Harbor Freight compression tester: injector or glow plug hole?
 

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.

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.

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.

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"?

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.

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.

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

The injectors are quite large and fill a gigantic hole. If the volume of the injector fitting for the tester is different than the volume of the actual injector (likely), then your results will be meaningless on an absolute basis.

Since the glow plug is much smaller, the error generated by the fitting for the tester will likely be much smaller as well.

I would suggest that the glow plug fitting would provide more accurate results.

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.

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

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.

Good point Brian. I'll take a closer look at the fittings and compare to an injector and a glow plug.

As a start, I did a test run and got the injector hardlines off/on without any problems. While in there, I got all the glow plug 8 mm nuts off without breaking anything.

Now that I can get the glow plugs off, I'll take the compression readings there. I'll still pull the injectors to pop test and clean and also take a look at the PC's. I want to see if the balls are all there. What else should I be looking for?

Nothing catastrophic will happen but the chances of your compression being the same with a used heatshield are minimal. Spend the 2 bux and get new ones.

The directions that come with the Harbor freight tester says to use the glowplug holes.

I don't think the size of the opening will affect the PSI that the cylinder is creating. When the cylinder is sealed and you are measuring the compression it cranks a few times and pumps up to pressure on the gauge. It shouldn't matter if the hole that you are measuring the compression in is smaller or larger. It will still only compress as much as the piston and ring allow. It might take slightly longer if the hole is larger but it will still create the same PSI. Some rigs are supposed to be checked through the GP hole and other the injector hole.