M116.981 smokes sporadically

[W109nut]

I just got my 1971 300SEL 3.5 M116.981 overhauled, all original parts, I´ve driven 2k miles. The engine runs good, I had to disconnect the air temp sensor because it was making it run very rough. Every 2 or 3 days it will smoke like crazy for a few seconds(we are talking big cloud of white smoke) and then it will run ok again......any suggestions? It does it @ operating temp, never when it starts, you are just driving along, suddenly you look on the rear view mirror and there it is, a thick cloud of white smoke..... takes 20 seconds and it's gone. You drive another 250 miles and nothing. I ruled out valve seals, since it always has clean starts, piston rings woud smoke almost all the time..... I'm thinking it could be a fuel inyection problem?? I have a brand new ECU, wiring harness, trigger points, pressure valves....they only thing I didn't get new were the inyectors. For some odd reasonn the air temp sensor (also brand new) messed up the engine so I disconected it. Could a bad inyector be causing this?

[Tomguy]

Trans modulator... that's what caused mine. When the diaphragm ruptures it will suck up ATF right into the intake, which causes nearly pure white smoke (oil has a bluish cloud). If this is what the cause is the test is kinda simple: Gun it until say 80-90, and then just completely let off the throttle. If it starts smoking at that second you let off, and if you give it some gas and it goes away, bingo.

[Robert Richards]

Idk name of part but It's sucking transmission oil in care. It's th vacuum house that gos from transmission to carb. to this small round part, I can't recall if It's attached to transmission or not but I think so. Sorry I can give better details . But change that part and ur fixed. It's been few yr.s when I changed mine or I have better terms and names for part.

[W109nut]

Thanks for your reply guys! But mine has a manual gear box....could it be sucking oil from somwhere else?

[mercmad6.3]

Newly rebuilt? Go for a long run,100 miles or more . It may just be old oil in the muffler. As the exhaust heats ,it burns some of it out . You need to get the exhaust really hot to burn it all out so a long run is the only to get rid of it. I would sort out the air temp sensor first though because it will run rich and ruin your engine really quickly.
The only other thing that will do it is brake fluid getting sucked into the via a faulty master cylinder .

[Carleton Hughes]

You have a standard crashbox. Does happen during deceleration when upshifting? wailing out in 2nd or just on high? I would be tempted to watch during upshift. Several things occur thus, when the throttle is closed manifold vacuum rises dramatically and will suck in any oil from intake valve guides which may be worn. From poorly fitted seals etc. The brake booster is a possibilty as mentioned above. A tell tale sign of which would be of course low fluid in the reservoir.
20 seconds is a long time in auto engine years. did you ever pull your plugs to see if the electrodes are burning clean or dirty?

[W109nut]

All clean! Engine had a complete oberhaul. It's been driven 2k miles since. The only thing that I noted unusual is that sometimes before it starts smoking, the engine seems to to misfire for a very short while, then it goes back no normal, but the smoke is there

[mercmad6.3]

Do a leakdown test on each cylinder. Smoke is only caused by leakage . You need to find the leak.