Should I re-gap my spark plugs???? - PeachParts Mercedes-Benz Forum

mike690003 · #1 08-05-2003, 03:34 PM

Hello Board,

I bought a new set of spark plugs today and I wanted to know if it is necessary to re-gap them. The plugs are the Bosch copper H9DC for my 300E. I think their factory gap is .32 . I have read a few posts dealing with people re-gapping their plugs. I have the tool to re-gap them, but I wanted to know the pros and cons of re-gapping spark plugs. Is it necessary? If so, what should I re-gap my plugs to??? .38?? .40??

Help!!!

1991300SEL · #2 08-05-2003, 03:39 PM

I've seen posts by one of the resident professional techs here saying that they go as high as .040 in his shop. Another pro once responded to this saying that .032 is std. factory gap and that was the way to go.

I set mine to .036 and believe I get a slightly smoother idle at this gap.

This is of course is one persons opinion.

mike690003 · #3 08-05-2003, 03:50 PM

My idle is at about 500rpm, but i gets a little bumpy when i am at a traffic light. Will the higher gap fix that??

1991300SEL · #4 08-05-2003, 03:54 PM

Idle speed is affected by MANY things. Spark Plug gap is just one of them.

mike690003 · #5 08-05-2003, 04:37 PM

Quote:

Originally posted by 1991300SEL
Idle speed is affected by MANY things. Spark Plug gap is just one of them.

I have cleaned all electrical connections.

cleaned idle control valve

new cap and rotor

new wires and plugs

new vacuum lines

I think I need to re-gap those plugs. It might fix my idle.

Its worth a shot right???

1991300SEL · #6 08-05-2003, 04:39 PM

Look at the thread below - Steve Brotherton's 1st post in the thread.

There's a mod that might give you an additional 50 rpms. If my memory serves me right, it works on earlier 103's that do not have the fuel pump relay incorporated into the MAS as they do on later versions.

http://www.peachparts.com/shopforum/showthread.php?threadid=22213&highlight=50RPM

mike690003 · #7 08-05-2003, 05:02 PM

Thanks for the link. The problem with my idle is that is sort of coughs, as if it is missing. It will idle smoothly at 500rpm and then it does this brief "cough" every 15-20 seconds. Im lost with this problem. Any suggestions???

1991300SEL · #8 08-05-2003, 05:26 PM

Shot in the dark - you might consider this post.

http://www.peachparts.com/shopforum/showthread.php?threadid=7048&highlight=intermittent+miss

The business end of the spark plug wires on the 103 motors is subject to being easily damaged when removed. I don't care what anyone says, it helps to have the special MB tool used to pull the resistor tips off of the spark plugs.

You need to pull them STRAIGHT off and not rock back and forth. That approach damages them.

I'm not saying this happened, just saying it's a possibility.

Some people also use dielectric grease in order to facilitate future removal. Here's another interesting post that discusses this and plug removal in general. The guy who wrote this post uses an approach to plug removal that I feel is damaging to the interior of the resistor(my opinion). Then again, he mentions that the resistor tips can be a bear to get off if they've been on a long time, so it's sometimes catch-22.

http://www.peachparts.com/shopforum/showthread.php?threadid=69738&highlight=grease

#9 08-05-2003, 05:38 PM

dont know gap spec, but...

i would think 500 idle is below spec, and should be adjusted up to proper idle spec. after proper (or desired) plug gap.

#10 08-05-2003, 07:55 PM

Have you had your injectors cleaned yet?

albert champion · #11 08-05-2003, 09:04 PM

i shall just say this...very few understand spark plugs. they are a very passive device. succinctly a spark plug is a high voltage feed through providing a spark gap with a design-intended rate of thermal transfer.

when you have problems that you think are caused by the spark plug, they could be. so check them out with a VOM or a MEGGER. make sure the insulator doesn't spin in the shell.

but most of the time the cause of the problem is upstream. distributor cap. coil. ignition cables. spark plug boot.

have someone put a scope on the engine. it will tell you what you need to know.

#12 08-06-2003, 09:44 AM

In the old days of points and condensors, etc., hot rodders generally agreed that if you had a starting, idle, or low speed response problem, you wanted to try a wider plug gap (among other things), but if you had a high speed miss, you wanted a narrower gap. Of course, fixing the one problem by means of gap changes often caused the other.

jay3000 · #13 08-06-2003, 02:05 PM

Quote:

Originally posted by ThrillBilly
dont know gap spec, but...

i would think 500 idle is below spec, and should be adjusted up to proper idle spec. after proper (or desired) plug gap.

THere is no adjustment for idle speed..

mike690003 · #14 08-06-2003, 08:13 PM

Well I guess I will have to take my car to Satan (The Dealer). The mechanics I go to do not have OBD I device, so I guess they have no way of telling whats wrong with my car. Time to pay $200 for a diagnostic.

ROGER E. · #15 08-07-2003, 12:44 AM

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It's more than one person's opinion. The shop foreman at my MB dealer told me that they use .036 for a smoother idle.

RLE/Seattle