Sherlock Ohms

GM Distributor Cap Sets Off Misfire Code

Mark Hicks
Page 1 / 2 Next >
View Comments: Oldest First|Newest First|Threaded View
Page 1/2  >  >>
User Rank
Non-Optimal design. Good enough for "Made-by-Monkeys"
Amclaussen   12/23/2013 10:46:38 AM
Another case for a "Made by Monkeys" blog!

(Thanks GM for keeping us entertained with your engineering blunders. BTW, thanks to ALL of them, as they keep lowering and lowering design practices!).

If the distributor cap is not simetrical (electrically speaking), as is the case with this design, it will always fail at that particular cylinder.  In other words, that Cylinder will always be the weak link in the chain.

The reason for this is the bad design practices resulting from a mediocre lay-out of the engine components inside a too-tight engine compartment.  Would the engine compartment have been designed with proper engineering, it woulndn't have required a flattened, side exitFlat, side exit distributor cap distributor cap to begin with...

But being that present day designs made extremely fast, by people that has very little actual experience (albeit posessing great AutoCad dexterity, that produces extremely compact-but failure prone designs!).

As for the cap and rotor design, OEM not always is the best. In this particular engine, you can replace the rotor and cap with a much better fabricated one (better materials and better overall dielectrical design) made by ACCEL, sold as a "performance" part, Part Number: 130141 "Heavy Duty".

That part will cost you 20 bucks (the garden variety, equivalent to the OEM is about 9.99).  Either you can buy two of the std. ones and change them sooner,or get the performance oriented one and expect twice (or more) service time. Amclaussen.

User Rank
Re: Non-Optimal design. Good enough for "Made-by-Monkeys"
Turbineman   12/23/2013 11:29:09 AM
Ah yes........moisture in the cap.  Back in the early 80's, my neighbor came knocking on my door at 7:00am looking for my help.  He was late for work because his car wouldn't start.  It had been raining all night and still coming down, so I suspected moisture was shorting out the distributor cap.  I pulled the cap and wiped off the interior and exterior, and put it back.  Engine still wouldn't start.  As this Ford Fiesta was approx. 5 years old, I suspected the cap material was no longer as dielectric as when new.  I pulled the cap again, took into my kitchen, and baked it in the oven at 250°F for 10 minutes.  Rushed it across the street before it could cool and put it back on.  Engine started right up.  I told him to pick up a new cap on his way home from work that night.

Ralphy Boy
User Rank
Ralphy Boy   12/26/2013 12:33:27 PM
Thanks for the memories Mark...

I was a VW freak back in the 70s. The list of awesome bugs and buses I owed is now flashing before my eyes. A 65' beetle, and a 60' delivery bus that had spent its first few years as a TV repair shop runner in good old Germany gave me the best street cred that I could afford at the time. I also owned a 60' Karman Ghia, and assorted other beetles.

One thing they all had in common while I owned them was that there were a couple spare distributor caps in the glove box.

Those carbon traces were a constant problem, and since I did a lot of woods cruzin I'm sure it was worse that the average strictly street driver would deal with. Sometimes I could wipe away the dust and the engine would get better for a bit, but usually the traces were forming on hair line cracks in the Bakelite of the cap. I'm guessing running way hot... then trying to float across a huge puddle is not the best way to treat a cheap used car.

Slightly different root cause but very similar in the cap failure issue. I will say that I've not changed a distributor cap in at least 25 years...

You also reminded me of hitting the rotor and cap contacts with some emery to clean up the burning and corrosion for good measure... Ahh... The good ole days.  

William K.
User Rank
Cooling down time?
William K.   12/26/2013 12:37:35 PM
My experience with distributor caps has always been that at engine shutoff they are a bit too hot to hange on to for very long, although they cool fairly rapidly. On my Chrysler products with the slant six they seldom had problems, and somight be replaced once at 60,000 miles on general principles. On my Horizons and Neon they did not ever seem to develop problems. Of course, all of those caps were as close to symetrical as possible.

Plug wires were a different sory, as they usually had to be replaced a bit more frequently, I never did figure out why some failed and some didn't. I did come up with a cheaptrick for checking plugs, which is to use a DVM multimeter to measure resistance between plug top and engine block. Anything less than open circuit meant time to either clean or replace that plug.

User Rank
Another GM experience...
kf2qd   12/26/2013 12:53:17 PM
Years ago we had a Chevy Monza Spyder with the 305 V8. Never did change the sparkplugs on the drivers side, but that's a different story...


Had the High Enewrgy Ignition with the coil as part of the cap. I was in college at the time. married with 2 toddlers so money was very tight. My wife had driven into town and the car just quit. I was able to get a friend to drive me to the car between classes and I figured out te spark was jumping through the rotor to the distributor shaft. So I looked in the car and found a candy wrapper and put it under the cap and fastened it back down. Drove the car for several days until I could connect with one of the instructors from the Automotive Department and got the advice to change the sparkplug cables. Seems that the in center of the cap was an area of low desity because of the design of the mold they were made in. A thin candy wrapper had better dielectric properties that the much thicker plastic in the rotor.


One area where GM did a good job was in the ignition points with the hex socket adjusmant screw. Should have hired more engineers like that one.

bob from maine
User Rank
Re: Memories...
bob from maine   12/30/2013 12:04:49 PM
I've had VWs and Volvos exhibit this problem. I also had a boat with two brand new 307 GM engines with Mallory low-profile distributors that had similar problems. The issue was the engine got hot when running (a good thing) and when shut-off the non-vented crankcase would cool and draw relatively cool air in from the outside through the distributor vent hole and moisture would condense inside the distribtor. Subsequent starting would cause arcing between the coil input and one or more spark-plug outputs inside the cap; this arcing seemed to cause breakdown of the bakelite and leave carbon tracks which would eventually create a permanent leakage path. The boat issue actually would cause the engines to quit during cold weather operation (<40 degrees) and required re-designing and sealing the entire distributors.

