Compression test for Mercedes om441? 16.9:1

[joispoi]

I pulled the injectors on my truck this morning and sent them off to get rebuilt. While I have them out, I figured I'd test the compression. Using an off the shelf hand-held tester form Napa, I held it to the injector socket while cranking. The residual oil in the cylinder popped, blackening my hand and the gauge. I was a "slightly" surprised.:tong

After cranking it enough to clear the cylinders, the readings I got were all around 125psi. I can't say if the gauge is accurate since I have nothing else to compare it to.

Does anybody know what the compression reading should be on this motor? I believe it's a 16.9:1 compression ratio but not 100% certain.

 

[John C.]

I've never been able to hold one of those hand held things in the bore hard enough that the compression didn't blow past. What you got is what I'd expect for an old Chevy 350 gas pot. I would guess you should be somewhere over 250PSI.

I used to make injector adapters our of old injectors. Make a gauge fitting from an injector line, pull the guts out of a bad nozzle and install it like a working nozzle back in the engine. Keep the shutdown in the off position and spin the motor for 20 seconds or so for pressure to even out.

 

[joispoi]

Thanks, John. You're right about the gauge. It's a relic from when I took the 350 out of a '72 blazer that I put into an '80 camaro. 250 psi is a long way off from any of the numbers I saw, but my attempt at testing wasn't very scientific.

I'm going to have to put the injectors back in before I'll have the time to track down a useable gauge. I don't have any injectors that I can re-appropriate to use as a gauge adapter and I've got 3 more days to wrap up loose ends and then I'm heading back to Connecticut for Christmas vacation.

 

[John C.]

Was the engine running rough or something that made you want to check compression?

Over the years I started relying on crankcase pressure tests before doing a compression test. Unfortunately now days they are going to positive crankcase ventilation like on cars and there is usually a vacuum on the crankcase. Those furnaces they stick on the exhausts make things tough also. If they work right you don't see any exhaust that might tell you there is oil burning, unburned fuel or coolant in the stack. So far I haven't seen a computer that could provide that information.

 

[joispoi]

The truck was running rough and lacked power. It was making a hell of a lot of noise- like a rod was going to let loose, but the noise was coming from the top of the motor.

I got the injectors back from the shop. All 6 nozzles had to be replaced. I put them back in the truck, changed the fuel filters and it no longer sounds like the same truck. I didn't have time to road test it, last night.

Nothing is visible at the dipstick in terms of crankcase venting. I think the rings and cylinders should be good. If anything were to cause low compression, I would first suspect carbon build up in the valves. The truck had a roll off flat bed with a crane on it. It could have idled for hundreds of hours throughout the years.