Author Topic: Cylinder 1 Misfires at Idle - Blue Smoke on Acceleration (Read 2972 times)

mBdrvr · « **on:** June 12, 2022, 22:52:11 »

My 250 engine was rebuilt about 3,000 miles ago. It has started to misfire at idle and produce blue smoke on acceleration after warmup. No smoke when cold. Runs OK at speed. No stumbling but produces bluish smoke.

Pulling the wire for cylinder one does not change the rough idle as it does with the others. Thus cylinder one is the culprit. The misfire stays on cylinder one when switching plugs and wires. One's plug fires when rested on the head.

My next check is the injector.

Any ideas or suggestions?

ja17 · « **Reply #1 on:** June 12, 2022, 23:50:11 »

Check compression on number one.

mBdrvr · « **Reply #2 on:** June 13, 2022, 00:05:37 »

1 = 145
2 = 127
3 = 127
4 = 125
5 = 131
6 = 130

MartinK · « **Reply #3 on:** June 13, 2022, 10:04:00 »

I think you may have a look at the fuel injection. The plugs are severely black. Did you check CO and HC parameters? Talk soon. Martin.

Benz Dr. · « **Reply #4 on:** June 13, 2022, 14:46:44 »

Those are your compression numbers on a fresh rebuild? They should be up around 160 - 170 PSI if everything is working right.

mBdrvr · « **Reply #5 on:** June 13, 2022, 19:55:28 »

I'm going to swap injectors to see if the problem moves with the injector.

Thanks for the input. I'll post here what I find.

mBdrvr · « **Reply #6 on:** August 03, 2022, 02:38:30 »

I found there was no fuel getting to cylinder 1. I traced it back to the fuel injection pump. I replaced the pintle in the delivery valve. Now cylinder 1 is getting fuel.

I still have plumes of white smoke out of the exhaust after a stop sign or light.

Any ideas?

ja17 · « **Reply #7 on:** August 03, 2022, 19:53:28 »

It may take some driving since the exhaust system may have filled with soot and unburnt fuel when the problem was happening. If so it should be better when starting with a cold engine and gradually gets worse as the exhaust system warms.

mBdrvr · « **Reply #8 on:** August 03, 2022, 20:26:24 »

A professional mechanic confirmed that it's definitely oil that's burning. Its better when it cold and worse as the engine heats up.

I've driven it about 100 miles and it persists. Should I take a look at the valve seals or drive it more?

ja17 · « **Reply #9 on:** August 03, 2022, 22:28:15 »

Check to see if the transmission fluid is low, if it is an automatic. A ruptured modulator diaphragm will allow auto trans fluid to be sucked up into the engine and burn off through the exhaust.

teahead · « **Reply #10 on:** August 03, 2022, 22:58:37 »

bad oil rings or bad valve seals.

Let's hope it's not the former. Not sure how to narrow it down to which one it is.

mBdrvr · « **Reply #11 on:** August 04, 2022, 01:17:02 »

The pro confirmed that its oil. The transmission and engine were rebuilt within 3,500 miles.

I've removed the valve cover to look for loose valve seals. I can see the wires wrapped around many of the the seals by looking through the springs with a borescope.

Are these in the correct position?

ja17 · « **Reply #12 on:** August 04, 2022, 04:39:27 »

Possibly a valve guide has come loose in the head. The oil drops into the exhaust port below and burns off in the exhaust system. With this situation, the oil never enters the combustion chamber and does not foul any spark plugs.

teahead · « **Reply #13 on:** August 05, 2022, 17:45:05 »

ya, you definitely do not have any fouled spark plugs.

the bad guide (were they replaced?) theory is sound.

mBdrvr · « **Reply #14 on:** August 17, 2022, 01:45:37 »

The leak down test showed air escaping through the oil filler therefore rings.

I did a wet and dry compression test with another gauge. Here are the results.

A mechanic I know asked if the engine ever overheated. It did once about 2 years ago (1,000 miles?) but didn't boil over. I revved the engine to get the fan to cool it down.

I think it needs rings.

teahead · « **Reply #15 on:** August 17, 2022, 18:27:30 »

well...that sux.

Maybe a ring broke. How could it be worn out on a rebuild?

Benz Dr. · « **Reply #16 on:** August 17, 2022, 19:50:10 »

Mid 170's is normal compression for a fresh 250SE. Compression is HP on our engines and it kind of looks like this one is lacking on both.

mBdrvr · « **Reply #17 on:** August 19, 2022, 17:20:53 »

Would overheating remove the spring in the rings? Are there better rings than the MB parts that I can use for the rebuild?

Charles 230SL · « **Reply #18 on:** August 19, 2022, 18:32:20 »

Paul, that valve seal doesn't look right in IMG_1421. If I'm interpreting the photo correctly, it looks like the spring holding the seal on the valve guide has pulled loose. I can see (what appears to be) the valve stem. I realize its difficult to see with a borescope but that valve seal just doesn't look right to me.

Charles 230SL · « **Reply #19 on:** August 19, 2022, 18:56:57 »

I've pointed an arrow at what appears to be the valve stem and I don't see a seal - just the spring/wire that holds the seal on the guide.
If you've still got the valve cover off, any chance you can get a better picture of that seal?

Shvegel · « **Reply #20 on:** August 23, 2022, 01:53:11 »

Having lived through this exact scenario I think Joe is right. Loose valve guide. A good puff of smoke leaving from idle. The new guides are stepped so if you have to pull the head do all 12.

News:

Author Topic: Cylinder 1 Misfires at Idle - Blue Smoke on Acceleration (Read 2972 times)