30" Compression Check on DD, by Genesis

First, note that this ONLY works for Detroits - engines with glow plugs or other starting assists will tell you little or nothing.

Second, block heaters must be OFF. The engines MUST be at room temperature. Batteries must be sufficiently charged to roll the engines over normally - use the parallel switch if in doubt.

You must calibrate expectations on the outside air temperature. Realize that Detroits ignite ONLY on compression heat - therefore, how they behave when lighting off tells you a LOT about their compression.

So, what you want it a completely cold engine and good power to roll them over.

Assume temperatures are in the 70s or thereabouts, and the overnight temps didn't go below mid 50s to 60ish. Basically, you need to know what temperature the BLOCKS are - shoot them with an IR thermometer if you need to. The following assumes the BLOCKS are around 70F.

Throttles are NEVER advanced on a start, so we have them at idle.

When the button is hit, the engines should roll over a few revolutions before catching. They should light off and settle down to a clean idle in less than 10 seconds - 5 is better. You WILL get some white smoke on the intial start - if you don't, then either (1) its the middle of summer and the blocks are 90 degrees, or (2) someone cheated and the engines are warm. Note that even a BRIEF start before you get there will queer these results - if you see NO smoke on a truly cold start, you've either got Jesus engines, its the middle of summer and the blocks are near 100 degrees, or someone cheated. If its not the middle of July or August, bet on the cheating. The reason for this is that Detroits run the rack to full fuel immediately when the starter begins turning the engine - so you are ALWAYS overfueled on a start. On a warm restart they come up before fuel pressure builds, so there's no white smoke. On a cold start the fuel pressure comes up first, you get overfueled injection into a cold cylinder, and the result is white smoke as it can't atomize and burn it all.

If compression is low on one or more holes, they won't all light off at once. You will get white smoke until they are all firing. If compression is significantly low you will get a LOT of white smoke and no light-off for a significant amount of time - 20 to 30 seconds isn't unheard of. The reason is that the fuel basically didn't light at all; you're seeing unburned, atomized fuel coming out the pipe.

You're looking for (1) the speed the engines fire up, (2) the amount of white smoke you get on start, and (3) how quickly it clears. If you get a good, clean start without hunting or misbehavior, a small to moderate amount of smoke on the start and it clears within 10 seconds or so, the compression is probably ok on all cylinders. Suspicions mount the more these metrics are "off" for the block temperature.

You should also get NO fuel on the water. If you get a tiny bit on the initial start but it disappears immediately that's not horrible. But if you get white smoke and visible fuel on the water that doesnt clear in short order then either you have a leaking injector or a cylinder that isn't firing.

The problem with this "quick test" is that misadjusted racks and bad injectors can produce these symptoms - as can low compression. HOWEVER, the odds of there being low compression and you getting a CLEAN startup when the engines are truly cold are pretty low.

BTW, check the engines for vibration once they're started up. A common "mechanic trick" if you've got an unscrupulous seller with one bad hole is to back off the injector for that hole so it doesn't fire at idle. This unbalances the engine and can be extremely damaging (since its unbalanced at ALL power levels!) but it will "hide" what would otherwise be a mosquito-control event on a cold start. The engine shaking on you from being out of balance will give it away - if you look.

With engine temps in the mid 80s to 90+ you should see almost no or even no smoke on start and immediate clearing. As temperatures drop from 70F towards freezing more time to light off and more smoke is NORMAL and its even a good idea in the cooler times to hold the STOP switch and crank for 5-10 seconds first to warm the combustion chamber with pure compression heat (no fuel going in) and also get the oil (which will be very thick) started towards the journals and turbos.

As an example a couple of years ago I looked at a 6-71 powered boat that was in an environment where the OATs went into the 50s overnight. We did a cold start test like this. It required upwards of 30 seconds, with white smoke rolling out of the mains - mosquito control heaven - before they fired and stayed lit. Even after 4-5 minutes, there was STILL significant smoke.

A full compression test came back with all cylinders just under minimums, and the deal blew up as the seller wouldn't make a proper adjustment for the trashed engines.

The key to cold starts is NOT pre-heating, although that's certainly good (from a standpoint of engine longevity and such) - its being able to roll the engines over at a sufficient RPM to get enough compression heat for them to fire. That means batteries in good shape and clean electrical bus connections to the starters.

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