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  #1  
Old 04-11-2005, 09:54 AM
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w126 Start Problem

I have a 1989 420 SEL with 155k.

About six months ago, I began to have problems when attempting to start the car - this was not dependent on temperature as our weather changed from about 20 degrees F to 70 degrees over the weeks to follow. The engine will not turn over without depressing on the accelerator. After the initial startup, I would need to keep my foot on the accelerator until the idle smoothed out. After that it would run just fine. If the car sat for a while, I would have the same problem again.

A local mechanic checked it over and said that the idle control valve was sticking. For $20.00 he cleaned the valve and everything went back to normal for a while. When it began happening again, I ordered a new valve. Once the old one was replaced, the car started like it should again but only for a short time.

Following the valve replacement, the battery light also came on. I checked the battery which was fine and was able to jump start the car. I checked the alternator and found that the voltage was low (even without any load) and this was confirmed by a local mechanic. I agreed to let him rebuild the alternator and again the car started fine for a short time. Since then, I have had the same problems when starting.

While searching this forum, I have found a number of possible problems. Unfortunately, I am not a mechanic and am not sure where to start or how to eliminate these as possibilities. They are:
1. OVP relay: can pulling it and checking for a 'rattle' determine whether it needs to be replaced?
2. fuel pressure regulator: where is it and how would I check it?
3. coolant temp sensor: I did not see this listed on the materials that I have for parts related to my vehicle...
4. adjusting the fuel mixture

To date the following have been checked or changed recently or with regular maintenance: plugs, wire set, air filter, fuel pump relay, idle control valve

Thank you to anyone able to offer suggestions.

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  #2  
Old 04-11-2005, 11:13 AM
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You did mention that the problem occurs across a fairly broad ambient temp. range.

Do you have a hard start problem when the engine is warm or when it's cold, or perhaps both?
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Last edited by 1991300SEL; 04-11-2005 at 02:25 PM.
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  #3  
Old 04-11-2005, 02:21 PM
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More from cold starts. If I turn the car off after having run it for a while and then attempt to restart it 10 minutes later, it will still struggle to turn over but it won't require as much gas from the accelerator to turn over and smooth out the idle.
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  #4  
Old 04-11-2005, 02:33 PM
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Quote:
Originally Posted by dtrain
it will still struggle to turn over but it won't require as much gas from the accelerator to turn over and smooth out the idle.
Depressing the accelerator pedal does not provide more gas. It provides more air.

If you remove the OVP and it rattles when you shake it(be sure flap is held closed), you need to replace it. There are other ways to test an OVP, but the "shake" test can be useful, especially if you hear rattling.

The fuel pressure reg. is tested with a fuel pressure gauge suitable for CIS injection systems. I'm fairly sure your system is CIS.

Engine coolant temp sensor is tested by measuring resistance.

I'd suspect that maybe your base mixture is off and that's why you're getting some relief by depressing the accelerator pedal.

If you don't understand all of these systems and how to use the equip. required to test, you'll need to pay a tech an hr. or 2 in labor to diag.

You may get this fixed easier/sooner/etc. doing that.
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  #5  
Old 04-11-2005, 10:41 PM
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hard starting 420 after sitting

Working from the apparent 'rich' condition on start-up from prolonged sitting (more than an hour), it sounds like gasoline is leaking into the intake manifold. (A single fuel injector would foul only that cylinder had you would have a miss, but the engine would start normally otherwise.) The leak into the manifold can be either : 1) the cold start injector is leaking, or 2) The fuel distributor diaphgram is cracked and leaking.

You can easily test (1) by removing the cold start injector, plugging the hole in the manifold with something, disconnecting the electrical wire to the injector, and starting the engine. A leaking injector will drip/leak fuel quite obviously. Even a drop every 20 sec will flood out a sitting engine.
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Old 08-24-2005, 05:49 AM
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start problem

I went ahead and allowed a local mechanic to look at the car. Since we are new to the area (Cleveland, OH), I asked around and found that this shop services most of the older mercedes in the area. I described the problem and asked if they could run a fuel check or related test to attempt to determine the problem. The owner told me they would take care of it.

The first time it was in they changed the spark plugs with Bosch copper plugs (I had the Bosch platinum plugs in when I dropped it off) and placed a 'miracle' bottle of injector cleaner in the gas tank. The mechanic told me to drive it for a little while and see if I noticed any improvement - there was no improvement. They did not do anything else.

The second time I took it in, they replaced the distributor cap, rotor, put another 'miracle' bottle of injector cleaner in the gas tank and adjusted the CO mixture. They had the car for a week before I got it back. There was no improvement...

Am I wasting my time with these guys? I stopped by and told the mechanic that the problem still exists and after two visits, the car has shown no improvement. He told me that cars this old have fuel problems like this and that they were attempting to fix itimprove it without having to replace some expensive items like the fuel distributor or a full set of injectors - if they ended up being the culprit. I asked what they would do if I brought it in a third time and he said they would attempt to adjust the mixture again and run a compression test to help determine the problem. Should I be looking somewhere else for a diagnosis?

Thanks in advance for your replies.
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  #7  
Old 08-24-2005, 06:40 AM
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fitters

The main problem today is that a lot of so called mechanics are really fitters, too young and lacking in years of experience. Change whatever the manual says ‘could’ be the problem for a new one, if that doesn't work guess and change something else. A TRUE mechanic should know his cars especially if it's a one make garage like MB, and be able to diagnose within 90% what the problem is without even looking at the car and he should be able to "mend" things too rather than throw expensive parts at the problem, but that was in days of old when you had to earn the title "mechanic" and it had some respect. These days a year at part time school and everyone’s an expert, NOT! Describe a problem with an air-cooled VW and I'll tell you the problem and if possible I can repair or even make the part, but I am nearly 60 and have 40 years at it, that's the difference, sometimes age is a bonus. Find someone who been working on these cars for the past 20 years and you should have it licked! Don’t ask me though, I’m new to CIS and my 16V is dead too.
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  #8  
Old 08-24-2005, 07:57 AM
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Join Date: Apr 2005
Location: Lafayette Indiana
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i have

an 84 500sec and i have had the same problem for all the three years that i have had the car. i have had all the things done that you have and i still have the problem. for me, it has become a rather minor annoyance that i have adjusted my driving to fit since we havent been able to cure it. btw i think it is a problem that is characteristic of many cis cars from when they were relativeley new.
1. always depress the accelerator partway when starting. this was suggested by a former longtime mb mechanic.
2. leave it run if you are going to be only 10 or 15 minutes stopped. this works in lafayette but might not work in a big city. theft.
3. a recent poster replaced the check valve at the fuel pump and solved what sounded like the same problem on a 300e on this website recently so i am going totry that on mine one of these days.

good luck.

tom w
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..I also have a 427 Cobra replica with an aluminum chassis.
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  #9  
Old 09-03-2005, 09:28 PM
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Location: Flushing, New York
Posts: 68
560SEL Won't Start After Timing/Rail Job .

I recently finished changing timing chain and all rails on my 1988 560SEL with
256K . Engine turns but it doesn't start . If I push down accellerator all the
way it starts somewhat with a lot of sputtering and backfiring from within the
engine but dies immediately if I release gas pedal . I'm quite sure that I put
back everything correctly, even timing marks on distributor gear before closing
timing cover . I'm using new battery and car seems to be getting gas . I loosened fuel lines at injectors and main line going into fuel distributor . A moment of Triumph turned into disaster

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