Rwiggins951

Ok, this one really has me at wits-end. Any guidance would be greatly appreciated. Whenever my 1990 300SL sits for more than a day, I end up with a nasty missfire/hesitation at mid throttle and a bit at idle. If you keep the throttle mashed, it will eventually clear out and run ok. At full load it is fine. The longer the car sits the worse it is for the first couple days of regular use. I have replaced the fuel filter and the cap/rotor/wires as well as tried another ECM. Same issue. Ignition modules are ridiculously expensive (even modern Ferrari modules are cheaper) so I have not tried that yet. Still I have never seen electronic parts misbehave more as a function of time sitting. I also replaced the coil. Any thoughts?
Thanks in advance!

Rod Wiggins
Cleveland Oh
1990 300SL
1989 190e 2.6 5-speed
1998 Lotus Esprit V8
1969 Porsche 911
2004 Land Rover Discovery II
1957 Triumph TR3 (project)

msmith66

I would always start with these cars by reading the codes out and clearing them before replacing parts. Have you done that?

Rwiggins951

I have actually. I receive an intermittent code for the O2 voltage. I have tried a few O2 sensors and a different ECM. Same code shows up from time to time. Does not seem to correlate to the hesitation though. Does that provide any clues?

Thanks,
Rod

msmith66

Rod, I don't know too much about the Jetronic system but you can learn alot reading here and benzworld. One thing that is simple but very important - check your car to see if it has the old style aluminum fuses. I think there are four fuse blocks in your car. If there are any aluminum fuses, change them out to brass/copper replacements. They are a huge source of problems for the older cars. Also make sure the overvoltage protection relay is seated and making a good connection.

I'll poke around the DVD tomorrow and see if anything jumps out at me. What was the flash code you read out? Since you had an ECU to swap out do you also have access to a fuel distributor you can swap?

I'm not sure how much adaptation the KE system does but its possible you have some intermittent electrical problems that are causing the system to over adjust then gradually find its way back. Seems like O2 sensors or more specifically the voltage supply to them might cause exactly that problem.

edcarls

I had the same experience on a Euro 500SEC. The car runs rough the first time I use it after a prolong period of non-operation. The cause was a faulty (leaking) fuel pressure regulator.

I hope your problem is as simple.

Rwiggins951

Thanks for the insights. I thought about the potential of the ECM adjusting due to faulty readings, but I am able to measure in a meter when the O2 sensor is acting up and it does not correlate the the hesitation issues. Also, there is such a direct correlation to time sitting that I have trouble connecting this with an electrical issue. I must also say that the hesitation is more of a distinct misfire on a few cylinders than something that feels 'saggy' like a overly rich or lean mixture. I still am looking at a fuel issue because of how bad it gets over time. The fuel pressure regulator is a great suggestion. I would just buy a new one, but I see that they are $400-500 for this car, so I will try to see if there is evidence of fuel on the wrong side of the diaphragm. The fuel distributor is a good call also. Unfortunately, I do not have access to a spare fuel distributor, but will try the test where I can pull all the injectors and spray them into 6 containers at the same time to compare flow. I just have been very short on time lately for such involved diagnostics.

The other thought is the ignition module, but I hear new ones are a couple thousand dollars and a used one is still expensive and most likely bad also. Still, I have never heard of an electrical component failing more as a function of time spent sitting. Heat or cold certainly, but never time. Has anyone seen an ignition module fail in this manor for these cars??

Another thought for a time-related issue with ignition would be the crank reference sensor. When the engine is at its coldest, I could imagine the air gap could be a few thousanths greater which could cause a loss of signal for a weak or misaligned sensor. Unfortunately I don't have the means to watch the output. Once the car is warmed up the gap could close and it would take a full day or two for the engine to be completely cooled. This is a long shot, but one of the few things that makes any sense. There are so many things that could go wrong and cause the hesitation, that I am trying to use this very consistent time-correlation to hopefully rule out a bunch of options. I see lots of posts about Mercedes missing/hesitating after sitting, but no one seems to have reported when they figured out the solution. Quite a mystery.

