380SL Hard Start & Poor Mileage

[timm9]

I wanted to start out by thanking the forum and all of the contributors. What an unbelievable wealth of knowledge!

I have searched and read until my eyes are blurry…

I have a 1982 380SL with 31,000 documented miles. The car is in excellent condition and runs beautifully with the following exceptions;

1) It has a hard time starting when warm, unless you apply the accelerator petal.
2) The gas mileage is poor. I’m getting approx. 10 to 11 MPG. I do live at 4000 feet elevation and I know that does play a role, but from several previous MB’s I expected better mileage. I do not treat/drive this car like my GT. It’s more like my baby! I have read quite a few post indicating that the mileage should be better.

After searching/reading previous posts it appears that it may be one of several items;

1) Accumulator
2) Cold Start Valve
3) Warm Up Regulator

Quite a few of the posts referencing hard starts after the engine is warm indicate that the problem was rectified after replacing the accumulator, but I did not find those same posts indicating poor fuel mileage. I have checked for vacuum leaks and all appears tight. While the cost of the accumulator is relatively inexpensive, the other suspects carry pretty hefty price tags. Has anyone else experienced hard starts and poor fuel mileage?

As always, I’m very grateful for any assistance and guidance!

Tim

[rocky raccoon]

If your car starts o.k. when cold, look for a temperature related problem. I suggest you check out the engine temperature sensor that inputs to your fuel injection computer. I believe your car has three temperature sensors; one for your temp gauge, one for your aux fan and the one I suggest above. On my 560SL, that sensor is located at the front top of the engine easily accessible by removing the air claner housing. It is the sensor with two wires. Yours may be the same.

The sensor is relatively inexpensive (I think I recall about $40.00).

[cth350]

Want to spend 250$ to find out what's wrong and fix it? Get the WUR rebuilt. As a low milage car, it's not been run too often. Your WUR is most likely the cause for your problems. You can prove it with some diagnostics that require you to have a CIS compatable fuel pressure tester. Presuming you do your own work, that's a handy tool to have.

-CTH

[slfan]

That's exactly how my 84 380sl acted when I needed a new O2 sensor. The
O2 sensor will only start working when the exhaust gasses have heated it up, so it is inactive on a cold engine. When hot, it would send a wrong signal to the computer which started "dumping" fuel......12 mpg. For a 380 that is way off.

[UT_Tech]

This car does not have a computer for the fuel injection and there is no temp sensor on your car which could cause this problem. There is a controller box which uses the O2 sensor to control the "lambda valve". This valve looks like an injector with two hoses on it. The lambda controller is a very dumb bit of analog circuits. The job of the labmda valve is to tweak the air mixture slightly richer or leaner for emissions control. The lambda vave can only move the air/fuel mixture a few percent and a failure in the O2 sensor/lambda system can NOT this cannot cause the engine to run rich enough to get 11MPG.

DO NOT drive the car running this rich. The engine block in your car uses a silicon/aluminum allow which wears out very quickly (a few hundred miles) if the mixture is rich enough to wash down the oil film from the cylinder walls. That happens at about an AFR of 11:1. A mixture that rich would explain 10MPG.

You have to check your control pressure. This is very easy to do with a simple tool. Email me at the office (support@unwiredtools.com) and I would be happy to lend you one at no charge. It takes less than 5 minutes to check control pressure. If the control pressure is less than about 3.2bar when the car is warmed up then your Warmup-Regulator is bad. A bad WUR is very easy to fix, if you fix it before it destroys your engine.

A bad fuel accumulator can cause a WUR to go bad by allowing pressure pulses to propagate through the fuel system. These pressure pulses cause calibrations shifts and metal fatigue failures in the little brass parts inside the WUR. That's generally why rebuilt WURs are not a good option.

The control pressure test I mentioned will also diagnose a bad accumulator.

[timm9]

Wow!

Thank you all for the responses! The car will be parked until I get to the bottom of this!

Steve, I have sent you an e-mail.

Again, a big Thank you to the "community"!!!

Tim

[ctaylor738]

Trouble is, when WUR's go bad, the result is high control pressure, which, while it makes for hard cold starts, also causes poor acceleration and poor cold running. Gas mileage might improve because the mixture is lean.

I would bet on a leaky cold start valve, a worn-out oxygen sensor, or maybe even a damaged o-ring in the fuel distributor plunger.

[ctaylor738]

This sounds fairly normal to me. You don't say what the CP was when you started the car, but it should have been in the 21-28 psi range. So if it was at 40 in 30-45 seconds, that's about right. Your warm CP is right on at 52 psi = 3.6 bar.

Next up is a check of the duty cycle/on-off ratio.

[UT_Tech]

Tim this is a key point so make sure of you results or if needed repeat the test. There was no residual pressure when you started the car because you fitted the gauge. When you removed the fuel inlet fitting there was also no residual pressure. We'll come back to that.

