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  #1  
Old 07-17-2017, 01:23 PM
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Compression Tester

I tried doing a compression test using the harborfreight tester but it is junk. What would you guys recommend buying/renting for a valid compression test? It will probably only be used once.

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  #2  
Old 07-17-2017, 01:26 PM
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Does the engine start OK in cold weather? If yes, then you've performed the ultimate compression test and no further testing is required.

What is the problem you are trying to solve?
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'87 124.193 (300TD) "White Whale", ~392k miles, 3.5l IP fitted
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  #3  
Old 07-17-2017, 01:37 PM
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The problem I have with this engine is it will start right up and run fine for about 10 seconds, then it starts missing until it warms up. Coldest start was probably 20F, it started right up but ran very rough.
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  #4  
Old 07-17-2017, 09:19 PM
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If the motor is running really rough, have you tried doing a diesel purge? I've included a link to the procedure - please let us know if you have any questions!

Mercedes-Benz W123 Diesel Purge | 300TD 1977-1985 | Pelican Parts DIY Maintenance Article
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  #5  
Old 07-17-2017, 09:46 PM
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Check the glow plugs, but valve adjustments will do the same thing. Valves too tight.
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  #6  
Old 07-17-2017, 11:28 PM
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Heres what I've done so far:
Diesel purge and new filters- it ran better for a day.
Valve adjust- exhaust valves were tight.
Replaced all fuel lines, primer pump, and fuel filter o rings- these were all leaking.
Replaced glow plugs and reamed- can see them working properly with the injectors out.

None of these have helped with the issue. It does run great once warmed up though and seems very happy at high rpms.
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Old 07-17-2017, 11:33 PM
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My engine is the same, runs HORRIBLE when cold but good when warm, missing at idle, lot's of blue, then white smoke. I'm guessing low compression for me.
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1992 Mercedes 300D 2.5 202,000 - Pure junk
2000 Mercedes E320 Black - 136,000 miles - Needs repair

Don't forget to grease the screw and threads on the spring compressor.
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  #8  
Old 07-17-2017, 11:48 PM
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The HF compression tester is actually OK. Obviously for professional use you'd want something better, but for the occasional home user it's fine. The 90 deg elbow for testing in the glow plug hole will fall apart, replace it with a JIC fitting from your local hydraulics shop. A well stocked auto parts or farm store may have it as well.

Add a chunk of clear tubing to your fuel return line, see if you have air in your system.

Have your injectors pop tested and rebuilt if necessary.

Check delivery valves and seals, look up the delivery valve thread. I chased "runs like crap when cold runs better but still poorly when warm" symptoms for years, came down to delivery valve seals.
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Old 07-18-2017, 12:27 AM
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Quote:
Originally Posted by Father Of Giants View Post
My engine is the same, runs HORRIBLE when cold but good when warm, missing at idle, lot's of blue, then white smoke. I'm guessing low compression for me.

Sounds like a bad glow or two to me.
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  #10  
Old 07-18-2017, 12:56 AM
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The harborfreight tester isn't working for me. It seems to be leaking from the pressure release valve right out of the box. After a web search it seems to be a common issue with them. I'm looking for something of a little more quality. Auto parts stores only seem to rent/sell gasser guages.
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  #11  
Old 07-18-2017, 01:02 AM
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Quote:
Originally Posted by t walgamuth View Post
Sounds like a bad glow or two to me.
I tried testing them, but I always get inconclusive results.
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1998 Ford Escort ZX2 5 speed - 279,000 miles My Daily

1992 Mercedes 300D 2.5 202,000 - Pure junk
2000 Mercedes E320 Black - 136,000 miles - Needs repair

Don't forget to grease the screw and threads on the spring compressor.
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  #12  
Old 07-18-2017, 02:07 AM
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If the tester is just leaking from the vent valve, isn't that just a standard Shraeder valve? I have the HF gage kit but forgot. If so, a regular car/bike one will work, but maybe yours just needs to be tightened. If endemic, maybe Ho on the night shift was lazy about tightening them.
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  #13  
Old 07-18-2017, 02:23 AM
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Quote:
Originally Posted by Father Of Giants View Post
I tried testing them, but I always get inconclusive results.
How are you testing them? Where are you testing them? What setting on your multimeter? You're not doing something right if you have inconclusive results man

Quote:
Originally Posted by Father Of Giants View Post
My engine is the same, runs HORRIBLE when cold but good when warm, missing at idle, lot's of blue, then white smoke. I'm guessing low compression for me.
Sounds like bad glow plugs. Low compression is not likely.

Quote:
Originally Posted by inzgary View Post
Heres what I've done so far:
Diesel purge and new filters- it ran better for a day.
Valve adjust- exhaust valves were tight.
Replaced all fuel lines, primer pump, and fuel filter o rings- these were all leaking.
Replaced glow plugs and reamed- can see them working properly with the injectors out.

None of these have helped with the issue. It does run great once warmed up though and seems very happy at high rpms.
It ran better for a day. Did the temperature vary from the previous cold start? Perhaps got warmer when it ran better that day?
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  #14  
Old 07-18-2017, 07:28 AM
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Quote:
Originally Posted by jake12tech View Post
How are you testing them? Where are you testing them? What setting on your multimeter? You're not doing something right if you have inconclusive results man


Sounds like bad glow plugs. Low compression is not likely.


It ran better for a day. Did the temperature vary from the previous cold start? Perhaps got warmer when it ran better that day?
Also warm and cold are based on operating temperature. Even when 90*f outside, it still runs horrible.
I had it set for resistance, but numbers kept fluctuating, it's been so long ago I forgot how I did it honestly.
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1998 Ford Escort ZX2 5 speed - 279,000 miles My Daily

1992 Mercedes 300D 2.5 202,000 - Pure junk
2000 Mercedes E320 Black - 136,000 miles - Needs repair

Don't forget to grease the screw and threads on the spring compressor.
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  #15  
Old 07-18-2017, 11:49 AM
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AutoZoo loans many tools free. They do charge you but refund the price when returned making it a free loan, not sure if a compression tester is in that program though, or any better than the HF item..

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