Chuffer only briefly appeared in the City of Fear story where he was promptly killed off in the first couple of pages by undead Transformers. He was unnamed in story but was later called Chuffer by Grimlock in The Transformers letters page. Several issues after this on the letters page Dreadwing named him Steamer but corrected himself later on after a reader pointed out the mistake.
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Given his 2 names of Chuffer and Steamer it’s likely he transforms into somekind of Cybertronian locomotive.