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Discussion Starter · #1 ·
Hi everyone I have a 2011 terrain v6 that I bought used recently. While trying to diagnose another problem I ended up taking the intake manifold off and seeing that the intake valves were absolutely cruded with carbon goo. I, being the foolish amateur I am, saw some YouTube videos of how to manually clean the valves (because there was no chance that sea foam/ intake valve cleaner through the throttle body was going to even make a dent in this) and got to work.

After cleaning the valves I can’t get the vehicle to start. After a lot of trying to figure out why I see now I’m not getting good compression on 5 of 6 cylinders, in the 60-90 psi range. This makes the most sense I guess as I’ve disturbed the pattern of crud on the valves. I was able to clean them pretty well, no carbon deposits on the bulk of the valves, but after looking with a borescope, the edges right near the seal were hard to get at with the valves closed and there is still carbon build up around there.

I’m now at the point where I’m ready to just take the head off do the timing chain, completely clean the engine, re-lap the valve seats etc. But, if I can avoid it I’d really like to. I have a feeling if I could get the engine to catch and start running, no matter how rough, that the heat and combustion would at the least allow the valves to seat better.

So I’m wondering if anyone else has experience or advice with this? At this point my plan is to clean the valves while they are open instead of closed so I can get at the mating surface, with a nylon brush and circulating hot solvent. I’ll have to flood the cylinder to do so but can pump the excess out afterwards, and evaporate with air. Is this crazy? Anyone have any better ideas?
 

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If I was in your position, I would pull the heads and have a machine shop clean and check the heads.
This is the right way to do it and will cost you less in the long run.

To flood the cylinders with hot solvent and pump the excess out afterwards is not recommend.
You will be running the carbon into the rings and cats, I doubt if you will get it all out.
Get a gasket set, timing chain set, water pump, take your time and clean as you put the heads back on and you will be glad you went that direction.
 
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Discussion Starter · #4 ·
When cranking the engine by hand to close the valves I found it very hard to crank at first and at one point I moved it backwards a couple degree backwards... are these engines that sensitive? What’s happening when it’s turned like that and is there anything I can do to correct it? I’m waiting on some backprobes so I can check the cam and crank sensors with my scope but I’m not sure what to look for necessarily.

Does losing compression like this make sense? I am not against doing the timing belt and everything (by the way did you mean oil pump or water pump? Both?) but I’m worried I’m not diagnosing the problem correctly. The vehicle ran before hand, I took the manifold off, cranked the engine by hand as described, cleaned with valve cleaner directly on the valves, got it relatively clean but when I tried to start it just cranks, it starts to catch sometimes and there is combustion but not enough to sustain. After checking the valves again I saw that some carbon stuff had been sprayed upwards into the manifold which made me think there is a bad seal and that’s when I did the compression test, although I’m not sure I entirely trust the cheap tester I got. I then did a much better cleaning using xylene with the valves closed, and then also did some brushing with a nylon brush over the edge of the valves and valve seal surface while they were opened slightly and none of this improved compression. Doing a wet test didn’t seem to make and appreciable change either. I took out the spark plugs and cleaned them and checked for internal resistance. I also verified I do have fuel by checking the schrader valve on the high pressure line. Battery has 13V after a deep cycle charge.
 

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Camshaft phaser no longer in the home position because the motor was spun backwards...one or both are probably damaged. Yea even a few degrees backwards could cause an issue.
 

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2011 GMC TERRAIN SLT-2 3.0L V6 (LF1)
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Yikes.
I don't have anything to add except I wish you good luck on getting it figured out.
I was going to walnut blast my valves this summer but after reading this post I'm kinda getting cold feet.
Don't wanna end up like this.
 

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Discussion Starter · #7 ·
Can I verify this by looking at the output of the camshaft and crank shaft position sensors? I can half understand how the timing can come out (although internally the cam phasers and crank shaft are connected by the chain... how does one but not the other?), but how does the phaser actually get destroyed? Not questioning what you’re saying, but want to understand. I guess it goes without saying i should buy new cam phasers as well when I get the timing chain.
 

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They are not designed to go backwards... There are pins that lock them into different positions, the inner part of the phaser is a separate piece than the outer part of the phaser connected by pins when the engine shuts off it locks into a home position. With no oil pressure along with spinning the motor backwards it could have come out of the home position or unlocked itself... Even with 1 bad phaser out of 4 the car may not start.
 

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Discussion Starter · #10 ·
That’s sort of what I thought, I’ve cranked it tons and tons as it sounds more and more like its going to start the more I crank, and I figured if it was timing I’d get a code but nothing so far. I do see a message on the gauge cluster that says engine poser reduced at the moment but I also have the intake off and have the map/maf/throttle sensors off.

is there any other way to diagnose them? The camshaft position sensor should be off shouldn’t it?
 

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Discussion Starter · #12 ·
Yes, when I say it won’t start I’m referring to when it’s completely assembled. I’ve cranked it lots assembled. Right now I have the manifold off, coil packs out, spark plugs out, to test compression. I’ve got one of the two valve covers off now I will take the over off tomorrow and check that the cam phasers are not moving backwards freely, but I have now seen one disassembled and I don’t see how it is possible I could have sheered off one of the thick steel pins that hold the inner sprocket to the main body.
 

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Discussion Starter · #14 ·
I’m using this as a reference:

But if there is no oil pressure the outer pin should be locked and the inner pin has no way of moving and wouldn’t be pressed upon during any cranking. There is only one position for the outer pin and I was cranking with the engine off, ie no oil pressure. I don’t think tearing the engine apart and replacing all the timing and cam phasers without knowing what exactly is wrong is a good idea. I mean timing does seem like the most obvious culprit but knowing in what way it is wrong may provide an easier solution. If is is just the cam phasers there is a way to replace them without taking off the entire front cover and doing any of the timing.
 

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What's the likelihood that, after this manual valve cleaning, you'd lose compression in almost every cylinder? Doesn't seem plausible to me. Maybe one cylinder ... but then it would still start and run, but poorly. Seems like it's something "common" that is wrong ... like timing, or even fuel delivery. I do remember years ago, with my '89 Mitsubishi, it got to the point where every morning it would require 20 cranks before it would start (LOL!). Every crank it would stumble more and more until it finally caught and ran. Then it was good the rest of the day. The next morning, the same thing. It was fuel delivery.

Could it be possible that your cleaning has blocked the fuel injectors, or changed their fuel spray pattern? Or could you have stretched or skipped a slackened Timing Chain somehow? Would it be easier to verify the Timing first, as opposed to pulling the heads off?
 
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