Hello All,
My Dad has owned a 88' 420 SEL since 2011. Prior to then it was owned by a close family friend since 91' and when he moved to Florida we bought it. It currently has ~110k miles and is driven around town by my father (~2-3k miles per year) who is retired and serves as their second car. We have enjoyed the car and take good care of it: annual oil changes, basic maintenance, including timing belt in 2012 (85k miles).
Recently we've hit a wall and are unsure whether we can keep it. Here is a summary and I'll try to keep short... in November the car has began to stall while in the driveway. My dad took it to a reputable indy mechanic who has done the following - note we have had a good leak from main seal and knew the transmission was giving issues. Turns out the pump was bad and might have also been due to running too low on transmission fluid (shame on us for missing this).

• Replaced auto transmission pump
• Replaced power steering pump and 5 drive belts
• Replaced O2 sensor, water temp sensor, thermostat,
• Replace fuel filter, oil filter, AT filter, crank seal,
  • Replaced spark plugs, confirmed
• Replace distributor cap and torot
• Replace E.H.A. valve

Even after all of this the car continues to not run very well: rough idle (also rough during cold start) and looses power while on the road. The mechanic does not feel the car is safe to drive and feels he has tried just about everything except the timing chain. Even though timing chain only has 20k miles he looked it and noticed the chain has a lot of play. He wonders if previous mechanic did not change the guides while doing the chain and improperly installed it.

His/our hope is by going ahead with changing the timing chain and guides we will solve the rough idle/running issue.

We have already spent a great deal at this point and my elderly Dad is at the point of giving up on the car. We all love it and I personally would hate to see it go. Curious if anyone has any advice they can share? The mechanic has spent a lot of time on it and has good experience with older Benz's. This one has even stumped him.
Any input is appreciated. Thank you in advance!

Best,
Nikhil

 

Hi,
i just did a timing chain change on my 560SEL.
Here are my thoughts about your issue: the timing chain which has been changed most likely involved new guide rails (crazy mechanic if not). The guide rails would not be responsible for a loose chain, but the chain tensioner would. Can you find out if that tensioner was replaced?
I do not believe that the timing chain would cause a rough idle if something was installed wrong 20k miles ago, I'd like to think issues would have shown earlier., but I'm not a pro mechanic.
The items I would check next in your case:
- fuel pumps: do they run? (not sure if the 420 has on or 2 pumps like the 560 does)
- bad fuel pump relay not providing solid power to the pumps (can be checked by pulling the relay and bridging contacts #7 and #8)
Let us know what you find out.
Good luck.

 

Hello Thomas.
Thank you for your input and advice. The car is presently getting a new timing chain along with tensioners etc. I agree with your logic above however the mechanic felt this would solve my issue. I too am not mechanically inclined and relying on their counsel. In reading the mechanics notes they have changed the fuel filter and tank filter. Notes also say they swapped the fuel distributor, replaced o2 sensors and spark plugs.

Your comment about the fuel pump relay is a good one. I recall doing this years ago. Thanks for the input and I expect to get news from the mechanic this week.

 

good luck and keep me posted. If you end up selling the car let me know… i might be interested.

 

OP.... You mention of fresh secondary ignition parts. Spark plugs, cap, rotor. Plug wires? Quality parts! No chinesium garbage.

No mention of ruling out vacuum leaks.

All VERY common.

Also common... crummy fuel pump relay or fuses that have poor contact in the fuse box.

Fuel pumps... yes; a pair of them are generally very reliable but worth testing. Volume and pressure. Check the pumps electrically with an oscilloscope. An open winding or worn brushes will jump right out. Those pumps when healthy and happy will easily fuel over 600 horses.

Spray patterns of injectors checked? Nozzle opening pressure correct? Injectors cleaned? Only way to clean them is on the bench with the correct tool. I have it. When injectors come out the plastic guys they stand in + all injector seals and the (silly) rubber air manifolds with plastic parts get replaced. There are many vacuum leaks to be solved just doing that.

If your guy is firing the parts cannon at the car he has no clue. Take it to someone who does.