I purchased a 1990 300SL a short time ago and have been working through all sorts of deferred maintenance items. I have most everything working, including the top. The only thing that doesn't work is the pneumatic central locking system, but thats not what I am writing about.

I am in the Los Angeles area and have to go through smog. My first try failed with an HC measurement of 141 with a max of 102 allowed.


I replaced the plugs, plug wires, cap and rotor. I gave it another go and brought HC down to 117



After that I firgured I should probably replace the O2 sensor. I will also add that the cats are only 2 years old. I havent gone back to have it checked again because of the whole Covid lock down.

I replaced the OVP and that solved several issues and improved the overall performance of the car. I also wanted to check the Lamba settings and using a volt meter saw values from 4.2 to 6.5 via the x11 connector(after the oz sensor change) I checked all the DTC codes and had none for CIS and 2 for the MAS, and got 2 and 3, fuel pump circuit and the TN interrupted (not exact verbage) The fuel pump code i find odd because the car always starts and I can hear the pumps running when I turn the key.

I took the MAS apart because I am not so bright. I didn't see anything obvious and it actually starts better with the cover off the MAS.
I also got a meter with the duty cycle mode. I just checked and with the key on I get a brief 70%(good for CA regs) and then it goes to zero. When warm and running it stays at 100%.

What should I be looking at next? I apologize for the wall of text. The car has 80k on it, but it has been a rough 80K

 

So, at X11 you measure 4-6 volts with the engine running with one instrument and a duty cycle of 100% with another? One measurement is wrong.

 

I am optimistically leaning that way. I will take some more readings when I get home today.

Semi related question. I always get code 2 (Fuel pump relay (circuit 87) not functioning) on the MAS relay. After I clear it it always comes back.
Despite this code, I always hear the fuel pumps and it starts and runs pretty well. How is that possible?

 

The MAS controller probably has a simple circuit to measure the current flowing through the relay contact, and somehow it is not working.

 

I finally had the opportunity to check voltages on the x11 port with both meters. I will be the first to admit that electricity is not my friend. In any event, if I use my original meter on the 12 volt range I get voltage fluctuations ranging from 3.8 to 4.5. If I use the 20 volt range it goes from 6.4-7.1. The meter with the duty cycle capability shows voltage at 6.7 with very little fluctuation. In Hz mode it cycles from 11.6 to 21.44. When I switch it to duty cycle it shows 100%. For a split second, before going to 100% it shows 48.7. The car runs well. What next? New meter? Thanks

 

IMHO, the 'test' results indicate the engine is running slightly RICH at low speeds, and just within upper limits at slightly higher engine speeds. The cause can be several things but: a) dirty air filter, b) dirty engine oil, c) clogged PCV system, d) catalyst, engine and oil not HOT for test, e) faulty oxidation catalyst f) faulty O2 sensors., g) faulty cooling thermostat or temp sensor--engine running too cool.

I suggest changing the oil, adding a can of TECHRON to a tank of Chevron high octane gas, checking the PCV system, cleaning the MAF and throttle butterfly, running the car 20 or so miles at high loads and speeds. 'Burning out' accumulated deposits and running the emissions test with everything HOT helps pass the emission test A slightly dirty throttle or MAF filament will cause a slightly RICH mixture and failing hydrocarbon emission test. A poor injector spray pattern in even a single cylinder will also cause high hydrocarbon, CLEANING EVERYTHING in the fuel and air metering/management systems is absolutely essential. Even deposits on the intake valve 'tulips' will cause poor combustion and high hydrocarbon emissions.