1996 S420 Hesitation problem
 

I have a great 1996 S420 with 174000 miles of nearly perfect performance. Yesterday the car started a pretty strong hesitation but is erratic. The car will run perfect then suddenly start hesitating and almost stalling. Speed seems to have no effect i.e. will do it at idle or running down the road at a nice clip. Took it to my mechanic today and says there are NO CODES showing a problem and he is as confused as I am. They are going to replace the fuel filter in the morning only at my insistence....any thoughts guys? how does the car do this and not throw a code?
thanks

no answers guy? had 23 folks read the thread bur no help. PLEASE GIVE ME SOME IDEAS

MAF
 

I would start at cleaning the MAF with electrical contact cleaner. If the problem persists after this, remove the electrical conection to the MAF if the car runs worse then it is not the MAF.

I had a bit of eratic idlke on my S500 and cleaning the maf sorted that straight out.

Check ignition coils and coil boots. They are usually the issue!!

If it's not the coils or boots, and the motor is running fine up to that point but suddenly starts to run rough, could it be that there is some emission control device that's introducing exhaust gases into the intake at a time that it shouldn't--like a faulty EGR system (valve or switching circuitry)?

I replaced the fuel pressure regulator on my S320.

Jack

Mine ended out being the fuel pumps with the erratic behavior. I have to take it in three times over the course of 4 months as it would never reproduce the issue while at the shop. I finally left my car at the shop for a week and a half until the mechanic could reproduce the issue on the spot and look for the trouble code. The wiring in the fuel pump only made it work off and on. And over the 4 months it got worse and worse.

S420 HESITATION UPDATE....STILL GOT THE PROBLEM
 

We tried cleaning the MAF sensor with the technician angry at me for suggesting and telling me I should take the care and work on it myself. Needless to say it didnt work, after further diagnosis he determined that it was a faulty MAF. WE replaced it and seemed to work but a day later the problem was back.
In essence the car will always replicate the problem when stone cold. Severe hesitation and attempting to stall but never does, Drive it a few blocks turn the engine off and starts right up and hesitation is gone. No problem starting even when cold. The car is showing NO CODES. Im am not mechanically inclined and live in a small town where we are pretty much dependent on this one tech who has always been fair and correct but now he is stymied also. ANY FURTHER IDEAS GUYS?

Have you tried cleaning the Intake Air Temperature sensor? On the E420, it's in the plastic housing right behind the left headlight in the engine compartment.

Any idea where it is on a S420? I want to add that the car runs perfectly when the problem goes away. Never a problem starting.

Well we cleaned the IAT with no difference, also added some octane booster but that didnt help either. Ive read up on the O2 sensor and the EGR and neither seem to fit the symptoms. The confusing thing is the fact that the hesitation comes and goes, seemingly only when the car is stone cold but even to get it to stop when it warms up you must turn the car off and restart it. ANY MORE IDEAS GUYS?

If the plug boots are old, you might want to consider changing these out. The plug wires are different from American cars because they are solid core and seem to outlast the life of the car, so they don't have to be replaced on a regular basis. The boots do need replacing.

I know it's like shotgunning parts at the car, but you have to decide whether you want to let the shop take your money for the labor of troubleshooting (and maybe get it right), or replace parts yourself that will probably eventually need replacing anyway. It is a balancing act most of the time, but I lean in favor of taking an educated guess of what part is bad, usually with the help of the more experienced members on this forum (and I'm certainly not one of them).

Do you think it could be a coil? I haven't yet experienced a bad one on my 119's, but I do hear they are prone to fail. I'm not sure how the heat variable would affect the output of the coil though.

does it have a cold start valve like older models.

If there are no codes. Write down all the parts to the fuel injection system.

Make an educated guess at what part or parts could be causing the problem.

Start replacing cheapest parts first.