Does the motor need to be TDC to do a boost leak test?

[fatness_42]

I am putting the intake manifold under pressure right now and I hear no bubbling no sounds of leak but it will not hold any pressure it just leaks out the exuast not around the manifold but out the tailpipe? Ok so I tried to set it at 9 ATDC and I could not get it to hold any pressure still leak out the tailpipe. I also tried 30 ATDC and about every other crank setting all with the same results. What does this mean? The head was just rebuilt, new valves, springs guilds the whole works.

 

[ROOKIE1]

Need to make sure the valves are closed . No overlap to where the are allowing air past .

 

[crimsondragon]

30* ATDC is recommended when doing a boost leak test.

 

[ROOKIE1]

Oldman posted this about a year ago .



Quote:
1. Disable your mbc.

2. Turn your motor to 30* ATDC to avoid valve overlap.

3. Start your test at the TB elbow and focus on area behind the TB first.

4. Spray soapy water at TB gasket, BISS, TB shaft on both sides, IM gasket, injector insulators, brake booster, afpr and all vacuum lines/connections.

5. Open your oil cap and listen for leaks. (PCV, valve seals/guide, rings)

5. Listen to your tailpipe for leaks. (EGR, valve overlap, jumped timing, bent/unseated valves)

6. Once all leaks are fixed, move the tester back to the turbo inlet.

7. Spray down the compressor cover (known leak), BOV return/flange (DO NOT TAP YOUR BOV LINE FOR YOUR MBC!!!), IC end tank/fins and all connections. Re- test.

8. Note that you will leak air into the crankcase through the turbo seal but do not panic, this is normal during a static pressure test as long as there are no shaft play.

The desired test result from the begining of the LICP (bypassing turbo) is around 20psi (on boost gauge) with the compressor set at 30psi, while taking no less than 30 seconds to bleed down to 0.

As a reference, my last test on my 500 mile new engine, I was able to pressurize the system to 25psi, bled down to about 16psi (my 1G bov) in about 30seconds, then took about 3 mins to 6psi and just kinda lingered there for a while. It's not easy to do but the point is it's possible. My next goal is 30psi After motor break in and Dodge modding my BOV. A boost leak test is one of most pita but important regular maintenace task, the key is patience and endurance

 

[the_mork]

Generally for a boost leak test it doesn't matter because you are mainly testing between the turbo and the throttle body and the throttle body will hold most of the air in. As for pressure testing the manifold that is a bit different, I can't find where I've seen it before to be entirely certain but I believe the motor should 30 degrees past TDC for all the valves to be closed but your other option is to attempt to cap off your exhaust.

 

[bane3d]

I'm sorry to ask a noob question, but how do you set it to *30 TDC? I've never really understood the whole *5 retard and so on.

d

 

[FORMONTOYA]

When I do mine I just use a screwdriver and an eyeball.

Turn the motor by hand until the screwdriver (plug removed in cylinder 1) is at its highest point, and then "eyeball" about a 30* rotation. It doesn't have to be 30* exactly...just roughly 30* ATDC to ensure that a set of valves are closed so the pressurized air is not passing through the head and straight out the exhaust.

DO NOT TURN THE MOTOR COUNTER-CLOCKWISE

 

[steve]

Generally for a boost leak test it doesn't matter because you are mainly testing between the turbo and the throttle body and the throttle body will hold most of the air in.

If that worked your car would never idle since it couldn't get any air with the throttle closed.
Since your car does idle the air must be getting past the throttle plate when it's closed and that will cause the intake manifold and runners to be pressurized during a "boost" leak test.

If any intake valves are open then your also pressurizing that cylinder and if the exhaust valves are also open the air is just going out the exhaust pipe. Each cylinder at the end of the exhaust stroke and the start of the intake stroke is in overlap where both the intake and exhaust valves are open. If you look in the manual or search here you'll see the specs for the cams and can note the crank degrees where at least one set of valves in each cylinder is closed. That's where you want to test at.

Bruce, used to tell people to test at TDC until I asked him to think about it. I pulled 30* ATDC out of the air based on the factory cam specs since people should be able to visualize where that is on the crank and it wasn't that critical that they get it exactly. Close is good enough. The exhaust valves close around 9* ATDC on a turbo car.

 

[the_mork]

If that worked your car would never idle since it couldn't get any air with the throttle closed.
Since your car does idle the air must be getting past the throttle plate when it's closed and that will cause the intake manifold and runners to be pressurized during a "boost" leak test.

