Leakdown test - Exhaust valves leaking

[Spetz]

Hi everyone,

I performed a leakdown test on a 6G75 3.8 Mitsubishi V6.
Engine has 140,000 miles. It is very clean inside though with no sludge or varnish, and crosshatching still on the bores.

The engine is on a stand, so I have easy access to everything.

The leakdown numbers are OK (10% or less per cylinder).

However, some of this is escaping from the exhaust valves.

On one of the exhaust valves I can hear the leak.
The rest I cannot, however when I spray brake cleaner into the port, it bubbles up on all exhaust valves so obviously some air is escaping.


My questions:
1. Is this normal, or should exhaust valves normally seal 100%
2. If this is normal, would it be worth lapping the valves?
3. Can this exhaust valve leak correct itself? In the sense, when the engine is warm, and/or combustion pressures are pushing the valves to seat more, could this essentially be a non-issue?

 

[clinebarger]

If the engine has sat for a extended amount of time....Rust will form on the valve seats of open valves. Have you checked valve clearance....Assuming they're not hydraulic?

 

[dumkid]

They're hydraulic on these Mitsubishis

Spetz, didn't you already ask this question in this forum?

 

[Spetz]

If the engine has sat for a extended amount of time....Rust will form on the valve seats of open valves. Have you checked valve clearance....Assuming they're not hydraulic?

The engine has not been sitting, and I have had a look with an endoscope camera, all valves etc look good.
Hydraulic lifters in this engine

 

[Spetz]

They're hydraulic on these Mitsubishis

Spetz, didn't you already ask this question in this forum?

I asked about a sticky exhaust valve.
I have remedied that, but in the process found out that all exhaust valves have a slight level of leak

 

[Rand]

They're hydraulic on these Mitsubishis

Spetz, didn't you already ask this question in this forum?

yes

Sticky exhaust valves

Hi everyone, I have a 3.8 Mitsubishi 6G75 on an engine stand preparing it for use. Using a leakdown tester to keep the valves up while replacing valve stem seals. I noticed a couple of exhaust valves are dirty from carbon deposits (presumably oil has been leaking through the seals), and only...

bobistheoilguy.com

 

[Spetz]

yes

Sticky exhaust valves

Hi everyone, I have a 3.8 Mitsubishi 6G75 on an engine stand preparing it for use. Using a leakdown tester to keep the valves up while replacing valve stem seals. I noticed a couple of exhaust valves are dirty from carbon deposits (presumably oil has been leaking through the seals), and only...

bobistheoilguy.com

I have remedied the sticky exhaust valve issue, but in the process found out that all exhaust valves have a slight level of leak.
Which is what I am asking about here

 

[Rand]

so none of that information is revelant to your new exhaust valve question?
since its related I would have just continued on in that thread.

I think leaking sticky exhaust valves are very relevant info to "leaking exhaust valves"

 

[Spetz]

so none of that information is revelant to your new exhaust valve question?
since its related I would have just continued on in that thread.

I think leaking sticky exhaust valves are very relevant info to "leaking exhaust valves"

No, it is not relevant.
The issue there was a sticky valve, and how that can be fixed.
The issue discussed here is the results of a leakdown, with no issue of sticky valves.
If I were to bring up a new issue, 10+ post down in the thread, how many people would see that thread and answer that question, instead of the first post?

In any case, let's focus on the issue, and not the perspective and semantics of relevance of one thread to another.

 

[clinebarger]

The engine has not been sitting, and I have had a look with an endoscope camera, all valves etc look good.
Hydraulic lifters in this engine

Slight exhaust valve leakage can be considered normal at 140,000 miles as the exhaust seats & valves get subjected to much more heat than the intake. With good leak-down results.....I'd run it.

 

[Rand]

No, it is not relevant.
The issue there was a sticky valve, and how that can be fixed.
The issue discussed here is the results of a leakdown, with no issue of sticky valves.
If I were to bring up a new issue, 10+ post down in the thread, how many people would see that thread and answer that question, instead of the first post?

In any case, let's focus on the issue, and not the perspective and semantics of relevance of one thread to another.

I felt it would be relevant and helpful to know the recent work done on the motor. Which you didnt mention in your first post.

Also if the valves were sticky due to leakage.. and the new post is about valve leakage seems like important facts to know.

 

[Spetz]

Slight exhaust valve leakage can be considered normal at 140,000 miles as the exhaust seats & valves get subjected to much more heat than the intake. With good leak-down results.....I'd run it.

The engine is on a stand and a bare long block.
Would it be worth the cost of head gaskets and head bolts to lap the valves would you say?

 

[clinebarger]

I'm spoiled with a good machine shop that will expediate a valve job if needed, If possible....Send them out & have the seats & valves ground/machined concentric.

 

[KrisZ]

I’m not sure what’s here to even debate. The engine is out already and the valves are leaking. Pulling the heads seems like a no-brainer at this point.

 

[Spetz]

I'm spoiled with a good machine shop that will expediate a valve job if needed, If possible....Send them out & have the seats & valves ground/machined concentric.

Here (in Australia), servicing the 2 heads will be about $2,000, so at most I would do a DIY valve lap

I’m not sure what’s here to even debate. The engine is out already and the valves are leaking. Pulling the heads seems like a no-brainer at this point.

As above, it is the cost. The leakdown results are still good, and the question is whether this is normal for all engines (especially at this mileage), or whether this is necessary to address

 

[fantastic]

The engine is on a stand and a bare long block.
Would it be worth the cost of head gaskets and head bolts to lap the valves would you say?

Yes, Clean them up & lap them good. Might as well change the valve seals if you can. Like mentioned Exhaust valves are under extreme heat & will deform over time causing the leak you see on all of them.

 

[Spetz]

Yes, Clean them up & lap them good. Might as well change the valve seals if you can. Like mentioned Exhaust valves are under extreme heat & will deform over time causing the leak you see on all of them.

Thanks,
Yes I think I will do that.
I am changing the valve stem seals, it is how I discovered the issue, as I could hear air escaping from one of the cylinders via the exhaust port.

I assume I will not require to skim the heads/block if everything is true? Since there was no overheating or head gasket issue to begin with

 

[fantastic]

Thanks,
Yes I think I will do that.
I am changing the valve stem seals, it is how I discovered the issue, as I could hear air escaping from one of the cylinders via the exhaust port.

I assume I will not require to skim the heads/block if everything is true? Since there was no overheating or head gasket issue to begin with

Good deal, As long as there's no issues with these engine's gaskets failing & your machine shop gives you some direction. You & them would know best on probability of gasket issues but you might do just fine w/o doing that machining work.

 

[Chris142]

When the engine is running it happens to fast that it doesn't have time to leak much .

 

[SC Maintenance]

I thought 10% leakdown was really good? Is this 10% purely from the valves, or total - ie you did a wet leak down.

Unless there was some other compelling reason to take the head off, I wouldn't personally.

FWIW - the instructions always say to do a leak down with the engine warm, but no one ever does.