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post #1 of 7 (permalink) Old 10-29-2002, 05:35 PM Thread Starter
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engine problems

ok i was gonna trade my car in and its in really good shape interior and all but the guys at the dealer said my engine has no compression. i have no idea what that means. my friend told me it could be a gasket or seal or vavle thats bad. but how do i know which one and what does no compression mean?
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post #2 of 7 (permalink) Old 10-29-2002, 06:11 PM
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if your engine had no compression, it wouldn't run. they mean it has low compression.

Some possibles:

they are lieing to talk you down on the trade in price,
you blew a headgasket,
your piston rings are bad and you are presurizing the crank case instead of the combustion chamber,
the pulgs are not seated, and allowing the escape of gases.

Matt "Looks Like Egon" Davis
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post #3 of 7 (permalink) Old 10-30-2002, 09:36 AM
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If it runs ok then it's probably a lot of BS but if it's running rough take it to an independent shop and have them run a compression and leakdown test on it... and that you want to see the numbers. Don't tell them a thing about what the dealer said beforehand. The numbers should all be fairly close to each other for each cylinder on each test. You can also buy a gauge and do it yourself.... If the shop doesn't want you to spend $1500 for a new engine or rebuild job then the dealer is jerking you around.
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post #4 of 7 (permalink) Old 10-30-2002, 01:12 PM Thread Starter
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ok what numbers would i see if i got a test done. can someone give an example. it kinda runs good but when i press the gas it hits 3500 rpms then kinda dies down to keep alive. i have no idea what im gonna do.
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post #5 of 7 (permalink) Old 10-30-2002, 04:28 PM
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I'm not sure what you mean by "dies down to keep alive"?? ..... do the rpms drop while you keep a constant pressure on the accelerator and the car is moving or is it more like the transmission is not engaged at first and the rpms jump until it grabs?

I don't know exactly what the numbers will be, they'll vary from one engine to the next even amongst the same type. Maybe there are some mechanics around that can give you specific experiences with the 3.8 but I haven't had to do mine yet and the last time I did this it was on a Chrysler 318 in the 70's. The key thing is that they're all pretty close to the same in each of the cylinders. Let's say for example that the peak compression is 400 psi in #1, 405 in #2, 403 in #3, 398 in #4, 410 in #5 and 400 in #6 then your car has "good compression". If #4 is 300 psi then that's an indicator of a problem, maybe a bad ring, a slightly bent valve or a head gasket leaking badly. The leakdown would be similar, say 5 of the 6 cylinders take 5 minutes to go from 400 to 300 psi and the 6th takes 1 minute. The one that loses compression in a minute has a problem, again maybe a head gasket, maybe a ring, maybe a valve seat.... Once you know there is a compression problem then you're pretty sure taking the heads off isn't a waste of time.
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post #6 of 7 (permalink) Old 10-30-2002, 06:23 PM
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you won't have 300 or 400 psi compression.your lowest cylinders reading should be 80 to 90% of the highest though and they should all read near the same all around.what year ,make and engine do you have?maybe some would know what the readings are supposed to be for your engine if they knew that info.
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post #7 of 7 (permalink) Old 10-30-2002, 08:25 PM
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All cylinders should be close in pressure. My former 84 5.0 Mustang usually ran around 90 - 120 Psi. I've seen 4 Cylinder toyotas run 130 Psi and it is a function of the combustion chamber in the cylinder head. Unless you have a problem, the pressure should be +/- 10 Psi of one another. Have you noticed any overheating? This is another tell tell sign of a bad head gasket. Also check the oil for water, I have seen a nasty mess because of the lack of this simple check.

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