HOW-TO: Solving M119 Stumbling at Ignition Distributor Caps

[gerryvz]

It is a common malady to nearly all M119 engines with dual-distributor systems, sooner or later, that the engine will suffer stumbling and cutting out. This condition is because there is inadequate air circulation inside the closed environment of the distributor caps, which causes condensation to form inside of the caps, effectively electrically "shorting" the flow of electricity to the spark plugs inside of the cap.

There is MUCH more background about this condition here on the forum, in this thread: FYI - Essential M119 Ignition "Tune Up" mods [Distributor cap vent / slot] | M119 Engine

The air circulation slots that Bosch and Beru cut into the edges of the distributor caps are inadequate and do not allow air to circulate into and out of the cap. This HOW-TO illustrates how to modify the distributor caps so that appropriate slots are cut to allow air to properly circulate. You will find that this modification, performed well, will permanently eliminate this problem.

Difficulty of job: 4 out of 10

Time required: 1-2 hours

Tools required:

• 3/8" drive ratchet and short extension
• 5mm Allen socket, 3/8" drive
• 5mm Allen wrench / hex key
• 3mm Allen wrench / hex key
• Dremel tool with 1.5-inch EZ-406 cutting wheel + EZ-Lock mandrel (more info here)
• Blue Loctite
• Brake cleaner
• WD-40
• Shop rag or shop towels

Lift up the hood into the vertical "service" position. This will provide much more room to work, and make the job easier.

The first step is to remove the left and right air intake tubes (aka "zoom tubes"). Set them aside on the sides of the engine compartment.



Then, remove the center engine cover by lifting it straight up, and set it aside on the edge of the engine compartment.



Then CAREFULLY remove each of the plastic ignition system covers on each side of the front of the engine. Use your fingers to GENTLY release the clip at the bottom of each cover, and then rotate the cover upward and off of the front of each cylinder head. Set the covers aside at the rear sides of the engine bay.



Removing the covers will expose the distributor caps and ignition wires, as shown below.



Carefully pull each ignition wire off of its terminal on the distributor cap. Each of my wires is numbered both with the factory cylinder number label (brownish band) and also a newer, white label that I placed on each wire in 2020.



As you pull each wire off of its terminal, use a Sharpie marker (light color preferred) to mark the number of the terminal where the wire goes. This will help immensely when it comes time to re-install the wires on the caps' terminals. Note the silver Sharpie marker numbers that I wrote on each terminal (under the rubber ignition wire cap) back in 2020. These numbers were still very clear. The "R" in the center denotes the coil wire terminal, as well as "Right" and "L" for "Left" side coil.



From there, take your 5mm Allen wrench, or 5mm 3/8" Allen socket, and gently/slowly loosen each of the three captive screws of each distributor cap, as shown below.



After loosening all three screws, remove the distributor cap from the distributor.



Here is the inside of my passenger side cap, immediately after removal. Note the electrical tracks between the four outside terminals and the main inside terminal, as well as the greenish dusty by-product of random spark distribution inside of the cap.



Repeat the same cap removal operation for the other side.



Here is the inside of the driver's side cap of my E500. Note that it is COMPLETELY PRISTINE inside after 8,000 miles driven.



Side by side shots of the interiors of my two distributor caps:



The nest step is to remove the distributor rotors. These factory/OE/Beru rotors are held to the aluminum rotor carriers behind them with three 3mm captive Allen bolts. NOTE: Bosch aftermarket rotors have a strange 3.5mm size captive bolt, and are VERY VERY easy to strip. Check the forum for information on how to successfully loosen and remove the Bosch bolts.



After loosening the three rotor bolts, carefully remove the rotor from the distributor.



Repeat the operation for the other side's rotor.



The last operation you need to do for removal, is a single 5mm Allen bolt that holds the rotor carrier to the camshaft. Gently remove this bolt and remove the rotor carrier, and the distributor insulator behind it.



Here is a view of the insulator. I had replaced this insulator back in 2020.



And here is the front of the cylinder head cover, behind the insulator. Note that I had replaced the camshaft seal back in 2020, and there was NOT A SINGLE DROP OR SMIDGEN OF OIL OR MOISTURE in this area when the insulator was removed.



Repeat the rotor carrier removal for the other side.



Again, not a single drop of moisture or oil seepage/leakage at the camshaft seal. Great news for both sides.



