I have a 1989 Chevy Suburban 2500 with a 454 TBI engine. Ever since I bought it, I've been plagued with an occasional misfire/ rough idle. So far I've tried changing the spark plugs and wires (cylinder #8 spark plug was cracked), and the dealership I bought it from replaced the cap and rotor and adjusted timing to get it to run well enough to pass emissions, but the miss is still there. They mentioned cylinder 6 and 8 were "weak" on power, but didn't elaborate further, and as far as I know, they didn't compression test them. It seems worse when the engine is cold, and will smooth out a little after it warms up. Does anyone have advice as to where I should start troubleshooting?

 

Well, in my own opinion the 454 was not the greatest motor GM ever developed, but it was a low-compression substitute for the great GM motors of the 1960's and early 1970's. That said, you need to put a vacuum gauge on it and run the OBD I codes. Could easily be a bad HEI ignition component, or a vacuum leak of some kind. Their
"scope" likely showed up cly. # 6 and 8 as "weak", but the only way to test that is either or both a dry/wet leak-down test. At that age of motor, anything is a possibility.