77 350SL - Warm up compensator - PeachParts Mercedes-Benz Forum

DAVID LEE · #1 04-16-2007, 08:05 AM

After reading the service CD (several times) and poring over about 20 or 30 threads concerning the warm up compensator, hard starting problems, etc. I would love for someone to explain the function of the warm up compensator when the car is cold, and when the car has warmed up. It appears that it performs several functions.

Additionally, I see what appears to be a fuel regulator between the fuel distributor and the warm up regulator. Am I correct, and if so, is the fuel regulator on the supply side, or the return side of the fuel going to the warm up regulator. What is its purpose?

Also, there is a vacuum line going from the thermo valve to the warm up regulator. Apparently, when the engine has warmed up (40C+) vacuum is supplied to the warm up regulator, but I can't figure out why. I'm trying to figure out why I'm having hard starting problems when the engine has reached operating temp, so I want to understand how this stuff is meant to work. The car starts perfectly when cold, and once started runs perfectly when either cold or warm. When starting the car "warm" it seems to "help" if I push the accelator down and leave it down. This would abviously open the throttle plate and ???

The Cd states that when the engine is shut down, that fuel pressure drops to below the pressure needed to force fuel through the injectors, yet it doesn't (unless I've simply overlooked it) say what component drains off the pressure to the pressure level below that of forcing fuel through the injector.

This forum is incredible and I know there is someone out there who has struggled through this problem and I'm counting on someone out there to help. I don't want to just throw parts at this problem, but rather properly troubleshoot the issue gaining an understanding of how the components are "supposed" to work.

The parts I've eliminated by either testing or replacing are - Accumulator, Fuel check valve, Air flow sensor plate adjustment, idle mixture adjustment. vacuum leaks, deceleration valve (since it is also connected to the thermovalve via vacuum line), heater coil within the warmup regulator, cold start valve. Someone will probably want to know what the fuel pressure values are at the various engine temps, but until I figure what the thread sizes are for the adapters needed to connect a fuel presuure gauge, I will be unable to supply those values (any help in this area would also be appreciated as well). As I said earlier, once the car starts, it runs absolutely flawlessly.

I realize I've asked more than one question and I apologize in advance, but I've pored over threads and Cd material for days and just can't seem to get the "how does this work" question answered completely. I've picked up several nuggets, but I'm not satisfied that I have the full picture.

thanks,

#2 04-16-2007, 08:51 AM

I can't help with the specifics, but I do know that the usual failiure mode for the warm up compensator is very hard cold starting. If the car is hard to start warm, I would look at two thiings:

1. Mixture adjustment, done with a 3mm allen key at the fuel distributer, using a CO meter to set the proper mixture.

2. Injectors that are leaking down and flooding the engine with fuel, they are thirty years old and this might be contributing to the problem. A fuel pressure gauge would confirm this, as the static fuel pressure would drop while the car is sitting.

If I had to guess, it would be leaky injectors, since the car runs so well and is hard to start when warm. What do the spark plugs look like?

DAVID LEE · #3 04-16-2007, 10:53 AM

Checking for leaky injectors was a suggestion made by an old experienced mechanic (not MB, but the concepts are the same). I guess the only way to check is to pull plugs and look for wet plugs just after stopping the engine OR should loosening the fuel lines from the fuel distributor to the injectors and checking for fuel under pressure be the test?

Again, this site is incredible.

thanks for the reply.

#4 04-16-2007, 12:11 PM

Fuel will be under pressure in the fuel lines, even with the engine stopped. What you are looking for with leaky fuel injectors is fuel dribbling(sic) out of the end that sprays into the cylinder head. They can be tested with the right tester, but only Bosch service centers typically have the proper test equipment. The injectors cost around $30.00 each, so it's usually cheaper to replace than pay someone to test them. The gauge setup for Bosch CIS fuel injection is kind of specialized, I forget the exact way they hook up, but I do remember they tie in to the car in two places and use a valve to switch from one test port to another.

