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  #1  
Old 07-12-2007, 09:29 PM
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Location: Niceville, FL
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Stumbling Problem Resolved

Finally about to get to the bottom of what was causing the stumbling on start-up. I replaced the fuel filter, fuel accumulator, injector seals, insulators and o-rings, fuel pressure regulator on the back of the CIS and the idle air circulation hose...all to no avail.

Turns out there were at least two injectors bleeding down. Replaced one on the right side of the engine and the stumbling was reduced by about sixty percent. Still need to replace an injector (maybe two) on the left side of the engine and I expect this to cure the problem.

Replaced a lot of parts trying to run this to ground. All of them, except for the fuel accumulator, had never been replaced. So, after 140,000 miles, it was probably about time. In the process, I might have saved myself some headaches down the road.

In retrospect, I should have replaced the injectors at the time the seals and insulators were replaced. Wouldn't have cost any more in labor beyond the $200.00 that I paid. Just another $250.00 for injectors. I may go ahead and do that anyway.

Hope this helps others with a fairly common problem
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'85 500SL (Euro) - 186,000 w/a complete restoration and engine rebuild at 154,000
'95 C280 - 174,000
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  #2  
Old 07-20-2007, 11:37 AM
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Location: Houston, Texas
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What is involved in replacing injectors?
Is it just unscrewing the fuel line and pressing the isolater and oring in then pushing the injector in?
A few details would be great, as I am considering doing this on my 87 560SL to tackle a rough idle problem....
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  #3  
Old 07-20-2007, 11:08 PM
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I watched the mechanic do most of the job on mine and think I could do it, but it took him 2.5-3 hours to get it done. Plus, if it can be messed up, I stand a pretty good chance of accomplishing that.

There are two decent size hoses that feed into the driver side of the fuel distributor, which have to be removed. Probably best to also go ahead remove the idle air distributor hoses. They are the black hoses that lie along side the valve covers and connect to each of the injector insulators/holders. They will have to be removed in order to get to the holder anyway, so best to disconnect them up front. Depending on the mileage, it's likely they will need replacing, too. There is also a hose that wraps around the fuel distributor that you might consider replacing. You can get the first two from Phil at Fastlane, for about $17.00 each. The one that wraps around is close to $50.00. A word of caution. There is a nylon fitting that mates each of the two long hoses to the wrap around one. I was going to just pop the connectors in and mate the hoses up, but the fitting is about 1/2 inch and it goes into a hole about 1/4 inch. I tried greasing the fitting and hose and heating it in order to get the hose to stretch enough for the fitting to go in. I am just not strong enough to force it in. It's worth what ever the mechanic wants to charge for that part of the job alone.

Then, it involves unscrewing each line from the fuel distributor and injector. Since I replaced everything but the injectors, I could probably just loosen the clamp on the manifold which holds the injector in place enough to pop the old injector and seal out without having to pry the insulator/holder out, too. However, you'll likely want to replace the whole assembly. In that case, the mechanic used a long pair of needle nose pliers with the tip bent at a 90 degree angle to pry the insulator out. He then cleaned the seat with a rag and made sure the manifold was clean so no dirt or grit got into the openings.

In all truthfulness, I'll probably pay him another $200.00 to do the job. Though it seems pretty simple on the surface, it is time consuming and there is plenty of room for a non-mechanic type (me) to mess things up.

Still, it is doable for anyone with a little mechanical skill and a good back.
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'85 500SL (Euro) - 186,000 w/a complete restoration and engine rebuild at 154,000
'95 C280 - 174,000
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  #4  
Old 07-21-2007, 10:18 AM
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Talking

thank you for the detail on the injector replacement.
I will have to study that a bit more to decide if I want to tackle....I guess I can always tow it to my mechanic if progress stops!
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  #5  
Old 09-22-2007, 10:46 AM
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When I made this post over three months ago, I thought the problem was solved. However, not the case--yet.

I've now replaced everything from the fuel pump/filter/accumulator forward--except for the warm-up regulator (WUR) to try and cure the stumbling problem. This includes injectors/holder/o-rings and fuel distributor. The car is running rich, as well as continuing to stumble. Start it and, after a couple minutes, the plugs are fouled horribly. The richness and fouled plugs are probably the cause of the stumbling.

Interestingly, I bought a rebuilt fuel distributor from John at SpecialT Auto. When it was installed, the fuel mixture got even richer than with the old FD. So, the mechanic is considering removing the rebuilt FD and reinstalling the old one. Don't know if John will let me return the FD for refund. So, I might be out $449.00 as a result. John says the problem is most likely in the WUR.

The car hasn't been started for about three weeks now, but has just been sitting in the mechanic's garage. Since the last start a new fuel pump (which I had on hand) has been installed. I picked up a new O2 sensor from Phil (Fastlane) and that, too, will be installed.

Went down and talked to my old Mercedes mechanic at Quality Imports in Fort Walton Beach. He, too, seems to think the problem is the WUR. He also said the car probably needs an O2 sensor, but a bad one would not cause the car to run as rich as it is. However, my current mechanic is balking at installing a rebuilt WUR. Says he doesn't want to throw parts at the problem. That's almost funny at this juncture, since just about everything in the fuel system is new at this point.

