Neil:

Nice write-up, but 2 of your links don't work and one requires a loginid/password.

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'91 300-SEL

Thanks '1991300SEL',

Just edited it to make the refs to AllDataDiy no longer links.

As I mentioned I think to properly view the one remaining link you need your own login. I've printed off the pages from this link and can quote extracts from them, but am too unsure of the copyright laws to simply post them......

Just had my second stall/refuse to restart episode of the week when I took the car for a 10 min spin after 3 days of non-use.

While stalling you could still hear the fuel pump under the car whirl for a few secs on turning the key - does that mean I can rule anything out? Did though try '1991300SEL's suggestion of jumping pin 1 & 2 of the MAS. This did indeed get the car to fire up with it idling pretty high for 10secs or so then suddenly dropping back to normal. Once idling normally the 'hiccup' in the idle was still present though.

Not sure what conclusions I can draw from this as it took me a good 15 mins before I took the MAS out, so it could be that the car was getting ready to restart anyway. Will be leaving the car unused for 5 days next week so if/when I get it to stall/not restart again then I'll try to jump the MAS pins immediately.....

Hi Neil

I could still hear the fuel pump as well, with the car refusing to start. The pressure guage arrived Wed. Am out of town till Sun. Will test fuel pressure and ignition Mon. (using in-room webtv).

-Robert

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89 300te 222,222
92 300e 190,000

I just discovered that the 7MM Torx bolt that secures the distributor rotor driver to the camshaft was broken. The rotor was loose. I don't understand how exactly this could cause the symptom but regardless this is a major find.

I performed the following tests prior to discovering the fractured bolt, none of which made any difference:

• Rigged-up a six spark-plug assembly and observed what appeared to be a mostly normal firing pattern during a stall episode

• I jumped #1 & #2 pins on the MAS. Pump ran.

• Sprayed carb cleaner into throttle body

• disconnected the ICV

• disconnected the EHA

Again, none of these made any difference what-so-ever. The motor didn't even give a hint of starting.

By this time I was almost at the end of the 20 minute stall window so I didn't have time to attach the fuel pressure guage. I did however remove the distributer cap just for a sanity check. That's when the discovery was made.

I suppose the loose rotor could have caused the occasional spark failure, which in turn could interfere with the coil and subsequently the EZL. The changing temperatures in the distributor could have caused a gap when the cam is hot but the rotor driver is cool. The ignition timing advances as the accelerator is depressed, and if the rotor driver is broken it is possible that the advance could be beyond the reach of the rotor's electrode. This is definately a theoretical stretch. Theory aside, obviously the rotor should not be loose and that is my next repair.

Neil - you might want to inspect this area and verify there is no wiggle in the rotor. There was considerable rust at the back of the driver and it was somewhat bound-up with rust. I could see how the broken bolt would be missed by a tech since the rust kept things together. It wouldn't hurt to remove/clean/replace the rotor and the driver, just to make sure it is properly seated.

Robert

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89 300te 222,222
92 300e 190,000

I never heard of a 7mm torx. They come is sizes like 20, 25, etc.

No matter.

Your discovery is a common problem on 103 motors. I wouldn't bother with any other diagnosis until you resolve this.

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'91 300-SEL

1991300SEL: Thanks for your tip on jumping the #1 and #2 pins of the MAS - saved me a bunch of time.

Seems bizarre to me that the bolt would break commonly, it doesn't appear to have any stress on it what-so-ever.

Do you have any more tips on R&R of the stud or have any idea what I should expect when trying to get the broken stud out of the cam?

Thanks,
Robert

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89 300te 222,222
92 300e 190,000

Robert:

I haven't had to drill out a broken bolt in that area yet. Hopefully someone who's been there will offer up some solid advise.

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'91 300-SEL

I have never drilled a stud out of a cam but have done it on an exhaust stud.

Use a crisp punch first to create as close as possible to the center of the stud. Then start with the best drill bit you can find. Smaller size is better and you can always graduate the sizes as needed.

I would attempt using an E-Z-Out and see if it will turn.

