om617 not spinning fast enough for compression test - PeachParts Mercedes-Benz Forum

stukreit · #1 03-04-2013, 02:39 AM

I acquired a turbo engine/transmission that had been sitting in a warehouse
for unknown number of years. Have it on the bench, taking parts off, like
ip, alt, turbo, etc, cleaning it up, getting ready for new seals all around.

Installed a known good starter. Hooked it up to a battery with jumper cables
and jump the starter with a screwdriver. With all injectors out, it spins about
1 to 2 revs per second. With a compression gauge installed in #1, it labors
over the compression stroke too weakly to give a compression reading.

Checked the shift linkage to make sure its in P. (tailshaft not turning during
tests)
valve cover is off. valves are adjusted. timing chain is off by >8degrees,
so planning to install a new chain. Otherwise, cam looks ok, nothing out of
order.
Battery tests good over 10 seconds on a load tester.
Suspected dry bearings, (had previously drained the crankcase) so I put
4 quarts in, and squirted each cylinder with wd40. No change.

Next step is to build a more stout cable system with appropriate lugs on each
end, remove the transmission entirely (need to do that for rear main seal anyway), and get starter bench tested.

cranking engine by hand on the front crankshaft nut: Its pretty stiff, but I can
do it with one hand.

Suggestions what to look for would be appreciated.

Also: I have a complete lower end from an engine that ate its vacuum pump
for lunch. Is there any interest in it or parts of it?

Thanks,

Stuart
Palo Alto, CA

t walgamuth · #2 03-04-2013, 06:49 AM

I'd add a second battery with jumpers, clean all cables and run a jumper directly to the starter bolt for assured ground in case of a poor connection for ground. Bench testing the motor would require the negative cable to go to a starter bolt probably to get good connection. the bosch starters unless abused last a very very long time.

vstech · #3 03-04-2013, 07:13 AM

can you check battery voltage while the engine is spinning? if it's under 10, the battery is not up to the task...

Volker · #4 03-04-2013, 09:21 PM

Check wiring and battery, even without oil should do better... I had once a car with very weak cranking on hot start. Turned that main engine ground was not seated well, so all engine current (starter, alternator, injection) passed on a 22 ga wire. When cold start, it worked OK, but at hot start, the wire got so hot the insulation burned off and engine cranked slowly or not at all...

moon161 · #5 03-04-2013, 11:49 PM

When I tried to start my new engine out of the car, it hardly spun and heated up the cables. I looked at pulling the positive cable out of the engine compartment, but figured I'd put it all back in regardless. Installed the engine and it started up fine.

So, it doesn't seem like the engine. A short, thick conduction path is what you need. If you measure the resistance between the two ends cable between the positive battery terminal and the starter post, this should give you an idea of the resistance you are shooting for. Probably a lot less than your typical 10' jumper cable.

Not that hard to drop it onto the mounts and a jackstand under the tranny. Hook up the positive cable, ground strap, and a remote starter switch, bottle of diesel if you want to start it.