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Old 03-22-2004, 11:45 PM
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Join Date: Sep 2001
Location: Former New Jersey resident, transplanted to Atlanta
Posts: 465
97 E320 Autolite spark plugs

According to Autolite's website the recommended OEM plugs for a 1997 E320/M104 is an Autolite Resistor 3924 , I was wondering if anyone here has used these plugs as the archives do not mention them........I also own an 87 300E/M103 and I use Autolite 104 plugs as my valve seals leak a bit.........Compared to ANY Bosch plug that I ever used on my m103, the Auotlite plugs last longer and tend not to foul as easily............I was wondering if the same holds true for the M104?.......Thanks in advance!

Currently Driving
2006 E320 CDI
1999 E300 Turbo Diesl
2002 ML500
1995 E320 Station Wagon

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1997 E320 Assassinated by Pine Tree
1987 300E Wife Killed Engine
1981 300D Stretch Limo Total Loss
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Old 03-23-2004, 12:50 AM
Mike Murrell's Avatar
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Join Date: Feb 2000
Posts: 2,580
Why use a Ford product in a German car?

My guess is the Autolites in your 103 motor run hotter than std. Bosch H9DC0.
Mike Murrell
1991 300-SEL - Model 126
M103 - SOHC
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Old 03-23-2004, 01:00 AM
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Join Date: Mar 2003
Location: Southern California
Posts: 2,225
103 engines call for a non-resistor plug and Bosch Super is the only readily available non-resistor type. Not sure about '97.

I've pulled and tossed 30K mile Bosch H9DC that still looked serviceable. Never had any problem with them.

My experience with Autolite is that their shell plating has poor corrosion resistance, so I don't care for them.

Bosch Super H9DC are not to hard to find for about two bucks each. The 103 OE H9DC0 has a "heavy duty" electrode, but they're tougher to find and more expensive.

Resistor type plugs in 103s seem to increase idle roughness and test emissions.

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Old 03-23-2004, 10:21 AM
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Join Date: Aug 1999
Location: Florida / N.H.
Posts: 8,804
104 ignition system has the resistors built into the plug connector/boot, so... adding a resistor plug only taxes the coil.
The coils on these sytems fire 2 plugs , in series. [waste spark system]. Adding resistor plugs can be a burden on the coil as now the system has to travel through 4 resistors . [ 2 connectors/2 resistor plugs.]
The wasted spark has little draw off the system b/c the piston is not on the compression stroke, but the resistance of the plug does , regardless of which stroke you are on .
This is also something to consider when using larger plug gap than spec .. [ the 2 plug/per coil series circuit means the spark has to jump the total gap of BOTH it is double the distance/gap..]

F8DC4 Bosch Super is the called for non-resistor replacement .

Last edited by Arthur Dalton; 03-23-2004 at 10:33 AM.
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Old 03-23-2004, 10:51 AM
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I tend to believe what Duke is saying. I had my 103 apart a few weeks ago and thought it would be a good time to screw in some new plugs. My wife was going to town and I asked her to pick up a set. She called me from the auto supply and said all they had were motorcraft. I told her to go ahead and get them.

The Bosch that I took out were opened up to about .044 or so, but even with fresh gapped plugs the idle quality got worse.

I need a set of H9DCO's and about fifteen minutes to get it right again.

Have a great day,
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Old 03-23-2004, 05:53 PM
Ali Al-Chalabi's Avatar
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Join Date: Nov 2000
Location: Knoxville, TN
Posts: 1,837
I would stick with the OEM Bosch plugs in the M104 engine. In my 97, the plug is the F8DC4. I change them every 15k miles just because I like to have a look at them and at 15k miles they look near brand new. I am sure they would still be serviceable at the recommended 30k mile service interval.

Ali Al-Chalabi

2001 CLK55
1999 Dodge Ram 2500 Cummins Diesel
2002 Harley-Davidson Fatboy
Merlin Extralight w/ Campy Record
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