We had a 2001 Dodge Caravan with a 3.8 engine and a little over 118,000 miles come into our automotive shop. Intermittently it was hard to start. When it did start, it ran rough, misfired, and sometimes under a load, it would even backfire through the throttle body.
We tested fuel pressure, crank and cam sensor signals and scoped both these sensors, at the same time confirming they were in sync and the timing chain was sound. After some experimentation, we found we could get it to act up by power-braking the engine and running the RPM between 1,500 and 2,000.
Anti-seize compound made a wreck of this 2001 Dodge Caravan.
The most challenging of these symptoms was the backfiring through the throttle body. For this to occur, combustion has to take place in a cylinder while the intake valve is open. This led us to take a close look at the spark plugs. On plug number three, we found a carbon track where the spark was jumping down the side of the plug. For this reason, we decided to pull the plugs for close inspection.
We found excessive amounts of anti-seize compound on the threads. This vehicle has a DIS ignition system, which uses the threads of the spark plugs as an electrical conductor. We later found out the owner of the vehicle installed the plugs six months earlier and used anti-seize compound. We installed new plugs and cleaned the threads in the heads, and the problem was solved.
We were still curious and confused. How could using excessive anti-seize compound cause the engine to backfire through the throttle body? We checked with three different spark plug companies to see what their ideas were on this subject. All three of them recommended against the use of anti-seize compounds.
They told us they already put a tri-valent zinc dichromate plating on the shells of their plugs to prevent the steel threads of the spark plug from seizing in the aluminum of the cylinder head. They also told us the problem with using oil or anti-seize on the plug threads is that it affects the torque setting. With aluminum heads, it is an important aspect of spark plug service these days to use your torque wrench to accurately tighten the plugs. NGK said that the concern is the threaded outer shell of the plug. If you overtighten the plug, there is a danger of stretching or even breaking the threaded portion of the plug.
The Cosworth Vega used a clean sheet of paper engine and was actually a very nice car. It used an aluminum head and block, but the block was conventional using sleeved iron liners. The Block material of the Vega was later vindicated BTW. The reason many of the Vegas smoked turned out to be poor valve guides, but like the fuel injected 327s of earlier days that got converted to carburettors, all new technology is suspect! The no liner block material has been used by several Porsches and Mercedes V-8s.
This is rather far from the original misfire caused by anti-sieze however. :-)
With all manner of electronic devices on modern cars and motorcycles we must be careful about how we service and modify them. A simple ground problem can create a very difficult to find problem.
The Cosworth Vega engine block was made of the high silicon content aluminum, but it did not have the iron liners in the cylinders. There was a special procedure for boring and resurfacing the cylinder walls. Once the bore and hone was done the cylinders were etched to remove a microscopically thin layer of the aluminum to expose the harder silicon for the wear surface. Once you exceed the bore spec, a cast iron liner can be installed and then bored and honed conventionally. A lot of current Cosworth Vega owners have made the conversion to the cast iron liners as the process for the original block is specialized and there are not many, if any, companies or shops who can do it properly. If you want more history of the Cosworth Vega, check out the Cosworth Vega Owner's Association's website at cosworthvega.com.
My Mistake the Renolds 390 block was extended to the "CosVeg." inrerestingly so few of the dealers were ready to etch the block that most repairs were sleeved. I knew 2 people that had them, and they were very rare with less than 4000 sold. Both of those units had been sleeved, probably by previous owners. The little car was great fun to drive, it had a better running gear than the standard Vega along with the Rev happy Cosworth heads. The problem was the price. "One Vega for the price of two!" was a common comment about them. The block did require some beefing up as at least one was reported cracked durring dyno testing.
The high silicon block went on to success in several other cars. Also some motorcycles.
I can see that a deposit of almost anything could lead to a potential backfire if it held enough heat to ignite the incoming mixture towards the end of the intake stroke. The mystery I see is the allegation about the plugs being so special that "they used the threads as a part of the circuit". The alternative woulld be a plug with two terminals, and I have not seen one of those on an engine. Undoubtedly, since the engine does not have a distributor, instead, as a cost savings, there are two plugs in series and the coil center is not grounded. This is a cheap trick that cuts costs and reduces reliability in half. Now if one plug wire falls off two cylinders don't fire, and there is a larger risk of the spark finding an alternate path to ground that becomes permanent.
Of course, steel plugs in aluminum threads is a poor choice, no way around it. Yes, they can be made to work, but even a good band-aid is still a band aid.
My method of avoiding the very real cross thread problem is to turn the plug in by hand until it seats. It is much easier to tell when the threads are not right doing it that way, and for the cost of having even one Heli-Coil installed it is easily worth the effort.
William, that yet another example of the "feels right" or "sounds right" way of telling whether things are right. Much of that is going away as technology solutions proliferate. In plants, diagnostics and prognostics are replacing the "check by listening" strategy for detecting flaws.
I don't know what kind of anti-seize compound has been used by others, but I used a copper-bearing anti-seize compound for the many years that I raced my Quasar D-Sports/Racing car at Road America and other Midwest tracks. I built this power plant, a 750cc Suzuki water-cooled 3-cylinder, 2-stroke that I routinely wound to 9500 rpm. I used three ignition coils (one per spark plug), each of which was driven by its own Hall Effect sensor. This engine had cast iron liners in its aluminum cylinder block and its aluminum head's threaded spark plug holes required anti-seize compound. I never had a plug (Champion and also NGK platinum electrode) fail to come free for service or replacement. I raced this car for years winning the 1982 Midwest Council DSR Championship.
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