QuestionDear Ron, I'd appreciate it if you could help me with a 1978 mbz 450 slc I
just bought. It has only 50k miles and the smog results were really clean.
Hjowever, it takes about 3 seconds to start but it's fuel injection. It also has
slight rough idle. Lastly the exhaust is not really smooth, making a puffing
sound.
Being a mechanic muself, mostly Japanese and American models. I did a
compressiion test, vacuum test and checked ignition timing, the compressiion
was virtually all even at 90 to 160 max. The vacuum was steady but low at
12 in. [is that normal for these engines?], As for the timing, it's on the mark
if I follow underhood label with vacuum connected at neutral. But if I follow
Chilton method it's almost a degree off with vacuum hoses disconnected and
plugged in drive. I also checked for vacuum leaks with propane but found
none. Could the timing be the problem of all these symptons? I'd be very
grateful you could share your wisdom on this. EGR also works fine. Thanks
Sincerely. Anthony
AnswerHello Anthony, sounds like you found a nice SLC.Your findings all sound pretty good.
Fuel line pressure probably should be checked - 29 -30 psi while running
At 50K and a 78 I would spend the money to get the injectors pop tested and refurbed, I suspect that is your issue. they can crust up and the seals can look good but suck air. Any rubber must be looked at as a suspect for sucking in air. You may also have a faulty cold start sensor(s) ( there are several) I would disconnect the one on the air cleaner ( if yours has it) and see what happens.
Remember also that ANY vacuum leaks affect these ol gals. I found on mine ( its a D jet as compared to your K jet) I run a bit advanced timing, runs better, you may try that. I just set my timing by ear :).
I am also assuming you have changed fuel filters and the CAT's look good ( not collapsed or rusted out)
Let me know what else I can help you with, I have some specidfic K jet information if you need it.