I get that, my understanding (from practical experience) has been that the throttle plate holds enough air back to pressurize the system enough to find leaks between the turbo and the throttle body, I wasn't saying that the throttle plate seals all the way. In my boost leak testing I've never bothered to turn the motor to have all the valves closed, sometimes they are and sometimes they aren't, when they are I'm testing the manifold as well, when they aren't I'm testing the rest of the system which is generally going to be more problematic unless you suspect problems with the manifold/injector seals/gastket. Keep in mind I'm considering this from a every other oil change standpoint and not a problem hunting one.

 

[V8KillerTSi]

When I do mine I just use a screwdriver and an eyeball.

Turn the motor by hand until the screwdriver (plug removed in cylinder 1) is at its highest point, and then "eyeball" about a 30* rotation. It doesn't have to be 30* exactly...just roughly 30* ATDC to ensure that a set of valves are closed so the pressurized air is not passing through the head and straight out the exhaust.

DO NOT TURN THE MOTOR COUNTER-CLOCKWISE

You can always use the timing belt cover and damper. It shows TDC.

 

[steve]

I get that, my understanding (from practical experience) has been that the throttle plate holds enough air back to pressurize the system enough to find leaks between the turbo and the throttle body, I wasn't saying that the throttle plate seals all

From my practical experience the throttle plate doesn't hold back enough to make a difference. If you have a significant leak behind the throttle plate it you won't be able to build pressure in front of it.

So "Generally for a boost leak test it doesn't matter because you are mainly testing between the turbo and the throttle body and the throttle body will hold most of the air in." is false since the throttle body isn't going to hold any pressure.

 

[the_mork]

I must be lucky then since I've always been able to perform boost leak tests pretty well without worrying about what the motor is turned to.

 

[crimsondragon]

A proper boost leak test should be done at the TB elbow. You never know when that intake manifold gasket starts leaking. Chances are it's most likely the throttle body gaskets though.

 

[steve]

I must be lucky then since I've always been able to perform boost leak tests pretty well without worrying about what the motor is turned to.

That a different issue. While this discussion has been about where to turn the crank to, the simple fact is that the engine usually stops running in a state where at most one set of valves is open. The odds of it stopping during overlap on any cylinder is pretty low.

 

[FORMONTOYA]

You can always use the timing belt cover and damper. It shows TDC.

I'm an old man and leaning over the fender while trying to turn the engine by hand sucks. It's just easier to see the screwdriver move and then just "eyeball" the rotation.

A proper boost leak test should be done at the TB elbow. You never know when that intake manifold gasket starts leaking. Chances are it's most likely the throttle body gaskets though.

The majority of my leaks are at the PCV valve, BOV base and throttle body seals (in that order).

 

[the_mork]

The majority of my leaks are at the PCV valve, BOV base and throttle body seals (in that order).

Same for me, except I also have problems with my J-pipe sometimes. I have never had a problem after the tb, in fact the only problem I've had besides the above was the throttle body to tb elbow gasket.

 

[Splitpi]

TB is my culprit for a small leak. But I'm too lazy to fix it and its not really noticeable.

 

[steve]

I see PCV, injector seals, EGR gaskets, and IM gasket leaks all the time (not on my car) and each of those is post throttle body. Once in a while I'll see a car leaking past the valve guides and seals. That's usually a head that had smacked the valves off the pistons and they didn't replace the cracked guides.

With the low quality of PCV valves available now and the number of rock hard or poorly fitting (Rochester derived injectors) injector seals I'm sure there are still a ton of people with intake leaks. It used to be you could get a PCV from the dealership that would seal but since they don't sell any new boosted cars using that valve their quality has dropped to the typical autoparts store crap. You have to test each before you can use one or change the system.

 

[erald01]

i just did my boost leak test the other day and noticed a leak by the elbow pipe that connects to the tb. Took it off put some rtv and there was still leakage but i noticed it came from the spring or somewhere around that area. is that a possibility?

 

[FORMONTOYA]

Throttle body shaft seals (2) are in the spring area. Don't forget to purchase the gaskets for both sides of the TB as well.

 

[Mike_GST]

Just trying to figure this out: why is recomended 30* after TDC?
Since the overlap happens between 21* Before TDC and 15* After TDC, wouldn't be the boost leak test recomended from 15* ATDC to 21* BBDC, in other words 15* to 159* ATDC crank angle range?

 

[steve]

Because it was easy.
You can see how far the crank moves for 10* by looking at the front cover, moving the crank roughly three times that far puts you into a safe area without requiring precision.