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1993 300e 2.8
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Discussion Starter #1
I’m having an intermittent misfire on cylinders one and six. I verified this using a timing light hooked up to the HT lead coming from the coil pack on number six cylinder. Initially, I found a loose connection on the coil pack (firewall side very loose) and thought that was the culprit. But the problem returned.

Here’s a list of what I’ve done so far. This is all within the past year as I have been sorting out this car. The wire harness is an updated version. My cruise control works. The car starts at 1100 RPM idle and pulls down to 900 within 30 seconds, then about 650 with in a couple minutes.

-Bosch F8DC4 spark plug
  • replaced both fuel pumps with Bosch as well as fuel filter.
  • New coil packs (Bosch, made in Slovenia), new HT leads (Karlyn)
  • Compression test last spring read 165 across all cylinders.
  • Rebuilt lower wire harness (positive terminal)
Like I said, the problem is intermittent. When I turn the car off and back on again it stops misfiring. Could this be my ECU?
I found that this was the issue on other threads that I researched.

I also read that the molded section of wiring that runs in the center of the engine near the coil packs can also go bad. The coil pack leads coming from the wiring harness do seem stiff. Should these have been rewired when the loom was replaced?
 

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2014 G550, 2000 SL500, 1995 E320 Cabriolet, 1980 TR8
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1,326 Posts
It sounds like you have an M104 with HFM, and if so, the molded portion that runs to the coil packs is part of the main harness so it would have been replaced when the harness was replaced. Is the harness truly "new" of was it rebuilt by someone? Does this issue pre-date the replacement of the harness?
 
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1993 300e 2.8
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141 Posts
Discussion Starter #3
Yes sorry. It’s a 1993 M104 2.8 L. No I’ve never had this issue before. I thought that I had a photograph of the Delphi replacement. I think it was 2003 on the sticker under the battery box.
In fact when I replaced all the coils it was just to get a baseline. What I pulled off seemed to have been good coil packs. The car’s performance didn’t change at all with the replacement of coils, HT leads, and plugs. The previous owner was running NGK BKR5EGP plugs.
 

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1995 E320
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2,558 Posts
I kept burning out the coil for the #1/ 6 cylinders. I reckoned it was because of it's location, at the back near the firewall; lots of heat.
It got to the point where I could change out that coil in less than ten minutes. I keep two spare (good, used) coils in my toolbox now.
 

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2014 G550, 2000 SL500, 1995 E320 Cabriolet, 1980 TR8
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1,326 Posts
I would swap the front coil back with the back coil pack and see if the issue changes.
 

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1993 300e 2.8
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141 Posts
Discussion Starter #6
I’ll try to have codes by the end of the weekend (family in town). It happened again yesterday after driving for about 20 minutes and my timing light didn’t help much. Now I’m not sure which is misfiring. Is it possible to test weather the plug is firing under the #6 cyl coil? The HT lead from cyl 6 to cyl 1 was firing consistently. When I turn the car off and back on it goes away but returns fairly quickly.
 

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'95 E300 DIESEL, '91 600SEL, '92 600SEL
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17,495 Posts
Are you sure the coil in question is grounded properly? In other words, is it seated securely on the studs?
 

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W124
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5,232 Posts
You may have been "all over" this already but with all the harness inspections and the initially loose coil connector, have you confirmed the cap covering the harness connectors is in place on all three coil connections? The caps prevent poor connections at the coil. Also, confirm the engine is solidly grounded to the chassis and as Sbaert said, you have the coil solidly mounted.
 

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1993 300e 2.8
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Discussion Starter #9
Thank you everyone. These are all important things for me to check over the next day or so. I thought I had everything pushed down on the studs firm but I will double check (I used a tiny bit of dielectric grease between stud and coils. Is that OK?). I will also double check the ground from engine to chassis. The whole ignition system only has 5000 miles on it tops. I will report back.
 

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'95 E300 DIESEL, '91 600SEL, '92 600SEL
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No grease between the studs and the coils. You want, correction need, pure metal to metal contact. There should be zero play and you should feel/hear something when the coil snaps into place.

And yes do check all ground straps.
 

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2014 G550, 2000 SL500, 1995 E320 Cabriolet, 1980 TR8
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I'm curious why the body of the coil needs to be grounded on a wasted spark ignition system like the M104. The primary circuit ground return is via the ECU and it's switching transistors. On the secondary high voltage side, current flows through the center electrode of one spark plug, across the plug gap, through the cylinder head, across the second plug gap, and returns through the center electrode of the second plug. So other than possible insulation dielectric issues, I can't see why a ground path to the coil body is needed.

