JDWilson

My 1990 MB 500SL starts perfectly when it is dead cold but once the engine has warmed up and you turn the engine off and let it sit for more than 10-15 minutes, the car is difficult to start. The starter spins but the engine doesn't want to ignite for 6-8 seconds. I have had two MB shops look at the car and neither can find the source of the problem. Fuel line is holding pressure, distributors are almost new, spark plugs and wires have been replaced. Coils have been tested. Car has a rebuilt fuel distributor, a new accumulator and other new parts. It will do this for hours after the engine has been turned off until the engine has totally cooled down and then, it will start perfectly (when dead cold; like it does in the morning after it's sat all night). I'm very frustrated because I've never had an automobile (or motorcycle) that has exhibited this behavior. One of the mechanics who checked it out (30 years experience as a MB mechanic) told me that he's seen this before with some other MB of this vintage and that I should just accept this as the way the car is going to behave. I have a problem with believing that statement. Am I wrong? Has anyone else experienced this problem that supposedly can't be fixed?

Pazo

Your description points to faulty CPS (crank position sensor). If it has not been replaced/checked, an new one probably solves the issue. However, since it is a rather common fault, your "two MB shops" should have spotted it - if(?) it is the CPS.

JDWilson

Pazo, thank you for the reply but that has been checked and is OK.

Kebowers47

Hard warm starting ?? The problem of difficult (6-8 sec. cranking) when heat soaking is either fuel or ignition related. A fuel problem can be either a 'vapor-lock' issue caused by a loss of system pressure on standing (faulty check valve at the pump outlet), or, fuel leaking past the fuel distributor or fuel injector (s) A pressure leak can be easily confirmed by temporarily installing a pressure gauge in the fuel line by the distributor and monitoring for pressure loss vs time.
An ignition problem likewise can be detected by testing for ignition sparking when cranking when cold and hot. A crank position sensor will remain dead when hot--not return to life after 6-8 seconds of cranking.

JDWilson

KEbowers47: Thank you for your reply. Unfortunately, those items have been checked by both mechanics (2 different shops). Also, within the last 60 days, a rebuilt fuel distributor was installed along with some other new parts. Both mechanics say that the line has been tested and is holding (I think they said, 50 lbs of pressure) with no problem. The ignition system was also tested by both shops and deemed okay.
This problem has stumped two, very knowledgeable, long-term MB mechanics but they're obviously missing something because if the car starts perfectly (like a new car) when it's dead cold and then the problem only starts when the engine is warm/hot, they're missing something. It's far beyond my skill set to find and there's only one other independent MB shop in the city (+ the MB dealer). I'm reticent to take it to another shop because all they're going to do is test the same things that have already been tested and then ask for more money for not fixing anything. It's frustrating.

JDWilson

I spoke with another MB mechanic who thinks that the fuel pump may be going bad. His opinion is that the pump gets hot after running the car and that when you attempt to start the car while the engine (& pump) are still warm/hot, an internal seal in the fuel pump may be leaking and after cranking for 6-8 seconds, it's finally pushing enough gas through to the engine so it will start. I have zero experience with working on fuel injected engines but he thinks that it's highly possible that this is the cause of the problem since everything else has checked out to be okay. The bad news is that his shop (& the other MB mechanics) in the city are closing today for a couple of weeks for the Christmas/New Year holiday season so I won't be able to find anyone to replace the fuel pump for several weeks. At least the car DOES start when warm/hot; it just takes 6-8 seconds of cranking to get it to ignite.

DMagee8030

JDWilson - I'm having the same problem with my 91 420 SEL. Please post the solution once you find it.

JDWilson

Will do. His shop will reopen next Monday and I have an appointment to drop off the car so he can install the new pumps (my car has two of them) and test it for a couple of days to make sure that he's nailed the cause. He told me that he's seen this before so he's pretty sure that this is the cure. I'll post once I get the car back.

JDWilson

My 1990 500SL has been at the shop since Monday. The mechanic was pretty sure that the problem was being caused by one (or both) of the fuel pumps but he replaced both and the fuel filter but it still doesn't want to start when the engine has warmed up and you let it sit for10-15 minutes (or more).
He called a couple of hours ago and wants to keep the car longer to see if he can nail down the source of the problem. He says that the line holds pressure (as it should) until you attempt to start it when the engine is hot and that's when the pressure drops enough to cause it NOT to ignite until you crank the car for 6-8 seconds.

Shadedmoon

These kind of car issues is very frustrating. Going for casual trips and having to take the taxi back home and strand the pearl at a darkening parking lot is no small issue.

