Leak Down results
01-11-2004, 01:47 PM
#1
Racer
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Leak Down results
Looking at a wonderful 1986 911 Carrera with only 34,000 original miles. The car is near concours condition. The previous owner had the 30,000 mile maintenance done including valve adjustments etc. The dealer also performed a leak down test and showed the following results:
150, 170, 160, 150, 165, 155
The car drives great, looks great, and seems to be a good buy. Are these leak down results o.k?
Thanks for the help.
Brad
Michigan
150, 170, 160, 150, 165, 155
The car drives great, looks great, and seems to be a good buy. Are these leak down results o.k?
Thanks for the help.
Brad
Michigan
01-11-2004, 01:53 PM
#2
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Location: Destin, Nashville, In a 458 Challenge
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What in percentages would that be. I may be confused about leak down and compression, but it seems that you want less than 10 percent leak down in any one cylinder. You have a pretty big variance with two. Thta may need a top end so I would be careful and wait for someone with more knowledge than me to chime in on this topic.
01-11-2004, 03:30 PM
#4
RL Technical Advisor
Hi Brad:
Those numbers are results of a compression test, not a leakdown test.
Compression testing displays the number of pounds per square inch of cylinder pressure at cranking speeds,.........Leakdown testing displays the percentage of lost cylinder pressure from a predetermined pressure (such as 80 psi).
The guideline is that compression test figures ought to be within 15 psi of each other. Leakdown test results should be under 5%.
Now,...Here is the important part:
Compression testing can show unsatisfactory results with leaking valves. This can make people draw incorrect conclusions about the health of the motor and over the last 30 years, I've seen hundreds of unnecessary valve jobs performed due to less-than-optimal compression test results. It can be a "red herring" unless other tests are performed and I'm sorry to say that many shops consider this one of their prime cash cows. The customer/owner doesn't really know any better.
Leakdown testing allows one to pinpoint WHERE the leakage is occurring. This is a critical followup to compression testing since this really qualifies the entire diagnostic procedures and puts everything into context. When performing the Leakdown test, one listens to where air escapes from and determine what is, or is not faulty.
1) Air escaping from the air inlet shows leaky intake valves.
2) Air escaping from the tailpipe(s) shows leaky exhaust valves.
3) Air escaping from the oil tank filler tube shows leaky rings as this vents to the crankcase.
If one sees some leaky valves but NOT leaky rings, then adding a large bottle of Techron to the fuel tank and following that up with a very "sporty" 100 mile drive will usually bring the compression figures up and leakdown numbers down into an acceptable range.
So, it should now be clear that any compression test figures and leakdown test results MUST be put into context for these diagnostics to have any value.
Regarding this Carrera you are considering,...
Get a leakdown test performed that shows not only the raw numbers, but WHERE the leakage came from. Then, and only then can you accurately evaluate the health of the engine. I'd bet that if the mileage is accurate, all it needs is a good hard long drive with some Techron in the tank.
I do hope this helped,
Those numbers are results of a compression test, not a leakdown test.
Compression testing displays the number of pounds per square inch of cylinder pressure at cranking speeds,.........Leakdown testing displays the percentage of lost cylinder pressure from a predetermined pressure (such as 80 psi).
The guideline is that compression test figures ought to be within 15 psi of each other. Leakdown test results should be under 5%.
Now,...Here is the important part:
Compression testing can show unsatisfactory results with leaking valves. This can make people draw incorrect conclusions about the health of the motor and over the last 30 years, I've seen hundreds of unnecessary valve jobs performed due to less-than-optimal compression test results. It can be a "red herring" unless other tests are performed and I'm sorry to say that many shops consider this one of their prime cash cows. The customer/owner doesn't really know any better.
Leakdown testing allows one to pinpoint WHERE the leakage is occurring. This is a critical followup to compression testing since this really qualifies the entire diagnostic procedures and puts everything into context. When performing the Leakdown test, one listens to where air escapes from and determine what is, or is not faulty.
1) Air escaping from the air inlet shows leaky intake valves.
2) Air escaping from the tailpipe(s) shows leaky exhaust valves.
3) Air escaping from the oil tank filler tube shows leaky rings as this vents to the crankcase.
If one sees some leaky valves but NOT leaky rings, then adding a large bottle of Techron to the fuel tank and following that up with a very "sporty" 100 mile drive will usually bring the compression figures up and leakdown numbers down into an acceptable range.
So, it should now be clear that any compression test figures and leakdown test results MUST be put into context for these diagnostics to have any value.
Regarding this Carrera you are considering,...
Get a leakdown test performed that shows not only the raw numbers, but WHERE the leakage came from. Then, and only then can you accurately evaluate the health of the engine. I'd bet that if the mileage is accurate, all it needs is a good hard long drive with some Techron in the tank.
I do hope this helped,
