cold rough idle 420 SEL 89
 

My new 235 000 km 420 SEL always has very rough idle for 5 to 7 minutes, till warm enoug, and then is perfect. Dealer's mechanics and I tried everything I found on that marvelous forum, change or tried new : All filters, sparks, wire, injectors, EHA, coolant temperature sensor,ignition control unit, adjust valves, tried few cleaners (BG 44k, for more than $40 can), leak test, compression test...Timing chain with a 12 degree extension have to be change in a few days but i expect no results on idle... Lot of canadian $$$! Nothing works except unplug coolant temperature sensor for those feuw minutes, what do a fair but imperfect job. Can someone explain what this mean ??? And if one find what is in trouble, I'l give him a very big virtual kiss !!!

I would have them check the operation of the cold start injector. Where you live, it would have to run for a while when cold.
Just a thought.

I had a cold running problem for one year on my 85 190E 2.3 - changed everything, but what finally worked was changing the oxygen sensor.

cold rough idle 420 SEL
 

Thanks to Benzmac and Agupta for answers. I went to dealer to change timing chain and tensioner. Suggest to check for cold start injector and O2 sensor... Said it could not be the problem. I read all about cold rough idle in this forum : 120 thread in fact. Also heard about a throttle switch adjustment that never had been consider yet. What and where is that switch ???

Benz-LGB (apr 2001, no 166 post) described exactly my problem and fixed it with valve adjustment on no 6 cylinder! So I asked dealer's mechanic to check that possibility. In fact didn't believe me, but scope it as I ask for. Surprise : No 2 cylinder had evident and fixed missfire. Tried new wire, change gas line on fuel distributor: no effect; was then sure that it was a mechanic problem.He made a leak test that show an audible leak in valve cover, not elsewhere! So made like for Benz-LGB car, adjust both no2 cylinder valves till no or minimum leak (75% to 15% leak success)!

So my mechanic think that there is a valve or valve seat minor worn, at least partly corrected by valve shim adjustment, and that maybe cause a secondary carbon build-up explaining idle misfire till that no 2 carbonated sponge plently saturated; it must clear in a while, he think. But the engine was now almost warm and correction impossible to confir; I was sure that this was the solution, like on Benz-LGB 1991 420 SEL!!!

Few days and hundred km later, it didn't cure yet and I experience cold rough idle again !? In fact cold idle is little less rough last few minutes less, but hot idle seem to be a little more rough, specially when stop with transmission on drive ! Not satisfactory to me. And in fact I use again an ownmade switch to unplug the coolant temperature sensor what do the best job roughness till now!

But what seem sure is that no 2 cylinder is involved, and maybe no 7 a little bit, and that the problem is mechanic, neither gas or fire control problem... It is a pressure leak on second cylindre, in fact valve, head or manifold gasket problem, but I don't beleive the possibility of a hole in piston or ring leak only because it is a Mercedes and not Lada!!! The challenge is to understand why metal warm cured the symptoms, and the answer would make final diagnosis...

Someone have ideas ???

Marcelb

Did Ever Solve this Problem
 

Did you ever solve this problem with the rough idle on cold start?

Cold Start Fluctuating RPMs
 

I recently acquired an 89 420 SEL. On cold start, the engine idles very roughly and the rpm fluctuates from about 500 to 1,100 in 5 sec. cycles until the engine reaches 80 degrees. It has 217,000 KM on it, about 160,000 miles. The maintenance history is unrecorded.

I'm starting with replacing the air and fuel filters. The fuel line had rusted through in one spot, but was patched using rubber gas line, no leaks. The previous owner also indicated that he had put in platinum plugs, which is yet to be confirmed. The timing chain probably has not been replaced, so it too, is on the to-do list. Valve seals have been replaced.

Any recommendations?

Have you checked the intake manifold seals? The M116 engine I illustrate this on is a 380, similar to your 420. Idle was very rough and intake leaks were detected by pumping inert white gas into the engine at the local independent mechanic's shop. Lots of hours to do, little $, similar symptoms to yours.

Intake manifold seals
 

No I have not checked the intake manifold seals.

I have now replaced the gas line, the gas return line, and the brake line, replaced gas filter, replaced platinum plugs with bosch w8dc, replaced oil and filter. Replacing these lines was a 6 hr. job, the rear end had to be dropped to get to the gas tank.

Things seem to be getting worse. On cold start (15c or about 60f), no start on first crank of 5 sec.. I can start it on first cold crank if I press down on the gas pedal while cranking. But then the engine races to 1100 rpm, and as I ease-off the gas the rpm drops to 500, and the engine feels like it is missing, rocking, but I apply more gas to prevent it from stalling, it runs smooth at 1100 rpm. I give it gas 2 or 3 times, especially if the rpm drops below 500, to prevent it from stalling. I have put in 2 cans of duralube advanced fuel system treatment in one full tank of gas, and run it 1/8 full.

When she is hot, above 60c, she runs very well. Just a little bit of roughness on idle, a little gentle rocking, hardly noticeable, unless you looking for it. Someone suggested replacing the ignition coil to smooth out this idle problem. I finally got the right coil, and it should be installed in a couple of days.

Any other suggestions? Replacing the seals looks like a very big job!

Solved
 

If anyone is interested, my problem was solved by replacing the O2 sensor, and adjusting the mixture. The mixture was set, on the lambda meter, to fluctuate between 4 and 5 at hot idle.