| Author |
Message |
Herbert E. Gault (Irfgt)
| | Posted on Friday, May 04, 2001 - 6:07 pm: | |
First of all if an engine has a computer controlled idle speed motor you would first have to disconnect it. For the shade tree mechanic you would then connect an accurate tachometer and either disconnect one spark plug wire and ground it, or preferably,disconnect one of the injector wires, and then read how many rpms the engine drops. On a perfect engine all cylinders should drop the same number of rpms. This can help identify a weak cylinder and allow you to begin your investigation as to why that particular cylinder is not hitting as hard as the rest. It could be valves, compression, valve timing such as a worn camshaft, or a fuel problem or vacuum leak at that particular cylinder. The only positive way to check for a worn valve guide is to remove the valve springs and measure the play or buy a two valve 308 with the factory guides. |
Arnaldo Torres (Caribe)
| | Posted on Friday, May 04, 2001 - 5:04 pm: | |
For the uninitiated like myself, cancelling a cylinder means unplugging the wire from the Spark plug?
When you say a cylinder's effectiveness, what exactly do you mean? Is it the ability of the remaining cylinders to keep the engine running, or its contribution to the smoothness of the running engine?
From this test, how do I know the oil control ring is busted? Should the engine die when cancelling a cylinder if it is busted? What about the valve guides?
Thanks,
Caribe. |
Herbert E. Gault (Irfgt)
| | Posted on Friday, May 04, 2001 - 3:41 pm: | |
Most people put too much emphasis on a compression test to determine an engines condition. You can have excellent compression and still have massive oil consumption if an oil control ring is broken. A compression test will not always pick up a worn valve guide either. Worn rings can actually cause an increase in compression due to the oil itself helping to seal the rings. The best use of a compression test is to confirm a dead cylinder. If an engine runs smoothly and there is not a feeling of a miss,then you can bet your money that the compression is good. The best quick test of an engine's condition is a power balance.With the engine running, cancel one cylinder at a time and note the rpm drop and compare each cylinders effectiveness. A vacuum gauge test can help determine valve condition and exhaust restriction or valve timing along with other readings if you are clever enough to figure out what it is trying to tell you. |
Arnaldo Torres (Caribe)
| | Posted on Friday, May 04, 2001 - 2:53 pm: | |
I was reading the Service Manual section of the ExpensiveCar.com on Vacuum Gauge Testing, and it occured to me that this might be a good Pre-Purchase Inspection (PPI) test for a Testarossa, if applicable. Does the Vacuum Gauge Test supersedes a Compression Test? or Should they both be performed on a PPI?
Any other engine condition tests that should be performed as part of a PPI?
Thanks,
Caribe |
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