blue smoke

oldae82
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blue smoke

Postby oldae82 » Tue Oct 25, 2022 5:00 pm

I have a 4ag I've recently put back together. I used Tomei 264.8.15 cams, intake and exhaust. I used weisco pistons, used, with rings, rods, and new rod bearings. The seller of the pistons was unsure of the compression ratio, thought it might be 10:1. He said the engine he'd installed them in spun a crank bearing and he lost enthusiasm for the project. He thought maybe 150 miles on the pistons, but I suspect more, looking at carbon. Pistons looked good, rings looked good, had sharp edges. All assembled and running. Engine is showing 215-220# compression without the butterfly open, warm engine, on all 4 cylinders. Engine shows 195-210# on cold compression test.

Problem is this. I was driving on the interstate at about 4k in 5th, went to slow down for traffic, downshifted, and was enveloped in a cloud of blue smoke. Further playing/testing on back roads shows blue noticeable smoke on hard acceleration. Engine sounds good, pulls hard to redline, and shows good compression. I am running 93 octane fuel. Any idea as to cause of this embarrassment?

Corey20v
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Re: blue smoke

Postby Corey20v » Wed Oct 26, 2022 1:04 pm

Bummer...
Valve stem seals tend to cause blue smoke especially after downshifts off throttle (high vacuum in the head). It will then smoke once you get back on throttle.
Usually a block should be machined to fit the pistons to get the right wall clearances, and the rings should be gapped to account for thermal expansion. There are rings for oil control, but i believe that would cause smoke all the time.
I dont think these cars suffer too badly from pulling oil back into the intake from the PCV, but it doesnt hurt to pull the intake apart a bit to inspect around.
1986 SR5->20V ST (WIP!)
Other toys: 1993 S13 Coupe, 1993 NA6 Miata, 1996 Z32 300ZX

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jondee86
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Re: blue smoke

Postby jondee86 » Wed Oct 26, 2022 2:12 pm

It is possible that the rings are not doing a very good job of keeping oil out of the cylinders
when the engine is operating under vacuum conditions. On the overrun with a closed throttle
and fuel cut operating, oil will get pulled into the combustion chamber and not burned.
Once you get back on the throttle, fuel is supplied to the injectors and combustion occurs,
burning off the oil.

Worn stem seals can also allow oil to get into the intakes and cause smoke, but this is usually
evident by a short period of smoke when the engine is started after sitting overnite. Oil
getting past the rings will give a big cloud of smoke if you coast in gear down a long grade
with the throttle closed, and then hit the gas. Similar situation to getting off the throttle
to coast (in gear) up to a stoplight and then catch the green without having to stop and idle.

Another check is just to let the car idle for ten minutes at normal operating temperature
and then rev it hard a couple of times. A big cloud of smoke means you have ring sealing
problems. Since your compression figures are good, you could just try beating on the engine
by getting out of the city and running up and down a few mountain roads with plenty of gear
changes and using the full rev range.

This is commonly known as an Italian Tune Up, and in this case it may help the rings conform
a little better to the bores. It is not a substitute for machine work but it will be more fun :)
Choose an area with little traffic; keep an eye on the oil level and temperature gauge, and
hope for the best !!!

Cheers... jondee86
The reasonable man adapts himself to the world; the unreasonable one
persists in trying to adapt the world to himself. Therefore, all progress
depends on the unreasonable man.

oldae82
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Re: blue smoke

Postby oldae82 » Wed Oct 26, 2022 6:49 pm

Thank you both. The reason the engine was torn down was loss of compression. The cause was broken lands on all four pistons; the land between 2nd compression and oil wiper. Still don't know the cause of that problem. The engine block had been bored .20 over and the hone marks were still in the bores. There was no scoring from the broken lands. I wasn't in a position to spend the money for new pistons, so I went the used route. I did some work with a micrometer and a dial indicator on the new pistons and the existing bore and they seemed to check out but I'm no pro. I'm assuming the oil is pumped up to or remains in the head and is drawn into the intake by the hose that runs from the intake valve cover directly to the intake plenum. I don't know if there's an alternative to this method. I can deal with the shamed-face look caused by the smoke as along as I feel pretty sure there's no major mechanical problem.

Since the purpose of the car, for me, is summed up by the Italian Tune Up, I'll try that method. South west Virginia has a treasure trove of twistys.

jm

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jondee86
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Re: blue smoke

Postby jondee86 » Wed Oct 26, 2022 10:41 pm

The cam cover vent on a 16V 4AGE is a bit deceptive. While it looks like it is extracting
into the inlet manifold, the fumes are actually internally channeled towards the intake
end of the manifold. They pass thru a hole cast into the throttle body, and exit in front
of the throttle plate.... so basically fumes exit at atmospheric pressure.

Standard practice was to vent the fumes into the intake ducting on most old school Jap
engines until they adopted the use of the PCV system.

Cheers... jondee86
The reasonable man adapts himself to the world; the unreasonable one
persists in trying to adapt the world to himself. Therefore, all progress
depends on the unreasonable man.

oldae82
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Re: blue smoke

Postby oldae82 » Thu Oct 27, 2022 6:17 am

Very interesting. Thank you. What does that tube from valve cover to manifold actually do?

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jondee86
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Re: blue smoke

Postby jondee86 » Tue Nov 01, 2022 1:23 am

It is the first pert of the fume venting setup. It connects to the second part that is a hollow
tube cast into the inlet manifold wall. The cast tube terminates with a round hole in the entry
face of the inlet manifold, and that matches a vent hole cast in the throttle body. The vent
hole opens out just in front of the closed throttle plate.

viewtopic.php?f=9&t=12011

Theoretically, at idle the fumes will get sucked back into the engine thru the idle air bypass
circuit. When the throttle is open the fumes get sucked directly into the engine. Point to note
is the RWD throttle bodies have the vent hole on the opposite side to each other. These days
throttle body gaskets have both holes, but some early versions only had one. Put it in the
wrong way round the vent is blocked and the engine will will pressurize the crankcase, possibly
causing oil leaks and blown seals :(

Cheers... jondee86
The reasonable man adapts himself to the world; the unreasonable one
persists in trying to adapt the world to himself. Therefore, all progress
depends on the unreasonable man.