I recently bought a 1989 Mercedes 300E that didn't run. Has a straight-six gas engine. Thanks to the miracle of Seafoam and a lot of luck, I was able to get fuel delivery happening again. I then switched out some incorrectly-gapped plugs and pretty much every other fluid. Fired her up, coaxed her through the first 35 seconds and VRROOOM!

Now I have to get this car emissions-tested. I'm going to burn through the old gas in the tank and refill with premium Chevron (which I hear is among the best). I may try the top-down valve cleaning thing or whatever it's called with Seafoam (hopefully managing not to hydro-lock my engine in the process).

Does anyone have any other emissions-test-passing tricks?

  • 2
    is this car in the US? If it is I thought cars that old didn't have to worry about emssions tests
    – Patrick
    Mar 15, 2011 at 18:46
  • 3
    In the US, it depends on local laws. For example, high population counties in Texas must pass emissions if they are 2-24 years old.
    – S_Niles
    Mar 15, 2011 at 20:52
  • 3
    This is highly-specific to the locality. Regulations vary by state and sometimes even by county / city. Mar 16, 2011 at 1:24
  • Glad I am in Yuma do to farms no emissions compared to Phoenix area which gets the Cal flow through and has states tougher standards. Oct 18, 2016 at 15:35

4 Answers 4


For a pre OBD-II vehicle such as this one, the best thing to do is get it running as smoothly as possible, using normal maintenance and repair procedures. This means no misfiring, smooth idle, smooth acceleration, not overheating, etc. The same stuff you'd expect to work well in every day driving. There should also be no check engine light. This vehicle will have some sort of basic self-checking, it just won't follow OBD-II standards.

If all of this is working well, then there is a very good chance you'll pass emissions. However, there are some conditions that won't show up in daily driving that could still fail you, and those have to be diagnosed on an ad hoc basis, after the test.

Mosts emissions tests will check NOx, CO and HC. A very basic rundown:

  • NOx - Oxides of nitrogen. This means the engine is running too hot (2500F +). This can be caused by poor cooling or fuel mixture is too lean. Modern vehicles also have an EGR system, which exists exclusively to reduce NOx. If the engine is running in the correct tempuratures, and NOx is high and everything else is normal, the EGR system is strongly suspect.
  • CO - Carbon monoxide. CO exists when there is an incomplete combustion of the air/fuel mixture. This can be caused by restricted airflow (dirty air filter), or a sensor that is reporting the incorrect airflow (MAF/MAP, TPS), or a faulty O2 sensor.
  • HC - Hydrocarbons. This is raw fuel, meaning the fuel didn't burn completely during combustion. This can be an ignition problem such as misfiring, or poor timing. Usually this will show up as a driveability problem.

There is a lot more to diagnosting emissions, as there are a lot of complex systems all working together, and all of the readings have to be considered together, not in isolation. For example, if all three readings are high, and the engine checks out and all underhood systems check out, the catalytic converters may be faulty.


In the past, I've owned a lot of cars that were... iffy... when smogged. Here is the list of what I've done in the past. A lot of these steps are only marginally effective, but there are times I've squeaked by with a margin of 1%, so I think every bit counts.

  1. Make sure no error codes were stored in the ECU. In some places, an illuminated check engine light or stored fault codes = instant fail. Your car is pre OBD2 so this is moot.

  2. Part of passing emissions is passing visual inspection. Make sure your vacuum hoses match the vacuum diagram that is on a sticker under the hood. Make sure emissions equipment (EGR, catalytic converter, etc) aren't missing.

  3. Make sure your sparkplugs aren't worn, gapped properly, and that your ignition timing is within manufacturer range.

  4. Have a new tank of premium unleaded before your emissions test. In your case, show up with about 2-3 gallons (more on that in a bit). This will ensure there is less weight over your drive wheels.

  5. remove all excess items from the car, especially the trunk - spare tire, etc. (bear with me here)

  6. inflate the drive tires (the rear in your case), to as high as you can safely drive on them. The goal here is the minimize load on the engine when the car is on the dyno. Less weight over the driving wheels + less rolling resistance = less load on the engine = possibly slightly lower emissions.

  7. Seafoam. See you've got that covered. Be very careful and only let the car take small sips of this (when ingesting through the intake manifold). Watch some youtube videos of how this works. YOUR CAR WILL MAKE A LOT OF SMOKE. When you're done with the bottle, shut the car off and let it sit 20 min. Then start er up and drive it very hard. MORE SMOKE WILL ENSUE. Make sure you swap plugs AFTER seamfoam, as this may foul your plugs. Afterwards, run a bottle of injector cleaner (like gumout regane or chevron techron concentrate) through a tank of gas prior to your emissions test.

  8. After the seafoam, it wouldn't hurt to have an oil change. More lubricity = slightly less load on engine = better for emissions.

  9. MOST IMPORTANTLY. make sure your car is fully warmed up prior to the test! gun it up and down the highway. When you show up at the inspections facility, leave the car running.

... and one more thing. I've never had to try this, but I've read about some shadetree mechanics that have run a small amount of denatured alcohol with their gasoline to reduce hydrocarbons, but YMMV and you will need to serious homework before attempting this.

  • I just recently moved into a city that requires emissions checks and found that not every place that does the checks is as 'detail-oriented' as other places. So one thing to think about is to take the car to someone who won't grill you on the little things.
    – Patrick
    Mar 16, 2011 at 5:01
  • @NoCarrier... brilliant, lol. You even mentioned my fav. I've heard Seafoam referred to as engine crack and think it's fitting. It's what I used to get this engine running in the first place.
    – codemonkey
    Mar 16, 2011 at 19:36
  • I'm not sure what inflating the tyres will do. Are emissions tests not carried out with the car stationary and in neutral? May 27, 2016 at 13:20

There is a chance your 300E has a Bosh Jetronic fuel injection system. These need their own servicing techniques. Key points are:

  • new injectors would usually be a good idea
  • the air path from the filter to the manifold needs checking for leaks as the Jetronic system doesn't like vacuum leaks
  • how old is the catalytic converter? If it's original, perhaps a new one would be a good investment

I can not point out any one specific item that will help your car pass.
But for your greater Phoenix area location I would start, after you run through your tank of fresh gas, by parting with the $17 dollars for the first test.
This will provide a reference of where your at on emissions, you might even pass.
Testing will provide areas that are off and can help focus repairs.

It will also show if your more then twice the allowed emissions which presents it own set of issues for passing and for waivers.

Another reason getting through the first test is that for the Phoenix area after two fails you are allowed a request for waiver. If the car can not be repair at a reasonable cost and everything reasonable has been done to lower emissions then the waiver is allowed if total repairs will exceed $450.
All cost related to repairs for 60 days prior to making the attempt to pass count toward this $450. So document the automotive parts you have spent so far (plugs, additives, parts changed).
Check myAZcar.com for details.

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