Measuring compression with a Harbor Freight compression tester, (using injector holes) and after checking 2 cylinders in the 300's, started getting low 100's, and when i went back to the cylinders that had tested in the 300's, those too were now in the 100's. The release valve took several attempts before it would release the pressure. Any idea what's wrong? The engine starts on a dime, and seems to run smoothly, so i'm inclined to believe the 100+ readings are bogus.



Thanks!

Which Hong Kong Fright tester were you using? The diesel one? The $29.99 one?

I bought ONE, count 'em ONE, tester from HF to test for fuel pressures. After I completely disassembled it, including the Schrader valves, thoroughly cleaned all the parts, removing the stray bits of sealant tape, rubber bits, etc. and reassembled it, it works, sort of. Never bought another.

Bottom line is, you get what you pay for. I triple-damn guarantee you have more than 100PSI in those cylinders if it starts without excessive cranking.

Pull apart the unit and give a though cleaning with denatured alcohol and compressed air. Try it again and if it doesn't work, take the P.O.S. back.

__________________
“Whatever story you're telling, it will be more interesting if, at the end you add, "and then everything burst into flames.”
― Brian P. Cleary, You Oughta Know By Now

I once used a brand new HF fuel pressure tester to diagnose a Chevy EFI engine. After a couple of readings, the gauge developed a leak. It sprayed gasoline at high pressure all over the engine compartment. The leak was internal to the gauge, and it came out every seam in the case. Caught it before it was a disaster and never used that thing again.

Certainly >200 psig if it starts easy. I have the HF diesel tester and works for me. I also use the injector holes, since have only measured when cleaning and popping injectors. Insure you are using the thick collar which comes in the kit. That is to give the same TDC volume as with the injectors screwed in. Without it, you surely could measure 100 psig compression. Re the gage kit, it simply has a weak Schraeder valve at the tip which acts as a check valve and another one to release the pressure. The gage itself might get a zero offset, but unlikely the sensitivity would suddenly change. Most have a rack & pinion gear which drives the needle and could skip a tooth. The tip Schraeder valve is very special, having a very weak spring. People sometimes install one for a bike or car tire and then wonder why their readings are low.

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

Used it about 6 times before the current episode, same adaptor in all cases. Have had it about 3 years, so can't really return it.


Harbor Fright owes me many free flashlites! (B.T.W.,the little cylindrical ones all have bad switches; the oval blue ones are better)

My experience is the HF testers don't last no more than a few rounds. So 6 is pretty good. What is important are the adapters. You can get a new gauge and hose. Preferably, a brand more reputable and use the adapters with it.


.

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1983 123.133 California
- GreaseCar Veg System

Any suggestions as to a better gauge to use with the H.F. adaptors that is relatively affordable?

O.T.C., Star Products, K-D Tools are all good brands.

Check the local pawn shops for deals on used Snap-On, Matco, etc.

__________________
“Whatever story you're telling, it will be more interesting if, at the end you add, "and then everything burst into flames.”
― Brian P. Cleary, You Oughta Know By Now

I have a HF compression test set which I highly modified that is 6 years old. The only parts I kept is the gauge and the adapters. I replaced everything in between: all HF fittings were replaced with JIC flare fittings and the hose with 1/8 nylon with push-connect at the adapters for quick connection/ tests.
I know my mods are good because after getting a reading, it stays put for hours and do not bleed down.

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85 300D turbo pristine w 157k when purchased 161K now
83 300 D turbo 297K runs great. SOLD!
83 240D 4 spd manual- parted out then junked

Lot of good brands out there I like OTC.
Lang and S&G also are another. Probably will spend over 100 bucks on them.

But, like funola stated perhaps rebuilding in between the gauge and adapters would be cost beneficial.

.

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1983 123.133 California
- GreaseCar Veg System

My HF pressure tester leaked the first time I connected it. I had no problem bringing it back smelling of diesel fuel.

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Greg Schwall

1983 300SD - 465,000 miles

Keep the hoses, buy a used quality gauge.

They're not a fortune.


https://www.ebay.com/itm/Motorite-Diesel-Compression-Tester-YUF-Nice-Kit/163056880888


Here's a Snap-on!
https://www.ebay.com/itm/Snap-on-Tools-USA-Kilopascal-0-7000-PSI-0-1000-Pressure-Gauge-Dual-Readout/263681584848

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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.

Easy to get a gage cheap, if you think that is the problem. Grainger, McMaster-Carr, ebay used. For gas, I don't think you need silicone filled. Most have a small sensing hole which will dampen any pulses. If the gage reading holds steady, I doubt you have a leak. Unscrew the Schraeder valve in the tip and insure it isn't clogged w/ debris and it moves freely. You should have a Schraeder tool anyway for tires and AC.

To clarify, are you certain you are using the correct thick collar to simulate the volume of the injector? Not doing so is the most likely reason for getting low readings. Try installing the injector and measuring thru the glow plug hole.

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

Will try cleaning the schrader valve. & keep eyes out for a non H.F. used unit.

The correct special Schrader valves used to be sold by NAPA, Champ and others, they're white on the plastic part .

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-Nate
1982 240D 408,XXX miles
Ignorance is the mother of suspicion and fear is the father

I did then what I knew how to do ~ now that I know better I do better