This has happened to me on other car brands. Does your car have a temperature sensor for the coolant that helps control fuel mixture when cold vs warm? A cold engine as you may know need more fueö than a warm one. A failty sensor can signal that the cold engine amount of fuel ahould be delivered to the engine when this will instead make the car hard to start or not start at all.

Is there any relay box that is controlling voltage/current to fuel pumps and injectors? This can be an issue too. Happened to my SLK of 2000. Bad relay or in my case bad soldering joints made the car impossible to start when warm. It also could die while driving at slower speeds.

Good luck, I sense you will get it sorted. Please keep us updated, cheers!

JDWilson

Shaded Moon; Thanks for the reply. I just picked up the car from the shop because he can't nail down the problem and at this point, I'm pouring money into a big hole.
All of those things that you mentioned have been tested and deemed to be working properly. If I only turn the engine off for 10 minutes or less and then start it again, it fires up perfectly; but, longer than that is when it doesn't want to ignite. The starter just turns, it will start after 6-8 seconds of turning the starter and I will smell occasionally smell a little gas (but, not every time this happens) The car never stalls or fails to start; it just is hard to start after the engine is warm or hot.
I only drive this car about three times a week to run errands, go to a Dr./Dentist appointment, etc so I don't do a lot of driving to one place, leaving the car for more than 10 minutes and then attempting to start it again; so, I've decided that I'm just going to live with it the way it is. Other than this problem, the car runs and drives perfectly with zero other problems The previous owner started a huge restoration on the car for several years and poured over $34k into it (I have all of the receipts) before losing interest and trading it to a car dealer for a 450 SL that he also needed restoration. I knew that I would need to spend some money when I purchased it for 7k to get it where I wanted it and have now put around 7k more so I'm now into it for $14k but just about everything has been rebuilt or replaced on the car so it drives like a new car (other than this hot starting problem).
I'm not going to pay any more MB mechanics to experiment and run the identical tests that have already been run by three different shops. If I could find an expert who KNOWS how to actually fix the problem, I would gladly pay them to do so but no more "testing" when each mechanic that I've taken it to runs the same tests and tells me that they can't find the source of the problem; and then hands me another invoice for doing absolutely nothing.
*Of the total $$ that I've spent for repairs/parts, I've spent around $700 for "testing and diagnostics). Yet, the problem is still there.
Thanks again for the reply.

Shadedmoon

Good the car always starts at least. Sounds like the "proper" mechanics have evaded this endevour so far although I know these are hard to come by these days. I've found two in 20 years.

Not too shabby to have a classic driving like new. If things would change there are some good ideas how to start search for solutions in this thread. The closest to these symptoms I have had are bad temp sensors on two different cars. Of course it could be a combination of small faults like slightly leaking injectors etc.

I dont knlw how these cars behave when stomping the gas pedal at startup. If this could add some extra air and make the life a bit easier on the starter and battery. Just a thought.

Enjoy the weekend. :-)

JDWilson

All fuel injectors were replaced about a month ago. Temp sensor was checked and found to be okay but we replaced it anyway since it looked original. That was also done about two months ago. Rebuilt fuel distributor, accumulators, etc. He thought that it highly likely to be a fault in one of the two fuel pumps but we replaced them on this visit; not the problem.

Shadedmoon

Okey so most componenta have been replaced. Sounds like methododic fault search monitoring values from the car comparing cold and warm start to help find any remaining issue. I guess engine compression and leak down test has been made and rhat lambda sensors if fitted have been diagnozed.

Sorry can't help more.

JDWilson

ShadedMoon: Yes, everything has been checked by the MB mechanics. One of the mechanics has been working exclusively on MB automobiles for over 30 years and he says that he's had several cars with this identical problem and has never found the solution. He has a friend who is also a long-term MB mechanic in another city who told him that he has also seen this problem.
It seems to me that if the car was designed to work a certain way, the cause of the problem could be found but since this is a very old fuel injection system that has not been used by manufacturers in a very long time, anyone who designed the system has retired or moved on and cares to spend the time to find the solution. I have also found other threads in various forums re. this identical problem with this fuel injection system.
Unless someone stumbles upon the solution and posts in on the web, the owners of MB cars with this problem (and have replaced all of the components that are known causes for the problem) will just have to live with the problem or get rid of the car. I really enjoy this car and it has had so much mechanical restoration that I will just tolerate this flaw and keep the car until I quit driving (I'm 72) or die and then one of my sons can decide what they want to do with it.

Shadedmoon

So the car has some charm, with quality much better than the newer generations.

Some issues are indeed tricky to solve.