Based on your symptoms, I would try to hook up a fuel pressure gauge and see what you get. Look on eBay, they come up every once in a while.

250 Coupe · #5 04-16-2007, 10:43 PM

To paraphrase the operation of the fuel distributor, when air flow pushes down on the air plate, the pin inside the fuel distributor is pushed up. The amount it is pushed up controls the amount of fuel delivered to the engine. Thus, the air/fuel ratio is controlled by the fuel distributor.

This would be enough if it were a stationary engine or a lawnmower, but we need finer control of the mixture, including a richer mixture during warm-up and full throttle acceleration.

One way to control the air/fuel ratio is to apply an opposing force on the top of the pin. This is the job of the warm-up regulator. The warm-up regulator is in series with a fuel line that delivers “control pressure” to the top of the fuel distributor. When the engine is cold, the warm-up regulator reduces the pressure applied to the top of the pin and allows more fuel to flow to the injectors. As the heater in the warm-up regulator heats, the control pressure is increased to normal.

When maximum power is demanded from the engine, it will need more fuel than is normally supplied. Low vacuum on the warm-up regulator will decrease the control pressure and provide the richer mixture.

The injectors themselves will bleed the fuel pressure down until they close. Any loss in pressure after that is due to faulty components.

The thing between the warm-up regulator and the fuel distributor is a pulsation damper. It smoothes out the control pressure.

Your SL may have the hot start valve, my 1976 450SEL has one. It is located at the rear of the fuel distributor and is a cylinder about 2-3 inches long sticking straight out the back. There is a temperature switch in the drivers side intake all the way to the rear and not far from the hot start solenoid. The purpose of the hot start solenoid is to increase fuel flow during hot starts as the C.I.S. injection can tend to vapor lock in the SL due to the close quarters and under-hood heat

I can’t tell you the thread sizes you need but some are straight thread with an o-ring so rather than mess around finding fittings, I bought a tester from JC Whitney. Keep in mind that it looks nothing like the picture. You might be able to rent one from NAPA or find on at your local version of Tool Town.

Michael

cth350 · #6 04-17-2007, 07:44 AM

To amply that last post. The WUR is vital to fine control of fuel delivery and as such, will give you horrible pickup when bad, presuming you can get the car going at all. There are several revisions of it, not to mention that each one is tuned to the specific motor type.

Mercedes published a thin grey manual that covers the diagnostics for the CIS systems. It goes into a little more detail than the service manual. Any of the Haynes manuals has the same info I think, as CIS is common to lots of 70s cars.

Get the fuel pressure tester. It's a gauge with two flexible hoses and a cut-off level between them. The kit I got from my local Napa came with a dozen fittings to adapt it to lots of different cars.

If you do frequent ebay, that grey manual shows up from time to time. There are at least three versions of it. Get the one with the right model year mentioned. There's also a "theory" one that is nice to have.

BTW, flashing overhead as I type this is a banner ad from Unwired Tools. They have a very nice after-market WUR that can be adjusted, unlike the factory one. It's also about half the price of the a new one from mercedes. Though a used unit can be found usually for a tenth of that. Do what you can to clean your original and make sure you know that it's misbehaving before replacing it with a new one.

-CTH

ctaylor738 · #7 04-17-2007, 08:02 AM

To answer one question:

The WUC has a vacuum line because it also controls enrichment. When you step on the gas, manifold vacuum drops. The WUC senses that through the vacuum line and basically bleeds less fuel to the return line, causing a richer fuel/air mixture. More manifold vacuum, as in coasting, and the mix probably goes a bit leaner.

DAVID LEE · #8 04-17-2007, 12:15 PM

Thanks so much fellows. I hadn't even thought of going to NAPA for the tester. I bought one years ago, but trying to find metric adapters is very difficult. I will check with NAPA.

Thanks for the theory on the WOC AND for the heads up on 3rd party replacement vendors. One thing I'm NOT going to do is simply start replacing parts. It may take me a while to figure this out, but I will figure it out (with help from folks like you).

