hello everyone

the 72 is running just fine now. starts right up cold or warm a little sluggish but can make 90 on the freeway with no problem. still has 4 of the old plugs.

what it does is after it has sat so a cold or cool start it starts right up but...then something strange. it's idle is about 700 rpm then it raises to 2000 then back down then up then down. it's like someone or something is pushing on the throttle but we're not.

i've released the after market cruise control. so what ever is creating this condition is from the motor itself. i'm wondering if the electronic injection is creating the condition.

any suggestions would be appreciated.

__________________
Thanks Much!
Craig

1972 350sl Red/Blk 117k
1988 420sel charcoal/Blk 140k
1987 420sel gold/tan 128k
See My Cars at:http://mysite.verizon.net/res0aytj/index.html

Pound it to fit then Paint it to match!

There is only First Place and Varying degrees of last!

Old age and deceit will overcome Youth and Enthusiasm every time!

Putting the square peg in the round hole is not hard... IF you do it fast enough!

Old enough to know better but stupid enough to do it anyway!

73 450Sl 107044
Starts perfect then surges like yours does for about 90-120 seconds, then smoothes out perfectly, this is well before it gets up to operating temp, also if I put it in gear it smoothes out...
I verified air mix and idle RPM.. What is the secret???

Thanks,
George

Sounds like either the setting for the idle air screw is set too high (what is the idle speed in neutral at normal operating temp?), or the auxiliary air valve is not closing properly during warm-up. Prompt cold start-up implies that the cold start valve (adds extra fuel) is fine.

See if closing down the idle air screw improves the surging. If the surge disappears but the warm idle is then too slow, I would say that AAV is very likely the culprit. Should be fully closed at ~70ºC, you can check by disconnecting the air hose to this valve and checking for suction.

Excellent info on the AAV (also used in D-Jet Jags):

http://www.jag-lovers.org/xj-s/book/AAV.html
http://www.jag-lovers.org/xj-s/book/AAVdisassembly.html

I rebuilt my AAV by disassembling, cleaning, and adding a shim to the thrust piece attached to the plunger on the themostatic bulb. Sometimes the upper half of the AAV cylinder has come out of the base a few mm and you can use a mallet to push it back and restore proper closure behavior.

Clean the trigger points, too! Idle should be 1500 or so out of gear cold, and constant. Will drop rapidly as the engine warms up.

I'd also check for vac leaks and the condition of the idle air hoses and crankcase vent lines as leaks here will cause all sorts of trouble, especially on the Y-shaped idle air hose.

A bad vac lock line, leaking locks, leaking AC system, or brake booster line, or even an emission control line leaking will cause idle funnies.

So will the need for a complete tune-up. If you still have points in there, are they new? They get dirty on high milage cars from oil seepage, and will make trouble.

So will a sticky mechanical advance -- has it been oiled regularly (did you know it needed to be oiled? A drop or two of oil on the felt pad under the rotor every 5000 miles or less). If it sticks, it can snap back and forth rather than moving smoothly, causing idle speed variation.

A leaking vac retard on the distributor can make trouble, too.

Peter

__________________
1972 220D ?? miles
1988 300E 200,012
1987 300D Turbo killed 9/25/07, 275,000 miles
1985 Volvo 740 GLE Turobodiesel 218,000
1972 280 SE 4.5 165, 000 - It runs!

hello

george: you hit it right on the head. up down up down put it in gear stops after it warms up doesn't do it again till it's sat overnight.

guys what is taking place is on a cold start the engine starts right up no problem but....it then proceeds to this rpm up down thing. the symptoms are... the rpm sits at idle for a moment then rapidly raises to 2000 rpm then suddenly drops back to idle of appx 700 rpm. then rises again to 2000 then drops back. it keeps doing this until i put it in gear. i've never let it run out to see if it will quit on it's own but george has and he says it quits in about 2 minutes.

so the really strange fact is if it was just extra gas being dumped the engine would cough and stutter with black smoke but it does not. it's just as if someone is pushing on the gas pedal then letting up and repeating this over and over.

so something is changing up and down. if somebody out there has a idea please throw it our way. at least 2 of us are experiencing the same thing.

