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1980 450SL named Freya. 202,000mi
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15 MPG and Sea Foam

Subject vehicle is Freya, my 1980 450 SL. I've owned this car for 15 years. In that time, I've never had better gas mileage than 14 MPG Freeway. Ever.

On my last two fill ups I am getting 15 mpg, in mixed but mostly freeway driving. I've never had gas mileage this good, and I'm pretty happy that I finally hit the 15 MPG mark.

I attribute this improvement in gas mileage to all of the work that I've done recently, mostly discussed in my thread on engine and smog diagnosis. In recent years,mileage had deteriorated to about 13, and this I attribute mostly to the bad fuel distributor which was causing cylinder one to run rich to the point of missing at idle.

However, the fuel distributor problem was somewhat recent, and not present years ago, when I was still only getting 14 mpg max. Some other likely performance related replacements that I think helped most were:

  • New oxygen sensor STRAPPED to chassis.
  • New injectors.
  • NGK Yttrium plugs

While the old injectors weren't that bad, I didn't pressure test them, and I suspect some were leaking down and despite using copious fuel cleaners in the tank, spray patterns were probably sub-optimal. But consider that 80,000 miles ago I was also only getting 14 mpg max.

I don't see the plugs really improving mileage that much, but there are some claims that support that for yittrium plugs.

I thus have reason to believe that the single biggest change that has resulted in the better MPG was the oxygen sensor and the strapping modification.

I had the O2 sensor replaced by an Indy 10 years ago, and that didn't affect gas mileage. Of course they didn't strap it to the chassis. In my most recent replacement, I found the exhaust system electrical resistance to be over 1100 ohms. Such a high resistance would lower the output voltage of the O2 sensor and with the old, analog ECU, tend to cause the engine to run a bit rich. Strapping it to chassis with a grounding strap and I find I am running leaner. No more "fuel" rich smell, and gas mileage now up to 15.

As such, I think it's useful for any one or three wire sensor to be grounded to chassis. Four wire sensors do NOT need this mod.


Yes, it is fun to pour Sea Foam down the throttle and see the big plumes of smoke. Nevertheless I doubt I will use it much in the future.

COST: while a can can be had on sale or online for $8, that's still a lot for essentially naptha alcohol and water. You can get a six pack of Techron Fuel Injector Cleaner at Costco on the frequent sales for under $14.

PERFORMANCE: I don't think Sea Foam down the throat does much. I have never seen it make a substantial difference in terms of carbon on plugs, nor in the catalytic converter nor other exhaust components.

Moreover, it is not substitute for a properly tuned engine. A properly tuned engine, driven at freeway speeds, should clear itself of carbon - more so than than dumping some "mechanic in a can" down the throat.

CONCERN: on two occasions, with the tank near empty, I added Sea Foam and a few gallons of gas. On both occasions, after driving 10 miles my fuel pump started buzzing very loudly. Annoyingly loud. The buzzing continued, but stopped after I had filled the tank complexly and driven through most of the untreated gas.

The pump no longer buzzes. And I will not put Sea Foam in my tank *ever again*.

PLACEBO EFFECT: in the same way "washing your car makes it run better", I think Sea Foam, especially the "down the throat" treatment has a strong placebo effect that cannot be discounted.

In short, Techron fuel injector cleaner at Costco is a better value. I don't plan on using sea foam again.

MORE IMPORTANTLY: I consider Techron (and to an extent Sea Foam) to be part of "preventive maintenance", and never ever for "fixing a problem". A can of cleaner will never replace the proper diagnosing and repairing of a specific problem. Fuel itself is a pretty good solvent, and most brand name fuels already have cleaners in them by federal law, to keep injectors clean.

I will tell you that after I REPAIRED the actual problems plaguing my car, it runs so very well, so smooth, so peppy, that I realize that nothing beats ACTUAL MAINTENANCE.
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