Mercedes Benz repair: Mercedes 300E fails smog, mercedes 300e, leak down test

Question
While I agree with the principle you have stated, I am wondering if you would still be willing to speculate on the problem.  For reasons that are beyond the scope of this forum, I cannot return the car to this shop to fix.

Thanks
Paul
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The text above is a follow-up to ...

-----Question-----
I have a 1986 300E 220k miles that had been passing smog for years with low HC, CO and creeping up on the NOx but passing.  I recently had some major work on the engine (valve job etc) and now I fail quite badly on HC and CO but NOx is basically nonexistent:

test/Max/Meas
@15 mph
HC/126/268
CO/0.79/7.63
NO/1097/87
@25 MPH
HC/101/278
CO/0.59/7.68
NO/927/91

Do you have any idea what could cause such a dramatic change in performance on this car?

Thanks
Paul
-----Answer-----
there could be quite a few causes for this, but....if you had engine work done recently and now it fails emmissions, then I would return the vehicle to the shop.

If your vehicle passed prior to the repair, then it is the responsibility of the shop that repaired it to make sure the vehicle is returned to you in the same or better condition. THEY SHOULD NOT CHARGE YOU FOR THE REPAIR OF THE EMISSIONS as it was ok BEFORE you had it repaired. You must impress upon them that they are required to repair a vehicle and correct any problems induced by the repair at no additional charge.

Answer
no problem....total speculation though....based on the numbers you provided and the repair work that was performed, I would be very suspicious of the exhaust valve (one or more) not seating properly. If an exhaust valve was not seating properly, it would allow for excess unburnt fuel and air to pass through to the exhaust system prior to combustion causing the emissions numbers you are getting.

Other things could be suspect as well, but based on the complete decrease of NOx and the extremely high HC and CO, I would most certainly have a cylinder leak down test performed on each cylinder first. Usually, any repair that negatively affects the emissions system is the issue. Any engine repair should have either a positive effect or no effect at all on emissions.

Of course I am just giving you an opinion as to the problem. It is very hard to diagnose a drivability concern such as this via the internet. Hopefully this will help and let me know what you find out or if you have any other questions.