gandersen

I've read through all the posts with idle issues. Still not sure were to go from here. I'm a new M/B owner. Always wanted one, so I got a fixer upper.
87 420SEL. Car starts fine. Then the idle goes up and down, between around 500 to 850. Almost dies, the engine rocks horribly, then it goes back up for a second or two, then goes back down. I've replaced the OVP, EHA, fuel pumps, even was stupid enough to take the injectors out and clean them. If I disconnect the power to the idle control valve, the rpms shoot up to around 1400.

Any idea's?

elolson

I have not seen that problem. You might consider picking up the factory service manuals. They go for about 20$ on line. They have trouble shooting flow charts that are pretty good. If you are up for it then that would be best unless you take it to a shop. If you look you will see that you might get only 1 reply or none from questions on this forum. Most folks just don't wrench on then benz's It take specialized equipment for the FI system and most folks don't want to pony up for a 1 time repair. I bought the fuel pressure kit for a 100$ and used it once and will probably never use it again is one example that I offer up.

Good luck and good luck with the car.

rcrc7

Give this a shot: The left bank rocker cover breather hose goes to the start valve fuel injector. Remove this hose and give it a good clean up internally. Its a ejector type of a design. While the hose is out, use a Q-tips cotton swap to clean out the 2 mm bleed hole inside, at the start valve side. Light air blast helps. Apply a small amount of silicone sealer to the "tower" where the smaller end of the hose sits to prevent air leakage when you reinstall back the hose. Try not to push too far down on the small end side of the hose. Allow the locking tap to do its job. Hope this help!

gandersen

I'll give the breather hose a shot this morning! Thanks. I replaced the brake pads on the left side last night, and afterwards started the old girl up. What caught my eye is that as the idle decreases, the economy setting starts to move towards the red, then it kind of stutters, and the rpms shoot up to 1000, the economy needle shoots to the far left, and then the process starts over again. Vacuum? Something opening and closing? RPM up, vacuum up, then all over again. Hmmmm...She's testing my perserverance!

gogi

economy gauge is indicating a vacuum leak or possibly a partially leaking vacuum hose.
Dave S.

JayTheCPA

Like eoloson, recommend getting a set of shop manuals, but use care as some of them require activation that eBay sellers do not always make available.


Heavy disclaimer, do not have the 420 but do have the 560 which is sensitive to vacuum leaks causing idle issues, but it is more common to cause a high idle (~1K), so am not sold it is vacuum. In my case, there are two critical elements to a good idle which is no vacuum leak and a good idle control valve; a vacuum leak will throw off the idle control valve and make it idle high. Another possibility (again disclaimer as this comes from my Taurus experiences), aside from vacuum leaks is a mass air flow sensor where if it gets dirty (or coated from an oiled filter) the idle will dope. Wish I had more direct experience with your 420, but at least some things to consider.



Given you already eliminated the idle control valve and the RPM spiked, it appears that the part is doing its job, but not necessarily out of contention yet. As rcrc7 mentioned, lose / dirty / clogged breather hoses will cause interesting results, so make sure these things are open and in their correct places. After checking the breather hoses, try to eliminate a vacuum leak as it is the least expensive / time consuming test and go from there; if you do have a leak, happy hunting as there are too many possibilities to list, but at least you can isolate systems per manuals.