After 2 years of troubleshooting, I finally fixed the hesitation problem in my ’88 260e. The problem only occurred in the winter with the temperature below approximately 40 degrees. The car would start fine but after driving about 5 minutes, would start to idle a little rough and hesitate badly when trying to accelerate. The hesitation continued until the car reached normal operating temperature then it would run fine again. The problem was so bad that I would take alternate routes to avoid the highway until the car warmed up.

The EHA turned out the be the culprit. Before replacing the EHA I had:

Replaced Spark Plugs and Wires
Replaced Distributor Cap and Rotor
Replaced Vacuum Hoses and Checked Meticulously for Vacuum Leaks
Tested the CIS Coolant Temperature Sensor
Replaced OVP Relay
Replaced Airflow Meter Pot
Cleaned and Tested Idle Control Valve
Cleaned Throttle Body
Checked Current at EHA

I suspected that the problem was in a component that controlled the engine during warm-up because the problem was definitely not intermittent and occurred only during the warm-up phase (after going into closed-loop operation). I narrowed components related to warm-up to the coolant temperature sensor, EHA, and ECU (my car doesn’t have an intake air temperature sensor). What made the problem difficult to find is that the current to the EHA tested normal during all phases of engine operation. Now, I realize that this just means that the ECU is supplying the correct current to the EHA. My EHA must have had a mechanical problem with the springs or diaphragm or whatever is inside so it could not adjust the fuel pressure according to what the ECU was telling it.

A word of caution though before shelling out the $ for a new EHA: Be sure to check as many of the other components as you can first. As far as I am aware, there is no way to test an EHA other than to replace it. According to the other posts I have read here, hesitation problems have been solved by replacing everything from spark plugs to fuel injectors.

I’m hoping maybe this will help someone else having this same problem.

John

i've been having the same problem with my 190e2.6 in the past 6 winters, this included. Happens only when starting the engine cold with the temps below zero Centigrade.

i let the car idle for about 5min, then rev it to around 1800rpm, at which point the rough running is encountered. I leave it there for around 10-15seconds, and the engine smoothens out. Seems like it's something mechanical, like you described - could very well be my EHA. But it's something I've learned to live with. I sometimes take the car through a warmup route before merging into busy and fast-moving roads.

on the other hand, there is another type of rough running that could be traced to the O2 sensor going bad, and this is *very* dangerous, as it could leave you without any power on a busy part of the road, where you have no right of way. And this is very intermittent and difficult to predict. You have a few seconds of sensing a drop in power relative to your gas pedal input, then it happens. I really dread this symptom ...