Theories Welcome

Hi Guys,

So, accepting any/all theories. My engine runs perfectly except for excessive blow-by (evidenced by crankcase pressurization but, no oil burning). Normally, I'd suspect bad piston rings but, I get great compression (172-175psi across all 4 cylinders), spark plugs are all clean and dry, valves are all in spec and I pull an excellent 20" vacuum on the intake manifold.

By ruling most other things out, I am now suspecting stuck piston rings. But, if that were true, would I still see such good compression?

My engine was rebuilt about 7,500 miles ago. The issue started about 2,000 miles ago.

Go figure...

Bob

PVC valve blocked, stuck, or not functioning for some reason would be my first guess. Or maybe one of the hoses to or from the PVC collasped.

Others chime in.

I'd like to suggest a theory, but I truly don't know what "blow by" is, or how you'd detect that. What symptom are you seeing?

Also, how are you detecting excessive pressurization? Oil pressure?

I'm really just confessing how little I know, but an education on those concepts/terms along the way would be good. Then maybe I could have something small to contribute, having just rebuilt my own TR4A engine.

thx

As bgbassplyr suggests, take a look at the P.C.V. system, including the valve and hoses.
If those items are not found to be the problem.
A compression leak down test would give an indication of ring, valve and valve guide condition. {If you really want to know the "Skinny" on them}
Once you know the rings and valve systems are ok, there really isn`t much else left to look at.
Excessive blow by is usually indicated by leaking seals {front and/or rear} and pan, valve cover and or timing chain gaskets, and/or oil vapors escaping through the oil fill cap {if vented to atmosphere}
If none of these conditions exist, were it me, I would forget about it and ..... drive it like I stole it. *SMILE*

Yes, us aircraft guys prefer leak-down testers.

They give you more info (but first, as suggested) check PCV hoses...........and be sure you don't have too much oil in the engine.

Here's a cheap ~Leak-down Tester~.

I know some of the guys here have built them too.

Another thought:

Do a stone-cold compression test (don't start or warm up the engine). See what reading you get.
Then put some motor oil (about a teaspoon) down each spark plug hole and see if compression changes a lot. If you temporarily get a lot higher reading, it's usually stuck or worn piston rings.

Thanks everyone. It runs great in every other way. So, as you suggest I drive it like I stole it...it's just my curiosity needs to figure this out.

I have cleaned, inspected and rebuilt my original PCV valve. That didn't work so I broke down and bought a new one. That didn't work either. For the time being, I am running a 1/2" hose off the valve cover down below the chassis to vent the crankcase and it completely solves the issue. However, as mentioned, my curiosity is getting the better of me. When I remove the oil filler cap with the engine idling there is definitely a "puffing" of air. But, isn't a little puffing of air considered acceptable?

I've been trying to get it in for a leak down test since Labor Day but, schedules, work, travel, kids.... The wet compression test shows about 180psi, not a big jump. I definitely suspect the rings but, not sure why I still see such good compression(?). I was running a very rich carb mixture for a while (by accident) so I suspect one of two things:
1) bore wash which destroyed the finish on the cylinder walls (how likely is this? And, if this is the cause, is there any easier fix than a complete tear down?), OR
2) carbon build-up from the rich mixture causing the rings to stick in the lands?

I am hoping it is 2). If so, any suggested tips on how to free stuck piston rings? I figure I can try this first - what do I have to lose, right?

Bob
PS: for kentvillehound, this is not related to oil pressure on the gauge. Blow-by is when excessive combustion gasses escape past the piston rings and go into the engine crankcase rather than passing out via the exhaust valve and down the exhaust system. This is what a leak down test will detect. This is evidenced by crankcase oil being forced out under pressure past every seal and gasket on the motor (as Kerry said) - not pretty!

PPS: too much oil? - hadn't thought of that one. I use 6 quarts in the crankcase. Engine is completely stock.

Bob-

What kind of vent cap do you have on your cover? I thought the 4A with PCV valve had a vented cap, it is kind of funny looking with a smaller cap on top of the main cap. That may be part of the problem.

Randy

Thanks Randy,

I've tried both the vented oil filler cap and closed oil filler cap. No difference. The vented oil filler cap is what I originally had (and is said to promote better fresh air exchange in the crankcase). But, thought I might get a better vacuum pull in the crankcase from the PCV if the cap was closed.

In the end, it didn't matter as respects my crankcase pressurization issue. I'm still thinking it's something funny with my piston rings.

Bob

I have a small 1/2" crankcase filter where you describe you have the tube below the chassis that solves the problem. You should be able to buy one for around $20. Check the K&N webiste for crankcase filters measuring an interanl diameter of 1/2" and give that a whirl. If that solves your problem, then plug the tubes to the carb and call it a day?

RJS said:

...My engine was rebuilt about 7,500 miles ago. The issue started about 2,000 miles ago...

Click to expand...

What break-in procedure was used? When engines get broken-in with gentle driving some think that can prevent the rings from seating well (or other problems).

I had some similar symptoms but improved things with the alternative theory of break-in which involves hard acceleration and then abruptly backing off the gas. I think the idea is to work the rings with the sudden changes.

Before you try it get & consider other opinions.

Although that did help, in the end I added a TR3 breather tube to the side of the block and abruptly ended the tendency of oil to ooze from every pore.

I had a rebuild where in one cylinder the rings rotated until the gaps lined up.

