Problems during warm up cycle

My 420 SEC has some problems during the warm up cycle as follows:
The car nearly allways starts first time from cold, whatever the weather or temperature.
Starts idling at about 1500 RPM, then quickly settles down to about 800 RPM. When fully warmed up, it idles at about 600 RPM in drive, with the footbrake applied.
Towards the end of the warm up cycle, there are instances of hesitation/loss of power to varying degrees. Occasionally it has stalled. After stalling, it is very difficult to restart. Once fully warmed up, the engine runs fine apart from occasionally the idle will increase to 1500 RPM, then drop to 600RPM.
After the car has warmed up, and then parking and switching the engine off, and then coming back to the car after between 15 and 30 minutes it has been very hard to start again.
The car has new plugs fitted, and the leads, distributor cap, and rotor arm have been checked and are all ok. The fuel tank has had two cans of injector cleaner added to it as well. Also the fuel filter has been replaced. I have cleaned the idle control valve although it did not look dirty.
Any suggestions regarding which part(s) need checking/replacing?
Thanks in advance,
John Hughes

Forgot to mention - Car does not have a catalytic convertor, and often goes into "limp home mode" - any suggestions?

I have now replaced the distributor cap and rotor arm, and the coolant temperature sensor. The warm up cycle is much improved, and the car generally runs much better, apart from what I now beleive to be one intermittent issue, which is that the fuel mixture is being intermittently enriched. Sometimes at standstill, the car idles beautifully, and then the RPM increases to 1500, accompanied by black smoke coming out of the exhaust. This intermittent enrichment also happens when the car is moving, indicated by hesitation and the occasional stall, and again black smoke coming from the exhaust. I have checked for vacumn leaks and could not find any. I am going to have the fuel pressure checked, and also the air mass potentiometer, by a technician. Do you think I am thinking on the right lines to resolve this issue, and does anyone have any suggestions for any other items to check. Thanks in advance for any guidance.

The Car had the air mass potentiometer disconnected by the technician, and I was asked to drive the car to see if the fault reappeared. The fault was much less evident on a run, however during a period of idling at standstill, the idle started to rise, with black smoke coming out of the exhaust. I then reconnected the air mass potentiometer, and disconnected the Electro-hydraulic actuator. The fault manifested itself this time by an intermittent high idle. Difficult hot starts remain with either of these devices disconnected. It is as though the ECU is getting an intermittent signal to go into cold start enrichment, or possibly full open throttle. I am wondering if the throttle switch could be at fault. I am going to clean up the connections on all of the electrical components of the fuel system, to make sure all the contacts are good and free from corrosion etc. The car has a new coolant temperature sensor. Any further suggestions regarding items to check would be most welcome. There is one sensor that I am unable to identify, which is situated in the air intake scoop, at the front of the car. Presumably this is an air temp sensor, and does it send a signal to the ECU? (the car does not have the external temperature gauge in the dashboard fitted.)

[papadude21]

Hi
I just had very similar problem with my 87 300E. If you haven't checked the O2 sensor you might want to check it. I replaced mine and problems went away.
Good Luck Vinnie

The Car is a European 420 SEC and is not fitted with a catalytic convertor, and therefore it does not have an Oxygen sensor. It is fitted with what appears to be an air temperature sensor at the very front of the air filter scoop. I will be investigating this shortly, although I do not think the ECU bases much of it's output on this sensor's input. Further suggestions would be most welcome.

[wmercer]

Have you considered replacing the spark plugs, not too expensive of a preventative maintenance and checking the spark plugs wires?

The spark plugs are new, and the leads have been checked and appear to be ok. They look like they are the original leads, and so I may well replace them.

Have you tried adjusting the fuel mixture. This made a world of difference on my hot starts. I turned it counterclockwise to lean it out.

The technician has set the mixture a little rich to "help the car along a bit". Once I have resolved the intermittent fault, I will ask him to reset the mixture to it's correct proportion. I removed the air temperature sensor last night and it looks like it's damaged. It is shaped like a felt tip pen nib and covered in black plastic film. This has eroded in part to reveal the metal beneath. I have ordered this part today (£13.00), and it should be available for collection tomorrow. I understand that if this sensor is faulty or has become disconnected, then the duty cycle will run at around 80% and fixed, which will give very poor mpg. I have not checked my mpg carefully, but the car certainly gets through some petrol.

Replacing the air temperature sensor has made no difference. The new part looked the same as the old part, with a small part of the metal exposed, so it must be designed this way.
Three other jobs that I will do before I give up and take the car to a dealership for a proper diagnosis are as follows;
1. The throttle linkage is filthy, and needs cleaning. Also there appears to be some play (is play normal?), and I will check the operation of all the joints, and how the linkage interracts with the throttle switch, and what effect this has on the idle. Perhaps the throttle linkage sticks, therefore the switch sends a signal to the ECU saying the engine is not at idle.
2. Check movement and position of the airflow sensor plate. Clean airflow sensor plate and inside of housing as this is dirty with some filmy type discolouration.
3. Replace air filter, which is dirty, and does not appear to have been replaced for some time. (Will hinder hot starts)
Any further suggestions welcome.

I carried out all of the above jobs, and there was a slight improvement in the overall running of the car. However, the intermittent idle problem remains, as well as the occasional enrichment. I re-cleaned the idle control valve, spending an hour running an entire can of carb cleaner through it until the fluid ran clean. The result of this was to give the car a more stable idle, but at 1500 RPM. (Slightly higher than before.). I then took the car to a technician, and asked for the throttle switch to be checked, and a full search for air leaks to be carried out. The idle valve was cleaned for a third time, this time under pressure. No air leaks were found, and all electrics were working as designed. The tech found the plunger to be sticking in the fuel distributor (removed in the search for air leaks), so this was cleaned. The net result of all this was a slightly variable idle of about 800 RPM in drive, and very occasional bouts of enrichment, only under full throttle. The technician told me that he believed that the solenoid windings in the idle control valve were on the way out, but he suggested that this could be lived with for the time being. The car runs better overall. However, a new fault arose, or maybe one that was being “masked” by other faults which have now been resolved. During deceleration, the engine would “kangaroo” between 1000 and 1500 RPM. My car is a European model, so I thought it might be a faulty decel dump valve that Steve Brotherton has referred to in other posts. This part is only GBP 10.00 approximately, so I thought I might as well order it. I decided to bite the bullet and order a new idle control valve at the same time. The decel dump valve is on back order, and will arrive next week. I fitted the new idle control valve last night, and now have a perfect idle of approximately 500 RPM in drive. Not only that, there was no “kangarooing” during deceleration, in fact no hesitation at all. I took the car out for a long and extremely enjoyable drive last night, on motorways and country roads, and she ran superb. There was just one instance of enrichment, but this is a separate issue that I will be addressing shortly.

I am fairly certain that the occasional enrichment is due to the plunger "sticking" in the fuel distributor, despite it being cleaned by the technician. The car is still very hard to start when hot, so I am going to order a new fuel pressure regulator tomorrow. As fuel pressure applies a counterforce to the plunger against the air flow force on the sensor plate, I am hoping that this new part will also resolve the plunger "sticking " issue. The fuel pressure regulator on the car at present appears to be original. If the new part does not resolve the sticking fuel plunger than I will buy a second hand fuel distributor. Does anyone know if the fuel distributor from a 380, 500, or 560 will fit my 420, or if the plunger can be somehow machined to stop it from sticking?