In retrospect the best approach would have been to take it to the Mercedes dealership and paid for a diagnosis with a CIS tester. This would have been $99.69 for an hour labor. However, the dealership tried to screw me royally for front-end repairs the last time it was there for an alignment. The front-end man listed $4,300 in repairs that needed done before he could align the car. My current mechanic did the needed repairs for $700.00. Since the front-end guy wasn't going to diagnose the current problem, I should have gone that route.

This has been an expensive lesson and it's not over yet.
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'85 500SL (Euro) - 186,000 w/a complete restoration and engine rebuild at 154,000
'95 C280 - 174,000
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  #6  
Old 09-22-2007, 12:39 PM
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WADR to your mechanic, has he put a gauge on the system and checked the control pressure? This will take about 5 minutes and determine if this is the cause of your rich running. You can get a rebuilt unit from Phil or German Star for $199

Also, normally you have to adjust the mixture after installing a different FD, but I'm sure the tech knows this.

Lastly, you asked in another post about the coolant temp sensor. There is one at the back of the right head, but it only controls idle speed.

Good luck,
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'66 200, '66 230SL, '96 SL500. Sold: '81 380SL, '86 300E, '72 250C, '95 C220, 3 '84 280SL's '90 420SEL, '72 280SE, '73 280C, '78 280SE, '70 280SL, '77 450SL, '85 380SL, '87 560SL, '85 380SL, '72 350SL, '96 S500 Coupe
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  #7  
Old 09-22-2007, 12:46 PM
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Chuck,

The mechanic says he has put a gauge on it and that it checks out good. I've also been there when the car was running and seen him adjust the mixture. It's perplexing, to say the least.

Idle seems to be good, except for the stumbling. Get it up to highway speed and it runs like a champ--even with the rich mixture. Fuel mileage is good as well.

I'll be sure to post the fix--assuming it does get fixed.

Thanks,
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Don
'85 500SL (Euro) - 186,000 w/a complete restoration and engine rebuild at 154,000
'95 C280 - 174,000
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  #8  
Old 09-22-2007, 01:57 PM
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OK. A couple of other things to check, and then I'll shut up.

Be sure the oxygen sensor is working and that the on-off ratio is reasonable, like 45-55%.

Check to see if the pressure dampener is leaking and gas is getting sucked in through the vacuum line

There is one, maybe two vacuum fittings on the WUR. Be sure that both hold vacuum on the WUR side.

Be sure the o-ring on the FD is good and not leaking.
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Chuck Taylor
Falls Church VA
'66 200, '66 230SL, '96 SL500. Sold: '81 380SL, '86 300E, '72 250C, '95 C220, 3 '84 280SL's '90 420SEL, '72 280SE, '73 280C, '78 280SE, '70 280SL, '77 450SL, '85 380SL, '87 560SL, '85 380SL, '72 350SL, '96 S500 Coupe
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  #9  
Old 09-22-2007, 03:41 PM
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Chuck,

By all means, please keep the ideas coming. I'll take them to my mechanic. He's worked on MB, but not exclusively. So, any help is appreciated.

Thanks again!
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Don
'85 500SL (Euro) - 186,000 w/a complete restoration and engine rebuild at 154,000
'95 C280 - 174,000
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  #10  
Old 09-22-2007, 08:24 PM
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Replace the ignition coil!
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  #11  
Old 09-22-2007, 08:28 PM
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Hey, I'm at the point where I'll try most anything. Just happen to have a spare coil. Thanks.
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Don
'85 500SL (Euro) - 186,000 w/a complete restoration and engine rebuild at 154,000
'95 C280 - 174,000
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  #12  
Old 09-22-2007, 09:25 PM
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I had a similar problem with my 1991 300SL and it looked like a fuel problem so I spend a lot of time learning about and troubleshooting the injection system with no luck.

I noticed that the problem would go away for awhile when I replaced the spark plugs. When I asked why the problem would go away with new plugs then return, one of the guys in the forum advised that the coil is bad and not producing enough voltage. When the plugs are new all of the electrode edges are sharp and square which produces a good enough spark at the lower coil voltage, as the plug electrode wears the edges round off and the spark is reduced.

Thus the lower coil voltage and worn plugs causes the problem. I replaced the coil many miles ago and the problem has never returned.

sldan
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  #13  
Old 09-22-2007, 11:05 PM
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I appreciate the detailed info. I replaced the coil two or three years ago and it turned out the problem was something else. In spite of that, I left the new coil on and kept the old one. I'll put the old one back on and see what results.

Thanks again,
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Don
'85 500SL (Euro) - 186,000 w/a complete restoration and engine rebuild at 154,000
'95 C280 - 174,000
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  #14  
Old 09-23-2007, 11:56 AM
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If this has already been discussed, I apologize. When was the last time you changed your spark plug wires? New ones stopped my slight stumble when cold and greatly improved performance when hot and at highway speeds.

You can have new cap and rotor, plugs, coil, and injectors, but if the wires that get the spark to the motor are breaking down,
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  #15  
Old 09-23-2007, 03:12 PM
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Mike,

You make a good point about the plug wires. However, mine were replaced less than a year ago. Doesn't mean they can't be bad, but it's not likely. The main problem at the moment, though I suppose it could be two if the plug wires are bad, is the rich fuel condition. Carbon build up on them after just a couple minute running is tremendous.

I use plug pliers when removing the wires from the plug, but the mechanic just yanks them off. As many times as they have been pulled and the plugs cleaned, they might well need replaced before all is said and done.
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'85 500SL (Euro) - 186,000 w/a complete restoration and engine rebuild at 154,000
'95 C280 - 174,000
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