Distributor Cap and Rotor Ignition

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distributor cap and rotor
Distributor Cap and Rotor

This post is for people that still have a distributor cap and rotor ignition like me instead of the more modern ignition systems such as direct ignition or coil on plug type systems. Some vehicles including my 2004 Chevrolet Blazer still use a distributor cap and rotor to fire off their spark plugs.

This type of ignition system can cause car problems like an engine misfire or shudder under load. Caps and rotors have been around for a long time and for the most part they are extremely reliable. There is no shame in having a distributor ignition.

Sure your friends may say your vehicle is old-fashioned as they point and laugh at you while you’re in the parts store buying replacement parts. When this happens remind your friends that your cap and rotor only cost $28 while their individual coil that has failed might cost $200. A direct ignition coil and module assembly could cost around 350 bucks.

Problems with Cap and Rotors

The reason I’m writing this article is that I continue to see a common problem that develops on vehicles that use a distributor cap and rotor ignition systems. This would be moisture and corrosion that can build up inside. Back in the early 1990s the 4.3 Liter engine was extremely susceptible to this type of corrosion.

Bosch distributor cap
Bosch Distributor Cap

Technical service bulletins were created by General Motors to make technicians aware that the vent on the base plate could become clogged and accelerate the moisture buildup and corrosion inside the distributor. Not only would this damage the cap and rotor but it could also affect the ignition module.

The module connections can turn green and even disintegrated if the problem is not addressed. The technical service bulletin informed technicians that they needed to check that the vent screen and the base of the distributor was clean and allowing moisture to be vented out.

Just about every distributor that I checked in the early 1990s was experiencing this problem. When the cap was removed it looked like it snowed inside the distributor. Often it wasn’t until the vehicle was towed in for a no start condition that the problem was identified.

More Problems with Rotors and Distributor Caps

You should also make sure the distributor cap is properly seated on the base plate and all clips and screws are tightened securely to provide a good seal to keep moisture out. It is a good habit to inspect the distributor cap and rotor and check for physical damage and also corrosion build up inside.

Other types of damage that needs to be inspected would be electrical damage from high voltage. You can check for burned metal terminals and also carbon tracking inside the distributor cap. Carbon tracking is the formation of a line of carbonized dust between distributor Terminals or between a terminal and the distributor housing. Carbon tracking indicates that high voltage electricity has found a better path over or through the plastic.

The result can be car problems such as a misfire or shudder under load due to the spark plug firing at the wrong time. You can check the outer Towers and metal terminals for defects. Although distributors are usually well protected sometimes you find cracked plastic that may warrant replacement. You should also inspect the distributor rotor for a buildup of corrosion on the tip that passes voltage to the terminals of the cap.

Although you can take an emery board to the tip of the rotor to clean it I usually recommend replacing it because a distributor rotor is only about six dollars. For more information about solving car problems and the latest posts from this auto repair blog visit the homepage from that last text link. I recently wrote a page on my other website about no starts and no spark. If you have a no start condition and are interested in learning how to test for spark take a visit.

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16 Replies to “Distributor Cap and Rotor Ignition”

  1. Eric Simandl

    Hi, I have a 2000 blazer with a 4.3 that has the moisture in the distibuter cap problem. What I was curious about was that I was told that the Replacement distributers have been redesigned for this truck. So, If I were to replace the distributer with a new delco, the problem would be solved. Is this true? Or am I resigned to replaceing the cap and rotor yearly. Thanks.

  2. Mark

    Eric: I really don’t know the answer to your specific question. BUT I can tell you that the bulletin I did read was about the vent screen in the base plate of the distributor would get clogged and accelerate the moisture build up problem. I have not seen the replacement/ upgraded distributor but I bet it has larger venting ports to promote better airflow and reduce moisture collection.

  3. Mark

    Tommy: Good point and thanks for including the link to the Mercruiser TSB. I liked the part about “explosion arresting ability”. The marine cap is different (I think) then the automotive one. On some years for cars it has a top mounted vent. From what I understand air was originally designed to enter the bottom vent and exit the top. As for the marine version maybe if its sealed real good no moisture gets in? I had a 4.3 mercruiser in a Four Winns and when I pulled the cap off it was corroded badly. I think sealed or vented it’s a problem (My opinion only).

  4. Robert

    I have a 1987 Nissan 200SX, 2.0 L engine. I went to the car on Wednesday night to get something to eat and the engine turned over but would not stay running. What could the problem be? I checked the fuel pump and it seem to be working properly but while under the hood I noticed that the #1 spark plug wire where it goes into the distributor had a lot and I mean a not of corrosion on the spark plug wire and at the #1 spark plug wire slot on the DC. I am thinking that the distributor cap and rotor are bad and that is why the engine will not run. The battery is only about 2 weeks old. Do anyone know what the problem could be, if so let me know “in the comments below”. Thanks

  5. Mark

    Robert: On a 25 year old car the likely causes of a no start condition would make for 1 giant list. Cap and rotor would be on that long list. For someone in your situation I recommend following a no start diagnostic chart in a factory manual. You can get them online from All Data or a similar product. You can get a 1 month subscription for $12.95 using this code in the coupon field (1MOTR2587). The no start diagnostic chart walks you through a logical process so you don’t waste money replacing good parts.

