Head Gasket Replaced -- MPG drop -- Questions

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Dear Fellow MB nuts,

I posted this as an update to my Head Gasket thread, but since this has now evolved into a seperate question, I'm reposting it here as a new thread.
Regarding my 87 300E with about 160,000 miles (or more) that through November, 2000, used 1 to 1.5 quarts of oil every 1500 to 2000 miles depending:

I did some research and found that the camshaft and stem seals were replaced in 1994, 7 years or approx 100,000 miles ago. Before doing my head I replaced the radiator, expansion tank and overflow tank myself, flushed the cooling system and still after 3 weeks found oil reappearing in the coolant.

So I had the head gasket replaced and the head redone with new Stem Seals, etc. During the job it was found that one pre- cat in the OEM exhaust was completely clogged and fused with oil (the metal mesh surrounding the cat fused to the inside of the pipe) ( I checked this myself and tried after the mechanic did to dislodge this mass with a screw driver -- it would not budge). Apparently, this is where the oil was leaking to. In any case I replaced the header and pre-cat to cat system (its one OEM piece) with a stainless piece manufactured to OEM spec.'s by TimeValve without pre-cats (the new cat I'm told is sufficient to pass inspection -- we will see). This whole job was expensive (I bought my own parts and had the specialists do the work (@$80/hr) coming in for a total cost at a little under $3,600.00. Not bad however for a new header-cat system, remachined head and stem seals, replaced stem guides, timing chain and timing chain tensioner. The head bolts were within spec.

The car performs amazingly well now, is much quicker and smooter, etc. and burns no oil whatsoever at this point. Job was done 1 month ago. But, after averaging 19 mpg before the job for 6 months, the car now is averaging 17 mpg since the job -- the mechanics' claim to be baffled and they readjusted my air fuel mixture (without charge). Readjusting the mixture did not seem to solve this decrease in mpg.

Do you guys have any ideas or suspicions as to why my car dropped 2 to 3 mpg from its previous average when is should be 2 to 3 mpg better -- at 20 to 22 mpg (as per MB rating on this car 20 city 25 highway)? (I do drive the car a little more agressively -- but drive the same roads the same distances and use the same gas and gas stations).

Marc
Rebuit and restored Obsidian Black/Palmino Interior '87300E with Euro Headlights (they are amazing). Wanted OEM 8-Hole 16" Rims with decent tires.

The oil you were burning probably made up for the other 2 mpg.

[Arthur Dalton]

If the pre cat was pretty fouled, you may want to do a voltage/cycle time test on the 02 sensor. That may be as badly
fouled as cat was.
Also, seeing the head was done , ask if any silicon gasket material was used anywhere.

[Kebowers]

What did you do to the head? Face the gasket surface? If so, you removed some metal that is equivalent to more than 100,000 miles of timing chain stretch/wear, and the cam timing is probably several degrees late- unless you carefully checked it and put in the offset camshaft key(s).

A real common oversight with aluminum head overhead cam engines is resurfacing just the head gasket surface without also resurfacing the top side at the same time to ensure the two sides remain perfectly parallel. This required completely stripping the head (removing all the guides, cam towers, etc.


The heads are very flexible, and often seem 'bowed', or even twisted when laying there in the open, but straighten out when torqued down. Unless the head WAS perfectly true and a uniform amount of metal was removed and the top and bottom sides maintained parallel, you now have cam journals that are no longer perfectly aligned. You will have to add shims (see the parts counter) under each bearing tower to get them back in place. A tight cam can consume a surprising amount of power to turn. It should be possible to turn it by hand when the head is torqued down (without the rockers or chain installed of course.)