Many thanks!!

msmith66

I share your thoughts about the crank position sensor. I thought about mentioning it but I thought that I would never be able to explain it and who would believe me that it might take a couple days to completely cool that part of the engine. The only problem I see with it is that people usually complain of a no-start when hot with a bad CPS.

You should be able to do a leak down test on the fuel system. I'll see if I can find a procedure for it.

Rwiggins951

Thanks for the support on this train of thought.

I ordered a Crank reference/position sensor. Looking through STAR there is a bulletin about the CPS harness in relation to hesitation issues and relative to engine harness replacement. Apparently they had some corrosion and moisture issues with the CPS harness that caused drivability issues including missfire. Also interesting was a note that mentioned that peak voltage produced by the CPS was 1.5 volts at cranking and rising to over 3 volts above ~3000rpm. This is consistent with my symptoms because the car will miss at idle and on as it is accelerating, but will clear up at higher engine speeds. After sitting for a very long period, I have also had some trouble starting the car. This was another troubling part of this issue. Most fueling or weak ignition issues will only be worse under full load/demand. However, for my car, once you push through the hesitation the car can make and maintain full power. I suppose this is consistent with the CPS, and not much else.

Also of note is that it looks like the transmission has been out of the car. I was concerned that whoever did the work had not reset the sensor gap correctly. However, this gap is not adjustable on the 104 the way it is on some cars.

I don't think my sensor has failed in the normal 'shorted winding' mode that causes the hot no start. I think it is more excessive resistance that is limiting the amount of voltage that is induced(inducted?) with the additional clearance of a cold engine. Once rpms rise and/or the gap is closed the sensor produces enough voltage to provide a solid signal to the ignition module.

...That is the theory at least. We should know by next weekend.

I guess I could monitor this if I had an oscilloscope.

Some people do crossword puzzles or soduku. We work on the constant riddles presented by these cars. ugh.

Regards,
Rod

Rwiggins951

So the new crank position sensor arrived. I was eager to see if this would solve my problems so I went out after dinner to do what I figured would be a simple install. Notsomuch. In addition to being a tad challenging to get to, albeit not terrible, the old sensor seems to be completely seized to the bellhousing. After fighting with it for about 90 minutes, removing various items in my way and pulling out every tool and trick I had for stubborn sensors I finally ended up breaking off the sensor head from the rest of the unit. Needless to say, I was frustrated. A few hours later and even more tools I have yet to get the remnants of the old sensor out. I have run out of time and curseword medleys this evening. Looks like I will either have to remove the intake manifold and oil filter housing or the transmission. Both options are rather frustrating given this should have been a 20 minute job. If I remove the trans and flexplate I should be able to drive the unit out from below. If I remove the intake and filter housing, I still have to use a drill and some combination of easy-outs, luck, and fiddling to maybe get lucky. In the interim, the car now does not run at all. Not what one would call progress. Typical for my experience with this car though.

Rod.
Trying to find my happy place.

Rwiggins951

The old crank sensor is out. Used a torch to burn out the plastic, then pliers to pull the hardened pin down the center. With the pin out, I could use a right angle drill to drill out the guts and then threaded a 14mm tap down into the remaining metal. It was very seized, but I was able to twist it out.

That is the good news. The bad news is that it did not change my misfire/hesitation issue in the least.

Running the car without the filter housing in place, I noticed a fuel leak at one of the main rubber lines to the distributor. I added a clamp and was very hopeful that I had found the issue. Nope. Same problem.

Incidentally, I disconnected the wire to the fuel distributor (EHV?) and there was no difference in the symptom.

Car is getting worse. Patience is low. Anyone solved this with their car?

It feels like a missfire with bad wires when they get wet, but the wires are fairly new. Second set in 1.5 years, and they do not show arcing in the dark when misted with water.

What does ignition module failure look like? Is it partial like this, or just a complete loss of spark?

Please help!
Rod

Rwiggins951

I tried wires and plugs today, in case the ones that I put in 6 months ago were defective. I have seen this happen. But alas, this made no difference. I cleaned the contacts on the cap and rotor also. Nada.

