Tom M'Guinn

Hi All, excuse the slight OT, but I'm looking for input/info on the CIS system. I'm bringing my father's '79 Mercedes 450SL back to life, slowly going through the systems. The motor generally runs fine and has about as much power at is supposed to (180hp vs. curb weight of 8 tons...). The issue is it has a slight hesitation immediately off idle, like from a stop light. Seems worse once the car is warm. It only happens when you gently accelerate from a stop. It I accelerate more aggressively from a stop, there's no hesitation. As part of the general updates, I've changed the plugs, cap, rotor, wires, coil, fuel injectors, air filter, and maybe a few other things I'm forgetting. I've posted on the Mercedes boards, but there are very few hands-on DIY types driving these cars (and/or the typical owner isn't much for computers). Since the CIS systems are so similar, I thought maybe I'd see what you all say.

jpitman2

It sounds like its been stored for a period, and CIS does not like that much. If it generally performs OK, I would give it some Berrymans or Marvel Mystery oil in the fuel tank, and keep up regular use. I have had good results from loading the fuel with oils as above, and bridging the pump relay to circulate fuel through the system for as long as you can stay with the car. The other things to do are change the fuel filter, and check the fuel delivery rate (cc/minute - for a 928 its around 1350cc/30 seconds). I have found the forums at mercedesshop.com helpful when I have visited them.
jp 83 Euro S AT 57k

Tom M'Guinn

Thanks! I'll check out meredesshop. I've been posting on benzworld. Nice place with a handful of hands-on types, but pretty limited tech info. It wasn't so much 'stored,' but rather ignored and driven once every month or two for years on end. Your comments ring true. I have a fuel filter ready to go in, but the water pump (and heater core) started leaking and pre-empted everything else. It actually does seem to be getting better overall as I drive it. I'll change the fuel filter and see if that helps. If not, I'll start researching fuel deliver rate adjustments.

jpitman2

I doubt you can adjust delivery rate - its an indication of poor health of the filter, pump and pipe work if its significantly lower than spec. If the pump is noisy its usually a bad sign. Your issue could also be a vacuum leak - CIS really dont like these either. With the engine running, try spraying something flammable (WD40 etc) around the inlet manifold, and see if the note changes , and if it does, try to isolate which area is involved. A leak is more consistent with your problem, except maybe a sticky air plate. Keep using it as much as possible.
jp 83 Euro S AT 57k

dr bob

Basics: Crud in the system is a popular cause. Little brass screens are fine when bathed in fuel but suffer when the systems sits dry for extended periods.

After that, basic stuff includes checking the fuel and control pressures, adjust as needed. It's tempting to start adjusting things blindly to try and solve what show up as driveability symptoms. Do yourself a favor and start off with spec pressures, and diagnose from there. Roger sells the pressure gauges you'll need, and you need to find either a CO analyzer or a wideband oxy sensor and meter for final tuning.

docmirror

CIs uses an idle air bypass circuit so that the big damper plate is almost fully closed at idle. I'm not sure on the MB CIS but probably uses a pipe around the throttle plate with an idle stabilizer to set idle speed. If someone has messed with the idle screw stop on the actual throttle plate, thinking this was an 'idle' setting like a normal car, they would be mistaken. The throttle plate should be fully closed at idle and all the air should be going through the idle bypass with the stabilizer valve regulating the idle speed.

Things that go wrong; Mis-setting the throttle plate screw. Air leaks in the large tube from the throttle body to the CIS body damper plate housing. Air leaks in the bypass tube, air leaks anywhere. Mis-adjusted idle stabilizer valve. ISV dirty. And more.

There is an idle adjustment screw, I just forgot where it is, and it only sets the RPM, there is another screw for mixture. Make sure no leaks, make sure the throttle is fully closed, and then mess with the ISV.

Edit; I just googled and CIS calls it the "Auxiliary air regulator" by name, and not the idle stabilizer valve like normal people. The idle screw regulates the amount of free air that can bypass the throttle plate.

this is VW, but the MB is basically the same, with more injectors. http://www.n56ml.com/CIS.html

Lizard928

Remove the fuel block and under it is a roller. Make sure that roller spins 100% freely. If it does not this can cause what you are referring to.
Also get a set of pressure gauges and make sure that your pressures are all within spec. From there make sure that the base CO is set correctly.

jpitman2

Docmirror - the 'aux air regulator' in CIS is only used in warm up mode - bimettallic strip and +12V limit its action in time, and on a hot engine it should pass close to zero air. It is not adjustable, unless it is removed and dismantled - BTDT.
The idle control screw allows air to bypass the throttle plate, but absolutely ZERO air should bypass the air valve plate on the fuel distributor, otherwise the mixture will be compromised. The ' big damper plate is almost fully closed at idle' because very small amounts of air are required at idle.

Colin - whats a "AFCO double adjustable inverted 1000/500" please?

jp 83 Euro S AT 57k

Tom M'Guinn

I sprayed started fluid and visually inspected. Only thing I noticed was that the idle did drop when I sprayed the fuel distributor between the top portion with all the fuel lines and the bottom section. It wasn't a big change, but clearly some fluid was getting in there. With the air filter off, I can watch the air plate and it is opening despite the hesitation, so seems like it's not sticking. There are soooo many vacuum lines everywhere on this car, operating everything from the HVAC to door locks to the transmission, etc. So very difficult to have confidence in the vacuum system. Trying to burn off some gas so I can change the fuel filter and accumulator and tank screen --then if that doesn't help, I suppose I'll check the fuel pressure and go from there.

Tom M'Guinn

Fuel block? Is that the fuel distributor with all the fuel pipes for the injectors?

tailpipe

Sounds like the WUR is out of spec... Sounds like crud in the metering hole.. Easy fix.. Remove the WUR, remove the lines into the top of the WUR so you can see inside..Drop in a few drops of muratic acid in the holes where the fuel lines attached.. Let it sit for an hour or so... Flush with hot water dry and reinstall you should be better... The only way to really check is to use fuel pressure gauges... Cold at the WUR should be around 15psi and about 40 psi when hot.. this is called the control pressures, kind of works like a choke on a carburetor... If there is to much restriction in the WUR and the pressure is over 45 psi it will cause your hesitation... Good luck

Lizard928

Yes.

Jpitman, That is the shocks on my car (inverted means less unsprung weight, double adjustable means independent adjustments to compression and rebound).