DAVID LEE · #9 04-18-2007, 02:26 PM

Just got back from NAPA and they have a fuel pressure testing kit that is made specifically for the Bosch CIS K-Jetronic system - very cool. So..this weekend I'll start doing some measurements and see what's up.

thanks,

cth350 · #10 04-20-2007, 07:13 AM

If you already have a fuel gauge that will accurately read 30-60 psi and all you need are metric fittings, go to the nearest hydraulic shop with the gauge and the a few lines (there are two size fittings you need to plug into).

-CTH

DAVID LEE · #11 04-23-2007, 10:25 AM

Well, this weekend's troubleshooting episode was very frustrating. I got the "specially made for Bosch CIS" systems fuel pressure tester out and begin making the measurements. The Control pressure is 3.7 at full warmup, which according to the CD is OK. But, when I went to test the System pressure, things went downhill fast. It appears that someone has already worked on the system and has "added" some extra fittings making it impossible to connect the testing unit to the system in the locations that you would normally connect to (check valve in the FD fuel input line for one). I finally found a location that the fittings would allow me to connect to, but when I started testing, I had a fuel leak. After messing around for another 30 minutes I discovered that one of the fittings was manufactured with a defect causing the leak. Therefore, I could not test the system pressure.

After 3 1/2 hours of playing around, I put everything back together again, and now the car runs terrible - extremely rich, so I've "broken" something while trying to find my problem (quite possibly the cold start valve).

I did learn something though. I looked at the control pressure with the vacuum line connected to the WUC, and with it disconnected, and observed how the vacuum affects the control pressure. Also noticed the effect of simple load on the WUC - again due to vacuum changes. I now understand the theory behind how, and why the WUC does what it does - which will be helpful in the future.

So, after I get some time (about 5 or 6 hours), and a replacement part for the tester, I'll go back to the beast and continue this saga.

Thanks 250 coupe for your "how does this work" input - it was very helpful, and I was able to observe your system description via the pressure readings.

I'll keep you posted on this, because someone in the future will be asking the same questions.

DAVID LEE · #12 04-25-2007, 08:37 PM

I finally got the parts replaced (and the car running properly) that were defective on the fuel pressure tester and made the following measurements when the engine was warm.

System Pressure: 5.6
Control Pressure: 3.7
System pressure at shut down: 2.1 which then leaks down to 0 after about 30 minutes.

While making the measurements, I disconnected and capped the fuel line from the FD to the cold start valve. This eliminated the CSV as the reason for the fuel pressure leak,and hence the warm start problem. I had already replaced the fuel check valve, and tested the accumulator. I insured there wasn't any fuel leaking back to the fuel supply via the return fuel line. As far as I can determine, there isn't any other possible cause for the pressure leakage other than one or more injectors.

Is my diagnosis correct? If so, I will go about the task of either replacing ALL the injectors, or going through some (yet unknown) process of determining which injector(s) are leaking.

#13 04-25-2007, 08:45 PM

It shouldn't leak down in 30 minutes. I had the same problem with my '77 450SL, and it turned out to be leaky fuel injectors.

DAVID LEE · #14 04-25-2007, 09:15 PM

John - did you replace all your injectors, or figure out a way to replace the bad ones?

#15 04-25-2007, 09:43 PM

I replaced all of them, I bought bew Bosch injectors for around $27.00 each from a well known aftermarket Merceds parts store. I figured that after more than twenty years, the old injectors didn't owe me anything. The injectors can be tested with a tool similar to testing diesel injectors, the local indy wanted one hour of expensive labor to test them. I figured the labor would pay for half of them anyways, so I replaced all eight. I changed them myself, and it was pretty easy, don't forget to lube the two injector seals so they don't tear. I used plumbers silicone grease for rubber.

It was the best money I spent on the car, after adjusting the CO with the new injectors the car ran great and started instantly hot and cold.

I also noticed that the oil stayed much cleaner because it wasn't being diluted with raw fuel.