W124 - Cold and Warm Engine Start

As I browsed a number of videos and watch how a W124 M104 engine starts, I noticed that mine does not have graceful starts. 

The engine runs after cranking and most of the time in one crank but as the engine starts there were some observations that are not what I saw on the other cars:

  1. On cold starts, the car engine revs to 850 rpm and idles there
  2. On warm starts, the engine revs to about 2100 rpm and sequentially reduces to about 800 rpm in 10 or so seconds
  3. The engine does not stop once it has started

Upon the replacement of the OVP relay, there was a change in behavior where, during the warm, the engine start and revs to 1200 rpm and gradually reduces to 700 or so rpm, instead of the earlier 2100 or so rpm start. This is an improvement but the cold start remains the same with no sign of improvement. After doing some reading online, I understood that one of the components that are involved in a cold start is the air intake temperature (AIT) sensor. 


According to dutchfixmycar.net, the air intake temperature sensor is responsible for detecting the temperature of the air that is coming into the engine so that it can relay the data to the car’s main computer. The computer then controls the fuel-to-air ratio the engine needs for proper combustion and ignition under those specific circumstances (and parameters). For instance, if the sensor detects that the air being sucked into the engine is warmer since the air is less dense the engine will need less fuel for proper combustion. When the IAT sensor begins to fail, one can imagine that it will cause significant changes in performance.


While some of the online forums were pointing my issue to a number of things that need to be checked such as the OVP, OVP fuse, fuel pump, fuel pump relay, throttle body, and even the AIT sensor.


With the exception of the throttle body (electronic throttle actuator) and AIT sensor, I have either checked or replaced all the other items and they seem to be in good working condition.

Today, I decided to check the AIT sensor to see what the issue could be. What I noticed is that firstly, the mount connectors are broken on both ends. After pulling it out, I noticed that the wire to the sensor was coated with a film of dust and oil.


I decided to give it a shot and pulled out the throttle body cleaner spray and sprayed it onto it, all of the dirt came off and the temperature wire became clean. Before I jump to replace the sensor, at least I can see how it works if it is still working. I also cleaned the connector pins. After that is done and the sensor is dry, I refitted the sensor back in the air intake housing followed by plugging in the connection socket back.


That followed by me starting the car. It did some improvement but I am not completely sure as the engine was started a few minutes earlier and would have somewhat warmed the engine. What was noticed was the cold start is now starting at 1200 rpm and reducing to about 700 rpm gracefully.


I have ordered a replacement AIT sensor and the new part has been redesigned. Will have it installed and test along with the replacement throttle body that was sourced. My mechanic did caution me that the current throttle body I have seems to have a bad wire loom, it is damaged and could be the reason why the car is behaving as such.


The fix did show some improvement over the idling and also the smoothness of the engine, but it is not quite fixing the cold and hot start issue that I am trying to resolve.


Update #2:

The throttle body has been fitted into the car and while we were at it, we also replaced the AIT sensor. The sensor had an updated design while the throttle body is from VDO, made in 2013 I believe. Upon installation, much to my dismay, the issue was not resolved.


In the process, we also found that the air cond relay which is seated where the ECU is has been removed leaving a bunch of wires to be fitted bypassing the relay completely. We do not know at the moment what was done or why at moment.


At this juncture, we do not know why the car is behaving like it does, what we do know is

  1. The air-cond compressor should only run 3 seconds after the engine starts
  2. The ECU may need to re-adapt with the new throttle body 
  3. The vacuum that goes to the gearbox has no suction
  4. There is a fuse (fuse 7 - 16a - red) that breaks the following circuit (bolded ones are of interest)
78Automatic Climate Control
16 (1)Automatic transmission electric:
   Kickdown solenoid valve
   Transmission mode valve
   Data link connector, terminal 16
   Throttle valve actual valve potentiometer
Blower motor
Heater/climate control system:
   Electromagnetic clutch
   A/C compressor
Auxiliary fan relay, terminal 86 Auxiliary fan relay (stage 2)
O2S (after TWC) heater relay module
Climate control system
Monovalve, duo valve
Auxiliary coolant pump
Fresh/recirculated air flap switchover valve (switchover valve block)
Rest relay module
Stationary heater control module
Bypass valve
Purge control valve/fuel tank vent valve
Upshift delay solenoid valve
Heated windshield washer nozzle
Heated windshield washer system
Kickdown cut-out relay module/compressor
HFM-SFI control module
EGR switchover valve 2
Resonance intake manifold switchover valve
Air pump switchover valve
Adjustable camshaft timing solenoid
Electromagnetic air pump clutch
Control/function indicator lamp
ASD/ASR/4-MATlC Backup lamp

This is being monitored and once the replacement sensor, as well as the throttle body, is fitted, there is very little left to be checked. I hope there is no wiring issue that I causing all of this.


Update #3:

The issue is not resolved in this episode and the continuation of this issue is documented here. 


Pictures:


The sensor as it was pulled out from the air intake housing

The sensor was dirty and coated with oil and dirt


Part number: 000 542 2818

Cleaned AIT sensor using throttle body cleaner

Another view of the cleaned AIT sensor

New IAT sensor - updated part

Replacement throttle body from VDO

Old throttle body, it's dirty but works fine

Another view of the throttle body

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