White smoke at high RPM

[dcraigbrown]

My 73 240z belches white smoke, only at high RPM. I've read enough threads here and else where to know that white exhaust can mean a blown head gasket but I don't see any water in the oil, no bubbles in the radiator when I applied pressure to each cylinder, and compression is between 140-150. I assume these tests are just backyard mechanic quickies and it still could be a blown head gasket. Are there some more likely culprits?

Thanks for any help

[Fun_in_my_z]

Bad rings could do it also. Define "high rpm? How high are we talkin here?

[d240zx2]

I'm thinking rings, but could be head gasket. Have you smelled your exhaust lately? You can definitely get a whiff of the ethylene glycol at the exhaust pipe if you've got a gasket failure.

And as Fun asked, what does "high rpm" mean in tach numbers?

Frank

[dcraigbrown]

Unfortunately the tach isn't working. A rough estimate would be ~5k.

[dcraigbrown]

I should also mention this car is in the process of being made road worthy. I just did the 280zx dizzy mod and am still playing with the timing, getting carbs tuned, etc. This isn't a situation where the car was runnig great and then the problem suddenly appeared.

Also, concerning compression. I'm in CO at about 6400ft and I found a chart that converts compression readings base on altitude, 150 is right around 180- so I guess I wasn't considering rings.

[ozconnection]

Its not auto is it?

[DeesZ]

The previous suggestions seem more likely, but have you changed the oil lately? If so, what did you put in? Using a lower viscosity oil than the previous stuff may be the culprit. From personal experience I have seen this happen. 30 years ago at about 75K I switched from Castrol GTX 10-30 to GTX 20-50.... about 10K later, when winter set in, I went back to 10-30.... what a mess with smoke at higher rpm... It looked like white smoke in the rear view mirror (but it must have been a very light blue). Went back to 20-50 and no problem. I'm really not sure if it was blowing by the rings or the valve seals, but the problem did not return as long as I kept 20-50 in it.... ran well for another 30K until I sold it, and for a good while for the next owner, then I lost track of the car (wish I had it back).... Just a thought....FWIW

[dcraigbrown]

Its a 4 speed.

I believe I put straight 30wt in it. Would 20-50 be considered a higher viscosity than 30?,

under normal driving conditions?

[m4xwellmurd3r]

20w-50 is 50wt oil at 100c so yeah, 30wt is a lower viscosity oil than 20w-50

multi grade oil is only really good for places that see cold temps (like, freezing cold) I live in arizona, single grade is fine.

(I run VR-1 20w-50 in my Z, but I'm in Arizona, higher viscosity helps)

How many miles are on it? as far as I know, a higher viscosity oil in an older engine isn't really a bad thing.

[dcraigbrown]

It has 206K on it, 100k on a rebuild.