Terrible Tom wrote:
>
> Jamie Calder wrote:
>
>> A while ago I posted a problem I had with my '93 318 where cyl 1 and
>> 7 don't
>> fire. A compression test shows there no compression at all in those 2
>> cylinders. Before I take the valve cover off for further inspections,
>> would
>> anyone like to speculate? Why those 2 cylinders at the same? Are those 2
>> always in the same position? What would cause 2 to go at once? What
>> would be the best case scenario? Rockers or pushrods, right?
>>
>> Thanks,
>> James
>>
>>
>
> 1-8-4-3-6-5-7-2 - is the firing order. The number 1 and number 7 are
> the front and rear two on the drivers side bank... Why the two are not
> getting compression, is a little odd.
>
> You either have a situation where the timing is off, and the piston is
> hitting TDC while you have a valve open, you have burnt/broken valves,
> or you have damaged pistons.
>
> Odds are - if the timing has not been fussed with, and the chain in
> intact, you have bad valves. I could see if you had no compression
> between two common cyls, (indicating a hole on the block or head)... but
> two opposites? Its not impossible to have fubard pistons... but it
> would take some serious destructive force to take out two of em.
>
> Why do I not suggest rings? Because even with bad rings, you should
> still have SOME compression, be it very low. If you have ZERO
> compression, I'm suspecting valves.
>
Going to add some more info here. As you mentioned valve train, you
could have a pushrod issue, however if the rods were bent, it would
cause the valves to open less, and you would hear clatter. You would
not normally see zero compression with bent rods.
Also, try this... squirt some clean engine oil into the spark plug holes
for 1 and 7... not a whole lot... half a teaspoon or so... and see if
that bumps up compression at all. If it does - you know its a piston
ring problem. If it doesn't you are back to either valves, holes in
something, or timing.
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