The time it takes to start having the issue is getting shorter, and the time it takes for the car to warm up enough for the issue to go away is getting longer. Also, when before it felt like it was running on 5 cylinders when attempting to pull away from a light, now it feels more like 3 cylinders. Not really going to be drivable much longer. At least when it gets really bad, it should get easier to diagnose. It runs pretty well at idle. Sometimes a slight miss, sometimes perfect at idle, but down to 3 cylinders when any load is applied.

I am understanding why Elvis shot his car. Although I think it was a pantera which I guarantee never would be even 1/8th as difficult to sort as this 300SL.

Next step is to pull all six injectors out and spray them into 6 identical recepticals. It just so happens that this car makes me want to drink, so I might have an idea what I can use to collect the fuel....

This will help confirm that the issue is not with the fuel distributor or injectors.

Still, the issue feels like it is ignition related. All that is left is the ignition module, some wiring to the module, and the flexplate. Crank Position Sensor should have been the fix dammit.

Rod

Rwiggins951

I read a post about an issue with distributor caps on the M104 engines. Supposedly, they can crack or have issues with the terminals that can cause a misfire.

I replaced mine with a new Bosch unit along with a new rotor.

Symptom is unchanged.

Now shopping for a good used ignition module. Dammit.

Rod

Rwiggins951

I installed the used ignition module. This was a very clean looking part. Supposedly came off of a well maintained car that was hit. The symptoms are unchanged. Still fine at idle and free rev, but with a distinct miss under load. Now down to as few as three cylinders. Miserable.

So far I have replaced every component of the ignition system at least once. Cap, rotor, wires, plugs (multiple times), coil, crank sensor, ignition module, engine ECM.

The next steps are to inspect the flex plate for anything that could be causing a problem for the crank position signal, and then to find the pin outs for the ignition module and confirm voltage, resistance, and continuity for the mess of wires that feeds the ignition module. I am at a loss.

Rod

Rwiggins951

So I put in a fresh set of cheap sparkplugs and the car ran perfectly for nearly a half hour. All conditions on my normal drive in. The first time it has ran this 'miss-free' in months. The old plugs look clean, but I will examine them closer once I get home. By the time I got to work, the car was starting to misfire just a bit.

Does this indicate a headgasket issue? It does use a bit of coolant, although I honestly have not checked the reservoir in a while. Also, the car had some cooling issues when I bought it (along with this hesitation) so head gasket is not hard to imagine.

I have a CO test kit. May be time to check. Although if it is just causing a miss (not sure why it would, but I hear missfire listed as symptoms of a head gasket issue) I suppose I may not see enough CO to show up on the test.

Thoughts??

Thanks,
Rod

Rwiggins951

Ok, so I tried a new set of injectors (including seals, holders, holder o-rings) and the car ran very nicely for about 30 minutes then started with the hesitation again. I have put a couple sets of plugs in it and tried some more types of injector cleaner. Sometimes it is no change. Sometimes I can make it half way to work before it acts up. It has been getting worse though. The car can cruise easily at 90+ but sometimes has stumbled so terribly when leaving a stop sign, for example, that it is dangerous.

I have also checked voltage at critical points of the ignition control (per the Star manual) as well as resistance to ground. I have had very little time to devote to diagnostics on this. It is also difficult because it does not act up at idle or free rev. Only under load at lower RPMs.

So we know my misfire is not due to the following replaced items:
Plugs
Wires
Cap
Rotor
Coil
Ignition Module
Engine ECM
Fuel Filter
Injectors

This car is crushing my very soul.

Rod.

tusabes

have you tried taking the car to a highly rated shop for some diagnostic time/ they may find something overlooked.

I am eagerly following this thread but unfortunately have no tips to add, i am thinking about buying a 300sl but this concerns me

Rwiggins951

The idea of a highly rated shop is a good one. The challenge is that the car only had the problem intermittently under partial load. I was a tech myself for many years, but I did not specialize in Mercedes. So I have most of the tools and decent diagnostic expertise (granted with little time to fully apply it to this problem) I was hoping this forum would provide a broader bit of experience specific to this car, than might even a Mercedes specialist in town. A friend of mine had a R129 500SL with similar symptoms and spent over $3000 in parts and labor at a very highly regarded independent shop nearby. They were still unable to solve the problem. For all of my frustration, I still have not spent that much.

