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WUR better, now engine races

Started by Feather535, 04 March 2023, 07:57 PM

Feather535

I've been working on cleaning, refurbishing and adjusting the WUR on a 1977 280SE (California model, K-jet) and finally got the warm and cold control pressures in spec with the WUR in a special test setup above the intake manifold and the fuel pump running (see my other thread on excessive control pressure). 

Today I installed the WUR back in its normal position, connected everything up and started the engine.  It started right away, but now there's another issue.  Instead of settling into idle, the engine races to about 4000 RPM.  I'm reluctant to let it rev that high with no load, so I turned it off, but I tried several times and it did the same thing.

This actually isn't a new problem.  I posted about the same issue back in October, before doing all the work on the WUR, and that time the cause turned out to be a disconnected vacuum line.

That line and the one on the WUR are both connected now.  The throttle linkage is correctly adjusted and isn't blocked or sticking. 

So where else to look?  Ignition timing?  It was correct before and shouldn't have moved, but I can't really check it with the engine behaving like this.  Other vacuum leaks?  I looked for disconnected lines and didn't find any, but this engine has loads of them.  Ideas?
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1977 280SE
1983 300D (sold)
1988 BMW 535is
1999 E320T (sold)
2009 E350T (wife's)

daantjie

Did you do anything to the fuel distributor?  To my mind the only way you'll get revving up like that is with a corresponding open throttle situation so check throttle butterfly and also position of the air flap at the fuel distributor.
Daniel
1977 450 SEL 6.9 - Astralsilber

raueda1

Quote from: daantjie on 04 March 2023, 08:05 PMDid you do anything to the fuel distributor?  To my mind the only way you'll get revving up like that is with a corresponding open throttle situation so check throttle butterfly and also position of the air flap at the fuel distributor.
Yeah, agreed.  I'm not sure what you could possibly do to the WUR, FD, or vacuum to do that.  Try disconnecting the throttle linkage entirely and see what happens. 
-Dave
Now:  1976 6.9 Euro, 2015 GL550
Before that:  1966 230S, 1964 220SE coupe, 1977 Carrera 3.0

ramiro

If you press the meter plate down you should see the throttle butterfly it must be fully closed , a vacum leak could not cause the engine to rev that high because it would get no fuel for the unmetered air.

I don't think that you can adjust the idle to 4000 with the idle screw fully out , but maybe check that also.

Feather535

The only thing I've done on the fuel distributor is to replace the o-rings in the pressure regulator, so I don't see that causing this problem.

I tried turning the idle air screw all the way in, but it didn't make any difference.

So linkage or a sticking throttle plate look like the most likely sources.  On the linkage, what is the purpose of the rods that go over the top of the engine and downward on the exhaust side, presumably to the transmission?

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1977 280SE
1983 300D (sold)
1988 BMW 535is
1999 E320T (sold)
2009 E350T (wife's)

ramiro

Yes that linkages go to the transmission.

rumb

"I tried turning the idle air screw all the way in, but it didn't make any difference."

Turning the white plastic idle screw should definitely affect idle speed. At 4K rpm you wont see anything happen.

Next there is the Aux air valve.  Good youtube videos on how to clean and calibrate. When working correctly it will be open when cold and when it warms up will be fully closed.  It can be tested by placing in a pot of boiling  water and examine what happens. The bulb can be crimped slightly to increase the closing amount, after your have cleaned it all up.


If you turned the screw in the hole next to the FD then that was not the correct thing to do.  THAT screw is the LAST thing you adjust after you have taken care of everything else.  If you did screw it in, then the only hope is to send it out to be calibrated.

As already mentioned look down into the throttle and confirm the spring is still attached and it fully closes the throttle plate.

4K rpm suggest to me that the throttle plate isnt closing. ATF is a good lubricant for all the linkages.(PS make sure your gas pedal isnt stuck to the floor - it happens --

'68 250S
'77 6.9 Euro
'91 300SE,
'98 SL500
'14 CLS550,
'16 AMG GTS
'21 E450 Cabrio

Feather535

Finally got a chance to look at this again. What seemed to be happening was that after starting the engine, it would idle at normal speed for a while--up to a minute, but often less--then the revs would abruptly increase.  Putting the transmission in drive or reverse would bring the revs back down to a normal idle speed. 

I confirmed that the linkage isn't sticking anywhere and the throttle plate isn't binding.

But after going through this sequence a few times, I noticed that the revs always started to increase when I took my foot of the brake pedal.  Pressing the brake again would restore normal idle. 

This is strange.  Now I'm wondering if the brake booster is creating a massive vacuum leak somehow.  I'm looking for a diagram of the booster to try to understand if this could be possible. 