__________________
Thanks Much!
Craig

1972 350sl Red/Blk 117k
1988 420sel charcoal/Blk 140k
1987 420sel gold/tan 128k
See My Cars at:http://mysite.verizon.net/res0aytj/index.html

Pound it to fit then Paint it to match!

There is only First Place and Varying degrees of last!

Old age and deceit will overcome Youth and Enthusiasm every time!

Putting the square peg in the round hole is not hard... IF you do it fast enough!

Old enough to know better but stupid enough to do it anyway!

a 350 '72 is in fact a 450 sl !!

You have either a vacuum problem (vacuum leak) or the temperature sensor that broke.

Attached Thumbnails

 

I've been working on this problem a while, here is what I did...
I have verified all temp sensors to the ECU and then ohm'd from the ECU harness back to all the sensors and observed them move to spec during warmup, these are most likely functioning, also I have TEED in between the MPS and manifold and put a digital vacuum guage and it bars out to spec (it sure sounds and acts like the air valve but this one is only a couple of months old, the old one was stuck open,tried the balpeen trick,didn't work) This '73 SL actually runs smooth on strart and after about 25 seconds will start hunting for almost 2 minutes and smoothes out, at this point the engine temp is about 30C or so....
Ofcourse if you start it and put it in gear (under load purrs like a kitten......
If you need specifics on any measurements let me know, I took copius notes on troubleshooting the DJET and have a binder about an inch thick....
Thanks for taking the time to help us on this one you guys!!!

The only thing that can make the engine rev up is more air, so I still thing you need to take a look at the idle air system. I've seen big steel ball bearings stuck in the blowby hose to cut the idle back!

Places to look:

Seals between the intake manifold halves (there are 8). Spray some carb cleaner down there, if the idle speed changes, take the manifold off and re-seal it.

Seals at injectors.

Manifold gaskets.

Y-shaped idle air hose from idle valve to manifold. A crack here can give you fits.

Other thoughts:

Is the mechanical advance working properly, and is the timing set correctly? A broken spring on the mechanical advance can make the idle surge. So will a bad vac retard diaphram or hose.

Fuel pressure fluctations from a bad damper or pressure regulator. A bad pump or relay will cause the car to quit, I think.

Good luck!

Peter

__________________
1972 220D ?? miles
1988 300E 200,012
1987 300D Turbo killed 9/25/07, 275,000 miles
1985 Volvo 740 GLE Turobodiesel 218,000
1972 280 SE 4.5 165, 000 - It runs!

Still sounds like too much air. Afmcorp, verify that the AAV is closing! If so, (George, sounds like you are sure that yours is okay) close the idle air screw until cold idle is no higher than 1500, and see what value it settles down to when warmed-up (~500 is okay).

Vacuum leak is a likely culprit if you can't get the idle speed down enough even with the idle air screw fully shut (and the AAV is closed, of course). I would venture that smooth running in gear implies reasonable injector/manifold seals (leaks here affect one cylinder at a time causing rough idle). Definitely check the breather Y-hose between the idle control and the intake manifold.

good luck!

Peter, just curious, re valve clearances in your 4.5, I'm surprised how much tight exhaust valves affect idle smoothness. Do you know why?

Tight exhaust valve cold become exhaust valves held open when they heat up, so you lose compression. Doesn't take much of a leak to mess up the idle.

Intake valves don't get so hot, so they "grow" less.

Peter

__________________
1972 220D ?? miles
1988 300E 200,012
1987 300D Turbo killed 9/25/07, 275,000 miles
1985 Volvo 740 GLE Turobodiesel 218,000
1972 280 SE 4.5 165, 000 - It runs!

