Compression test - What did I do wrong?
 

I did a compression test today using a diesel compression tester I bought at Harbor Freight. Engine was fully warm. I removed all of the injectors and heat shields. I cranked with the throttle all the way down until the gauge didn't move anymore. Here are the readings:

Cylinder 1: 200 psi
Cylinders 2-4: 220 psi

I tried it a second time on one cylinder after installing the other three injectors but that didn't make a difference.

These numbers can't be right. It's my understanding that once you're under 300 the car won't run well at all. My engine is tired but it's not that bad.

Any ideas?

Thanks,

Scott

Not impossible numbers. Could be right. When were the valves last adjusted?

Freshly adjusted valves and a new starter. My gauge also goes up to 1,000 psi. Maybe the gauge isn't accurate? Also, the attachment I used for the injectors wasn't screwed in all the way. I only screwed it in until it snugged a little bit because I wasn't sure the thread pattern was the same. Would this have thrown the numbers off?

Scott

down to 220 it will run fine but below that and you will begin to have trouble starting it. cold. i am not sure about warm.

even very worn diesels will run fine once started.

tom w

I think I would borrow another compression tester and re check before doing anything drastic, but those could be your numbers.

Is the adapter supposed to screw all the way down into you fuel injector hole? I don't think the threads on mine were the same as the injectors so I only put it a little way in until it started to snug. Maybe it was leaking and causing the numbers to be low??

Scott

An indirect injection diesel should not be able to start at all below 250psi.

according to my old chiltons from early seventies 218 i believe, cold is the minimum.

i tested a 220 once that had only 185. it would run but was tough to get started.

on the threads you should be able to get five or six turns in before it bottoms against the rubber gasket. you can feel the gasket which will feel much different than a stoppage from being cross threaded.

good luck

tom w

I tested my car fully warm. Also, what rubber gasket are you talking about? My adapter is a piece of metal that screws into the fuel injector hole. There's nothing in those holes except for the heat shields which I removed for the test.

Scott

i think your compression tester would have a rubber gasket to seal it off.

if it doesnt and should have you would get false low readings.

tom w

Bingo. I knew those numbers couldn't be correct.

Thanks,

Scott

I use the HF CT all the time, it does not have any gasket... steel against steel I use it on the gp holes though. I leave the injectors in.
John

I have a harbour freight set in the garage a site member was kind enough to purchase for me. I thought the adapters supplied only gave you the adaptor for the glow plug holes on the old 616 and 617 engines but might be wrong. I kind of thought one of the adaptors was for volkswagon diesel injectors. I am pretty sure the thread pattern is different as well. Also you just might want to see if your chain is stretched as you say your engine is tired. It might effect compression readings if off quite noticably. Your compression sounds fairly even if low all things considered. I guess it would be pretty hard to start cold if the numbers are close to true without using the block heater. If it starts up pretty well with no block heater use in the cold. Then your gauge readings must be wrong. You might try adding a small amount of oil to the cylinder and doing another compression check to see if the value is a lot higher. I have heard of 100 lb gains sometimes with a little oil. That test may also clear the gauge pretty well. Let us know the final results of what you find with the compression tests.

I just remembered... the HF tool has a really sucky bleed off button, if it's not fiddled with it keeps leaking. look at it closely.
John

Update:

I went out to the garage and tried it again. It looks like the HF kit does have an adapter for the MB fuel injectors. I screwed in the adapter, made sure everything was tight but the adapter leaked and I couldn't get a reading. So, I tried using the glow plug holes. I was able to get readings on cylinders 1 and 2. I had to use a different adapter for 3 and 4 because of the IP and that adapter leaked and I couldn't get a reading.

I guess quality control at HF isn't that good and I've got a bad tester. I'm sure other people have them and have had better luck than me but I wouldn't recommend it to anyone.

Anyway, the readings I got on cylinders 1 and 2 were 270 psi and 320 psi, respectively. I'm not happy about the first reading but I guess the engine still has some life left it it. I'm thinking cylinders 3 and 4 will read like cylinder 2 because they had the same reading as number 2 yesterday when I was using that other fuel injection adapter.

Scott