Replacing distributor cap on GM truck.
Ignition System Repairs:

Replacing The Distributor Cap And Rotor On A 1996 GMC Yukon or Chevy Tahoe/Suburban

May Also Apply To GM Full-Size Pickup Trucks

 

In This Article:

After removing a few parts to reach the distributor area, the spark plug wires are removed from the cap. The distributor cap is removed, then the rotor is removed. A new rotor and cap are installed, and the spark plug wires are replaced.

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Skill Level: 2 (Basic) Time Taken: About 1 Hour

By Bruce W. Maki, Editor

Start:

December 2009: About two months ago, my 1996 GMC Yukon would sometimes refuse to start. It seemed to only happen when the weather was damp. I tested the fuel pressure at the test port, and it was good at 60 PSI. I suspected the ignition system was the culprit because when cranking, the engine would fire just for a split second, as though one or two spark plugs was sometimes getting adequate spark. I wiggled all the spark plug wires where they connected to the distributor cap, but still the engine would not start. Same thing when I wiggled the wire at the ignition coil.

I tried heating up the ignition coil with a heat gun, but it didn't help. When I used a heat gun to warm up the distributor cap, the engine finally started with just a few seconds of cranking. If I used the truck every day, I never had a problem starting it, and it always started when the engine was warm. But... if I went a couple of days without using the vehicle, AND the air was damp, I could NOT get the engine to fire up unless I warmed up the distributor cap with a heat gun. In northern Michigan, in December, this was a real nuisance.

I simply needed to replace the distributor cap and rotor. These parts just wear out after a few years and the high-voltage blast of energy no longer has enough strength to make a good spark that will ignite the gasoline inside each cylinder.

In order to reach the distributor cap, I removed this part of the air ducting. This required a 10mm socket.

This plastic duct part doesn't go anywhere, it's just an empty chamber.

Removing air duct to reach distributor cap, GM truck.

 

Location of distributor at rear of engine, GM truck.

The distributor is the item circled in green. To make access easier, I removed the vacuum hose (red arrow) that ran from the intake manifold to the power brake booster.

This distributor cap was really dirty. I already wiped off much of the dust. The dirt may be a factor in why this truck was hard to start in damp weather... when wet the dirt may be conducting some of the electricity to ground, resulting in poor spark.

Note: Be careful when removing that power brake vacuum hose from the engine. The hose connects to a plastic fitting that connects to the upper intake manifold. This fitting is plastic and has two "ears" that engage the manifold body. When I tried to twist the hose off, one of those plastic ears broke off. The fitting is designed to be removed from the engine by rotating it a quarter-turn.

This picture shows the general area of the distributor on GMC and Chevy trucks (green arrow). The distributor is at the back of the engine, just a few inches in front of the firewall.

On this GMC Yukon, I could not reach the distributor while standing at the side of the truck. I used a small stool to climb up into the engine compartment. I placed some foam gardener's knee pads on the metal structure just in front of the engine. Then I could kneel down and reach the distributor.

General location of distributor in GMC or Chevy full-size trucks.

 

Removing spark plug wires from distributor cap.

I used a silver Sharpie marker to write the cylinder numbers on the ends of the spark plug wires, so I wouldn't mix up the firing order. The cylinder numbers on molded into the cap.

Then I removed the spark plug wires. Instead of pulling the wires off, I used a pair of bent needle-nose pliers to push on the end of the rubber boot in the direction of the red arrow. The wires just popped off.

 

Then I removed the two screws that hold the cap to the distributor. These screws required a Torx T20 screwdriver.

Removing Torx T20 screws from GM distributor cap.

 

Front screw for distributor cap on GM truck.

View of distributor cap screw (red arrow), looking almost straight down.

The front screw was loose, while the rear screw was so tight that I thought I might strip the head. I sprayed some penetrating oil on the rear screw and worked it back-and-forth until it turned smoothly.

Stripping out a Torx head on a distributor cap screw would be a real pain in the ass. Such a problem would require removing the distributor so the screw could be grabbed with vise grips, or the damaged screw might need to be drilled out.

 

Then I lifted the distributor cap straight up to remove it.

Lifting cap from distributor.

 

Inside of old distributor cap. A view of the inside of the old distributor cap. There was a lot of fine dust inside the cap, and a ring of corrosion around the rim. That corrosion felt kinda oily to the touch, which made me wonder if it was a trace of anti-freeze.

 

With another truck, I've had problems with a coolant leak letting traces of anti-freeze into the distributor cap and causing the exact same problem this truck had... difficulty starting in damp weather with a cold engine. The anti-freeze would absorb moisture from the air and turn into tiny droplets of conductive liquid, which would ground out the spark and prevent the vehicle from starting. I could solve the problem by heating the cap with a heat gun or hair dryer, and the truck would start just fine.

 

Close up of the inside of the new distributor cap. Note how clean the terminals are.

Even after just a few thousand miles, the terminals become corroded, but this corrosion doesn't create as much problem as the dirt or tiny cracks on the inside of the cap.

Inside of new distributor cap.

 

Top of GM truck distributor cap with cylinder numbering.

This is the new distributor cap that I used, which is NAPA part RR207SB.

The cylinder numbers are molded into the plastic near each terminal.

In this picture the distributor cap is oriented as you would view it from the front of the truck.

 

I used a Torx T15 screwdriver to remove the two small screws on the rotor, then I just lifted the rotor off the distributor. Removing distributor rotor from GM truck.

 

Old distributor rotor showing wear and corrosion.

The old rotor had become quite corroded at the tip (red arrow).

Some people just clean up the rotor tip with emery cloth, but I prefer to spend the extra ten bucks and buy a new rotor.

 

I installed the new rotor, which came with new screws. Installing new rotor on distributor.

 

Installing new distributor cap, GM truck.

Then I installed the new distributor cap.

However... I had a small problem. The front screw would not tighten. Upon closer inspection, I discovered that the mounting tab (which is plastic) had cracked, allowing the screw to slip.

I applied a dab of medium-strength thread-locker (Loctite) to the screw threads before replacing it. I let the liquid dry for a couple of hours before I tried starting the engine.

 

In hindsight, it would've been smarter to apply a small dab of J-B Quik (or J-B Weld) to hole in the broken screw tab. This would surely hold the screw tight... but it might be a b-i-t-c-h to remove the screw later.

 

Next, I re-connected the spark plug wires. Actually, I took this opportunity to replace the spark plug wires. I had replaced the spark plugs about 6 months earlier. Now I have a completely new ignition system (except for the ignition coil).

Before installing the new spark plug wires, I applied a small dab of silicone dielectric terminal grease to the inside of each boot. This should help prevent corrosion and make removing the plug wires easier next time.

Spark plug wires on GM distributor cap.

 

Re-connecting vacuum hose to engine, GM 5.7 V8.

Then I re-connected the vacuum hose (red arrow) that supplied the power brake booster.

I replaced the piece of air ducting and the repair was complete.

 

More Info:

Tools Used:

  • Basic Mechanic's Tools
  • Sockets: 10mm
  • Torx T20, T15 Screwdrivers
  • Channel-Lock Pliers
  • Needle-Nose Pliers

Materials Used:

  • Distributor Cap: NAPA Part # RR207SB
  • Rotor: NAPA Part # RR256SB
  • Spark Plug Wires: NAPA Part # 3137
  • Penetrating Oil
  • Silicone Dielectric Grease
  • Medium-Strength Thread-Locker

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© Copyright 2009 Maki Media Group LLC

Written January 9, 2009