The manual shows the 3.0 litre gap to be .042 - .046 and the 4.0 litre gap to be .052 - .056. I just installed Autolite Double Platinum plugs with .044 gap and the engine idles lower, feels like there is less power, and seems to use more fuel. The original Motorcraft single platinum plugs show a huge gap of at least .056 or more, probably due to electrode wear. Could the manual and the parts stores be wrong about the gap on the 3.0 litre ?
The plug gap on my 1999 3.0L (FFV) is 0.044-0.046. The non
FFV 3.0L might be 0.054-0.056.....
Bob, could be that's where a lot of the confusion is coming from. And here's something else to make it even more confusing for everyone: my 1997 Ford Factory Manual says use a 0.042 - 0.046 inch gap for the Vulcan and .054 -.056 for the OHC Duratech! I don't believe the Vulcans in the Rangers are any different than the Vulcan in my Taurus, so why is Ford recommending different gap sizes from one year to the next for the Vulcan?
I contacted my local Ford dealer and asked the same questions to their service department.
They were aware of the conflicting information but could not explain why different model years of the same 3.0 engine would have different plug gaps, nor, could they explain why FFV and non-FFV engines of the same year might have different plug gaps.
They did say that if presented with conflicting information, check the sticker under the hood. The microscopic print on the sticker will give the correct information for that specific engine.
I think it has to do with timing. The larger gap takes longer to build up enough energy to cross; therefore, ignition takes place a bit later. The engine with the larger gap spec is probably timed a little ahead of the smaller gapped engine. Either that or the larger gapped engine has higher cylinder compression.... which also effectively advances timing. I'd also speculate that as an engine ages the compression reduces and, consequently, the timing retards. Therefore a smaller gap should be used for older engines with high miles. ..... I'm just guessing here, but it sounds good.
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