280se 4.5 runs rich , idles poorly, accelerates poorly.

280se 4.5 runs rich and poor idle, partial load. 

Attempt 1. Car came in, ran poorly and stalled, hard to start. Changed fuel pressure regulator (original measured 4.5 bar), ran better but ran rich…not a smooth idle. Idle hunts when warm. As a sporadic cutout. Just basic checks were done. We knew we had vacuum leaks.  13 hg or .44 bar. All the rubber is deteriorated in the engine bay and well as the wiring harness. Compression was good.

Attempt 2 .Customer brought the car back and said it ran poorly at idle and a stop under load. The car runs and drives poorly under partial load, idle , full throttle is ok. We have not installed our portable CO meter on a road test but we can do a plug read and it is running rich under partial load and idle with multiple plug reads.

History: The car is a high mileage, high heat car. All of the wiring is brittle. All rubber hoses are brittle. They shatter in your hands so work has proceeded cautiously. We tested, CO is at 8 percent and smells of gas, vacuum leaks at injector and intake. Vacuum measured 13 inch HG (.44 bar) this is too low. Idle speed screw when fully screwed in is at 1000 rpm. Can not adjust idle any lower. please note we have 4 280sel 4.5’s in inventory, we pulled one out to use as a reference with 35k miles that runs perfectly, we are using this as a reference for compression, vacuum warm up etc. the car is 100 percent unaltered original.

We used Dr. D jet checklist as best a we could. all sensors ohm'd correctly. MAP sensor ohm good but it has been adjusted. throttle potion switch was tested by the idle speed screw on the ECU worked fine at idle.. Driveability on the car is poor under load.  Car never stalls out.  Occasionally we get a complete cut out of the system and then it comes back, happens rarely but is annoying. We call this an electrical hiccup indicating a ground or power drop out probably related to wiring?

Compression tested the car compression is good  as follows, engine at 170f : 1=130psi, 2=143, 3=143, 4=124 (checked 4 times and is correct) 5=150 psi,6=140, 7=140 psi, 8=142 psi.  Adjusted valves are all good, valve train is good.

Changed points to .3mm, checked dwell to 28 degrees at idle ok. Dwell is steady.

Changed plugs to WR9DC Bosch gapped to spec.


Cracked electrical connection on 4 injectors, old injectors leak externally.  Ordered new Chinese blue injectors (part number 0280150036) from Ebay.  Installed them. new seals. Rewired injectors. started car. the injector seals increased vacuum to 14 hg (.47 Bar).


Changed distributor cap Changed wires to correct Beru. Timing was set to 5 degrees AFTDC with vacuum attached. No change.  

Car burns oil at idle (blue smoke). Valve guide seals?

Checked the cold start system, slightly wet at seat, changed, works fine  Idle air solenoid is stuck. Idle goes up and down. Replaced with  good. 

Manifold air pressure OHM’d out good.  The seal is broken.  Must replace this? 

Changed ECU with correct part number from a 4.5.  No change. 

Checked fuel pressure running 34 psi. A rest 32 psi.

Ok Infrared checked exhaust manifold temperature all are at 420 degrees except number one cylinder at 200 degrees.

Tested injectors while on fuel rail. All spray nicely while cranking. 

Repaired main engine wiring harness under battery completely damaged. No change  

We can adjust the idle from the idle screw on the ECU. changes idle correctly but runs rough still  no change.

Remove distributor rotor and install the correct rotor. The 3.5 rotor is not correct. No change

Check vacuum at idle again and record pressure 14 hg of mercury (.44 bar)  too low.  

Check and clean grounds, we do not know where all the ground points are but will go underneath the battery tray. All ok 

Check throttle position switch at 12 and 17 pins. The connector will break and it is brittle. not tested we will use a better probe.

Exhaust temperature measured by an Infrared gun. All the temps measure 440 degrees except cylinder 1 reads 200.

We are going to remove intake and replace the 8 rubber gaskets so the intake leak is fixed. Does anyone have any recommendations?.
 

Hi Unknown,

I would like to greet you with your first name but then you must leave it in signature.

A lot of issues that you mention and I will pick out only 3 to start with:

1. Can you measure CO and HC per cylinder on engine running in idle and partial load?
2. Can you swap MAP sensor 0 280 100 100 from a well running engine? Ohm is just not enough and A/F mixture is heavily determined by MAP sensor
3. Is engine now vacuum tight? An EVAP tester can show that.

Hi Dr- D jet Volker This is Neil From Star Motors, We will change the intake manifold gaskets (8x seals) today, the vacuum at idle is 13hg or .433 bar. This is to low. We will use the correct map sensor from a running correct car. We will test the CO and HC under partial load and idle. Thank you for the recommendation will post the results. Neil

Hi, I guess this one wants to fix things around first. Burned intake valves disrupt the work of the MAP sensor, the engine will never run well. Pulsation in the engine intake due to uneven cylinder pressures is also a nuisance. With K/KE Jetronic this is also a problem, even more so. Take the engine out and fix everything. My guess is that any other work without repairing the engine will not make it run well. Findings from practice - the engines that went worse got significantly worse after adjusting the valves in case they were set on fire. In 15 years of repairing Mercedes with K/KE systems as well as pre-flood purely mechanical systems and Zenith 35/40 INAT or 175 CDT, there has never been a valve fire. 90% of engine running problems that no one has tuned before have been fixed by fixing the engine, not the fuel system. After assembly, the engines ran like Swiss watches again. Of course, I changed all the rubber parts of the intake, ignition cables, spark plugs, etc. But they are not the problem. What is the problem when someone instead of diagnostics immediately takes a 3 mm hexagon or a screwdriver and turns, adjusts. Prohibition of selling tools to these people.

Hi, I agree 3 an 4mm hex wrenches should be banned.

Thank you Dr. Djet.

This car is fixed.. we changed the intake manifold gasket and all hoses, every one was brittle and worn out. Started car up decent vacuum still missed and high CO.

We took a MAP sensor from a low mileage unaltered 280se 4.5. installed it and it came to life. Changed plugs drove the car to et rid of unburned HC. set idle correctly and it run like a watch. perfect idle smooth acceleration.


Set CO 3.89 percent, read the plug and the are no longer black, but tan. The moral of the story is change all the old rubber if you can. No vacuum leaks.

The MAP SENSOR CAN NOT BE TOUCHED. If it is you are in no mans land. The old MAP sensor ohm'd correctly and vacuum tested correctly but it had an altered cap.

Hi Neil,

to be honest with you I have seen only 1 MAP sensor on US cars in past years in my workshops that was still untouched and correct. All others were detuned or even broken. Sometimes Type 3 MAPs also break of failing aneroid cells. But mostly it happens as your citizens dream of making it it a EURO engine when they detune MAP sensor. But it will never work, only ruin engine. EURO engine - like my 450 SL - have higher compression and other camshafts. That is how they come to 225 hp.

If I may wish something for Xmas: Please my American friends, leave MAP sensors and ignition distributors untouched !

Neil, if you want I can buy the broken MAP sensor from you. Don't throw it away.