  6. George

    I purchased a new rotor for my vehicle and noticed that the contact is a strip of black coated metal that is riveted to a stainless steel spring strip that presses against the carbon center button inside the distributor cap. Testing with an ohmmeter shows that the black coating on the contact strip is totally non-conductive – infinite ohms. My question is, should the coating at the strip edge be scraped or filed away before rotor installation, or will the spark be enough to jump through the coating and burn it off?

    • Mark Gittelman

      George: Interesting test. I have never taken a meter to a rotor. I can say that I have installed a lot of rotors and know what your talking about when it comes to the coating. I have never had to scrape a rotor before or after installation, so I’m thinking just put it on and give it a try?

  7. javi

    I have a 98 Chevy k1500 in the cold my chevy does not like to start It takes a couple turns . Then she turns over in the hard rain she doesn’t start. I have read some articles and I am starting to believe that it is condensation. Getting on my ignition parts is there anything I can put on them so it doesn’t get condensation on them. When the truck turns it’s strong . But in the dry weather turns over no problem. But in rain or cold it really has trouble turning over. Any Suggestions would be greatly appreciated. Is there some kind of spray to stop condensation?

    • Mark Gittelman

      Javi: The distributor has a vent screen on the base plate and a vent on top of the cap. This is supposed to let some air flow through to prevent condensation. Its common to have the vent screen clog up a little with corrosion and make the problem worse. This is especially true for the 4.3L v6 which GM issued a technical bulletin for. The problem with sprays or dielectric grease is they are only temporary solutions.

  8. Greg

    My dad has a 89 chevy 1/2 ton 4×4 with a 350. I have replaced 15 rotors in the distributor. Finally bought new distributor complete with module cap, rotor and it still burns rotors its a TBI. Help me with this please

    • Mark Gittelman

      Greg: The only time I have seen repeat rotor issues is when performance parts had been added to the ignition system. Things like a super coil or MSD parts. Theoretically a spark plug gap that is to large could also cause a burnt rotor situation. Finally if you are using the made in china standard ignition parts (most common) you might try another supplier.

  9. Jarrod P

    I have a 98 1500 5.7 Vortec and struggled for months to keep it running down the road consistently through all whether. Sunny and 70 it would be awesome! Raining, gray, and cool outside it would run and start like crap or not start at all and it’s like it would surge power. Turn the key on the headlights were dim, left blinker light would be dimly lit,airbag light would come on, the door chime would go off randomly, just really weird symptoms. It would burn a cap and rotor every 2 months and misfired horribly. Well after about 6-8 months of trying to figure out why it would do this even after replacing almost all electrical ignition parts and spending hundreds of dollars and time on the side of the road I’ve finally found the source of what will be probably thousands of guys problems with the infamous Chevy misfire and no start?

    My truck had a sound system put in it which I thought might be causing my problems too but oddly it helped me figure out the problem. I had took the fuse out of the inline block on the power wire to the amp of my sound system due to my what seemed like a surge problem. Thinking that it was just trying to draw to much power and causing my truck to freak out. Well it wasn’t. I was on the side of the road with a truck that wasn’t running right so i removed the fuse on the power wire to eliminate the option of that being the problem. Got it running ok again and then plugged the fuse back in to the power wire and next to me was a big pop and a spark next to the battery on the body of the truck where the bolt for the ground from the battery is grounded. That was the epic wallet saving sign I was waiting for! So naturally as a guy that fixes all his own vehicles I checked it out. The ground was not only loose but GM didn’t do a quality check that day on there work. I found that the ground from the battery to the body of the truck was poorly secured and no paint wasn’t scratched away at all for metal to metal contact. It’s like GM just drove a bolt through the inside of the body and said good enough. Well might be good enough for a little while but not forever. I scratched the paint away very well cleaned it and put a lock washer under the bolt and secured it. Not a problem ever since. Runs great! No misfire, no hard starts, no strange symptoms of lights dimming or coming on when they shouldn’t be. Now granted the moisture I believe is still an issue with these motors this could potentially save someone lots of money and headaches. So check your grounds! Simple very cheap (free) fix to a daunting problem.

  10. Andrew

    I have a omc boat volvo penta 1997 4.3 L engine. I’m told it’s a Chevy s10 engine. Engine was running fine until I completed a low pressure power wash on the engine with a degreaser. It’s a two barrel Holly with a cap, rotor, and ignition control module. I was careful to not get water in the works and I covered up the carb and distributor. Changed the coil and inspected the distributor no build up of carb etc. The spark plugs are in good condition too. And I have notice an orange spark when I completed a test. NGK Plugs. But like I said I replaced the coil. I cleaned the spark plugs with a plumbers torch and reset the gaps to spec. Fuel is fine and pumps works too. All that I haven’t changed was the ignition control module. Is the a bench test for this? Does a faulty ICM produce a poor spark? I don’t think so.

    • Mark Gittelman

      Andrew: Short answer is I don’t know? I think despite the engine being a pure Chevrolet product that the ignition system is modified for marine purposes. I worked on a Mercruiser 4.3L and it had an external module that said Thunderbolt Ignition on it. I’ve never seen the Volvo Penta version. A stock GM ignition module is inside the distributor.

  11. larry weinberg

    Have 97 5.7 in a 1500 4×4. Seems engine gases are reason cap fails. Design flaw. Not large enough to burn off small particles thru upper vent? Bottom line what is solution?

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