For the V8 126s:

High idle can be caused by a dirty idle control valve. This can often be fixed with a long soak in brake cleaner. If that does not work it should be replaced.

Rough idle is often caused by vacuum or air leaks. Check for broken or disconnected vacuum lines in the engine bay. The injector seals should also be checked.

Know when the timing chain and guides were last changed. If you don’t know and the car has over 100,000 miles, replace the chain and guides.

300 SE/L:

Stalling is often due to a failed fuel pump relay (MAS controller in 90 – 91 cars).

Hard starting cold and hot with gas smell under the hood can be a bad EHA.

M103s must have a good cap and rotor to run correctly. Check for cracking or burned points. The distributor oil seal can also leak and foul the components.

Keep a quart of good oil in the trunk and check the oil at every fill up. It is normal for the engine to use oil, especially at highway speeds. Also at highway speeds it is normal for the engine to run near 4000 rpm.

Whoa, I like the sound of that. Neither my Bimmer nor Benz have a working cruise control. I tried used parts in the Bimmer - no luck - could not find anyone to rebuild it. New parts around $1,000. I want it, but not that bad.

I'll do a search for threads on w 126 cruise control but do you have any other info on what sort of solder connections to look for and where? I guess it might be obvious, but who knows.

I figure if I mess mine up, I'll try a boneyard item -- if it doesn't work, I'll send it in as a core.

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1986 300SDL, 362K
1984 300D, 138K

As for tips, don't have many yet but, most all of you experienced hands know about the Alda adustment on the injector pump but when I was a really green newbie, I was wondering why my diesel was not torquy like diesels are supposed to be.

I somehow found a tutorial on adjusting the Alda, and Oh Mah Ghod, that was a good day. We have ignition and power, Houston.

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1986 300SDL, 362K
1984 300D, 138K

If I may add to this..... Also replace your antenna seal if you have water in your passenger rear floorboard. I know it sounds impossible, but I was getting water in my trunk and right rear passenger floor. I replaced the antenna seal to stop the leak in the trunk and instantly, my water problem in the floor also went away. Now my floor is DRY and has been for a long time. Luckily, my floor is still like new - no rust and still paint covered, so I was glad to get this fixed. I suppose water was flowing over somehow and dripping into the floor panel on the passenger rear.

All great tips. Thanks folks

The '126' is IMHO, the BEST EVER BUILT. However, the rubbery 'undercoating' can crack or get torn, or water find its way through a panel form the other side--and terminal rust cancer can kill the car--especially the passenger side firewall and front bottom of each rear wheel-well.

Inspect the lower front passenger firewall and all other areas at least once a year and faithfully clean-out and repair ANY cracks or tears in the coating. Rust is VERY hard to detect under the rough coating. Some other models have front upper spring perches falling off due to rust at the perch/frame(body) joint hidden under the 'rustproof' rubbery coating. Same issue around rear window, rear door quarter windows, under door gaskets, trunk gasket, inside bottom edge of trunk lid,etc. All the crevices MUST be kept clean, dry, and rusp proofef. LPS #3 is BY FAR, the best after-market rust preventive for inside seams and body cavities. (I kept a 1971 Chevrolet VEGA ( a well proven rust bait) RUST FREE for over 15 years in a highly corrosive industrial environment by LPS#3 sprayed into all areas and seams (took out ALL screws and bolts, exterior and interior that penetrated body panels and cavities, foamed/sprayed generously with LPS#3 aerosol cans). Result was a quieter, absolutely rust free car.

Remember, the 126 'frame' is thin sheet metal formed into complex shapes, and only a little rust is needed to greatly weaken the structure.

Great tips! Thx.

Why you wanna save a POS Chevy Vega. Not even the Cosworths worth the price of admission. God save us from the 70s, Gremlins, Pacers, Chevy Vegas and Pontiac Astres, Pinto woody wagons, Audis with sticking accellerators and Firestone 500s, Chrysler Cordoba with Hoor House red velour interiors and Corinthian leather halo vinyl tops and fake wire wheel hubcaps, and Cheap Trick Live at Budokan (actually I liked them) but you geddit right?

God help me the 70s. I have *NO* recollection of the Ford years.!!!

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1991 560 SEC AMG, 199k <---- 300 hp 10:1 ECE euro HV ...

