Acceleration Hesitation
 

Hello,

I have a 1987 300E with about 195K miles. Purchased used last summer with no records. I have a problem with a hesitation, sometimes a small backfire in the engine, only when accelerating from a stop. If I accelerate when the car has any momentum, there's no problem. It's not terrible, but it bugs me as I believe there should be no hesitation on a good running engine. I've replaced: Plugs, Wires, Cap, Rotor, Injectors, OVP. Each part replaced seems to have improved it a little, but it's still there, and I'm getting frusterated. The hesitation doesn't seem to happen when the engine is cold.

Also, the SRS light has started to illuminate...it started when I was moving from FL to NC during the drive it was intermittent. Now it stays on for about 10-15 seconds after engine start.

My questions are: What causes the SRS light to illuminate?
What should be my next move in trying to diagnose the cause of the hesitation?
Could the two be related in any way? (Someone suggested that a bad OVP relay could cause both to occur.)

Thanks for any answers and/or advice!!!
TC

Anyone?

Well, anything that would cause the self-diagnostics to conclude that the SRS will not activate as it should. Could be anything from an accumulator that won't hold charge to a firing squib with incorrect resistance. Airbags have a stated 15 year life, after that you're supposed to have an authorized repair facility evaluate them, and replace parts as needed.

Most common cause of SRS light is worn brushes in steering column.

As regards acceleration hesitation, be sure all vacuum connections are good, particularly hoses to valve cover. The rear one can get hard and brittle.

Do a search for fuel mixture adjustment. You might have to check and reset your fuel mixture.

You may want to install a new oxygen sensor then follow the previous suggestion. A new O2 sensor on a KA (CIS with frequency valve) or a KE (CIS-E with electrohydraulic actuator) fuel injection system does wonders. IMO-at 60K the sensors frequency has degraded, at 90K the switching time is too slow and it is time to install a new one. I have had lots of people argue about replacing the sensor however they sing a different tune when they experience decent engine performance with good fuel mileage. :D

You may have another issue but an O2 sensor at this point may be a good investment.

For your hesitation and backfire, look for bad hoses, particularly idle air supply. These get hard and brittle with age, developing cracks and sealing poorly at fittings. Replace them all if in doubt. Note that any air leaks here will allow unfiltered air to bypass the airflow meter, causing a lean mixture. This will be only noticeable at idle and off-idle acceleration.

If your hoses are relatively new, then the airflow potentiometer regulates acceleration enrichment and typically develops a wear-thru spot at idle, with age. This usually also causes erratic idle.

Steve