FiFi was grounded in 2006 because her four R-3350-57 engines, even when running at less than the rated 2,200 hp, consistently overheated and exhibited poor reliability. This problem plagued all B-29s during World War II. FiFi's new engines are hybrids that combine more advanced versions of the R-3350. The new engines are carburetted, rather than fuel injected, and combine the gear reduction nose case, rotating assembly, cylinders and power section of the C-119 Flying Boxcar's -95W engines with the supercharger and accessory section of the Douglas AD-1's -26WD radials. The hybrid engines produce higher power and run cooler. FiFi flew again on August 5, 2010.