User Rank
Re: Another GM experience...
Amclaussen   12/30/2013 3:56:58 PM
Your comment made me remember a demostration at school: I was studying Chemical Engineering around 1974, and at a Physics Lab class the teacher was demonstrating the dielectric properties of different materials with a high voltage generator.  It was very enlightening for me to actually see how badly some common plastics behaved under the high voltages around 100KV that the demo was using. For example, vinyl (PVC) insulation on common jumper wires and alligator clips was completely useless above one or two KV...  and some pieces of "lucite" plastic withstood easily the full output of the test rig. A couple of years later, I built a small scale Electrostatic Precipitator using a two inch diam. copper pipe, about 18" long, with a metal guitar string along the center of the pipe. Under the pipe I placed a plastic cup where they used to sell 8-piece sets of Autolite sparkplugs during those years. That package made of good quality High Density Polyethylene.  That plastic cup withstood the 3 KV easily and served to receive a small duct where I connected a very small centrifugal blower from a miniature vacuum cleaner.  A pair of fellow students kept blowing cigarrete smoke one after the other so to provide an almost continuous stream of an extremely fine dust or smoke flowing into the precipitator.  At the top, a common F14Y Champion sparkplug served as the top insulator, once the bottom electrode was removed.  A strong light from a 100 W bulb illuminated any smoke exiting the apparatus.  Switching On and Off the high voltage to the guitar string provided an electrostatic field strong enough to completely precipitate the cigarrete smoke, chalk dust from a blackboard and every conceivable dust!  This project was reviewed by the same Physics teacher and won his approval.  In that exercise, the HDPE plastic of the Autolite brand sparkplug package withstood many Kilovolts with easy, even at a thickness of less than 1/32"!

User Rank
Re: Cooling down time?
JimT@Future-Product-Innovations   12/30/2013 10:17:17 PM
When I was in High School, the standard tune-up mantra was ALWAYS; "Points, Plugs & Condenser".  If a car really needed help, you would throw in a new distributor cap. That was pretty much all you ever needed to make a V8 scream, unless you got more sophisticated and borrowed a timing-light from the Auto-Shop tool crib.   Condensation and corrosion was pretty much an expected thing you had to deal with.  Plugs & Points were ALWAYS burnt and corroded; it only stands to reason the distributor points would be, too. So I'm not too surprised to head the distributor points had a corrosion bridge.

User Rank
Distributor Cap
RobLewis   1/6/2014 6:19:33 PM
Distributors are so 1990s, don't you think? 

User Rank
Critic   1/7/2014 10:01:59 AM
Thanks, Mark, for an interesting article.

I have learned that a bad PCM/ECM can cause random diagnostic codes.  I had an ECM in an F-150 fail, and various different codes appeared, although there was nothing really wrong (except the computer itself was bad).  The connector to the engine wiring harness protrudes through the firewall of this truck, and a gasket was supposed to protect the computer from splashes.  However, when the computer was installed (carelessly), the gasket got pinched, so the computer got soaked repeatedly and corrosion eventually killed it.  The moral of the story:  sometimes codes are wrong, so verify the problem/complaint first, if possible.

Other ways to find a misfire:

1. Use an engine oscilloscope.  This is probably the fastest and easiest way, but not everyone has a 'scope.

2. Disconnect one spark plug wire at a time and see if it makes a difference in the way the engine runs.  Be careful not to shock yourself, and remember that a test like this can damage a weak component (sometimes a good thing)!

3. Inspect the spark plugs.  One that is wet with fuel or a different color than the others indicates a problem.  An experienced mechanic can determine a lot from looking at the plugs.

I had a Ford engine that would constantly oil foul one spark plug, about every 200 miles.  I knew that the engine was mechanically sound, so it was a mystery why there was so much oil in the one cylinder.  I ultimately discovered that someone had mistakenly installed two PCV valves on the engine, and one was bad, so lots of oil was being sucked into the intake stream.  The geometry of the plenum is such that most of the oil ended up in one cylinder.

Page 1/2  >  >>
Partner Zone
More Blogs from Sherlock Ohms
When your car keeps dying, sometimes you have to sit down to solve the problem.
What should have been an easy microcontroller upgrade opens up a mysterious web of problems.
Sherlock Ohms highlights stories told by engineers who have used their deductive reasoning and technical prowess to troubleshoot and solve the most perplexing engineering mysteries.
Sherlock Ohms highlights stories told by engineers who have used their deductive reasoning and technical prowess to troubleshoot and solve the most perplexing engineering mysteries.
Sherlock Ohms highlights stories told by engineers who have used their deductive reasoning and technical prowess to troubleshoot and solve the most perplexing engineering mysteries.
Quick Poll
The Continuing Education Center offers engineers an entirely new way to get the education they need to formulate next-generation solutions.
Oct 10 - 14, Embedded System Design Techniques™: Getting Started Developing Professional Embedded Software
SEMESTERS: 1  |  2  |  3  |  4  |  5  |  6 |  7 | 8 | 9 | 10

Focus on Fundamentals consists of 45-minute on-line classes that cover a host of technologies. You learn without leaving the comfort of your desk. All classes are taught by subject-matter experts and all are archived. So if you can't attend live, attend at your convenience.
Next Course September 27-29:
Sponsored by 3M
Learn More   |   Login   |   Archived Classes
Twitter Feed
Design News Twitter Feed
Like Us on Facebook

Technology Marketplace

Copyright © 2016 UBM Canon, A UBM company, All rights reserved. Privacy Policy | Terms of Service