--------
1977 280SE
1983 300D (sold)
1988 BMW 535is
1999 E320T (sold)
2009 E350T (wife's)

raueda1

Great observation, really crazy!  I honestly have no idea why a massive vacuum leak would cause racing but nothing surprises me any more.  So why not confirm it by removing the large nylon booster tube, plug the manifold hole and see what happens?  I guess you could also pull a vacuum on the booster and see if it holds.  I've got a booster if you want it.
-Dave
Now:  1976 6.9 Euro, 2015 GL550
Before that:  1966 230S, 1964 220SE coupe, 1977 Carrera 3.0

Feather535

That's what I was thinking, too.  I'll try both ideas and report back.
--------
1977 280SE
1983 300D (sold)
1988 BMW 535is
1999 E320T (sold)
2009 E350T (wife's)

PosedgeClk

Quote from: Feather535 on 11 March 2023, 09:25 PMThis is strange.  Now I'm wondering if the brake booster is creating a massive vacuum leak somehow.  I'm looking for a diagram of the booster to try to understand if this could be possible.
The brake booster operates off vacuum. Listen for a hiss around the drum. It's easy to plug the line with a vacuum cap while you troubleshoot.

The WUR could come into play here if you're running a really rich or really lean mixture and then you have an odd vacuum leak that sends the engine into a sane AFR range. I did a mod to my WUR once upon a time which involved adding a new seal to the chassis and a new diaphragm. The diaphragm wasn't cracked, but it was stiff. I recall pressing out a post and turning that on a lathe and threading it. If I needed to pull it out, I could just turn a screw at the end. If I needed to push it back in, I could back out the screw and tap the post with a hammer. That was enough to make all the needed WUR adjustments.

As for the nylon screw in the MFI, I don't recall one for adjustment. I recall a nylon sealing screw which has to be removed to access a coaxial screw below. Once the first screw is removed, you have access to the second screw like 1cm below, and you have to be very careful about making adjustments here. If you go more than a quarter turn either way, you have no idea where the set point should be and are now headed toward Jupiter. I am in a similar boat and bought a CO sniffer a few years ago but have had other projects take priority lately. My problem is an idle too low that is temperature dependent. It runs great on a cold day for a little while, and then that is it.
1979 450SEL 6.9

Feather535

I found the cause of this problem, and it was totally unexpected.

I disconnected the vacuum brake booster and plugged its port on the manifold, but the same thing happened: the engine started to race as soon as I took my foot off the brake. 

This was puzzling, because the booster should have been the only connection between the engine and the brakes--or so I thought.  But then I realized that, on this car, there is another connection: cruise control.  The cable that activates cruise control was slack, as it should be, when the engine isn't running, but I disconnected it, started the engine, and it didn't race.

I didn't know the cruise control was even functional on this car and don't know why it was doing this, so the problem isn't exactly fixed.  But with it disconnected, I can get back to working on other issues.
--------
1977 280SE
1983 300D (sold)
1988 BMW 535is
1999 E320T (sold)
2009 E350T (wife's)

ptashek

Possibly the control valve in the vacuum actuator is stuck open, and if enough vacuum builds up the diaphragm pulls on the throttle cable?
1993 "Pearl Blue" W124 280TE
1988 "Arctic White" W124 200T
1979 "Icon Gold" W116 450SE

raueda1

Quote from: Feather535 on 15 March 2023, 10:33 AMI found the cause of this problem, and it was totally unexpected.

I disconnected the vacuum brake booster and plugged its port on the manifold, but the same thing happened: the engine started to race as soon as I took my foot off the brake. 

This was puzzling, because the booster should have been the only connection between the engine and the brakes--or so I thought.  But then I realized that, on this car, there is another connection: cruise control.  The cable that activates cruise control was slack, as it should be, when the engine isn't running, but I disconnected it, started the engine, and it didn't race.

I didn't know the cruise control was even functional on this car and don't know why it was doing this, so the problem isn't exactly fixed.  But with it disconnected, I can get back to working on other issues.
Nice find!  Interesting that the problem was indeed related to the throttle linkage, though in a way that nobody anticipated.  Live and learn!  The odds of ever getting the cruise control to work properly aren't good.  Might as well disconnect it entirely and cap the line.  And now on to the next item....    Cheers,
-Dave
Now:  1976 6.9 Euro, 2015 GL550
Before that:  1966 230S, 1964 220SE coupe, 1977 Carrera 3.0

Feather535

Quote from: raueda1 on 17 March 2023, 09:10 AMand now on to the next item....   

On the the next, or back to the same...

So after all this, I could finally test fuel pressures with the engine running and the cold CP is still too high--about 3.5 bar now. 

This is a big disappointment, because I had it down to 1.0 cold and 3.7 warm, which is just right for this car.  The curious thing was that the longer I ran the fuel pump or the engine, the higher the CP went, even with the WUR'S electric heater disconnected.  On removing the WUR (for at least the 10th time) I found sediment in the inlet screen again.  So it's back to step 1: cleaning the WUR (again) and the rest of the fuel system.
--------
1977 280SE
1983 300D (sold)
1988 BMW 535is
1999 E320T (sold)
2009 E350T (wife's)