This is my kid's car. Here's the entire serious of checks and the symptoms. I haven't worked on the car at all until recently; I got all my info from Kev, my son.

The first thing I told him to check was the fuel filter. He did, and it was partially clogged. he replaced it.

He then told me no fuel was coming out of the tank. I told him to look at the repair cd's to make sure he was looking at the correct line. He's too smart to do that, so he pulls the tank. It was clean, the screen was clean, he was checking at the fuel filter outlet. The line coming from the tank flows fuel freely.

I am sure this is not a fuel problem.

He had the car at a buddy's shop. They put a real battery load tester on the battery and it failed miserably. Replace battery. Thats seemed to help. He went to pick up the car and a block away from the shop, the car dies again.

They test the alternator. It is marginal. They replace the marginal alternator. Seems fine, until he drives away.

So now I get involved and visit the place he has the car.

here's how it goes:

Cold start, fires right up, idles fine, and at around 50c coolant temperature or so, the idle jumps up to about 2200 rpm or so. It runs like this for anywhere from 15-45 seconds, then abruptly shuts off. It doesn't spool down slowly; it shuts off.

If restarted immediately, it jumps up in rpm almost bright away, and dies after about 15 seconds.

While in this process of dying, I sprayed started fluid into the air valve. The engine did not pick up at all in RPMs and died. So, something electrical.

What could cause the engine to shut down electrically AND cause the rpms to jump up for 15-45 seconds? Does anybody here have the suspect controller we could borrow and try out, and then buy if needed?

If after the first event, the car is allowed to cool down, it takes longer for the process to occur. So, somehow, this is related to coolant temperatureas well.

He had a known good OVP and tried it. Nope.

What do you think the issue could be? I am not around to help him, but I'd like to save him some money. He's going to tow it to a Euro-car repair place. I can just see the $$ racking up and he doesn't save money like kids should.

 

I have posted the entire problem on the S class forum here, and have had some responses. 1987 420SEL. I am thinking the operation of the FPR is the same on this car as other models that vintage. Maybe a non-126 tech could help. I hope.

Problem - car starts fine, idles fine, hits about 50c, seems to go into closed loop, the idle jumps up to the 2200rpm range, and after 20-30 seconds it dies. If restarted immediatly, it will almost immediately jump up into the high idle and die almost instantly. If we wait a while, and restart, the time to race/die is longer.

Jumped FPR. Car runs fine.

Tried a known good FPR in place of existing one, issue is the same.

I have the schematics for this car; FPR is shown on page 106, 108, 108/1, and 110. Many things are feeding or are fed by, or just passin' through the FPR.

Does anybody know, or can send a document, on how to troubleshoot the various items connecting to the FPR socket that could cause the above problem? I am thinking both FPRs are ok, but something connecting to it is somehow disconnecting it.

Or am I completely off and need to buy a new FPR? I still don't understand why, if the FPR is bad, it races like it does when in closed loop.

What if I left the FPR in, let it get to 50c, and when it races, pull the plug on the "thermoswitch" to trick the car into running cold? If it settled down in cold mode, would that indicate anything?

Thanks in advance for your time. This is a tough one for me. Besides, it's my son's car and he's driving the 202 now. Need to get him back into his car.

 

The only thing I can answer at the moment is if u pulled the plug on the thermovalve the 420sel ecu does not recognise an immediate temp change as a fault and does not switch off other units so the car runs on a "drive home mode only", I have a similar problem, mine starts, idles high and dies at any temperature.
If I push the air sensor plate (Round disc in front of fuel distributor) down slightly my car keeps running although somewhat roughly. I think my problem is fuel pressure related, just wondering if your vehicle fault could be pressure related, can u try pushing down on the air sensor plate, just an opening of an eighth inch should do to see what happens, the 420 engine will not flood with gas so dont worry about that, lets see if it keeps running and perhaps get other peoples input in hopefully identifying and solving this problem.

 

The problem was fixed shortly after the above posts.


Multiple issues - bad air distribution hoses, a failing IVC, both causing high idles, and finally, the EZL which solved the dying problem. Engine runs great now.

I was able to find a good used ICV and EZL.