Rough Starting

Question:

I have an 86-300E that requires several seconds of cranking when cold to start.  It dies immediately.  Then it starts the second try and keeps running, but VERY ROUGH.  It gets better the longer it idles.  After about 20 seconds or so it’s better but still rough.  It is fine after completely warming up (i.e., several minutes).  Any suggestions?

Answer:

If you are sure it is not:

• old spark plugs (use only Bosch H9DCO regular set at .8mm - do not use platinum plugs - they foul and cause rough cold idle)
• old spark plug wires (change at 100,000 miles )
• old rotor and distributor cap
• carbon on intake valves (add LM Ventil Sauber or Techron to the fuel tank and take car on sustained high speed trip to clean it out)
• bad fuel

Then:

There is a fix for 300E's that involves a special part costing $70 that will smooth out rough idles (once all other causes are ruled out).  Or you can fix it for 15 cents.  The fix is to snip the wire to the injection system engine temp sensor B11/2 (rear of cylinder head on left side)  - it is a green wire with a red stripe.  Then splice in a 470-ohm resistor in series.  This has the effect of telling the injection computer to enrich the mixture just a bit at idle.