TDC question

[black_eclipse96]

okay i have the head of the car and the cam gears are lined up , but the some of the valves aren't closed. im i doing something wrong here.

 

[SlowChild3]

Air comes in exhaust goes out...

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[Benshooter]

Need a little more background here or I'm going to have to ask some really stupid qustions...

Like, are your valves bent perhaps? Do you have adustable cam gears?

 

[68442]

You should be fine. Consider its not recommended to do a boost leak at tdc because of overlap. This is why you have to hold the exhaust cam gear to get them to line up.

 

[steve]

okay i have the head of the car and the cam gears are lined up , but the some of the valves aren't closed. im i doing something wrong here.

Why do you expect the valves to all be closed when the dowels are up and the cam sprockets timing marks aligned? They shouldn't be, in cylinder 4 both sets are open because it's at TDC intake stroke and the valves are in overlap. I forget is any of cylinder 2 and 4 will also be open off hand.

 

[black_eclipse96]

okay, background. i ended up breaking a balance shaft belt and decided to replace all timing components and eliminating the balance shafts as well. Then i was like why not do the head gasket while im at it. All the timing marks line up, so i don't think i have any bent valves. i guess i just came to conclusion that TDC means all valves are closed in the head. Fell kinds of stupid for asking the question now

 

[crash89]

Nahh man TDC (top dead center) just means that the number 1 piston is at the very top of its stroke. Yes you will have valve overlap and some valves may not be shut when you do this. Doesnt mean your doing it wrong. You really cant do this wrong if you follow the VFAQ and line all the marks up. Just remember to keep the exhaust cam slightly advanced so when you pull your pin out of the auto tensioner it will line the marks on the cam sprokets up!.

 

[steve]

Just remember to keep the exhaust cam slightly advanced so when you pull your pin out of the auto tensioner it will line the marks on the cam sprokets up!.

If you put the timing belt on correctly nothing moves when you pull the pin.

If any sprocket moves when you tension the timing belt using the tensioner pulley you put the belt on wrong.

The easiest way to be sure is to start with the exhaust cam and work clockwise making sure at each step that the belt is tight so you don't wind up off a tooth at the end. To aid in getting the belt on the right teeth I move the sprocket I'm fitting it to back half a tooth from it's timing mark so the belt slides on easily and then move the sprocket back to the timing mark. It should be impossible to move past the timing mark without pulling everything else too if the belt is on correctly. If it does more past just do that sprocket again back up one tooth on the belt. When you get to the tensioner pulley there will only be the slack between the crank and exhaust cam left which is what it's there to take up.

 

[romeen]

Why do you expect the valves to all be closed when the dowels are up and the cam sprockets timing marks aligned? They shouldn't be, in cylinder 4 both sets are open because it's at TDC intake stroke and the valves are in overlap. I forget is any of cylinder 2 and 4 will also be open off hand.

Not trying to be nit picky but I believe it's TDC of the exhaust stroke when valves are in overlap. I think you mean valve position in #4 when #1 is at TDC (?).

 

[steve]

Not trying to be nit picky but I believe it's TDC of the exhaust stroke when valves are in overlap.

Is it? It kind of both since intake stroke follows exhaust stroke and the exhaust valves don't close until after TDC. IIRC it's about 10* ATDC when the exhaust valves close on the turbo cars.

 

[romeen]

I just recently pressure tested the cylinders. When #1 is at TDC both the intake and exhaust valves of #4 are open (leaking air into intake and exhaust). When #4 is at TDC of the intake stroke #1 is letting air into both the intake and exhaust. Same correlation comparing 2 and 3.

 

[steve]

I'm guessing you didn't understand me.

When we (I) say TDC without qualifying it means TDC compression stroke cylinder 1 and as I said and you noted cylinder 4 is at the same time at the top of the stroke between exhaust and intake where the valves are in overlap. Cylinders 3 and 2 are at the BDC on the start of their compression and exhaust strokes respectively, what I didn't remember off hand was the timing for when the intake valves close on the just finished intake on cylinder 3 and then the exhaust opens on the exhaust stroke for cylinder 2, so I couldn't say if any of their valves were off the seats too.

For boost leak pressure testing the only important thing is that we aren't in overlap on any cylinder and for your leak down test the cylinder in question needs to be at the TDC between compression and ignition. But the OP just wanted to know if any of the valves should be open when the cam timing marks align and the answer still is yes.

 

[romeen]

I'm guessing you didn't understand me.

True.

Why do you expect the valves to all be closed when the dowels are up and the cam sprockets timing marks aligned? They shouldn't be, in cylinder 4 both sets are open because it's at TDC intake stroke and the valves are in overlap. I forget is any of cylinder 2 and 4 will also be open off hand.

This threw me off (it sounded like entire sentence was referring to #4) but I'm glad for the clarification.

 

[NOSLO2PT0]

If you wanna make sure none of the valves are bent, pull the cams out, lay the head on it's side, and pour water into the ports. Watch the bottom of the head and see if any water comes out. If it's a really slow leak, most likely that seat needs to be lapped. If it pours out, most likely the valve is bent. Repeat for the other side.

 

[steve]

They shouldn't be, in cylinder 4 both sets are open because it's at TDC intake stroke and the valves are in overlap.

Perhaps if you read the red text as one adjective describing cylinder 4 when the cam sprockets are aligned to their proper timing marks it might make more sense to you.

Mike (NOSLO2PT0) makes a good point about water testing the valve seating. Since the OP has the head off it's not too difficult to do.