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.

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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1984 300D Turbo 200K

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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1984 & 1985 CA 300D's
1964 & 65 Mopar's - Valiant, Dart, Newport
1996 & 2002 Chrysler minivans

Go on eBay and get a quality used one.

I've seen Matco leak down tester last go for $44 - $50ish

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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.

I ordered an old Mac tester kit off ebay. In the meantime, I found out why the engine is running rough. I pop tested the injectors and one is "peeing". They are also out of balance, so I'll work on rebuilding them.

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1984 300D Turbo 200K

Good find! That WILL mess things up.

Dan

It very well could be imbalanced injectors that are leaking causing your rough running from a cold start. Fuel leaks can do that as well. Read DieselGiants page on checking glow plugs. Make sure you hit range on the meter till it zeros out so it doesn't jump around everywhere.

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Only diesels in this driveway.
2005 E320 CDI 243k Black/Black
2008 Chevy 3500HD Duramax 340k
2004 Chevy 2500HD Duramax 220k

So it turned out 3 nozzles were peeing, they were all Bosch India nozzles. I put new Monarks in, and balanced to within 50 psi. Then I did the compression test and got 310 to 350 psi cold which seems good. The reason the harborfreight tester can't get a reading is there needs to be a special light spring schrader valve in the test fitting. The Mac tester has this so after the engine is cranked, the guage holds the compression reading until you release the pressure.

So the engine has much more power now. However it still has the cold start issue. I think it might be related to oil in the turbo, which I recently discovered.

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1984 300D Turbo 200K

So a quality used compression tester should be obtainable, buy from vendors who offer returns, DO NOT BUY FROM PEOPLE WHO REFUSE TO ACCEPT RETURNS, PERIOD.

I've gotten a lot of used quality tools for cheap.

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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.

When doing a compression test, you want accurate numbers. Cheapy testers will give you cheapy numbers. Snap-On or nothing. Find a shop that has a Snap On compression tester and pay them to run the compression test.

You'll thank yourself in the morning.

"It ran better for a day." -It just seemed to run better for the rest of the day after the diesel purge was done. The next day's cold start was back to rough running.

I tested the harborfreight guage with compressed air and it is holding pressure. Narrowed the leak to the injector fitting. Should I be doing this test with or without the heat shields in?

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1984 300D Turbo 200K

Long time ago I got one from JC Whitney. It is still a good tester, uses the same adaptors as Snap-on, (same dimensions, anyway the Snap-ons fit in it) Dont know if they still sell 'em, dont know if they're still around, even, , , Had it for 30+ years.

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"I applaud your elaborate system of denial"

I gave the HF tester one last chance to redeem itself since the guage seemed to not leak. Wrapped the quick connect fitting threads with teflon tape, used copper sealing washers, and torqued the injector fitting to 70 nm. Still not working. The tester can't be exchanged at the store, so rather than buying another junk set I'll look for a good used one on ebay.

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1984 300D Turbo 200K

If you believe it is only the Injector adapter that is the issue you can purchase them seperately on places like eBay,

Also the setup uses schrader valves like are like the ones on Tire Valve Stems and they can often be the cause of problems.

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84 300D, 82 Volvo 244Gl Diesel

You should be able to return or exchange your HF compression gauge. You must have the receipt though.
You have to tead the instructions! It tells you to use the supplied copper washer if you use the inj adapter.
The injector adapter will give you a lower reading than the glow plug adapter due to the extra volume in the inj adapter, provided both are sealing well. Absolute accuracy is less important than relative accuracy in a compression gauge.

I've found that there are too many poor quality fittings in the HF set, each being a potential leak source, that I have modified mine with my own quick fittings and hose, so that it is quick/ easy to use and accurate. I can take a compression reading in less than 2 minutes per cyl via the glow plug holes of a 617 except #5, which takes longer because of tight access.

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85 300D turbo pristine w 157k when purchased 167,870 July 2025
83 300 D turbo 297K runs great. SOLD!
83 240D 4 spd manual- parted out then junked