Model Specific Questions and Info > MK2 Golf/Jetta and Audi 5k
Heat Shield Usage for Compresson Test
thomas m:
I understand that, in general, injectors should be installed with new heat shields.
But can used heat shields, that are removed, be used with the HF adapter for the compression test??
Or, if new heat shields must be used, can they still be considered "new" and installed with the injectors?
I guess it comes down to using 1 or 2 shields per injector installation.
What's common practice??
libbydiesel:
Yes, you can use the old heat shields during a compression check. Replace them when you're done with the test. The design of the Harbor Freight compression tester and adapters are not great. Don't expect accurate results. It is ok for comparative testing but not good for seeing if an engine is within spec.
ORCoaster:
I have never had trouble with the HF tester kit I have. But then I don't have a way to test the gauge to 500 psi to see if it reads that. I do have a compressor that gets to 150 easy and the HF gauge shows 150 if I hook it up that way. So I have to have faith it is sufficient to tell me if I am in spec.
So.... It is ok for comparative testing but not good for seeing if an engine is within spec. Libby says.
Is in dispute here. Just how would you determine if the HF gauge is giving you the proper PSI reading?
Compare to another pricier manufacturer? Has Anyone ever done that?
I will agree without actually doing a test for accuracy on the HF tool your readings are for comparison purposes only. But I also trust it to be the number I need to know for spec purposes.
libbydiesel:
I have both a high quality IDI compression test kit and I also have a Harbor Freight kit. The higher quality tester I have, has an adapter that very closely mimics the IDI injector and has a schrader valve right at the top to maintain pressure in the adapter. I have tested both with air pressure of 100 psi and the HF gauge reads 30 psi lower. The HF adapter is also hollow without any schrader valve at the tip. I believe the check valve that maintains the pressure in the hose is within the elbow where the adapter connects. The result of the design of the adapter and the location of the check valve is that the unswept volume is greatly increased. Increasing unswept volume lowers compression. The combination of a gauge that reads low and the increased unswept volume will result in a lower compression pressure reading. If trying to determine if an engine is within spec, the result could definitely be misleading.
libbydiesel:
Here are some pics. On the left is the 'higher quality' adapter, HF on the right. I do not remember the brand of the higher quality version as I got it ~20 years ago and there are not any markings on it. :P
Notice the schrader valve at the very tip of the adapter.
Here is a pic of the HF adapter that shows the fairly large hollow area that runs the length of the adapter:
Without a schrader valve in the tip of the adapter, that added volume of air is the same as increasing the combustion chamber size and will significantly lower the compression pressure.
For testing the two gauges, I set my air compressor to 100 psi and ran that pressure into each gauge. The higher quality tester kit gauge read spot on 100 psi. The HF guage read 70 psi. I did not test at typical diesel compression pressure and it might be that the HF gauge is adjusted to compensate for the added volume of the adapter. I doubt that for $20 per kit they went to that sort of trouble, though. When I got the HF kit, I also looked at the 'professional' kit they sell for ~$110 and the adapter and check valve setup is no better in the more expensive kit.
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