Montgomery, AL Part IV - Motor Home Project - Replace Spark Plugs and Wires
Montgomery, AL - Events of Thursday, October 18 to Monday, October 22, 2012
On our last few travel days, we noticed a slight mis-fire in the engine of our motor home. The 8.1 liter GM engine used in our motor home is notorious for burning spark plug wires because of the high heat caused by heavy loads. The internal parts of the engine are cooled by water circulating through the engine and radiator. However, heavy loads can also cause external parts of the engine to run hot. Although there haven't been many reported problems with Tiffin's gas-powered motor homes, many manufacturers do not provide large enough grill openings to allow sufficient air flow around the outside of the engine to dissipate heat. Heat causes the insulation on the spark plug wires to deteriorate over time. The deterioration of the insulation causes electrical leakage, which in turn causes the spark plug to mis-fire.
On our last stop in Chattanooga, Paul climbed under the motor home and took a look at the plug wires, but he didn't see any signs of deterioration. On our drive to Montgomery, the mis-fire seemed to be a little worse, so Paul got more serious about checking things out. This time, he removed several of the spark plug wires and checked the resistance with his volt-ohm meter. One of the wires had resistance that was higher than the others. High resistance is a sign that the wire has deteriorated, so Paul decided to bite the bullet and replace all the wires.
While he had the wires off for testing, Paul also pulled out one spark plug to check it out. The spark plugs are platinum-tipped and are supposed to be good for 100,000 miles, and our motor home only has about 69,000 miles on it. The spark plug Paul removed didn't look that bad, but the electrodes were somewhat worn. Therefore, Paul decided to replace the plugs while he was at it since he was going to have all the wires off anyway.
Unlike the special exhaust gasket Paul had to track down a few weeks ago, the 8.1 liter engine in the motor home uses the same spark plugs and wires as the GM pickup trucks. Paul's online research indicated O'Reilly's Auto Parts a few miles up the road had the wires and plugs for a Chevy pickup in stock. Gotta love the internet. We stopped to pick them up on Thursday on the way to Walmart.
As soon as we got back to the motor home, Paul got to work removing the old plug wires and plugs. If you turn the front wheels of the motor home all the way to the right, you can climb up behind the left front tire and work on that side of the engine. Turn the wheels to the left to work on the right side.
Once you wiggle your way up in there, there is enough room to sit upright or kneel and reach all the plugs without too much difficulty. It is, however, pretty dirty under there with road grime on every surface, black brake dust on the sidewalls of the tires and greasy steering linkage that is hard to avoid leaning against.
After Paul removed all the wires, he did a visual inspection. None of them looked that bad. There was no visible cracking of the insulation and no signs of carbon-tracking that would indicate electrical leakage. Then he checked the resistance to confirm his previous findings. The resistance of two of them was indeed higher than the rest. One was about 3,000 ohms, and one was about 13,000 ohms while the rest were all around 1,100. Paul's research indicated normal resistance for plug wires the length of ours should not exceed about 2,500 ohms. The resistance of the all the new wires was about 500 ohms.
Paul didn't mark where the old wires came from when he took them off, but he thinks the one with the highest resistance was from the #7 cylinder (rear cylinder on the drivers side), which is also the spark plug that is adjacent to the broken exhaust manifold stud that we had repaired a several months ago in Kentucky. It is possible the heat from the exhaust leak at the broken stud caused that wire to deteriorate.
The metal cylinders at the top of the photo above are the heat shields that go over the boots at the ends of the wires where they fit over the spark plugs.
After a repair like this, Paul always likes to take the vehicle for a test drive, but that wasn't practical with the motor home. After replacing the spark plugs and wires, the engine started and ran fine at idle and when revved. Therefore, Paul decided to just keep his fingers crossed and wait until we left Montgomery a few days later to see how the engine ran under load rather than pack everything up for a test drive.
We hung out around the motor home over the weekend and left Monday morning for our next destination of Summerdale, AL. We're happy to report the motor home engine purred like a kitten and the mis-fire in the engine was completely gone. In fact, the engine ran more smoothly and felt stronger than it has all summer. Our gas mileage had also dropped about 6% this summer, so it will be interesting to see if it improves with the new plugs and wires.