A question on blow-by

[Z-Luke]

I have a question on the blow-by or crankcase fumes coming out of the top of the valve cover on my car. I noticed that the car doesn't burn any oil or smoke at all when the hose is disconnected from the air intake. The amount of 'steam' coming out of the valve cover starts out as almost none at all, to lightly and steadily rising out like a kettle with the lid open coming close to boil. It comes out steady and not in puffs with each stroke. It also steams out the oil disptick tube if you pull the dipstick.

When connecting the hose to the air intake, the car noticably idles rougher, and the exhaust gets a little stinky and it smokes a little on throttle. Smells like oil/gas combo, not sure if it's moisture in there too I can't tell. I know that it is just directed into one carb, so I'm wondering if I should try to get it to steam into the middle of the air intake instead where it bends into the front carb. I figure the rough idling is from the stink going into just one carb.

I'm okay with this much blow-by (maybe its just leaking past the valves, not sure) because the engine has pretty good snot and the smoke isn't really noticable when fully warmed up and under steady load. I'm just wondering if I should try to evenly distribute the blow by or if I have a bigger problem on my hands. Maybe some seafoam or heavy diesel oil will help get the gunk off the valves.

Thanks!

[ajmcforester]

OK I've felt stupid to many times by giving advice to find people haven't done the basics first. This can be several things, and I can help you start to diagnose the problem. First more information is needed.

How many miles?

Was the engine rebuilt, and when?

If harden seats have not been put in have you used and lead additive?

What color is the smoke?

When was the last time the valves were lashed (check and adjusted for cam clearance)?

Was their a lot of adjustment?

When was the last time a compression test was done if a all?

Any coolant or other fluid loss?

Since you mentioned oil smell what weight do you use?

Any engine based mods including electrical upgrades?

Edited April 20, 2010 by ajmcforester
one more thing

[Z-Luke]

How many miles?

?54,000. Whether there's a one or a two infront of that....I don't want to know really.

Was the engine rebuilt, and when?

number on the motor is L24-090472, which does not match the vin (XXXX62)

thats all the info I have on the motor.

If harden seats have not been put in have you used and lead additive?

I have not used any oil additives, oil was changed with penzoil 10-30w recently.

What color is the smoke?

It's a hazy blue, kinda smells like a mix of oil and gas. Stinky. Doesn't blow any smoke when the crankcase breather tube is polluting straight to atmosphere.

When was the last time the valves were lashed (check and adjusted for cam clearance)?

Was their a lot of adjustment?

I did the valve lash a few weeks ago, although I went .05mm too loose (car cooled off before I got the hang of the adjustments). When I did the adjustment, I noticed that sometimes i couldn't even fit a .10mm feeler inbetween some of the intake valves. To me they seemed too tight.

When was the last time a compression test was done if a all?

I have a compression tester, but I'm afraid of what I might find

Any coolant or other fluid loss?

If there is, very minimal. Just checked the oil and its full, still quite clean. It smells vaguely of gas, just a hint. Maybe some gas is getting in my oil, or it could be just my messing with carbs. My sense of smell isn't very good, but against the smell of the oil still in the container I bought it in, it smells a little bit of gas by comparison.

Blow by would put gas into the oil - seems reasonable?

Since you mentioned oil smell what weight do you use?

10-30W Penzoil yellow container. It's fairly cool up here. Dips below zero overnight still.

Any engine based mods including electrical upgrades?

A previous owner put 3-2 headers on. All else stock - dizzy, points, everything.

Edited April 20, 2010 by Z-Luke

[ajmcforester]

You have done more than most so far the next step is to do a compression test. I was recently in a discution helping someone do this and get the best results I hope this link will help. Also you might want to do a leak down.

http://www.classiczcars.com/forums/showthread.php?p=324506#post324506

What concerns me is a rule of thumb I was given of replacing the rings every 120,000 with these cars and ?54,000 does not sound good.

Lead additive is not for the oil but the gas these engines have brass or bronze guides and seats on the valves. What lead or lead substitute does is protect the guides and seats. When you put harden aluminum seats it fixes this problem.

Also how did the valve seals look?

My guess right now is either head-gasket, valve guides, and/or rings. However lets do the checks before taking the engine apart their are other probabilities, also getting some data before doing that will let you know ware to look.

[Z-Luke]

Just a quick update, I went and got myself a can of sea-foam. I warmed the car up to full operating temperature, turned off the choke completely, pushed up on the nozzles to make sure they weren't sticking (the rear carb is notorious for this).

I let the vacuum tube suck up 1/3rd of a can in quick spurts while gunning it. This was hard to do, so I ended up turning in the high idle screw and dropping the vacuum tube into the can. Stinky smog cloud!

Then I shut the engine off, and while it was still hot poured in 1/3rd of the can in the oil. Interestingly enough, the engine was still "steaming" from the inside when I took the cap off. Not pressurized, but still a-steamin.

Then I went to the gas station. The car was having trouble holding idle after ingesting all the sea-man-foam. I poured in the rest of the schmutz and 20 bucks of gas. Took it out on the highway for a nice long 30 mile cruise, and when taking the turn to come back I opened the throttle as far is it would go in second gear. Around 5000 rpm, where the car usually wheezes out and refuses to go further without missing, there were two INCREDIBLY LOUD CANNON FIRES out of the exhaust. It could have been the sea-foam. Not sure.

Then I came home and disconnected the breather tube I was talking about earlier. The one that throws the carbs off because of the stinky going into just the front carb. After disconnected that, there was a remarkable increase in power, off-the-line driveability, and smoothness! Impressive increases, in fact. However, when stomping on it in first gear, I impressed everyone in the neighbourhood with my 2.4L 6 cylinder anti-ship smooth bore explosion pipe once more. KAPLOW!!! It was so loud it sounded like it was right behind my head.

Anyways, the car still drives fine, not sure what gives with the explosions. Could be the sea-man. Just figured I'd update since I have now put in the additive.

Edited April 21, 2010 by Z-Luke

[Z train]

[quote name=ajmcforester;

What concerns me is a rule of thumb I was given of replacing the rings every 120' date='000 with these cars and ?54,000 does not sound good.

Who ever told you this has their thumb up their a$$.:stupid:With proper maintenance,L-motors can go 300 thousand miles before "needing" rings.

[Z-Luke]

after reading through the FSM a bit more, I think I may have a bad PCV valve causing excess pressure in the crankcase.

I'll look into it. The problem is getting parts for a 1970 240z isn't particularily costly, it also isn't very quick.... looking at 3 weeks at least to get a new valve. Might as well order a new alt, pertronix and tune up kit while I'm at it.

[ajmcforester]

Replacing the PCV valve is a good idea. I ruled it out since you stated you stated you notice one half of the engine was worse than the other. Back fires are usually caused by lean conditions, with the carbs on the Zs you can get some great explosions if one is really rich and the other is lean.

By the way the friend that gave me that advice about the rings, only raced his car so it might be a little soon for a street car, but if the rings didn't set correctly they would be done by then also it gives a good indicator when to start checking compression as routine maintenance. With how these blocks are if you catch it early enough you can get away without re-honing the block when putting in the new rings.