One cylinder compression test very low and burns oil

Chaud

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#1
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MAKE: Pontiac
MODEL:Firebird
YEAR:1994
MILES:210K
ENGINE
:V6 3.4L
DESCRIBE ISSUE
The car burns 1Qt of oil in 75 miles. Checked compression on all cylinders as follows
155, 135, 130, 35, 110, 160. Then I squirted some oil in #4 and it jumped to 40 but still quit low. I did not do any other cylinder test with oil.Now my question is
1. Is this low compression on #4 due to a valve problem or Ring/piston problem?
2. how can I determine for sure what is causing the low compression on #4?
3. Knowing what other cylinder read is it a good idea to have this problem fixed or install a used or rebuild engine. The idea being which is more economical and practical.
I will very much appreciate your help. Thanks
Chaud
 
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#2
compression loss as you said is either - piston related (rings piston itself), Valves or gasket...
Sometimes the spark plug appearance will lend a clue... Engine vacuum - OR blow by out the oil fill tube-

I like thee compression tester* where you use shop air into the spark plug hole... Simple explanation -with the engine is OFF safer quieter - the air will escape thru th damages area- you will hear pressure leaking to oil pan, intake manifold, or exhaust manifold... or the mating surface head to block...

*My colleagues will know trade names of these it escapes me right now...
 

billr

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#4
Understand, you can't be certain of where all the problems are. You could have both a bad valve and bad piston/rings, but the leak-down pressurizing probably couldn't tell you about the bad piston/rings, too much air would be blowing past the bad valve. With 35 psi, a burnt valve is my guess, pistons/rings and gaskets don't usually leak that bad.

None of those compression readings are nice, of course. If you have confidence in the compression gauge, then now is time to think seriously about how much you love this beast. An engine out of a junkyard is always going to be questionable, and rebuilding yours may cost more than the car is worth. I recommend using a local shop to do the machining and such on your engine, not getting a "rebuild" off the internet.
 
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#5
On your # 4 you have pretty well determined that that cyl has bad valves. That is because you put oil in to temperately "seal" the rings. If they were bad, compression would have moved up a lot. It did not, so probably bad valves.

Of course, that very high oil usage and high mileage makes one skeptical of rings and valve guides . That engine sounds very tired and probably requires a complete overhaul or replacement, which may not be worth while???
 
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#6
Have a compression tester with quick coupler spark plug hole adapters, just screw that into the suspected cylinder when its on top dead center on the firing stoke. Hit it with 150 psi from my air compressor and you can hear or see where the air is escaping.

Can hear air coming from the exhaust pipe, exhaust valve, TB, intake valve, oil dipstick hole, piston blowby, bubbles in the coolant, head gasket.

Could be a badly carboned up piston ring or intake valve. Would pour SeaFoam into the TB until the engine kills, wait ten minutes and take it for a drive. Ha, do this at 3:00 AM on a dead road after the drunks go to bed and before normal people wake up. Could correct it, 35 psi is not as bad a zero that you would get with a hole in the piston. Could be lucky, but don't know until you try.

A quart every 75 miles is drastic, sure you don't have an oil leak?
 

Chaud

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#8
Thank you all for your great inputs. Looks like the problem is more than one place. If I get the valve / sleeve fixed then it is still going to burn oil may be not that much because all plugs were oily and #4 was worst because its gap was completely plugged with carbon deposit. This means it needs rings too. I called a local junkyard it has 3 engines and willing to check compression on all and give me the best for $400. Now should I be looking compression 155 or better on all or it is asking too much. Thanks
Chaud
 
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#9
When checking out the salvage yard engine you want to pay close attention to the condition of the plugs as you take them out. Plugs tell a lot about the condition of the engine and if any leakage etc is happening with gaskets etc.. Compression is important but so is the condition of the plugs. That is if they have not changed them out recently.
 

billr

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#10
Compression of 155sounds kind of low for that, I would be looking for 180 minimum. Does anybody know what the lower service-limit is for that engine type?
 

Chaud

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#11
Have a compression tester with quick coupler spark plug hole adapters, just screw that into the suspected cylinder when its on top dead center on the firing stoke. Hit it with 150 psi from my air compressor and you can hear or see where the air is escaping.

Can hear air coming from the exhaust pipe, exhaust valve, TB, intake valve, oil dipstick hole, piston blowby, bubbles in the coolant, head gasket.

Could be a badly carboned up piston ring or intake valve. Would pour SeaFoam into the TB until the engine kills, wait ten minutes and take it for a drive. Ha, do this at 3:00 AM on a dead road after the drunks go to bed and before normal people wake up. Could correct it, 35 psi is not as bad a zero that you would get with a hole in the piston. Could be lucky, but don't know until you try.

A quart every 75 miles is drastic, sure you don't have an oil leak?
Your test is wonderful and it is the answers to all my questions. I will do that but have couple of questions.
1. How can I bring low-pressure cylinder to TDC. What I am thinking is to rotate the timing pully (crankshaft) to 240 degrees from the TDC . The 2nd method is to insert a straight wire into the spark plug hole and rotate the crank. Wait for the moment when it goes upward, stops and then just start moving down. If none of the methods I described any good then what would you suggest to get #4 onto TDC?
2. This car has Fuel injection and no TB. So where I am going to pour SeaFoam. Will it be ok to pour into #4 hole and put plug back.
Yes, the car does drip a little oil when parked but not a puddle.
 

Chaud

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#12
When checking out the salvage yard engine you want to pay close attention to the condition of the plugs as you take them out. Plugs tell a lot about the condition of the engine and if any leakage etc is happening with gaskets etc.. Compression is important but so is the condition of the plugs. That is if they have not changed them out recently.
Great point. Thanks
 
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#13
Didn't say it was going to be easy, V-6 with a FWD vehicle, jammed, distributorless ignition, no timing marks, but sure has a throttle body.



That hose from the air cleaner behind the battery can be removed bend upward to pour the SeaFoam into it. These engines are typically 8.5 compression ratio, 150 psi would be fine.

Can hook a compression gauge up to that cylinder, have fun, and rotate the crankshaft CW until you see the highest pressure. I use an 18" long ratchet wrench for this with a six point socket on the harmonic balancer bolt. But with some can't even to do this, have to remove the RF wheel and the inside fender cover, but if this is the case, need another soul to turn it so you can watch the gauge.

Or just skip it and try the SeaFoam first. May have to do this a couple of times, exhaust will eventually come clean.

Those oil drips can add up in a hurry if dripping when parked, what about driving on the highway? Have the steam clean the engine first to even learn where it is dripping from. Seen engines with a puddle, crankcase will be dry.

Changing an engine isn't easy either on cars like this, that young kid, The Old Man did this recently, had to raise the front high enough to drop the engine from underneath. Alldata.com would give the procedure.
 

billr

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#14
Skip all further testing. With low compression and using 1 qt/75 miles, this engine is pretty tired. It's going to have to be replaced or overhauled no matter what you find with further testing; and, if you open it up for overhaul, the problems can be more accurately determined.