Thread: 3.5 with no compression ?

One of my first thoughts when I witnessed the motor turning over freely on the starter was that it is normal due the low compression (8.13:1 ?) of the motor from factory.

Originally Posted by bee utey

Many moons ago I bought a very cheap Holden Statesman V8 that had missed out on a lifetime of oil changes. It too had no compression. The camshaft had eaten out the bottoms of the lifters to the point that when oil pressure was lost the lifters bled out and no longer opened the valves enough to run.

If the lifters were gone that would stop valves opening.
That directly would have no effect on compression. Unless a valve was stuck open. Buggered lifters wont do that.

Originally Posted by DAMINK

If the lifters were gone that would stop valves opening.
That directly would have no effect on compression. Unless a valve was stuck open. Buggered lifters wont do that.

It would if the inlet valves weren't opening, in the same way as a closed throttle does.

Originally Posted by pop058

One of my first thoughts when I witnessed the motor turning over freely on the starter was that it is normal due the low compression (8.13:1 ?) of the motor from factory.

That sounds reasonable. My crappy old ute's 3.5 spins over like crazy on a fully charged N70Z with a late D1 Bosch starter fitted, despite it being a 9.35:1 car motor. Perhaps it's just something simple like a failed ignition module or a burnt out rotor button.

Originally Posted by bee utey

It would if the inlet valves weren't opening, in the same way as a closed throttle does.

Interesting. I would have thought compression would remain if valves stuck or no cam etc.
Each stroke comp going up and down. Perhaps pushing the comp past the rings out of force and perhaps even drawing air in past the rings from the crankcase.

My understanding is compression is a fixed number. No relationship to throttle position.

Originally Posted by DAMINK

Interesting. I would have thought compression would remain if valves stuck or no cam etc.
Each stroke comp going up and down. Perhaps pushing the comp past the rings out of force and perhaps even drawing air in past the rings from the crankcase.

My understanding is compression is a fixed number. No relationship to throttle position.

Theoretical Compression Ratio is a fixed number (unless you bend the con rods). Actual cylinder cranking pressure as measured by a compression tester is a variable depending on lots of factors, cam timing, wear, cranking speed, throttle opening, temperature, altitude etc. The spark doesn't fire anything unless conditions are right.

Pulls the plugs and perform compression test.start with the basics.

Originally Posted by bee utey

Theoretical Compression Ratio is a fixed number (unless you bend the con rods). Actual cylinder cranking pressure as measured by a compression tester is a variable depending on lots of factors, cam timing, wear, cranking speed, throttle opening, temperature, altitude etc. The spark doesn't fire anything unless conditions are right.

Agree totally man. Not trying to argue the point. Just trying to give the best advice to the problem at hand.
Generally speaking compression is a fixed number. So if he has lost all cylinders (which i suspect he has not!) then its something other than cam, valves etc one can surmise.

Again i am taking the OP on face value here and trying to discuss the compression specific issues. The OP stated the previous owner just parked the car and that was it. No mention of bending rods or dramatic stuff like that.
Thats why i started with the obvious. Do a comp test. Wet and Dry. Might bring up some interesting numbers. Perhaps a couple cylinders are dead or none. But its paramount to start there.
Even before checking oil. As no oil pressure could dry out the rings and perhaps lower the comp? Unlikely but gotta be checked at this point.
The wet dry comp test will possibly identify valve/cam issues also or atleast give a direction to look in if a couple of cylinders show bad numbers.

If you believe it to be a compression issue there is no substitute foe a proper compression test. However, pull one plug and stick your thumb over the plug hole and give it a crank.. Disable the ignition first, of course... I'm with bee utey. My 3.5 always sounded like it had zip in the comp department, but it ran just fine.