Is is a WUR problem (M110/280SL ´81)? ...going mad

[MikeDK]

Hi, hope to get som advice. I am going slightly mad - and poor...


My issue: My 280 starts prompt when Cold. When warm it starts up with great difficulty - after turning starter 30 -45 secs... After 5 sec. it runs perfect and smooth - stabil rev. and no issues at all.


I have tried: All new sparks and wires. New Injectors. Tried starting warm with pedal to the floor - and no pedal - no difference.


My mechanic have tried: meassured fuel pressure - all OK. Changed airmass-something. When warm - a spray of fuel in the air intake: it starts promt.


Could it be the Warm Up Regulator ? ...my mechanic cant see the logic, but cant on the other hand come up with else to try...


Anyone with suggestions - og similar issues ?


Besides from that, I am a happy, (poor) Danish owner of a beautiful 280SLC 1981 with 150 miles on the clock.

[okyoureabeast]

Try replacing the fuel accumulator.

It's a cylindrical container that lives near the fuel tank under the rear of the car. It's designed to hold fuel pressure in the fuel lines to prevent vapor lock (ex: gas gets hot, turns to vapor, engine can't spray it, car has difficulty starting).

They do tend to go bad and if yours hasn't been replaced recently, I would check there first.

It's nothing more than a spring pushing against a valve. The fuel pump overcomes the spring. It also serves to dampen the vibrations in the fuel system which can also damage the WUR (takes a long for this to happen).

Bought mine for $100 USD, it's very easy to replace(socket, phillips head driver, and a vice grip to close off the fuel line), just make sure you don't get gas dripping on your arm and be careful to do this repair when the fuel tank is low in case you end up cracking the fuel hose line!

[Diseasel300]

If the fuel pressure is correct, you can rule out the WUR. It's job is to control the fuel pressure. If the pressure is where it's supposed to be, it is clearly working properly.

As suggested above, fuel accumulator. These older mechanical injection systems are more sensitive to it working than the later KE-Jet systems. If it goes bad, it'll start just fine cold, but when hot or heat-soaked forget it. The accumulator keeps pressure on the fuel system to keep the fuel from boiling into vapor in the injection system. My SL would require 2-3 30 second crank attempts before sputtering to life when parked hot and restarted after 5-10 mins. Replacing the accumulator (which was full of fuel) solved the problem.

You may also have an issue with a vacuum leak somewhere in the manifold or elsewhere in the car's vacuum system. If the leak is large enough it can cause mixture problems that will be masked at cold starting due to the cold-start injector firing at that time.

[Frank Reiner]

MDK:

Prior to replacing the accumulator test it for a diaphragm leak (the accumulator is a can divided into two sections by a diaphragm; on one side is fuel, and on the other a spring). To test the diaphragm, pinch off and remove the small diameter leak line from the accumulator. Then run the fuel pump by cycling the key on/off. If fuel squirts out of the fitting on the accumulator, it is probably due for replacement.

There is another, often forgotten source of hot start faults: The check valve in the fuel pressure regulator. The regulator is screwed into the fuel distributor (FD), and when it is removed, a second, smaller diameter valve will be found inside the regulator piston; it is called a "push-valve", and the o-ring that seals it may be defective. If the push-valve o-ring leaks after the engine is shut off, fuel pressure in the FD and the injection nozzles will drop to zero, and the fuel will percolate to vapor. All the vapor must be purged before the engine will fire up.

[MikeDK]

Thanks so far... I will pass it all though to my mechanic. Seemes to be a good idea to start with the valve in the fuel pressure reg. and continue to the Accumulator. Will get back to the thread if it solves my issue...

[junker68]

Auxiliary Air Valve - I disassembled, cleaned and lubed. Fixed my exact same problem, lickety split.

[MikeDK]

Solved. It was a little valve at the fuel pump - fuel ran back when warm... total: 12 nan hours at 140 USD/h - and a defective valve at 30 USD...

[Diseasel300]

Originally Posted by MikeDK

12 nan hours at 140 USD/h - and a defective valve at 30 USD...

Time to find a new mechanic. Someone is taking you for a patsy or they're incompetent. There's no way it should have taken 12 hours to find a leaking check valve. That should have shown up as fuel pressure dropping as the car sat hot.

[okyoureabeast]

Originally Posted by Diseasel300

Time to find a new mechanic. Someone is taking you for a patsy or they're incompetent. There's no way it should have taken 12 hours to find a leaking check valve. That should have shown up as fuel pressure dropping as the car sat hot.

Probably it's a mechanic who billed his "troubleshooting" time going over a massive and long diagnostic process.

When he could have pinpointed the problem a lot faster by simply googling the symptom and the car model.

Either way, mechanics bills are the entire reason why I do all of my own car work.

[1982gwagon]

This is an old thread, but the problem keeps cropping up. I had the same symptoms.

The fuel pump check valve can be bought for about $19 at https://www.pelicanparts.com/More_Info/0000900110.htm?pn=000-090-01-10-M14&bc=c&SVSVSI=2477

Simple job to replace.