You can see the moisture on the outside and the inside of the passenger side insulator, corresponding to what I found inside the distributor cap from that side.



More views of the inside of the passenger side distributor cap.



Now, it's time to cut the slots into the distributor caps. Using your Sharpie marker, make some appropriate slot markers where you will grind the new slots into the caps.



Then, put your cap upside down into a bench vise to hold it securely, in preparation for cutting the slots.



Then, carefully cut the new slots. Make sure to make them deep enough so that you go 1-1.5mm into the outer covering of the distributor cap. This will ensure that there is enough airflow into and out of the caps.



After slotting the distributor caps, clean everything off with a rag soaked in brake cleaner. The caps will be dusty from the slotting action.

Then take some WD-40 and spray a light coating on the inside and outside of the insulators, as well as the recess in the front of the cylinder head cover where the insulators fit. This will help displace any moisture, at least until the WD-40 evaporates.

Then, re-install the distributor rotor carriers onto the camshafts.



Then, install the rotors back onto the rotor carriers. I used a small dab of blue Loctite on each of the three rotor screws, on both rotors. Bad picture, but you get the idea.



Then re-install the distributor caps, and re-connect the distributor wires to the caps, paying attention to the numbers on the caps' terminals and the ends of the ignition wires.


Re-install the plastic ignition covers, center engine cover, and the zoom tubes, and you will find that the moisture problem in the distributor caps is eliminated forever !! (At least, until the next time you need to replace the caps).

Enjoy a successful repair.

 

[nocfn]

Great write up, as usual GVZ! Two things that are important to note, if I may, Be darn sure your rotor bracket is properly positioned over the alignment key. If you look at the mating surface you will find the nub on the cam and and the slight cut out on the bracket, One should double check during the install process that the rotor does NOT slip off the nub during the process. The alignment gap can be enlarged over time on the bracket and you do not want it to slip off the worn down alignment key on the cam. Lack of precision can be catastrophic, especially if the rotor bracket is already has an elongated alignment notch from multiple installs.

The second offer is take a small bit of fiberglass insulation perhaps form a spare piece in the attic and stick it over the passenger side cap, then slide the vanity cover over the cap. I have found that has worked since that cap area gets more forced air from the fan than the driver side does. YMMV

 

[gsxr]

Fantastic writeup! Thanks for sharing this, Gerry.

On the few cars that I've performed this modification on, that previously would have intermittent issues, the vent/slot modifications have cured the misfiring 99-100%. As I've mentioned repeatedly, the oil leaks must also be completely eliminated from the cam solenoid and exhaust cam seals. If there's the slightest trace of oil at the lower edge of the cap, the misfiring issue can still pop up occasionally, even with the slotted caps.

Your car appears to be completely dry due to your recent top-end refresh. I suspect many/most people have leaking cam solenoids and are not aware of this. If the cam solenoids are original and have never been disassembled/resealed (or, replaced) as shown here, it's 100% guaranteed they are leaking oil. The leak is very small, but it doesn't take much oil to cause issues two inches away from 20kV arcing. The exhaust cam seal leakage is, IME, far less common.

Photo below of one of my Beru caps after adding the slots.

 

[emerydc8]

Wow. Great HOW-TO as always!

I haven’t done this mod on either car yet due to the dry climate here in Arizona, but lately it has been fairly humid (62% currently) and I noticed condensation on the windshield this morning. Maybe it’s just the power of suggestion but the engine has not been idling as smoothly with the higher humidity. I can see how a more humid climate would adversely affect these caps and rotors. Mine have less than 1000 miles on them and when I installed them in June I even removed the insulators and thoroughly wiped them down with WD-40. I just ordered the Dremel EZ Lock EZ456 from Amazon (thanks for the link @gsxr) and will put this on my TO-DO list. Thanks for the write-up @gerryvz.

 

[gerryvz]

It's an easy "weeknight after work" type of job. The difference is night and day.

 

[emerydc8]

It's an easy "weeknight after work" type of job. The difference is night and day.

I ran into an obstacle right out of the gate. Am I missing a special attachment for this Dremel? Maybe a special drill chuck? The hole in this EZ-lock is too large for the existing chuck.

 

[gsxr]

I ran into an obstacle right out of the gate. Am I missing a special attachment for this Dremel? Maybe a special drill chuck? The hole in this EZ-lock is too large for the existing chuck.

