code. Might as well test it and do the cleaning anyway, just have to be careful not to get cleaner in the canister.

DaveH71 Enthusiast

DaveH71

Posted
Posted

can you get some pics of the sludge, maybe on your finger or something. If its coolant it should have a sweet smell to it.

Lv1G6He3V2 Enthusiast

Lv1G6He3V2

Posted
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Posted

Sorry, no pics as I said we were just trying to get it back together before the rain hit. But if you take a paper towel and wipe the grease/soot off the inside walls of a grill, it's very similar: sticky, slimy, and nothing that comes in contact with it will come clean without a solvent.

 

Thanks for mentioning the smell; it will be something to try later if the stuff reappears on the back of the throttle plate.

 

Recently I cleaned the EGR valve and replaced its gasket (which came with the intake gaskets anyway). The carbon residue around the EGR valve was just a dry black powder that came clean with ordinary hand soap.

 

After replacing the intake gaskets and EGR gasket, idle LTFT's dropped from 25%+ to 14%. Each number is what the computer settled on after a reset. Idle STFT1 constantly fluctuates +/- 1% and STFT2 constantly fluctuates +/- 2%. Pressing the accelerator causes the STFT's to instantly decrease; I saw values from -5% to -10% but did not floor the pedal. I conclude 1) the leak is mainly at idle, and 2) the O2 sensors are not being fooled by a faulty MAF sensor.

 

Come to think of it the brakes felt weak after I had my local shop put on new wheels and do the alignment. At the time I thought it was the larger wheels, but my brakes operate on engine vacuum, and at some point I was told I had a vacuum leak ... could a hose/line have been damaged or just not connected properly?

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