1995 E 420, 170k "The Red Plum" (sold)

2015 BMW 535i xdrive awd Stage 1 DINAN, 6k, <----364 hp

1967 Mercury Cougar, 49k

2013 Jaguar XF, 20k <----340 hp Supercharged, All Wheel Drive (sold)

Does anyone have an idea what would cause this car to run smoothly and with full power 2 or 3 days in a row, and then start with a rough idle that does not clear out --- and continuous rough running and missing that day, regardless of the engine temp? I know some one of the dozens of little parts on that engine is sticking or something when the rough running occurs, but cannot figure out what it is. The car is in mint condition, has 98K on it, good oil and gas, etc.etc. -- taken care of extremely well, but I need to find the cause for this errant behavior. Ideas anyone?? Thanks in advance.

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Ben Harrison Carter
1999 Mercedes ML320 87K
1992 Mercedes 300CE 89K
1995 Corvette 29K -- Sold Dec 09
1989 Mercedes 420SEL 99K -- (Sold 4/08)
1968 Mercedes 230S (106K) (Sold 9/06))
1976 Mercedes 450SEL 130K (Just sold - 06)
1961 Mercedes 220Sb (sold years ago)

A rough idle only – the engine is smooth and pulls strong over 1200 rpm or so – is often a vacuum or air leak. If the engine has a rough idle and misses at wot check for good spark. There may be a problem with the cap/rotor. The plugs and wires s/b checked, too. I had a car with a very poor idle and miss and it would come and go. The insulation on one of the spark plug wires was bad and the spark was jumping to the dipstick tube! As a quick fix I put a piece of fuel line between the wire and dipstick and the engine ran smooth again.

Make sure you have plenty of $$$$ in your wallet as well.

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2004 Toyota Sequoia Limited 4wd
1991 Lincoln Town Car Executive
1991 Cadillac Sedan DeVille
1988 Mercedes 300SEL
1972 Chevrolet Caprice Kingswood Estate 9-passenger wagon
1973 Pontiac Grand Ville
(Prior MB's: 1974 240D, 1985 380SE, 1984 190D, 1993 400SEL)

There's a long discussion of this on another thread, but it seems pretty common for the 300 SEL. Oil blows up through the valves and dribbles down the block, mostly over the #1 cylinder. The #1 plug will foul early and often. I'm betting that getting the valve seals replaced will drastically reduce my oil consumption (quart/750 miles, no smoke), keep my idle smooth, and prevent my having to replace the #1 plug 5 times as often as the rest of the set.

Tim
'89 300 SEL, like one of my kids

I survey, transport and repair common issues with W126 and W116 and carry a crash kit for common repairs.

Adding to the above posts: I make 1" X 1/2" x (questimate) 18.5" long to the
bottom of a knotch, sticks just incase the white nylon window slider fails. This way I can hold the window up and lock the car! I will measure them as soon as I replace several regulators.

If there is any crud on the positive battery terminal: Note a large battery can be fun to lift from this dark hole from @!X!!
1. brass terminals will be coraded past wire bursh fix. Use a Lesile battery terminal reamer or similar device and expect to remove a lot of brass. Some have taked 15 minutes per terminal!
2. wash everything under the hood. Battery acid and crud has a hunger for steel and aluminun goodies.
3. clean the leaves and other crud from under the battery tray. Wash this area.

Keep a 13/16" deepwell spark plug socket and wrench in the trunk of all sunroof cars. Now if the AC fails, you can manualy open the sunroof.

Power steering filters are never changed by DIYs. Change this NOW.

If there is any sign of DIYers, drain and repace brake fluid with NON DOT3 fluid.

If fuse box has the paper listing proper fues size per circuit, verify all fuses. DIYs love to bix & match. Remove negative battery terminal and clean fuse connections. If paper list is not in the box, GET ONE NOW.

Diesels:
Don't toss out the used per-filter before looking at it's condition. Check for any sign of alga growth. If found, drain tank and clean strainer. OH WHAT FUN!

Change the filters and keep spares in the trunk.

Verify prime pimp is in working order.

Verify battery condition and size. 50% of the diesels I have survey'd / bought have had a new SMALL battery.

Can someone please tell me why a failed vacuum pump will wipe out my engine as mentioned in this post?

Chunks of the metal linkage fall into the timing chain = broken chain and bent valves.

Diesel Engine Mechanical:

1982-240D-Vacuum System
http://www.peachparts.com/shopforum/showthread.php?t=105201

Vacuum pump lesson learned
http://www.peachparts.com/shopforum/showthread.php?p=872760#post872760

Vacuum Pump Gasket Sealer & Torque
http://www.peachparts.com/shopforum/showthread.php?t=138153

Does my 1989 420sel have a vacuum pump? If so where is it ? The only vacuum pump i know of is in my trunk,it opens and closes the doors.