When the gauge was on the car and you first started the engine it's normal for the fuel system to take a few seconds, perhaps 5 seconds, to pump up the control pressure. During this initial pressure rise did the pressure rise up to 40+ immediately (within a few seconds), or did it rise to 20+ PSI then rise slowly to its final value over a minute or two?

When the car was fully warm and you saw 52 PSI were the vacuum hoses connected? If the vacuum hoses are disconnected the control pressure should drop to about 40PSI. Does it?

[timm9]

I (think) I have finally resolved my issues and thought I would post a follow up for the benefit of all. Hopefully this will save someone time and frustration!

First of all, I owe a huge debt of gratitude to one of the Moderators on this site. They have for all these past weeks since my first post, painstakingly helped, advised, educated, and dealt with all of my questions, inexperience, frustrations, etc. You know who you are and I will never forget you!

As you can see from the original post, I was dealing with high fuel consumption (11 MPG) and very rich fuel mixture. No matter what I did (at the direction of an extremely helpful and knowledgeable Moderator) the duty cycle/on-off ratio would not change and was running between 75/80%. I won’t go into all the details because if I were to post all of the things that were tested, retested, etc. this post would be a small book.

After everything that was checked, replaced (including a new brain see attachment), and rechecked, it appeared that I still had a large vacuum leak. After several weeks of coming home after work and spending hours each night trying to find the leak, I had reached a new level of frustration. Out of desperation I drove the car to a shop 50 miles away that had a smoke machine. After $100.00 for the smoke test the leak was finally found! The problem was well hidden below the Mass Air Flow Sensor and Fuel Distributor (see attached PDF item #14) and caused by what the CD refers to as the Air Guide Housing. As you can see in the picture (attached) there are no less than 6 large cracks with the largest being approximately 2 ½ inches long. There is no way to see the cracks without the smoke machine. After questioning the previous owner, I feel these cracks have been in existence for well over 15 years! Keep in mind that this car was inspected before purchase, and also transported the car on a trailer to a specialist in classic Mercedes. The second person did find injectors that were leaking but no one has ever found the cracks in the Air Housing Guide. I now have what I believe to be normal vacuum, and I still need to replace the Accumulator and get my Duty Cycle adjusted but it appears that the cracks in the Air Guide Housing were creating the high fuel mixture and poor mileage. There may still be minor issues to deal with but I wanted to report back on the progress.

If you find this issue with your car it may be helpful to note that the first replacement Air Guide Housing did not properly fit into the newly designed plate. I had to remove the new Air Guide Housing and return it. The second new Air Guide Housing was not much better. I ended up placing it into the plate and applying clamps for a week before reinstalling it. To avoid doing this job a third time, I applied a small amount of super glue around the forward rounded edge (see photo) of the new plate to insure it sealed properly until it was bolted down.

In closing I wanted to again thank the above referenced Moderator whom I now refer to as the Master! Your kindness, patience, and goodwill will never be forgotten! I also wanted to recognize the fantastic members of this forum that take the time and effort to help out people that they have never even met! I very actively belong to no less than 11 forums pertaining to Heavy Equipment, Fishing, Boating, Mercedes Benz, Snow Removal, Welding and IT. This forum is by far the most helpful! I’m proud to be a member and hope that someday I will be knowledgeable enough to Pay It Forward!

Tim

[GregZ]

I am struggling with some similar problems except my car runs waaay to rich (black smoke rich). No vacuum leaks (fixed all of them, and inspected the air housing seal as well (I had it off to replace the TPS). My WUR is good my injectors are good, my cold start valve is not leaking, so I am going to replace (and overhaul) my fuel distributor next.

My question is though: after you fixed the brain problem (and before you fixed the air guide housing rubber), was your car running much better?

Oh, yes, I am gathering all components for a megasquirt conversion. I really dislike this CIS...

Thanks

[ctaylor738]

What is your system pressure?

What are you seeing for control pressure hot and cold?

Have you used the duty cycle to set your mixture? What are you seeing there?

Is your oxygen sensor OK? A low reading will cause the brain to enrich.

Usually IME, a FD problem causes a miss or roughness rather than a rich mix.

[Strife]

This was a very interesting post. Can anyone with experience say if these cracks are typical at this age, or did something unusual happen to this engine?

Operating at 70MPH, where is the vacuum needle on an 85 380SL? I would say mine is at 5/6th left on flat level road.

[GregZ]

I cannot run (do not want to run) the car long enough for it to reach operating temperature. It runs way too rich (a lot of black smoke). The control pressure is about 3.2 bar, and I get the fuel pressure to raise to the right level within about 10 seconds. The car is idling so the oxygen sensor will not effect the 60-40 preset duty cycle.

[GregZ]

plunger is free as a bird I have not checked the o-ring yet.

I remember reading in the manual that the TPS in idle will switch off the closed loop frequency valve control and it will be set to 60-40. Are you sure that it goes into closed loop after the oil switch turns off? Even if it did, could a malfunctioning brain cause such richness?

Thanks for your help!