I have the same problem with my 1971 350 SL.
But I have fix the problem.
I put a brass pice with 7 mm hole , in the rubber hose from the supplementary air valve to the idle speed air distributor.
The idle are now approximately 700 from the engine are cold to normal operating temperatur. Before the brass pice, the RPM goes up and down from 500 to 1500 before the engines has normal operating temperatur
It works for me.
From Norway
Arvid

This rising/falling idle is ever so prevalent in 3.5 motors. I am sure it is a programming issue with the set speeds of the decel cut-off function.

The reason for the up/down idle is that the closed throttle decel cut-off is initiated at around 1800rpms. That means that at 1800 rpms with the throttle closed the fuel is cut untill the idle speed drops to 1200 or so, where fuel delivery resumes.

If you wish to see how high it would have idled, pull the throttle switch wire. Without seeing the closed throttle switch the engine will idle where ever the air quantity places it. No more up/down at any speed.

The 3.5s either had a lower threshold of decel cut-off or they are more sensitive to failed aux air valves. The 3.5 also has different timing relation to idle speed with its higher compression.

__________________
Steve Brotherton
Continental Imports
Gainesville FL
Bosch Master, ASE Master, L1
33 years MB technician

thank you all for your responses.

steve: would the same hold true on Chaosmosis' and my 350sl? both have the 4.5 motors mine is a 72 and Chaosmosis' is a 73.

once the engine warms up a mite or is thrown into gear the up down cycle is over. runs just fine.

thanks to all for any help.

tks
craig

__________________
Thanks Much!
Craig

1972 350sl Red/Blk 117k
1988 420sel charcoal/Blk 140k
1987 420sel gold/tan 128k
See My Cars at:http://mysite.verizon.net/res0aytj/index.html

Pound it to fit then Paint it to match!

There is only First Place and Varying degrees of last!

Old age and deceit will overcome Youth and Enthusiasm every time!

Putting the square peg in the round hole is not hard... IF you do it fast enough!

Old enough to know better but stupid enough to do it anyway!

Many cars do this surging thing. All the early EFI cars such as these MBs and all BMWs will do it. The 3.5 is just more susceptible. The car has to be perfect or the fast idle gets to the cut-off point. The reason it stops when you put it in gear is the idle speed drops below the threshold.

All you have to do to stop it is adjust the idle cold till it doesn't do it. Then when warm it must still idle high enough.

__________________
Steve Brotherton
Continental Imports
Gainesville FL
Bosch Master, ASE Master, L1
33 years MB technician

As Steve states, the hunting idle during warm-up was a problem with the D-Jetronic engines, particularly the M116 3.5 in the 280SE3.5, 280SEL3.5, 300SEL3.5, 350SE, 350SEL, 350SLC and 350SL (our 350SL here in Oz was a 3.5, not a 4.5 like the North American deliveries).

My 350SLC suffered the problem. Replacing the auxiliary air valve did not completely fix the problem. With age, numerous other slight vacuum leaks probably combine to raise the cold idle speed to the fuel cut-off speed, resulting in the hunting. I considered the possibility of a temperature switch that disabled the closed throttle fuel cut-off until the engine reaches normal temperature (this would also prevent the hunting that occurs when decelerating at low speed with a cold engine). The easier option that I used was to place a restriction in the hose ahead of the auxiliary air valve. With some experimentation the correct size restriction can be chosen that will result in an acceptable cold idle speed around 1000rpm without an unacceptably low hot idle speed. My M116 has worked fine with this mod for many years now.

__________________
107.023: 350SLC, 3-speed auto, icon gold, parchment MBtex (sold 2012 after 29 years ownership).
107.026: 500SLC, 4-speed auto, thistle green, green velour.
124.090: 300TE, 4-speed auto, arctic white, cream-beige MBtex.
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201.028: 190E 2.3, 4-speed auto, blue-black, grey MBtex.
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