Compression test results.

[laserjock]

Well, with Big Red rolling, I spent the day cleaning the garage and ended the day by pulling the engine out and doing a compression test. It's been sitting for close to a year without being turned over. If I believe the odometer it has about 186k miles on it. Pulled the first glow plug... Oh crap they are Autolites. Well luckily, I think they were recently installed. No bad tips. They all came out pretty easy thanks to antisieze on the threads. So after having to modify my adapter, here are the numbers.

1. 300. 295
2. 340. 340
3. 300 300
4. 310. 310
5. 300 300
6. 340. 335
7. 340. 340
8. 340 340

Overall I think it's pretty good. The lower ones I'm not 100% convinced they are accurate. I had to use a right angle adapter on some of the cylinders to not have to pull the injector lines. Even if they are that's an average of 321 which puts them all within 10% easily.

Comments?

 

[icanfixall]

Personally I feel those numbers are kind of low. My rebuild had 530 lbs of cranking compression. I recall mid 200 was the low limit to a working engine.

 

[oztinker47]

Was this done with the engine warmed up? If not I'd expect them on the low side, especially after sitting awhile.

 

[laserjock]

Engine was stone cold. It's been out of the truck for almost a year.

 

[icanfixall]

Warm or cold engine. The results will be the same. We have a considerable amount of oil being sprayed up under the pistons from the cooling jets. So the cylinders will be wetted with oil and the rings are well sealed.

 

[OLDBULL8]

Did you crank it over at least 5 times on each cylinder, has to be fast cranking.

Cold engine will crank slower than a hot one will.

 

[laserjock]

Yeah I let it hit about 6 times and noted the pressure then let it go to max. Always about the same. Charged battery in between.

 

[OLDBULL8]

My book shows 260 to 440 for a 7.3. What ya might do after an install is run an Auto-RX treatment thru it, does wonders for a diesel. That really cleans an engine up and de-carbons the rings.

 

[laserjock]

So it looks like I'm on the mid to low end of acceptable. Given the mileage, that seems reasonable. I think that's the motor I'll tear down and check everything but for now, I think it's on to body work. I can turn wrenches in the snow but body work not so much. Sad to be worried about snow in August.

 

[laserjock]

Hold the phone on this one. I may have uncovered the reason for the low numbers when I tried to test the other engine. I think I screwed up my adapter.

Stay tuned.

 

[Black dawg]

My 7.3 blows right around 225, I wouldnt be too concerned with those numbers. I have never done a hot vs cold on one of these diesels, but on gassers, the hot number can be significantly lower.

Go to a racetrack that has a compression psi rule, and watch how long guys will warm up thier cars before they pull them out of the trailer. One particular engine I had would blow 250 dead cold, but only 180 warm.....right at the track rule.

 

[riotwarrior]

Your numbers are consistent. I doubt there is an issue with fitting.

Though lower that what one wants that in no way suggests this engine cant be overhailed with fresh rings and bearings to bring it back into decent spec.

The only telltale is to dissassemble and measure pistons and bores for taper wear and out of round.

If within spec a d cylinders hone out ok I would think easy 100K plus of usable miles if looked after and regular erl n filter changes...


JM7.3CW

 

[79jasper]

Shouldn't these read higher when warm?

Sent from my SAMSUNG-SM-G890A using Tapatalk

 

[laserjock]

Well, you pretty much nailed it Al. Adapter was apparently sealing okay. It looks prettier now though.

[emoji6]

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It feels better when it tightens down now. Nice and smooth. Few minutes with the lathe.

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Now on to more interesting info (maybe, is to me anyway).

I tested engine number two.

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Got out all 8 autolite glow plugs [emoji15]

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Couple were starting to swell.

Here are the results.

1. 180
2. 200
3.
4.
5. 225
6.
7. 220
8. 220

Notice there are blanks. They aren't zero I just couldn't get to them without pulling the fuel system and I didn't really want to do that this instant.

As you can see, she is tired.

I haven't pulled any valve covers yet but I think I've made up my mind. Engine 2 will be my core. Between compression test numbers, SCA data ( there was none in it I could tell vs the other one was just low) and the little info I have or can glean from the histories of the truck I think it makes sense to keep my original engine in tact as a drop in spare (for now) and ship the other one off as a core.

With numbers that low id say she needs a complete bore/sleeve job...pretty much the works. So I think I'll let someone else do that.

Appreciate your thoughts.