After losing sleep over whether my upper timing chain guides were slowly being eaten by my timing chain, I decided to change them out. To my surprise, there wasn't a scratch on them (picture 1), but they were very brittle and I had to break the little teeth on the guides in order to get them off (I was told to expect this, so I shoved rags under the chain to prevent the pieces from falling into the bottom of the engine).

Anyone considering doing this job needs to know that there is a rubber seal on each of the guides that either needs to be transferred to the new guides or replaced (picture 2). The seal did not show up when the dealer pulled the guides up on their computer. It didn't show up on the valve cover gasket kit either. We finally found the part on the head gasket kit. I had to special order the seal for each guide (they are different).

The tech at the dealer told me that he has pulled valve covers off to find that someone had changed the guides but not transferred the rubber seals (he said the seals are for blow-by to pass through the valve covers.

It took me three hours to do both guides and I even took my time. The hardest part was removing the 5 mm Allen bolt on the aft right side of the valve cover next to the fire wall. It is really tight. If you haven't already replaced them, you will almost certainly have to replace the hose coming off the right valve cover back there (p/n 119 094 71 82) and possibly the one that connects to it (p/n 119 094 55 82). There is also a white nylon coupling to fit the two hoses.

The manual said to torque the two Allen bolts on the guides and the valve cover bolts to 9 NM.

On the driver's side, I had to separate the fuel line going to the fuel rail in order to get the left valve cover off. I thought that fuel was really going to spray out, but only a few drops. No big deal.

My question to the forum is related to the slack in my timing chain. I have attached pictures of the right (pictures 3 and 4) and left (picture 5) between the cam sprockets. On the right side, I could pull up on the chain with about 5-10 pounds of force and get it to move about an inch. When I released it on the right side, I could feel the tensioner extend to its full travel and there was still a little slack in the chain between the sprockets.

I'm trying to figure out if a little slack is normal, or should I be looking at changing out the tensioner and timing chain. I suspect that if the chain was really lose, it would have hit the inside of the chain guides, but I didn't see any evidence of that on either side. I have read that these chains last 350,000 miles if you take care of the engine, but I know others have disgreed. I was thinking that my guides would have been shot after 242,000 miles, but they seemed fine. Now I'm wondering if I really need to do the chain or let it go. There is no racketing noise coming from the engine yet.

Attached Thumbnails

         

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1993 400E, 256,000 miles (totaled)
1994 E420, 200,000+ miles
1995 E420, 201,000 miles

Not sure but that doens't look like a lot of slack to me. Thanks for the write-up - very good.

I have no other experience with overhead cam engines, so I'm not sure how much slack is normal. I hope you're right.

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1993 400E, 256,000 miles (totaled)
1994 E420, 200,000+ miles
1995 E420, 201,000 miles

I'm doing this job right now and found two ways to help get that last bolt out. First is to pry the engine over to the left to give more room from the side firewall. I used a hydraulic porta-ram with the wedge attachment and placed it between the exhaust manifold and frame rail. I was able to get almost an additonal inch of clearance. Second is to cut down a 5mm hex bit so just a 1/4 inch protrudes from the bit holder and use a 1/4 inch u-joint and extension with a socket wrench. I'll post pictures if anyone wants it.

That's exactly what I ended up doing. Although, the job made me realize how useful a set of 1/4" drive Allen wrenches would be in my toolbox.

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1993 400E, 256,000 miles (totaled)
1994 E420, 200,000+ miles
1995 E420, 201,000 miles

I believe you can check chain stretch by rotating the engine to 45° BTDC on the #1 cylinder compression stroke. In this position, you should be able to insert the 6mm (?) lock pins into camshaft flanges. If not, turn the engine very slowly until the pins can be inserted, and read how far off the timing indicator is (for example, if the pins will insert at 42° BTDC, the chain may be stretched 3 degrees). The problem is, I believe that worn or damaged chain rails can skew this reading... the chain could be fine, new rails may pull the timing back into spec.

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