Haasman

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'03 E320 Wagon-Sold
'95 E320 Wagon-Went to Ex
'93 190E 2.6-Wrecked
'91 300E-Went to Ex
'65 911 Coupe (#302580)

Just had the distributor cap off to have a look - I seem to have 3 of these small Torx bolts holding the rotor in place - sound right? Anyway everything seems secure but did notice the following:

1) Each of the six small metal contact points in the the cap seemed to have a deposit on them - almost like iron filings. Wiped them all clean.

2) Base part of distributor (red plastic bit still attached to car) has a chip in the out ring (near the bottom) – significant?.

Put everything back together and tried to start the car (first time in 2 days). Guess what: It would crank but not fire up? Tried 3 or 4 times over a 10 min period and eventually it spluttered into life. From then on business as usual: hiccup when idling and poor accel at low revs. After leaving to idle for 10 mins on the drive it sorted itself out and I took it for a pleasant 10 mile spin – not problems at all.

Strange thing was that this is the first time ever that I've had the non-start problem from cold. Was it a coincidence that I'd been in the distributor or could this be the heart of the problem? Time to get a new one (distributor that is, though wife does say car!)?

Grasp the rotor firmly and try to wiggle it out. If you remove the three Allen head screws the rotor will rotor will come off. Behind the rotor lives the "driver", a metal part that attaches to the cam with a single bolt, and the rotor at the other end. It is this bolt that is broken on my car.

I think it is more than a coincidence that you had the initial stall associated with removal of the cap. And my similar symptoms seem to be associated with the cap also. A prior posting indicated these symptoms were remedied with cap and rotor replacement.

I doubt the chip is significant but the filings are suspicious. I think the bigger question is where did they come from? If I had to guess I'd take a look at that bolt.

I'll keep everyone informed.

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89 300te 222,222
92 300e 190,000

I think it's fixed!

I replaced the broken $3 T-40 Torx bolt that secures the rotor driver to the cam. I figured the cap and rotor needed to be replaced since a loose rotor would have probably damaged the contacts, so I did those as well as the plugs and wires The car sat for five days and I drove it to the spot where it has reliably stalled 5 out of 5 times. This time no stall, no hesitation, just a perfectly smooth, responsive engine.

Thanks to all who helped out!

Robert

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89 300te 222,222
92 300e 190,000

Brilliant news!

Have just been away for 5 days: took the car out last night and same sorry story! Should have a chance to investigate as you suggested tonight. Will get a new cap and rotor on order (other bits are new)....

One question: the central metal pin in the cap - is it a fixed length or spring loaded? Have seen both designs (think the sping'y version is the newer type).

Cheers,
Neil

The center electrode is always spring loaded. As somebody who has worked in an auto parts store I can tell you that I've never noticed one that wasn't spring loaded. I once had a terrible time with BMW 2002 where the center electrode looked fine but was stuck. Took me for-ever to find that problem.

While you've got the cap off, I highly recommend removing and inspecting the Torx bolt. It only takes a small amount of torque to remove it and should only be lightly torqued when replaced.

BTW, one of the sections of the plastic fan has the blades farther apart to make the removal of the cap easier.

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89 300te 222,222
92 300e 190,000

Went to my local parts supplier (German & Swedish, UK). Based on my car model they sold me a cap (made by Beru in Germany) and rotor arm (made by Bosch in Spain). Total cost £80'ish.

On opening the cap's box though discovered that the central electrode is NOT spring loaded but fixed (same as the one currently on the car).

Went back into the shop to discuss and all they could say was that this was the one for my car (the computer said so!), and if my existing one had a fixed length electrode then that proved it....

Next went on to the Merc dealership: There they said they'd actually recommend replacing 3 parts: cap, rotor and bracket (think this is what sits behind the rotor arm) - total cost £190'ish.

They couldn't say though if their cap would come with a spring'y or fixed electrode. Also they'd have to order all the bits in and I'd need to pay in advance...

Dilema
Robert/anyone,
1) Is the spring electrode version of the cap definitely available for my model (if so I presume it'd be what Merc supply for the extra cost), and if so is it that superior and so worth holding back for?
2) Should I be replacing cap, rotor and bracket (as Merc suggest) or is just cap & rotor likely to be sufficient to sort me out (if indeed my problem is the same as Robert's)?