Thanks Geo,

That's a new theory worthy of serious consideration. I can definitely admit the engine has been babied and driven gently. I think I'll give it a try since I have nothing to lose. Not to mention, I have heard similar advice for solving stuck rings. Namely, drop it in 4th gear at 30mph and floor it to 60mph, repeat. I also heard taking it for a good 2 hour run at a sustained 70mph could help too.

Unfortunately, our driving season is coming to an end here in the northeast so I may have to wait until spring.

Oh, hey, any validity to the "bore wash" theory? I've heard about bore wash but, find very little info about it and find it hard to believe my overly rich mixture was enough to destroy the ring-to-cylinder wall seal. The engine still ran albeit with poor mileage and sooty exhaust.

Bob

I've never done this but have heard that it works.

Pour a little Bon Ami, kitchen/bath cleanser, into each cylinder and fire it up. This is supposed to reseat the rings.

Jim

Bon Ami sounds pretty drastic. Not sure I want to go down that road. As I mentioned, the engine runs perfect in every other way so, the risk/reward factor doesn't sound in my favor. I'm thinking first option is simply to drive it hard for several hundred miles to see if the rings re-seat or un-stick. Possibly even a little Marvel Mystery Oil in cylinder or crankcase.

Bob

A couple of things crossed my mind after reading "rebuilt 7,500 mi ago & the problem arising after 5,500 miles of easy driving"
Question #1 who did the rebuild? And .... did they provide the proper ring end gap and orientation to one another during assembly?

I ask because 5,500 miles even with an excessively rich fuel mix isn`t enough to stick a ring with carbon deposits!

The consistency in your comp. testing so far, indicate that the rings are in tact {not broken}

Assuming ..... that seals and gaskets are leaking oil profusely, indicating excessive crank case pressure.
One can only guess at the cause,
I have seen compression rings installed upside down believe it or not! It does make a huge difference!
As well as all ring gaps aligned perfectly in a row, as well as aligned with the wrist pin! {which is a definite no no.}
I have also seen cylinders that were tapered beyond recommended tolerances which allows for good compression when rings are in the new to newer range, yet allows excessive blow by too.
Assuming the seals and gasket areas are dry.
Some "Puffing" out of the oil filler cap is normal As long as it isn`t cause for misting the engine compartment with oil residue. Or causing excessive oil consumption.
Assuming the engine was built with a race application in mind ....... these engines are normally built with excessive tolerances to allow for the expansion of rotating parts caused by heavy loads and prolonged heat conditions.
they are as a rule "Oil eaters" especially when used in a normal daily driving application.
There are just too many variables to diagnose a blow by condition, without knowing ALL of the pertinent information.
P.S. Bon Ami Ewwwwweeeeee! Although I have done that {It does work for a while} to an old tired engine in order to get a few more miles out of it before SCRAPING the entire vehicle. I don`t think I would be doing that to a vehicle I intended on keeping around.
I wouldn`t be beyond trying the water misting de coking trick if I thought I had a stuck ring or sticking valve {very carefully you don`t want to bend a rod or valve stem!}

OK, assuming your oil level is correct and you've compared cold/hot compression readings for excess differential:

How are your exhaust valve guides?

Does your car use valve guide seals? (I forget).
And if not, can they be retro-fitted?

I'll chime in with: I agree with Kerry that there are a LOT of variables.

Not so sure the steady 70 MPH thing ~or~ the third/fourth gear procedures are best... any engine I ever built for street running was broken in with a couple initial starts and run to normal operating temp, allowed to cool to ambient, then oil/filter changed and taken out for several runs with RPM variation not exceeding 3~3.5K RPM until it saw 500 miles or so. No abrupt RPM changes, just not any steady RPM for long.

The cleanser trick is only applicable for a race engine, where tear-down is coming within a few hours' running time. It'll chew things up really well!

Ring gap too large or all lined up is a real possibility, but short of pulling it apart no way to know.

Sea Foam or Marvel for a few oil changes couldn't hurt, either.

A leak-down test is about the best diagnostic, or a "smoke tester".

Hi Everyone,

Happy Thanksgiving! Just ran 5 miles with my son in a local Turkey Trot so I can PIG OUT today and not feel guilty.

Thanks for all the input. I really appreciate the collective contribution. I agree there are just too many variables so, much of this is speculation. But, that's OK. I'm just trying to get some ideas of possible causes. As I said, the motor runs perfect in every other way (so long as I vent the rocker cover with a 1/2" hose, not PCV) so this is not a critical issue.

Just two quick items:
1) No one's said much about the possible bore wash issue. Is that b/c it is folk lore?
2) For the past several thousand miles I have run a Pennzoil 15W-40 diesel oil (for the high ZDDP levels). Is it possible the high detergent levels in this oil had some effect?

In the meantime, I'll just keep driving it and eventually have a leak down test done.

Bob
PS: are piston rings designed to float and naturally rotate on the land? Or are they fixed once installed?

My WAG on this is that the piston rings just did not seat properly when the motor was re-built.Did you lube the rings in any way when doing the assemble???They should be clean and dry when installed!!!If you lube them even a little they can require years to seat,or might never.
MD(mad dog)

MDCanaday said:

...If you lube them even a little they can require years to seat,or might never.

Click to expand...

Indeed, it's my understanding that this is the condition the Bon Ami treatment is meant to address. I believe Bon Ami cleanser is specified as it does not scratch metal -- what is does do is absorb oil from the cylinder walls for a few strokes of the engine and thus simulate the conditions of the dry start that MD describes.

I'm not saying do this -- just offering my understanding of the principle of the thing.