Boost issue... Wondering if I have engine issues.

EagleDave

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Dec 6, 2020
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Hello all!

OK so I have a boost leak... I know I have a small one on the gasket of my aftermarket Y-Bridge.. but that's not what is concerning me. While doing the boost test, I noticed a lot of air coming out of downpipe connection at turbo.

Lets back up, I had a broken up pipe at egr bellows and am in the process of replacing it. Currently down pipe is out of truck and new up pipe is installed.

Back on topic... So I start reading on Mitchell OD and the factory procedure to test boost is to install tester on hot side after turbo (which I am doing) and to remove the cat and install a J-46091 to the exhaust. According to the manual "Failure to secure the J-46091 will cause excess air leak and hissing noise during the test" Since exhaust is disconnected, I put a Fernco 3" rubber cap over the exhaust part of turbo (fits perfectly). So now the boost leak is slower but I noticed air coming out of the PCV port that attaches to the turbo inlet horn (my intake horn is off right now). I can put my hand on the part to block the air, and when I remove it, you can hear the air whoosh out.

I've read a mix of reports that since the duramax doesn't have overlapping valves, you shouldn't have air leak out exhaust while testing boost.. Well I do... and I'm wondering why GM says you need to block off exhaust to test?

Secondly, why is air coming out of my PCV connection when I'm applying 30psi to tester?

Truck is a 08 LMM... EGR and DPF delete...

I was driving last week and noticed getting on it at 70ish, it would downshift and ran a little rough... no smoke out tail pipe though... After about 50 miles are so, the rough running after downshift went away. No noticeable smoking, idles just fine. Did lack a little power on acceleration, but that may have been because the huge hole in the up-pipe not allowing enough exhaust into turbo? I did notice one time my oil dipstick was out of the tube by about 1/2". I ignored it and just pushed it back in. Only happened once though.

Today, with tester on, pressurized to 30psi, takes about 12 seconds to go from 30 psi to 20psi.

I'm actually nervous about a cracked piston because the dipstick thing. But there is still good power and no white smoke. But why is air coming out of pvc tubing when air is supplied to boost tester?

Any ideas?
 

EagleDave

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Dec 6, 2020
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Tested it again while oil fill cap was off... I can feel air coming out of oil fill neck. So obviously some of the boost is getting into the crankcase.
 

2004LB7

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Dec 15, 2010
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How would you know that all the exhaust valves are closed when doing the test? I would think air would leak past the ring gaps pretty easy in a static test but have never done a pressurized exhaust test like that before
 

EagleDave

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Dec 6, 2020
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How would you know that all the exhaust valves are closed when doing the test? I would think air would leak past the ring gaps pretty easy in a static test but have never done a pressurized exhaust test like that before
I don't... Many people say there is no overlap in the duramax, but I see a lot of people saying there is.. and if there really wasn't any, why does GM say you need to block exhaust to do the boost test? So Yes, I assume some exhaust valves are open, some are closed and there is overlap... thus air from test leaving exhaust...

The test I performed came from the service manual. There is actually 2 tests listed. Both inject air at the hot side inlet after turbo. The one test is to test intercooler and you place a block off plate at intake heater. Pump system to 30psi, turn air off, and leak down should not be less than 10psi in 15 seconds with no noticeable air leaks. The second test is the full system test and you need to block exhaust. You pressurize the system with 30psi of constant air and it should hold at least 23 psi with no leaks. If there are leaks or it holds less than 30, then it fails. Mine held, but I had a leak at Y--bridge which I will tackle in the summer... it's just a small leak and it's a pain in the ass to do.

In regards to the air slipping past rings... that would occur too, especially with an older engine. I just feel like mine has a awful lot, but I'm new to diesels and I've never seen this before, however, I've never specifically looked for it either. If I would have observed the same stuff a year ago during a boost test, I'd probably not be concerned.

I'm probably way over thinking this. The truck ran fine except for the lower power which I'm attributing to the broking up pipe. When you first start car at idle there is a little smoke but it goes away, and it idled fine. The oil is down two notches on dipstick after a few thousand miles.

My MBRP downpipe was supposed to come Saturday. It's currently lost in USPS world so I'm heading out today to buy a PPE downpipe. I will get the truck back together tonight and do some tests. See how it idles, etc.. and report back so other people can see this if they have the same problem.

The thing that concerns me, which I didn't think was a big deal until I read about cracked pistons, was my dipstick being pushed out of the filler by about 1/2 inch. To me it wasn't a big deal and just pushed it back in.

Maybe one of my pcv vents is bad and all the block ventilation is forced out the other one plus the dipstick? Do pcv vales go bad and how do I test that?
 

zakkb787

<that’s not me...
Sep 29, 2014
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Granite Falls NC
You’re going to leak boost unless all your exhaust valves are closed. Low power was definitely your up pipe. Ask me how I know... I’d say it’s fine. If you really want assurance, do a compression test. Super easy. Pull glow plugs and get the kit from HF and go to town. Sure it’s not gonna be the most accurate test, but you’ll be able to see if one cylinder is way down.
 

EagleDave

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Dec 6, 2020
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Ok everyone. Truck is back together and it seems like I was worried for nothing.

I noticed the dipstick just pops out on its own without engine even running. I think I bent either the tube or dipstick itself during EGR delete and it has a little spring action that pops it out.

I installed everything except for the TC heat shield. Started truck up and ran great. Little smoke out tail pipe but it was also 31 outside. After awhile it cleared up. Tool oil fill cap off and no smoke or pressure out of it. Drive it for about 30 - 40 miles and great boost and plenty of power.

the only issue I have is the small boost leak at aftermarket y-bridge. Installing that part was the worst thing I’ve ever installed on my truck so that will either get ignored or fixed when it’s warmer. It’s such a small leak that I’m not that worried about it.

I will also soon post my procedure for replacing up-pipe and down-pipe. Lot of people said it’s a nightmare job in the LMM. I found it to be the opposite. I’d say, if I were to do it again, it would probably take 3 hours to do both. Maybe I did things differently and made it easier. Idk. I will create a new post soon as there isn’t much info on up pipe replacement for the LMM

thanks everyone for your help!
 

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