Hi All,
I may be at risk of repeating a message or thread that has already been passed along. However, having read thru some of the strings I see that there is an ongoing concern over whether any modification to a vehicle will invalidate your warranty. (Deleting the EGR is not included in this discussion as it is mandated in many States by other laws)
I found this article after having read the original text. This provides some good examples for guidance on how this is applied to you and how you can effectively combat dealerships that are trying to scam you...
Here is how the act reads as per the SEMA (Specialty Equipment Market Association) website (Specialty Equipment Market Association ). See also the link at the bottom.

Magnuson-Moss Warranty Act, Title 1, __101-112, 15 U.S.C. __2301 et seq. This act, effective July 4, 1975, is designed to "improve the adequacy of information available to consumers, prevent deception, and improve competition in the marketing of consumer products . . .." The Magnuson-Moss Warranty Act applies only to consumer products, which are defined as "any tangible personal property which is distributed in commerce and which is normally used for personal, family, or household purposes (including any such property intended to be attached to or installed in any real property without regard to whether it is so attached or installed)." Under Section 103 of the Act, if a warrantor sells a consumer product costing more than $15 under written warranty, the writing must state the warranty in readily understandable language as determined by standards set forth by the Federal Trade Commission. There is, however, no requirement that a warranty be given nor that any product be warranted for any length of time. Thus the Act only requires that when there is a written warranty, the warrantor clearly disclose the nature of his warranty obligation prior to the sale of the product. The consumer may then compare warranty protection, thus shopping for the "best buy." To further protect the consumer from deception, the Act requires that any written warranty must be labeled as either a "full" or a "limited" warranty. Only warranties that meet the standards of the Act may be labeled as "full." One of the most important provisions of the Act prohibits a warrantor from disclaiming or modifying any implied warranty whenever any written warranty is given or service contract entered into.

This means that, under the provisions of the Magnuson-Moss Warranty Act of 1975, an automotive dealership/carmaker cannot void your warranty because your vehicle has been modified with aftermarket parts. They (the manufacturers) have to prove that the failure was the direct result of the installed aftermarket part. Unfortunately, too many folks have gone to a dealer to have warranty service performed on their modified vehicle only to have the dealer refuse to cover the defective items. The dealer usually states, that because of the aftermarket parts installed, the warranty is void (without even attempting to determine whether or not the aftermarket part caused the problem). This is illegal...period.

Keeping Your Mod's Warranty Intact - For Dummies