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.   


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.   

[ORCoaster]

Now all that makes perfect sense but I don't remember my HF adapter looking like that.  So I will have to dig it out and study it.  But yeah, there seems to be a lot of that variability in the HF or other Cheap knock off tools.  Sometimes they are decent and sometimes they are trash. 

Mine was much better at registering the proper pressure as I stated.  Didn't think about the extra unchecked volume as you have shown. 

I will have to dig deep for the case that contains the kit.  Have been fortunate enough to not need it for a while now.  Will take a few days to find time to do that. 

Thanks for the reply. 

[thomas m]

My HF kit, in the red box, also has compression adapters for the glow plug holes. The Schrader valve is right there in the tip of the adapter thereby eliminating the extra volume problem of the HF injector adapter. There is also an elbow adapter that will (or might, as I have not tried it) allow for the limited space around the glow plug holes and behind the pump. However, I seem to remember someone saying that there is some reason not to use the glow plug adapter to check the compression. I don't know why. Maybe just an access issue???

[libbydiesel]

Interesting.  Which HF kit do you have?  I looked at the glow plug adapters of both of the kits they currently sell and they are hollow in the same way as the injector adapter. 

[libbydiesel]

As I mentioned, I looked at the adapters in the 'professional' kit also and they are all hollow. 

[thomas m]

Interesting.  Which HF kit do you have?  I looked at the glow plug adapters of both of the kits they currently sell and they are hollow in the same way as the injector adapter. 

My kit is the HF US General 93644. All of the chrome glow plug adapters, short and long, have the Schrader valve at the tip, nearer the piston. I'll try to get some photos up.
If the accuracy of the HF gauge is questionable, McMaster and Grainger have some better quality (I guess) gauges for $12 to $25 and, yes, for a lot $$$$$ more. I would rather get a better quality gauge rather than another comp test kit which could be spendy and possibly no better.

Adapter to gauge connector on far left.    12 mm x 1.25 (3 on left) and other longer adapters.


Closer look at the Schrader valves in the tip of the adapter:


Gauge connector screws on to top of glow plug adapter. There is a 90 degree elbow for this in the kit.

[libbydiesel]

I see.  Yeah, that is an older kit.  They no longer sell that one.  It's unfortunate that they have gone to a worse design. 

I'm not sure what to say about the accuracy of the gauge.  It might be fine and it might not.   

[thomas m]

I'll try to find a source of high pressure air and check out the gauge. Maybe a welding or gas shop. If it's not way out of whack, it's probably ok for comparison purposes.

[ORCoaster]

I found my kit and it is the same older one that thomas m has.  The GP take-offs have the valve in them but the adapter that goes into the injector area does not.  The threads on the injector adapter are finer than the GP take-offs so you can't use them there and get the benefit of the valve when using it. 

You put on a collar that takes up some of the volume like the injectors would but then you have the problem Libby mentions with the extra in the lines. 

So with my kit, I can get two readings from any individual cylinder,  interesting idea.  One a ballpark the other a measured? 

I would like to see if it makes any difference by just measuring the 4th cylinder as it is most accessible.  The problem with that is A. no time and B. Rain for the next two weeks likely.  So, not gonna do it. Now. 

But now I really am curious and might have to do it in the future.  I Will post if I get around to it. 

[thomas m]

Reference above Libbydiesel's injector adapter photos with and without schrader valve and this older thread showing TylerDurden's injector/adapter comparison photo with spacer and copper washer on the HF adapter.
http://vwdiesel.net/forum/index.php/topic,34751.msg329232.html#msg329232

It looks like the HF adapter could be tapped for a Schrader valve thereby eliminating or minimizing the "unswept" volume inside the adapter. This assumes that there would still be a good seal between the injector adapter and the heat shield.

I did a quick search on Amazon and there are Schrader taps available for various sizes.

Comments

[libbydiesel]

It is great that the glow plug adapters have the check valve at the tip of the adapter and that is a marked improvement over the current HF kit.  Bear in mind, though, that the glow plug adapter is missing the entire shape of the glow plug element.  Without that, again the unswept volume is increased.  The linked thread in your last post touches on that at the end of the thread.  For the IDI engines, an adapter that has the schrader valve at the tip an mimics the injector size will give a more accurate and higher reading than a glow plug adapter that is missing the geometry of the heating element.  Unfortunately, for the TDI engines, I have only ever seen glow plug adapters and they are similarly missing the element tip.  As is demonstrated in that other thread, it would be possible to measure the volume of the glow plug element and do the math to figure out the compression pressure difference, assuming the gauge reading is correct.