JDWilson

**Problem solved. After much searching and tracing, I discovered that someone (before I purchased the car in Sep of 2020) has changed out the MAS control module to an incorrect model that was made for a 6 cylinder engine. Three independent garages that I took the car to couldn't discover the problem but after I decided that the MAS control module may just possibly be the problem, I discovered that he part number on my MAS module was not the correct one for my car. Long story short; I purchased a used one on Ebay, plugged it in and the problems have totally gone away.

Shadedmoon

That calls for celebrations I guess. Good find!

seismic744

Hi, I'm having the same issue with my 1992 500SL. I've tested the resistance of my crankshaft position sensor and even installed a new one and still am having the hard start issue. Once started, it runs smoothly. I checked my existing MAS module, and it's the 011 545 85 32 version. Wondering which one you had and determined it was for a 6 cylinder model. I've done some searching and I found one (008 545 87 32) that is also supposed to be for my 500SL with the 5.0 V8 engine, but I'm just not sure if I should try swapping out the existing one. Thanks!

JDWilson

As it turns out, I had more than one problem. Problem #1 was the hard starting after the car had been started and ran in the mornings.......and then allowed to sit for more than around 12-15 minutes. *That problem did not completely disappear but I found a workaround that works every time; If the car has sat for more than 15 minutes after being started that day, I turn the starter over for just a second or two and then turn the key back to the zero position. Then, I immediately do the same thing again...........then, on the third time, the car will start within a few seconds, every time on the third attempt. Don't know why but it works. I got tired of throwing money at "experts" who couldn't fix it. **Problem #2; The MAS module. On your MAS module, there will be the letter M, followed by several numbers. My car has a M119 engine (V8) and M119 is printed on the module. Now that I have the correct MAS module, problem #2 completely disappeared. What was happening: When I got to rpm's up to around 5500, the car would stutter & jerk violently. I discovered that my MAS module was for a 6 cylinder model and had a different engine code stamped on the module. I also posted these problems on another forum and received a reply from one of the members that he had seen my post and someone had also installed a 6 cylinder module in his V8 model so this wound up solving his problem.
After installing the correct MAS module in my car, it pulls perfectly all the way to redline. I thought that the MAS module had also solved problem #1 because the hard starting went away after installing the correct module.........but, after a couple of days, the same problem returned and that's when I started trying different methods to get around this problem. (A piece of advice from me re. this subject; if any mechanic tells you that they can solve this problem, ask them to guarantee that they can fix it or you do not have to pay them a dime. If they won't give you that it writing, don't waste your money on paying them to test, test and then run more tests because that's what I did and they never found or fixed the problem).
**My car is a 1990 model and here is the correct code for the MAS module for my car. **Correct MAS Control Module for 90 Mercedes R129 500SL

***Module part number: 0115458532 (OEM)

Engine model #M119 -V8/5.0 liter engine
****I found some info when I was researching this matter and it said that this was the correct MAS module for a 1990-1993 model 500SL so it's most likely also the correct part number for your car but it wouldn't hurt to check with a Mercedes shop and see if they're showing the same number.
Hope this info helps you with your problem.

dolucasi

I would say the proper diagnostic testing for this would be to hook up an EHA current meter an make sure the proper fuel enrichment is being sent during cranking.

I'm fairly confident you will find that the proper enrichment is not happening.

I believe each engine type (from 4 to 6 to 8 cylinders) have a different warm start EHA current spec. Maybe in some cases there is a fault and the enrichment is not happening at all.
First find out what the spec is for an 8 cylinder car. It is most likely much higher than a 2.6L car.

My 2.6L 6 cycl engine requires 60-64mA. And I get that every time I start the car hot. I know because I have installed an EHA current meter on my dash.
Car will start after a fraction of a second of cranking every time, hot or cold.

At a minimum, do a search and find out what the spec is.

Cheers!

Mich1ael

I have a 92 500sl that does this too and my car has less than 10k miles.

Mich1ael

My car did this twice today after a long drive from cape cod to the new London ferry and once before on a short drive. Odd

dolucasi

With such a low mileage car I would worry about issues other than the missing/incorrect fuel enrichment in this post. But you can check it to eliminate that possibility which should have been done way back during the OP's nightmare post years ago and was missed by many mechanics. Easy check really if you can do electrical measurements. Will take you 2 minutes.

But extra low mileage generally means car sat for extended periods which causes other issues in the fueling system.
So once you check out the EHA current mentioned in my post you can dive into other problems diagnosis.

Also make sure the ignition is indeed working with hot start. Again, do not start replacing things. Acquire a $20 strobe light and make sure the ignition is firing as you are cranking away.
My sears strobe light a acquired 40 years ago still works like a charm. It was of course a must with the old style adjustable breaker point ignition, and now a diagnostic tool.

- Cheers!

Mich1ael

My 380sl does this too sometimes. It’s a mercedes thing. What is op post from a while ago?