87 420SEL, starts and only runs a few seconds

Problem not found - SOS

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; 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, 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.

Looks like all the various checks are ok, until the idle races. It then drops to 45psi.

Elrojo, I sent a PM to you and mclare about the parts.


Here's some pof what I sent:

He has the car at a friend's dad's shop, but they are not familair with Benz. They have been doing a lot of searching here, mbworld, mercedesforum, and peachparts, plus, they ahve aldata.

They have determined the issue to be either the fuel pump relay or the OVP. Elrojo suggested the idle control , the Fuel pump relay, and a control under the pax carpet

I was hoping that one of you had extra working used parts, or, between the two of you, the parts.

I hate throwing parts at it, but looks like the thing to do at this time as I am out of town and can't help. Even though on this one, I am lost.

Thanks everybody for staying with us through this. My son is working on this issue without my being there.

Thank you ElRojo and Mclare for trying to find some parts for us.


Here's what he tells me today. If the problem occurs, and the cars dies, and the engine is restarted, it will race and die again, as stated above,

He swapped the OVP out, the Fuel pressure relay out, and some other controls out, same problem. Then he jumpered the fuel pump relay. No more problem.

So, something is dropping out the run signal to the relay.

Any ideas????

Again, thanks everybody!!!!!!!

He used one from an alledged running car that is being parted out.

Any idea what all controls the FPR and the items in series with the control? If it were a simple relay, it would only need 4 pins - control +, control -, power supply in +, and power supply out +.

I just spoke with my son and they are tracing it out now, but any guidance would be appreciated. They are using aldata schematics, but none are experienced techs.

And the FPR came from a known reputable source. I am guessing it is not the FPR. What is odd is increase in idle for a while before it dies. It runs at a high rpm (2000-2500) for at least 5 seconds before it dies. When I was in town and stopped by a couple of weks ago, it ran at high idle for a good 20 seconds before it quit.

When it started to quit, I sprayed ether into the throttle hole with no increase in idle. Kinda leading me to beileve it is electrical in nature. If it still had spark, but no fuel, the ether should have kept it running. Or not? When jumpered, does the FPR feed current to anything else?

1987 420sel, all stock

My son had the car towed over here.

I took one of the Fuel Pump Relays and one of the OVPs apart. Both clean.

I plugged in both open ones, attempting to watch the contacts vs dying.

The car started right up. It's been in the 20s here the last week or so, and it had not been started for a few days.

Idle is about 1100 cold, charging at 13.97 and dropping over time to about 13.75 and staying fairly steady there.

It is fine through 50c, 60c, but up around 70c, the idle jumps up to about 2200 and stays there for a while. Then, the idle starts to settle down, but the the FPR contacts open and it dies.

I did this a few times, the last couple with some ether. I can keep it running with ether, so not an ignition problem.

Then restarted, and the idle jumps around 1100-1500 all on its own. Never settles below 1100. When it jumps up to about 2200, it will do so for a while, then the FPR kicks out and car dies instantly.

Then I took the fpr out, and jumped it so it would run the pumps without the fpr. It eventually goes to 2200-2500 and does not die, nor does the rpm drop.

So, the car will continue to run with fpr jumpered, but the idle speed is very high, up to 2500 rpm.

When at 2200 rpm, disconnected the black box in the footwell, and the idle jumped up. Reconnected, dropped back down to the 2200.

Tried a different one, same indications.

Any Ideas?


I've searched all sorts of terms and must have spent 15 hours reading old posts. I can't seem to find one relating to this issue.

Help?????