But putting that question aside for the moment, in this case I would suggest:

Swap the affected coil with another one in the engine and see if there is a change.​
Swap the spark plugs of the affected cylinders with two other cylinders and see if there is a change.​
Check the ECU connectors to make sure they are securely seated and the pins looks good and are clean. Maybe a little electronic circuit cleaner wouldn't hurt on these.​

With no results from those, I would have to suspect an intermittent issue with the ECU. Not very common, but possible given the age of the electronics on these cars.
 
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1993 300e 2.8
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Discussion Starter #12
So here are the codes from pin 8:

code 23:
Ignition output 2 or ignition coil for cylinders 3 and 4 (so I was wrong about cylinders one and six apparently)

code 36 purge control valve open/short to ground

code 39 EGR switch over valve open/short to ground

I switched coil packs from cylinders 3 and 4 to cylinders 2 and 5 (cleaned up grounding pins on all 3 packs) I left the spark plugs in place to determine whether it’s just a coil pack. I unplugged and plugged back in ECU connections. i’ve never done this before so I didn’t want to pull them completely out. I was able to see the pins under and they looked clean.

The engine ground strap that I found and cleaned came from the power steering pump to the chassis. Other than the transmission to chassis, is there another one from the engine that I’m missing?

Attached is a picture of what all of the plugs look like. I don’t see any plugs that look like a rich burn. This is from cylinder 3.
2639823


The car starts and idles fine but I won’t be able to do a proper run in until tomorrow To see if the problem comes back. to see if the problem comes back. I will report back.

I also wanted to note that I have a new O2 sensor (about a year old).
 

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2014 G550, 2000 SL500, 1995 E320 Cabriolet, 1980 TR8
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1,326 Posts
Interesting assortment of codes. Are you sure your newer harness was installed correctly and/or is not damaged? It will be interesting to see what the coil swap produces.
 

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1993 300e 2.8
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141 Posts
Discussion Starter #15
My OBD 1 pushbutton tool would not let me clear the codes. I held for eight seconds after each code to try to clear them. Hmmm....So I disconnected the battery and I will let it sit for a bit before and see if the codes are still there. The. Take for a 20-40 minute spin and report back.
 

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1993 300e 2.8
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Discussion Starter #16
Interesting assortment of codes. Are you sure your newer harness was installed correctly and/or is not damaged? It will be interesting to see what the coil swap produces.
Good question. Because I am not a mechanic I can’t tell whether it was replaced correctly or not. Attached are some pictures of what the loom looks like. The Delphi date code is January 17, 03.

I just had the battery disconnected for an hour and a half and I still can’t get the codes to clear. My OBD tester won’t clear the codes either when I hold it for eight seconds. I’ll have to put it through a drive cycle and see what happens. I’ll have to put it through a drive cycle and see what happens.
2639886
2639887
2639888
2639889
2639890
 

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Discussion Starter #17
Well, nothing to report as of yet. Not even one hiccup. Been driving around town with aircon blasting, ran a couple errands over an hour and a half. Plus another one hour run. Only thing I haven’t done is take it on the highway for a spell.

Unrelated ( I hope), was a brief time of cold air coming through the center vents and hot coming through the outer vents. This occurred after I did a passing down shift from 30 mph-60 or so. Ran the tac up to 5k. It went away and all the vents / recirculation buttons seem to operate normally.
 

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The vents are vacuum operated. There are vacuum pods for the vent system, a vacuum reservoir to store vacuum, and your engine creates the vacuum when its running. When you stomp on the loud pedal, vacuum drops considerably. This sets up a condition whereby the systems that require vacuum no longer have it. The reservoir's job is to store enough vacuum to buffer the various systems in those instances when the engine vacuum is nil. The reservoir is behind the right (passenger side) fender. You might have someone test it before digging into it. I suspect your vent issue was a momentary issue related to your passing maneuver. A vacuum system working at 100% would not have experienced this.

Back to your original issue, I had asked if the three harness connectors that plug into the three coils still have their round plastic cap on them. Did I miss your response?
 

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1993 300e 2.8
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Discussion Starter #19
The vents are vacuum operated. There are vacuum pods for the vent system, a vacuum reservoir to store vacuum, and your engine creates the vacuum when its running. When you stomp on the loud pedal, vacuum drops considerably. This sets up a condition whereby the systems that require vacuum no longer have it. The reservoir's job is to store enough vacuum to buffer the various systems in those instances when the engine vacuum is nil. The reservoir is behind the right (passenger side) fender. You might have someone test it before digging into it. I suspect your vent issue was a momentary issue related to your passing maneuver. A vacuum system working at 100% would not have experienced this.

Back to your original issue, I had asked if the three harness connectors that plug into the three coils still have their round plastic cap on them. Did I miss your response?
Thanks for the heads up on possible vacuum issues. The plastic caps seem to be intact. I enclosed a photo to be sure. All three look like this.
2640187
 
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