CA Smog

[Targa Me]

Hello Everyone,

The good news is, my 1984 3.2 passed CA smog test this past Saturday. But the not so good news is, I did a compression check while I was in the middle of the tune up and discovered my compression has dropped by 25 to 30%. My average compression is 120 psi. (358,000 miles on the clock.)

It may be time for a rebuild in the not-to-distant future.



PS: I had the engine out to repair some oil leaks.

[ClickClickBoom]

Compression is a trend monitoring tool. Plenty of reasons as to why it could be low. Hows the oil consumption, if I remember correctly the factory says 1 qt in a thousand miles is within spec. I would submit the type of driving will dictate oil usage. A 1000 mile trip with 4 engine starts is vastly different than a 1000 miles done 10 miles at a time.

[Targa Me]

I haven't monitored oil consumption lately, but I will.

[yelcab1]

Then again, at 325K miles, something is tired.

[Trackrash]

Agree, the compression test is just one tool. If all your cylinders are similar in numbers that is a good sign. Lots of factors can change the overall number, like was the throttle held wide open, was the motor warmed up, all the plugs out, fresh battery, etc, etc.

If you suspect the motor is tired, the oil consumption, as mentioned, is a good indicator. You should do a leak down test. That is another tool you can use to determine the motor's health.

Then again 358K miles is really impressive, and it will be just a matter of time till it needs a rebuild.

[Targa Me]

When I performed the compression test the throttle was not wide open, the engine was stone cold, all the plugs were removed and the battery was freshly charged at 13.6 volts.

I wonder what are the advantages or disadvantages to wide open throttle vs closed, and engine cold vs warm?

Can someone enlighten me on that?

[yelcab1]

OK, that explains the lower numbers.

hot engine seals the rings against the liners and hence higher compression.
wide open throttle allows more air into the intake while cranking, hence higher compression.

[Targa Me]

Thank you, Yelcab1.

I'll try it again and see what kind of numbers I get. I'll get back to you.

[Quicksilver]

If the engine has never had a valve job or a major rebuild I would expect that it REALLY needs a valve job. A normal valve job can be a simple reaming of the guides and a grind. If you wait until too late it will require all new guides and all new valves.

[Targa Me]

I figure, with 358,000 miles on it, if I pull the heads for a valve job, I might as well rebuild the entire engine.
Those crank and rod bearings have to be near the end of their life.

[porschenut]

^^I agree. Do the whole thing and do it once.

[996AE]

At 155 mi I couldnt pass CA smog so top end started. New P&Cs, then what the heck we are this far do the bottom end. Shop said the bottom end specs were good, no leaks. But I went ahead and spilt case. Sure enough bearings good, crank looked great, gears all showing no wear.

I still went ahead and all new bearings, chains, gears, oil pump flowed and rebuilt etc...

The while you're in there stuff ads up fast.