I want to take a compression reading soon. Has anyone used the Harbor Freight tester in the injector holes? One of the adapters looks like it will fit. I want to take a peek at the prechamber also and access looks easier through the injector holes.

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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

I have done it from the injector but that requires a new cush washer. When I do the PC's and new injectors I will check at the GP.

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http://superturbodiesel.com/images/sig.04.10.jpg
1995 E420 Schwarz
1995 E300 Weiss
#1987 300D Sturmmachine
#1991 300D Nearly Perfect
#1994 E320 Cabriolet
#1995 E320 Touring
#1985 300D Sedan
OBK #42

Good point about needing the crush washers. Couldn't the old washers be used to take the compression readings instead of wasting a new set? I'd use new washers after I pop test and re-install the injectors.

ps, are the official name of the "crush washers" "heat shields"?

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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

AFAIK you dont use the heatshields (aka crushwashers) at all with the injector port. The kit has its own. I did it the injector way and it was a PITA.

__________________
http://superturbodiesel.com/images/sig.04.10.jpg
1995 E420 Schwarz
1995 E300 Weiss
#1987 300D Sturmmachine
#1991 300D Nearly Perfect
#1994 E320 Cabriolet
#1995 E320 Touring
#1985 300D Sedan
OBK #42

The tester I have is a U. S. General, made in China...oh well. Anyway, it has a fitting that will work on the gp orifices. Don't have an old gp around to measure threads on. I've heard 'em referred to as both heat shield, and crush washer. When I replaced gp's on the SD, I did not remove injector lines, tight area to work on, yet won't need to bleed fuel or replace washers. I had taken my injectors out to have tested, that was on the, now dead 300 D.

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83 SD

84 CD

I must have pulled the injectors on my 240D 3 or 4 times since I've had it through all the work I've done to get it running right. I always reused the old heat shields. I know its not the proper way but its not like you are going to have some catastrophic results if you don't. If I was rebuilding my injectors, then yes I would replace the heat shields. If I had to pop one out real quick for something else then no I wouldn't. I'm sure many here will disagree but I thought I would add my experience with them.

Scott

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Scott
1982 Mercedes 240D, 4 speed, 275,000
1988 Porsche 944 Turbo S (70,000)
1987 Porsche 911 Coupe 109,000 (sold)
1998 Mercedes E300 TurboDiesel 147,000 (sold)
1985 Mercedes 300D 227,000 (totaled by inattentive driver with no insurance!)
1997 Mercedes E300 Diesel 236,000 (sold)
1995 Ducati 900SS (sold)
1987 VW Jetta GLI 157,000 (sold)
1986 Camaro 125,000 (sold - P.O.S.)
1977 Corvette L82 125,000 (sold)
1965 Pontiac GTO 15,000 restored (sold)

I am faced with the same question with the HF kit. From my reading I have decided to go with the GP hole...faster and easier it seems. I also am trying to limit the number of times I pop the injectors in and out. And I also want to do a sanity check on my glow plugs.

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'85 300D, 'Lance',250k, ... winter beater (100k on franken-Frybrid 3 Valve Kit)
'82 300D, 'Tex', 228k body / 170k engine ... summer car
'83 300TD Cali Wagon 210k, wife's car

funola, does the compresson tester you bought at HF also have the fitting to fit a spark plug hole? I am in the market for a tester, mostly because I am trying to check up on an old gasser that has been sitting in my yard for years... but I would love to know the compression on my 300D as well.

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1985 MB 300D/T

Fell off the WVO bandwagon when people started trying to charge $2.50 a gallon for dirty vegetable oil.

I have the US general unit #93644

This model should work in the glow plug. It has the adapter I have checked. The far right silver adapter is the Mercedes glow plug size.

http://www.harborfreight.com/cpi/ctaf/displayitem.taf?Itemnumber=93644

Dave

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1970 220D, owned 1980-1990
1980 240D, owned 1990-1992
1982 300TD, owned 1992-1993
1986 300SDL, owned 1993-2004
1999 E300, owned 1999-2003
1982 300TD, 213,880mi, owned since Nov 18, 1991- Aug 4, 2010 SOLD
1988 560SL, 100,000mi, owned since 1995
1965 Mustang Fastback Mileage Unknown(My sons)
1983 240D, 176,000mi (My daughers) owned since 2004
2007 Honda Accord EX-L I4 auto, the new daily driver
1985 300D 264,000mi Son's new daily driver.(sold)
2008 Hyundai Tiberon. Daughters new car

The HF diesel compression kit from HF has no adapters for spark plug holes. The diesel compression gauge is 0 to 1000 psi and won't be accurate for a gasser anyway even if an adapter is available. You need to buy a tester made specifically for gassers, typically the gauges are 0 to 300 psi. In short, one cannot be used for the other.

__________________
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

Brian is right. It's a matter of compression ratio between top dead center and bottom dead center of the piston. If the adapter you use in any holes changes the volume, the absolute compression value will change correspondingly.

I tool these readings using the HF compression tester on a Peugeot TD engine.

187,900 miles, Harbor Freight compression tester via glow plug holes:

(1) 460 psi
(2) 460 psi
(3) 460 psi
(4) 460 psi

205,000 miles Harbor Freight compression tester via injector holes:


(1) 430 psi
(2) 430 psi
(3) 430 psi
(4) 430 psi

The Peugeot TD engine happens to use the same glow plug and injector adapters as the Mercedes 617 engine. I took some pics of the adapters and will post them later. The injector adapter definitely add more"volume" and I believe accounts for the 30 psi lower reading. One cab argue that it is due to wear but I don't think so. I guess I can take another reading via the glow plug holes.

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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

Pic below shows the glow plug adapter next to the glow plug. The increase in volume when taking a compression reading is from the glow plug tip that is missing with the adapter.




Pic below shows the injector adapter next to an injector. Note the large hole at the adapter which fills the entire length of the adapter. This volume is much greater than a glow plug tip.

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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

I just bought the compression tester from HF.. item # 93644 and it looks nothing like what they have online.

It has like 6 silver fittings for glow plugs, two large fittings for the injectors, 7 adapters to mate to the air line, a hollow bolt with threads, the guage itself and the crappy manual...

which and which for the om617

I dont have a glow plug out to compare

It's one of those seven adapters. You'll have to use the 90 degree additional adapter to get the hose on with some glow plugs. I think I found that I could use the 90 degree adapter with all the glow plugs if I remember correctly. The one pictured earlier in the thread is the glow plug adapter.

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1977 300d 70k--sold 08
1985 300TD 185k+
1984 307d 126k--sold 8/03
1985 409d 65k--sold 06
1984 300SD 315k--daughter's car
1979 300SD 122k--sold 2/11
1999 Fuso FG Expedition Camper
1993 GMC Sierra 6.5 TD 4x4
1982 Bluebird Wanderlodge CAT 3208--Sold 2/13

ah.. well.. I know the glowplugs for the om602 are the same exact ones for the 617... so the adapter should of course be the same

thanks

http://cgi.ebay.com/ebaymotors/DIESEL-GAS-COMPRESSION-TESTER-W-ADAPTER_W0QQcmdZViewItemQQcategoryZ43989QQihZ009QQitemZ190203607838QQrdZ1QQsspagenameZWDVW

exact same tester I bought for $25 at HF