Low Compression, Bad Rings, Engine Rebuild?

[Hawkman]

I can't frikkin' believe it. I bought a one owner 91 LS400 from some old couple that had a folder complete with all service records and repairs. It had 120k miles on it. I've got 125k miles on it now and it just started acting funny during acceleration. Come to find out the #1 piston has a compression of 130. Mechanic is saying normal is 180. He replaced the spark plugs and it runs better now, but not 100%. He said the cap/rotors/wires were replaced last year, but whoever did it didn't replace the plugs.

So in order to get the thing running right again, it would require a rebuild? Bad piston rings in piston #1. Scheize!

What happened to these things being supposed to last 200K plus miles??? I'm very disappointed. :cries:

What would you guys do? I was hoping to have this thing for many years as my work car...not just 5 months.

[jadecuir]

Are you sure it's bad rings? When I checked compression on mine ('90 LS, 270,000 mi.), I thought I had low comp. on a cylinder, then I screwed in the hose to the gauge a little tighter, and it read 180 psi. If it still reads low, it could be a sticky or burnt valve, or one is out of adjustment (tight). The test for bad ring seal is to squirt a little oil in the cylinder and recheck compression. If it's still low, it's a valve. The oil seals the rings up for a few minutes.

John

I can't frikkin' believe it.  I bought a one owner 91 LS400 from some old couple that had a folder complete with all service records and repairs.  It had 120k miles on it.  I've got 125k miles on it now and it just started acting funny during acceleration.  Come to find out the #1 piston has a compression of 130.  Mechanic is saying normal is 180.  He replaced the spark plugs and it runs better now, but not 100%.  He said the cap/rotors/wires were replaced last year, but whoever did it didn't replace the plugs. 

So in order to get the thing running right again, it would require a rebuild?  Bad piston rings in piston #1.  Scheize! 

What happened to these things being supposed to last 200K plus miles???  I'm very disappointed.  :cries:

What would you guys do?  I was hoping to have this thing for many years as my work car...not just 5 months.   

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[monarch]

To determine if the compression leak is due to leakage past burned exhaust valves & valve seats or worn / broken piston rings a cylinder leak down test should be performed Valve trouble is more common than piston trouble on Toyota engines.

It's possible the old, misfiring spark plug on cylinder #1 caused combustion chamber temperatures on #1 to soar to the point of burning the two exhaust valves and valve seats causing the compression leak. The solution in that case would be to remove the cylinder head and have a machine shop replace the burned valves and repair any valve seat damage and then compression should return to normal.

130 psi is suggestive of substantial, but not immediately fatal engine damage to cyl. #1

If I owned your car I would first check the exhaust valve clearances, especially on cyl. #1.

If the clearances on #1 are moderaely tighter than the factory spec of .010"-.014" then adjusting them back to spec. would make the engine run smoother and might restore some of the lost compression which in turn might allow you to delay the head work for along time. However, the spark plug on cyl. #1 might tend to wear & foul faster than the others until the cylinder head work was performed.

[sha4000]

To determine if the compression leak is due to leakage past burned exhaust valves & valve seats or worn / broken piston rings a cylinder leak down test should be performed  Valve trouble is more common than piston trouble on Toyota engines.

It's possible the old, misfiring spark plug on cylinder #1 caused combustion chamber temperatures on #1 to soar to the point of burning the two exhaust valves and valve seats causing the compression leak.  The solution in that case would be to remove the cylinder head and have a machine shop replace the burned valves and repair any valve seat damage and then compression should return to normal.

130 psi is suggestive of substantial, but not immediately fatal engine damage to cyl. #1

If I owned your car I would first check the exhaust valve clearances, especially on cyl. #1.

If the clearances on #1 are moderaely tighter than the factory spec of .010"-.014" then adjusting them back to spec. would make the engine run smoother and might restore some of the lost compression which in turn might allow you to delay the head work for along time.  However, the spark plug on cyl. #1 might tend to wear & foul faster than the others until the cylinder head work was performed.

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same thing i would have said if i could have gotten it into words you sure sound like a technician on the low

[Hawkman]

Thanks for the responses! I got the car back today. Yes, oil was squirted in to see if it was the valves. Compression went up to 170 after squirting in some oil. All other cylinders were at 185 except for cylinder #5. It was at 175.

All spark plugs were replaced and the car is running much better now. Whoever the idiot was that serviced it last year put new caps, rotors and wires, but didn't replace the plugs. The plugs were burnt!!

Now that I know the rings are in fact bad in cylinder #1, what are your recommendations on what should be done? Just use good oil and keep it for as long as it will live?

Thanks again.

[monarch]

I personally don't consider the oil squirt method to be a reliable indicator of bad piston rings. The cylinder leak down test is much more foolproof. However, another simple method is to simply remove the spark plugs on cyl #1 & #2 after driving 300 miles or so. If the spark plug electrodes on #1 are black and oily and those on #2 are free of deposits and discoloration, then you could more confidently conclude #1 has damaged piston rings and / or cylinder bore damage. But if #1 has a similar look to #2 then the rings on #1 are still probably OK. If this test reveals cyl #1 to be OK then I'd also recheck the compression on #1 after driving the 300 miles, because it might have improved considerably on it's own. On the other hand, if compression is still 130 psi then I'd keep driving until the engine becomes rougher running and / or compression drops below 130.

As far as what to do about the damage well it migh be a toss up choice between paying to have the cylinder rebored and refitted with new pistons (including possibly the other 3 cylinders on that side of the engine to make the engine properly balanced) or installing a good used engine out of a Lexus that was totalled in a traffic accident or installing a used 50,000 mile engine imported from Japan

[Hawkman]

Sounds like a plan.

Any other comments and/or suggestions? Any oil additives on the market that could possibly help, or would it be detrimental?