Puff out intake on old diesel

[druid]

Hi. I have a few ideas about this, but I thought I'd ask here to try to get some more input.

I have an OLD Volvo MD6A 10hp 2cyl diesel. It kinda runs, not sure about the power, but smokes quite a bit. Now, one thing I noticed when I disconnected the hose from the valve cover to the intake: There was no "puff" out the valve cover side (indicating no real issue with compression getting past the rings), but there was substantial "puff" on the intake. I can't tell exactly when this puff occurs (compression stroke, intake stroke...), but at starting there's a lot of smoke comes out the intake manifold, so first guess would be the exhaust is not getting out.

I had the valves re-seated recently by a reputable company, so I doubt the puff is from compression leaking past the valve. But I'm wondering if somehow the intake valve isn't closing completely...? Or the exhaust valve isn't opening enough, or there's enough blockage in the exhaust that the exhaust doesn't get out before the intake valve opens? There is some buildup in the exhaust manifold, but still lots of passage.

I'm now looking at the rocker arm assembly (cuz it turns out one of the studs holding it down is broken), and will confirm that the valves do indeed open and close properly. Next place is to clean out the exhaust passages and see if that helps.

Any other thoughts? What would cause a "puff" in the intake manifold?
druid

 

[Texas Lake Sailor]

If you have a broken stud in the rocker, is there enough play that your valve timing could be off?? Have you put a compression gage on it?

 

[kloudie1]

Exhaust valve clearance way too loose (busted rocker stud?) so the exhaust doesn't all get out before the intake valve opens..
Intake valve clearance set too tight so the valve never seats all the way.(less probable)

 

[sailaway78]

Valve Lash

I'm thinking that because it doesn't have any smoke from the breather tube, you more than likely have a valve problem. The broken stud is probably at least partially the cause. My thinking is that fix the stud, set the valve clearances and you will be good to go.

 

[RichH]

First, Check for a 'sticking' intake valve.

 

[druid]

First, Check for a 'sticking' intake valve.

Ah... THAT's a possibility. I believe they did replace the valve guides, so they may be a bit tight. Also, this engine only has (can't remember the name - little rubber things that keep oil from going down the valve stem) on intake valves and one was missing.

I'm assuming it would only stick for a fraction of a second - how do you test for that?

"I'm thinking that because it doesn't have any smoke from the breather tube, you more than likely have a valve problem. The broken stud is probably at least partially the cause. My thinking is that fix the stud, set the valve clearances and you will be good to go. "

That's my thinking as well. Unfortunately the studs are no longer available, so I have to make one up from a bolt and dies. On my List of Things To Do....

druid - it's a "project" engine!

 

[wonkodsane]

Also, this engine only has (can't remember the name - little rubber things that keep oil from going down the valve stem) on intake valves and one was missing.

Valve stem seals!

 

[Deleted member 78819]

Just dealt with that. Had to clean out the mixing elbow... too much back pressure on the exhaust.

 

[RichH]

Ah... THAT's a possibility. I believe they did replace the valve guides, so they may be a bit tight. Also, this engine only has (can't remember the name - little rubber things that keep oil from going down the valve stem) on intake valves and one was missing.

I'm assuming it would only stick for a fraction of a second - how do you test for that?

Vacuum gage on the intake manifold. The gage will 'drop/bounce' each time the faulty valve attempts to close. You 'feel' each intake valve while you watch the gage.
An air pressure leak down test can also be used to find the offending valve.

 

[druid]

Vacuum gage on the intake manifold. The gage will 'drop/bounce' each time the faulty valve attempts to close. You 'feel' each intake valve while you watch the gage.
An air pressure leak down test can also be used to find the offending valve.

Don't need a vacuum gauge - I can feel the puff with my finger. Question is can I run it slow enough to see if it's puffing when the intake valve is trying to close (which one?) or open (indicating exhaust problem) or in compression stroke...

A leakdown would only work if it's not seating properly.

It DOES idle down to about 500 rpm.
druid

 

[Jalepeno]

Just arrived around midnight at the Nanaimo boat basin, chock-a-block full but I diid get the last spot.
Can you use your decompression levers (if you have individual ones) to isolate the suspect cy;inder?

 

[druid]

Just arrived around midnight at the Nanaimo boat basin, chock-a-block full but I diid get the last spot.
Can you use your decompression levers (if you have individual ones) to isolate the suspect cy;inder?

Aahhhh.... THATs a good idea!

"Nanaimo boat basin" - you mean Newcastle? Like Silva Bay, it's choked with locals, many of "questionable" seaworthiness. Too bad - used to be a good destination: love the bunnies that come out around sunset (and if you're lucky - a deer!)

BTW: I hear there's a guy in Sidney that deals in well-used Volvos that may have a stud for my engine - do you know who that is?

druid

 

[RichH]

"A leakdown would only work if it's not seating properly."

Precisely .... if you're getting an excessive leak when the suspect valve is supposed to be fully seated - you've identified the offending valve. A sticky valve is not seating properly over the total cam interval in which its supposed to be fully closed.
Such can also be attributable to a valve that has been adjusted 'too tight' ... and thats the easiest 'fix' to accomplish.

Suggest that you carefully/precisely 'readjust' all the intake valves and see if that makes a difference - FIRST.