Besides, that is part of the challenge, right? Perhaps it is ego, but I don't know that their techs are any more cleaver....the hope is that they have already solved the problem before on someone else's diagnostic dime, and that I can just take advantage of that. Better is to find the information in a community like this...

It turns out, that the real answer came in a Mercedes club article....

msmith66

So, are you going to let us in on the secret? Or, at least give us a link?

Jim Sharpwasakr

I have a similar problem with my 1999 SL500.
If the car has been sitting for a day or 2 it starts with a terrible miss. It acts like it is running on 4 cylinders and the check engine light comes on. It clears partially by shutting the engine down and restarting it 2 times or so but the check engine light stays on. On the next shut down and restart the check engine light usually goes off.
Any help wil be appreciated.

Rwiggins951

Sorry for the 'cliff hanger" I have been out of town. You saw all of the parts that I had replaced in an effort to solve an issue that has been with the car since I purchased it about a year and a half ago. All the while I said it felt like a spark issue, much like a bad wire. Despite my intuition, I started replacing fuel system parts because I thought I had replaced all possible ignition components. I see many many folks posting similar complaints about their Mercedes of this era, but yet there were no solutions. I finally got to where my extensive internet searches were starting to return my own posts with pleas for help.

Well. I finally found the true cause. Thank goodness I joined the Mercedes club. Right there in the September-October edition of THE STAR was a tech article entitled "Ignition Misfiring on 1990s engines". Sure enough he was dealing with exactly my problem on exactly my car, a 1990 300sl. Interestingly enough, he says this issue also affects the M103 and M119 engines in addition to my M140.

It turns out that the plastic/ceramic insulator disk that rides behind the ignition rotor can and does actually go bad over time. I had looked at mine, but did not see anything obvious, and could not imagine that it could ever go bad. Further, I never saw it offered as an ignition replacement part offered by aftermarket parts dealers. If I had, I am sure I would have purchased it.

The article shows photos of this disk and of centimeter long lines on the back side of the disk that look as though they were drawn with a pencil. These tracks are evidence that the spark has been leaking through this disk and grounding to the head instead of firing the cylinder. From the front, my disk looked fine. However, once I removed the disk, I could clearly see these lines on my disk. In fact, mine were a bit worse than what he showed in his photos.

The part was only available from the dealer. It looks like a small Frisbee made of under a dollars worth of plastic. So naturally, the dealership gets something north of a hundred dollars for it. $125 to be exact.

12 minutes to install, and the car runs better than it ever has. I have driven it every day for a week with no hint of a single miss.

Problem solved. I am guessing lots of Mercedes' out there are in need of this.

Rwiggins951

P.S. The part number was A 104 158 00 88. The car is still running beautifully. Not a hint of the hesitation.

Regards,
Rod

Wulmserine

I have to pick up on this issues even if the thread is old.

I had the misfiring, stalling issue happening after about 3-5 miles driving (not highway/lower revs).

This here completely solved it for me. I changed spark plugs which let it run smoother. I check the dust cap ( A 104 158 00 88) and it was actually almost broken. It fell apart when I pulled it out.





Old





New

MBs Only Now

I know this is somewhat ancient history for you. I think you've likely helped many people by documenting the issue you had here.

I was hoping you could tell me how many miles in you were with your engine before this issue came up? My 92 300SL has 54k on it, and I just replaced an obviously bad cap and rotor. Other usual suspects have been addressed. I'm still having some of the symptoms you've described. Like yourself, my suppressor/insulator cap looks fine from the inside, but I haven't pulled it to check out the back (working on cars in my parking spot is a bit frowned on in my building, so I've been hesitant lol). My 300CEs m103 has had the original cap back for three times the miles (not to mention a head gasket that has never been an issue). It's interesting that these early distributor cap equipped m104s have this goofy thing occurring. Here in Toronto the Stealer wants $200 for that plastic disk, so I'm likely going to pull the thing to look before buying anyway.

Wulmserine

Mine happened at around 70K. I suspect it depends not only on mileage but also on weather/climate, but I'm not 100% on that one.

MBs Only Now

Thanks for the reply! More now points to it being a possibility. I'm pulling it out today to take a look.