w126 420SEL...rough while cold...out of ideas
 

Car has 335,000km and is a 1987 420SEL

I got it about 2 weeks ago and have been working on figuring out the roughness and general hesitation of the engine. It was really bad at first but still "driveable", however it felt like it was going to die at stop lights or while idling in drive. It was a bit smoother in park at higher rpm.

I replaced the following things in this order:
1. Spark plugs - no difference
2. Ignition wires - little bit of improvement
3. Cleaned corrosion from distributor cap and rotor - little bit of improvement
4. Removed all fuel injectors and took them for testing and cleaning...one was clogged but is now spraying correctly. Smoother idle after reinstalling them
5. Found 2 or 3 disconnected vacuum lines in the engine, connected them properly - Car idles 95% smooth and fires on all cylinders ONLY after driving for 10 min, parking the car, turning off the engine and restarting it. WTF
6. Cleaned idle air piston thing - Slight improvement at warm idle


When I cold start the car it will run like crap and the engine will run very cool no matter how you drive. It will stay under 80 degrees celsius the whole time and feels like 1 or 2 cylinders might be missing, but not misfiring out of time or anything. While this is happening, the transmission refuses to kickdown properly. 0-60mph time in this situation is around 11.5 seconds

After driving it around, and letting it idle a bit, if i turn off the car and let it sit for 2 minutes, then turn it back on, it will then fire on all cylinders and idle very smoothly. The temperature then goes to 90-100 degrees celsius. Kickdown works properly and the car has full power again. 0-60mph in this situation is 8.5 seconds.

I've run out of ideas for what may be causing this weird pattern.

Coolant temperature sensor?

If the engine will not warm up properly that is the symptom of a bad thermostat, replace it with an OE or OEM. This may or may not resolve the rough running/misfire but needs to be done so it reaches operating temperature on the first start-up.

Good luck and keep us posted!!!

We might well ask, "What would respond to a period of heat soaking?"
Among other things, mostly mechanical, there is also a group of thermal switches and vacuum valves mounted in the water outlet manifold at the upper front of the engine, the highest point in the engine of the water system. The water at that point in the engine will rise in temperature when the engine is stopped. One of those valves controls vacuum to the EGR valve. If the EGR valve is being held open by faulty vacuum control, or is itself sticking open, the result will be 1) rough running, 2) low power output, 3) low water temperature due to low power.


qw, try this: pull the vacuum line off the EGR valve on the left exhaust manifold, and plug the vacuum line. Start engine, and if running rough, tap on the EGR valve to see if it will close (if stuck open).

Sounds like mixture too lean at cold idle, may just need the mixture reset, take it to someone to check it out , when its cold the mixture is in fixed mode open loop , when warm its using sensors to adjust mixture, you may just need an adjustment due to age , lambda need to be set hot fully warmed up , then checked cold