Jon, yes, you need the Dremel EZ402 EZ Lock Mandrel to use the disc. My Dremel kit included this mandrel:

https://www.amazon.com/Dremel-EZ402-EZ-Lock-Mandrel/dp/B000FBLRWE

 

[emerydc8]

Thanks Dave. Just ordered.

 

[gerryvz]

You can also get them in person if you have an Ace Hardware store near you; they stock them.

I haven't driven my E500 for about the past 2+ weeks, but will be driving it later today to run errands around town. I'll report back as to if there are any stumbling or running issues. Today is a drizzly / rainy day, and the current temperature is 72F and humidity 95%.

 

[gerryvz]

Tools required:

• 3/8" drive ratchet and short extension
• 5mm Allen socket, 3/8" drive
• 5mm Allen wrench / hex key
• 3mm Allen wrench / hex key
• Dremel tool with 1.5-inch EZ-406 cutting wheel + EZ-Lock mandrel (more info here)
• Blue Loctite
• Brake cleaner
• WD-40
• Shop rag or shop towels

Also noted in the list of tools required, per above at the beginning of the HOW-TO.

 

[emerydc8]

I skimmed right over that. Thanks Gerry.

Has it been your experience that letting the car sit in humid conditions without driving it makes it worse than driving it every day? It seems that a lot of the humidity-related distributor problems occur after the car sat for weeks.

 

[gerryvz]

When I had the problem, during the rare times when I drove the car say a day or two apart, I still had the issue. For example, parking the car at the airport and then returning a couple of days later - still had the problem.

There didn't seem to be a time factor as to when the problem was triggered. I'd say possibly it could be a bit more weather-related?

All I know is that I **never once** had the problem when I lived in very very humid Texas, and very very humid summers Maryland, UNTIL I parked the car outside. With the car always parked in the garage, even many many weeks and a few months not driving it, I never had the problem. Then when it was outside (under a car cover), consistently had the problem.

I'll be taking it out on errands in about 2.5 hours, and will report back. Should be interesting. Rain has dried up, but it's currently 74F and 91% humidity outside.

 

[gerryvz]

OK, took the E500 out on a four-errand jaunt, out and about for 90 minutes or so.

Car started right up no issues. Drove 4-5 miles to the first stop. Did my errand, back in 10 minutes to the car. Started right up again and took off to the next errand. About 5-7 minutes headed to the next stop, from a traffic light I got a very brief stutter/hiccup. Then at the next light, it happened again. And then nothing after that. Did the third and fourth stops, zero issues and everything ran 100%.

Overall, the stutter/hesitation was down about 95% from what it was before.

I am 99.9% sure that the reason for the hiccups today was that I only cursorily cleaned out the offending distributor cap, and I did not clean the inside of it or sand it, or otherwise try to remove the electrical traces on the inside.

What I'm going to do is to replace the offending cap (on the passenger side) with a good, used cap that I removed from my 2020 Top-End Refresh and kept. This used cap has no traces or other evidence of any electrical anomalies on the inside of it (note that I still have a brand new set of MB/Beru caps, never used). I will do this quick replacement over the weekend and see if there is any change.

The humidity has lessened today (still very and tangibly humid outside) to around 75%, but temperature has increased to a cluster-indicated 82F outside.

Will report back.

The lesson learned for others, in the future, who do this job and re-use their caps that had problems, is to make some efforts to not only wipe clean the insides, but take some emery paper (not @emerydc8 paper) or very fine sandpaper, and use it on the insides of the caps to remove the electrical traces in the diamond pattern between the center post and the four electrodes on the outer part of the cap inside. Then spray the inside of the cap liberally with WD-40 and then wipe everything clean of any dust.

Again, the problem was almost alleviated just with the modification alone, but not quite. I should have made an effort to clean that bad cap.

 

[emerydc8]

I was considering doing the emery trick to some older terminals but wondered if they are coated with anything (brass?) that might do more damage than good if they were sanded down to clean them up.

 

[gerryvz]

FYI - I'm not talking about sanding the brass terminals themselves (though you certainly can). I'm talking about all the real estate BETWEEN the terminals and the center post, on the inside of the cap (domed area).

 

[DerFuror]

You may be on to something regarding the car sitting outside. My car is kept inside the garage 99.9% of the time when not in use (with the relative humidity being whatever it may be in there). The car usually is covered.
Through all my years of ownership, I have not experienced the stumbling phenomenon. My caps are not slotted.

