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· Registered
89 190E 2.6 x2, 99 SL500 Sport
2,019 Posts
I failed by 10 points on the HC's earlier this year and passed the second time.

In order to pass I first had my MB mechanic lean out the mixture slightly. He's been working on Mercedes for almost 40 years and said that you have to get it "just right" When you lean the mixture out the NO will increase and HC will decrease. You have plenty of leeway on the NO side to do this.

Second pointer from him was to put 40psi in the rear tires since the engine will have to work slightly less on the rollers.

Also it's important that the engine must be heated up. When I went back to the smog station I had to wait for 10 minutes. Luckily the smog guy I use was nice enough to run a pre-test for me. When he first got it on the machine the HC was around 130 @ 15 mph. After a couple minutes of running and heating up the cat, the HC dropped down to 52 on the 15mph part of the test. He then started running the actual test and the car passed.

· Registered
5 Spd 1987 190e-2.3
29 Posts
My car failed its first smog as well.
I got it to pass the second time by getting the catalytic converter nice and warmed up, drive for a half hour or so. I can second adding some pressure to the tires as well.

Pickup some Techron additive and run that through, either the tank before or during your test. I can't recall if mine had Techron during the test or not, but I remember it was either during or the tank previous. I found this quote online and it's attributed to Chevron
Techron Concentrate does lower emissions but may not lower enough to pass the test. If you use mostly Chevron gasoline you should be getting enough of the Techron in the gasoline such that you don't need to use the Techron Concentrate. If you have used some inferior gasoline and all of a sudden used Techron Concentrate, there might actually be a short period of time when deposits that may have formed in your fuel system are being loosened by the Techron and driveability may suffer a bit until all the deposits are cleaned out of the system.

Using Techron for a time before emissions test may lower hydrocarbon emissions because it cleans valves and combustion chamber deposits. We don't recommend going for smog test while Techron Concentrate is at its highest dosage in your tank. Run a regular tank full of gasoline thru first.

Before you go in for an emissions test, it is best to make sure the emission control system, especially the catalyst, is fully warmed up You should drive a few miles on a freeway or highway at cruising speed to ensure everything is warmed up before you take the car in for testing.
I've heard good things about this
additive, but I'd only use that as a last resort.
Good luck, and remember the most important thing is to warm up your cat converter.
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