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  #1  
Old 12-18-2006, 12:40 PM
Bigpete123's Avatar
300ce
 
Join Date: May 2005
Location: Seattle
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Question Bad to run car w/o air filter on?

Hi,

I'm trying to track down what sounds like electrical arcing under my air box in my 1990 300ce. When I rev the engine, I here the "snap" or "pop" sound of arcing, but it's somewhere under the air box. Would it be bad for me to remove it and run the car in park just for testing?

My car has a slight hesitation when I accellerate from a stop. I think it might be bad wires/rotor/cap or ignition coil, but I really can't tell without removing the air box.

I'm a noob with mechanics, but I'm just trying to do my own work cuz I'm tired of paying $95/hr for my indy to do everything.

Thanks!!!

Peter

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  #2  
Old 12-18-2006, 12:56 PM
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Peter, you can run the engine with the air filter/housing off for a brief period, but you want to avoid driving the car with the air filter/housing off. Dust/dirt into the intake will not do the engine any good.
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Old 12-18-2006, 01:00 PM
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300ce
 
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Great, thanks for your response. I don't feel like replacing all of the ignition wires, coils, etc just on a hunch... especially because they are relatively new.
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Old 12-18-2006, 07:15 PM
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Have you tried looking under the hood when it's dark outside?
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Old 12-18-2006, 11:36 PM
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300ce
 
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yeah, in fact there has never been a better time than now in the Seattle suburbs! I was out at my parents' house where they haven't had power for days. Pitch black. I could see some very very subtle arcing in a few spots. It was erratic. My dad was looking too, and as soon as I pointed out a spot to him, it would go away.

So the next step is to take the air box off and take a look in the dark like that again. I live downtown, so it's impossible to find a spot to do it here!
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Old 12-19-2006, 12:01 AM
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It's a great idea, in fact, doing it for a while will allow the engine to give itself an overbore, with corresponding power gains .

But seriously, DON'T do the above - it's OK for a (very)short time during diagnostics, but on a modern engine don't expect the car to run properly - some cars have sensors built into the AC housing and their faulty input(s) will get the computers completely confused.
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Old 12-19-2006, 11:21 AM
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300ce
 
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cool, thanks. I guess my question then becomes, how do I figure out where this problem is? I don't want to just replace the rotor, cap, wires, and ignition coils when I could just replace the actual cause of the problem. I'm tired of throwing all of my money into this car!!!
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Old 12-19-2006, 04:27 PM
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Location: Dallas
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On my '91 SE with the same engine, I get a "click" sound from the ignition module on the left fenderwell, if I rev the engine in neutral. It clicks just as the revs begin to increase. I assume it's a relay; it has a more mechanical click sound, and it's done it for 15 years with no apparant problems.

But, sure, you can remove the air filter and run the motor in the garage.
Just don't drop the cat down the intake or shovel sand into it.
You can temporarily disconnect the air temp sensor on the filter housing, and the hoses, with no issues at all - no fail codes, etc.
But don't drive it that way, it might suck all that dirt from the underhood pad right into the engine - ouch.

DG
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Old 12-19-2006, 05:28 PM
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ElectroMechanical relays DO produce a spark/arcing when used (sometimes, depending on design)
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Old 12-19-2006, 05:42 PM
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Join Date: Dec 2006
Location: Auburn California
Posts: 127
Old plug wires can get leaky: the high voltage can jump to near by grounds. You may hear it but seeing it is more likely. Try a tarp or two, and try it with the motor warm, should do it more hot. I've never seen a benz relay that you could see into. They are in metal or black plastic cans, so you could rear ticking but you'd never see the internal arcing. And it would be hard for a relay to sound like plug wire leakage. The two sounds are very different.
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Old 12-19-2006, 07:02 PM
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Yeah, I agree - what I hear in the relay is definitely not a spark arc.
This is a mechanical "click", and spark arc is a definite "snap".

If he suspects wires, One thing he might try is grounding each wire solidly one-by-one, and see if anything changes (pull the wire off the plug and stick a screwdriver in it, and ground the screwdriver). If you have a leaky wire, it might stop arcing if you take the plug-gap load off it.
Then try floating each wire open one-by-one to see if it changes.
Might help isolate the offending wire.

DG

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