I hope I didn’t just jinx myself via this statement.

 

[sheward]

I believe so as well. My car that lives in the garage has never had the problem that I can recall. The car that stays outside a lot, covered, exhibits the problem from time to time.

 

[gsxr]

Parking inside a garage should definitely help. However, two of my cars which is always parked inside still had the misfire issue after being parked 1-2 weeks... until I replaced the cam solenoids AND slotted the caps. These are the cars we drag race all summer, in a dry/arid climate where daytime humidity is typically between 10%-30% and temps generally between 75-95°F.

The work was done in early 2024 (details here). Since then, both cars have been 100% free of misfires, for the entire 2024 summer race season, and now the 2025 summer race season. Previously, the one car in particular would misfire every time on the first drive after being parked 1+ weeks.

 

[gerryvz]

OK, took the E500 out on a four-errand jaunt, out and about for 90 minutes or so.

Car started right up no issues. Drove 4-5 miles to the first stop. Did my errand, back in 10 minutes to the car. Started right up again and took off to the next errand. About 5-7 minutes headed to the next stop, from a traffic light I got a very brief stutter/hiccup. Then at the next light, it happened again. And then nothing after that. Did the third and fourth stops, zero issues and everything ran 100%.

Overall, the stutter/hesitation was down about 95% from what it was before.

I am 99.9% sure that the reason for the hiccups today was that I only cursorily cleaned out the offending distributor cap, and I did not clean the inside of it or sand it, or otherwise try to remove the electrical traces on the inside.

What I'm going to do is to replace the offending cap (on the passenger side) with a good, used cap that I removed from my 2020 Top-End Refresh and kept. This used cap has no traces or other evidence of any electrical anomalies on the inside of it (note that I still have a brand new set of MB/Beru caps, never used). I will do this quick replacement over the weekend and see if there is any change.

The humidity has lessened today (still very and tangibly humid outside) to around 75%, but temperature has increased to a cluster-indicated 82F outside.

Will report back.

The lesson learned for others, in the future, who do this job and re-use their caps that had problems, is to make some efforts to not only wipe clean the insides, but take some emery paper (not @emerydc8 paper) or very fine sandpaper, and use it on the insides of the caps to remove the electrical traces in the diamond pattern between the center post and the four electrodes on the outer part of the cap inside. Then spray the inside of the cap liberally with WD-40 and then wipe everything clean of any dust.

Again, the problem was almost alleviated just with the modification alone, but not quite. I should have made an effort to clean that bad cap.

Just as an FYI, I drove the car on Monday the 65 miles to Dulles Airport and back home on Thursday of this week. The car had sat for several weeks without being driven. Pretty much all freeway driving the entire way, with a stop in Bowie, MD on the the way to the airport for a gas fillup.

Zero hiccup or any problem at all. The car ran extremely well in all types of traffic. It was raining solidly the entire trip home.

I have not replaced the cap yet with the electrical short "tracks" on the inside, but will get to this soon. I did see my brake lining lamp flicker a few times, so it looks like I need to replace the front brake pads. Fortunately I have a set of Akebonos on my parts shelf. Yet another job to get to. I also need to replace my original front struts with the new MB pair I have down in my basement.

The custom cat-back exhaust I had installed back in 2004 has also pretty much given up the ghost - some internal rattles in the back can. I will replace that unit with the "Brabus" unit I purchased back in 2017 from @5thscaleracer. It's been waiting for this. Oh well -- 85K miles and 21 years on an exhaust isn't too bad. Looking forward to the Brabus-pipe exhaust from 5SR.

 

[gsxr]

Gerry, what cat-back exhaust do you have installed that has started rattling?

 

[gerryvz]

Gerry, what cat-back exhaust do you have installed that has started rattling?

I had a Walker Dynomax pre-muffler and muffler installed at an exhaust shop back in Portland not too many months after I bought my car. It has three-inch tubing and mandrel bends. I also had them weld on a single, oval pipe in the back, as was the fashion in the early 'oughts with many MB models.

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The past year or so I've noticed a rattling on the interior of the rear muffler, especially at startup.

The system has worked well over the years, no droning and very very small increase in volume over the stock system. A bit deeper/throatier.

I'll probably cut both mufflers open after I remove them and see what they look like on the inside. Being a cat-back system (I also purchased a Flieẞen manifold-back section, with cats, from @TimL some months back), it should be something that I can easily install myself.