Rough Idle when hot

[Oreo]

Hi Guys

My 200 carb 115 engine is starting to idle rough when the engine gets warm. Coincidentally, it started doing this after an oil change to 5W-40 synthetic and new spark plugs. I never made the connection but my mechanic suspects it something to do with the oil. Only way this can happen is if its burning oil but its not. Still runs smooth, only starts to miss at idle. I cannot compensate this with idle mixture and I'm now thinking its an ignition problem.

Any theories?

Oreo

[mzsmbs]

since you just messed with plugs, that's where i'd start. check your connections. make sure you have no loops or crossing. if that doesn't get it then try to look and see if you maybe bumped something loose, like vacuum connection....

[mzsmbs]

are the new plugs the same as the ones that you replaced? how about the gap?

[Oreo]

Hi

Plugs are the same factory gapped to 0.8mm Bosch W7DC. I'm going to do a visual check from plugs to distributor cap later. Yes, vacuum is a good point too.

Oreo

[300SDog]

Thats a good idea to retrace your steps.

You can also try cutting back the wires 1/2" with razor blade where noise suppressor spark plug caps screw onto them..... and you can do the same thing where wires meet the distributor cap, depending on whether you've got screw fittings there. I also used to clean the contacts on the rotor and inside the distributor cap as part of basic maintenance while adding a drop or two of thin oil to the distributor advance.

On changing over to synthetic, your next oil change should come soon as synthetic oil purges and suspends old residue from the galleries. Why did your mechanic think synthetic oil caused the problem...... valves maybe??

[Oreo]

Hi Dieseldog

He believes that the oil could be too thin. 5W40 and running just at 1.2 bar at idle. Another thing I just thought of is that I put some Copaslip (antiseize) on the plug threads before screwing them it. Maybe this is insulating the grounding? I will be checking the resistance of each plug wire. This is a regular misfire I can hear in the exhaust. Do you think I should be checking valve clearance too?

Oreo

[Mark DiSilvestro]

Also check for vacuum leaks. Old vacuum lines get brittle (including the thick plastic one connected to the power brake booster) and one may have been bumped loose or cracked during the recent plug and oil change.

Happy Motoring, Mark

[Oreo]

Hi Mark

Just logged in to report my findings but yours is the most accurate long distance diagnosis!

I checked the plugs - 1 & 2 black, 3 & 4 very white.
I open the hose at the booster - vacuum hiss so that's OK. Trace the hose carefully and lo and behold! At the check valve connection, it was almost cracked in half. I do a quick patch job with some Hylomar and teflon tape, start the car and had to readjust the idle mixture back down (I've been trying to compensate). Test drive and everything is smooooth again.

Will get a new hose when my supplier opens tomorrow. Thanks to all once again, it was really driving me nuts the last few days!

Oreo

[300SDog]

Originally Posted by Oreo

Do you think I should be checking valve clearance too?

Oreo

No, valve clearances usually cause loss of power when the engine is cold and can cause slow starting too. Am wondering how the mechanic relates change to synthetic oil in this equation...... If slippery oil doesnt effect the valve train, then how else can synthetic oil effect performance except at reducing engine friction in general.

I'd like to hear your mechanic elaborate on his theory of oil adversely effecting engine performance.

[Tomguy]

Even if no smoke out the exhaust is apparent, your engine may still burn a (slight) amount of oil.

5-40 is too thin for these engines IMO. Lower oil pressure (so stuff like cam oilers dont squirt as well as they should) and the thinner viscosity can increase friction. Thinner oil flows away fast and doesn't coat as well as higher-grades. 5-40 is OK if you live in a cold area during the winter but NOT for summer driving! At operating temp your oil is nearly as thin as water, or oil that was dilluted with gas - not good!

However, it wouldn't cause rough idle... just increased engine wear, higher operating temp, and a crapload of black gunk probably that accumulated around the inside which the thin oil is flushing out.

[norbtx]

I beg to differ. 5W-40 works just as well as any oil in high temperatures, its a multi weight oil, thus the 40 rating. What you are describing is a straight oil. If it were a straight 5 weight, then you'd be correct, it would be too thin at running temps. But having the 40 means that it works just as well as other 40 weight oil at high temps.

[300SDog]

Welcome back Tom, what do you run in your 108 4.5? Synthetic oils in Vintage MB's is a subject that hardlly gets covered here. Am certain it will slip right past primitive engine seals on 111's. Old timers awhile ago cautioned me against running synthetic in my 114. Yet it worked well in my 116 twincam 2.8 last Summer, seeming to make the engine run quieter.

[Oreo]

Hi DieselDog

My mechanic suspected that the engine was burning the thinner oil and gunking up the plugs. He hasn't worked on the car a long time so he was just guessing too.

My experience with synt oil: Although thinner at 5W40, the engine is quieter and feels more powerful, more willing to rev. My last fill was a 20W50 dino (just to finish up some leftovers) and the difference is quite significant. Oil pressure with the dino used to start high but rapidly dropped when the engine warmed up. The dino starts lower but is more stable. When hot, Norbtx is right - the pressure for dino 50 and synt 40 is not significant, but you can hear the difference. Overall I'm quite pleased with synt and will continue to use it.

Oreo

[Tomguy]

DDog: Depends on weather, budget, etc. I'm gonna fill her up with Quaker State (dino) 10-30 when I do the next change. Right now I have Mobil Semi-Synthetic ("Drive Clean Plus") 10-40 in her. I'll let you know if I notice a diff. I haven't driven her much at all since I got back from a decently long trip (~1 hr each way, the longest I've taken her thus far since I basically rebuilt the top end and replaced a ton of stuff). The kingpin bushings must've worn out on the trip because, on the way back, she pulled to the left HARD. Came back and found a TON of play in the LF wheel which was NOT there before. Need to redo that now >_<

nortbx: Well, that's what oil companies WANT you to think. I beg to differ. 5w40 means its thinnest viscosity is 5 and thickest is 40. Viscosity goes DOWN at higher temps, not up. Test it yourself - nuke an ounce or two of 5w40 for 30 seconds, then do the same with 10w30. You'll see the 5w40 is a LOT thinner!
*edit* fix typo!

[norbtx]

Have you done the test yourself? How could you possibly measure viscosity of oil at those high temps without lab equipment? Microwaving oil for 30 seconds probably won't raise the temps to engine temps anyway.

I also don't subscribe to "Oil Company" conspiracy theories. What would they gain?