my mother has a 92 roadmaster, 5.7 engine, has 255k miles. it has a hesitation off idle,slight miss at idle. at 50-60 mph, cuts out badly,surges. at idle, sometimes idle speed will fluctuate by 400 rpm(watching tach) what do i need to do? o No: i also noticed rust on the inside of the oil dipstick, and sometimes shows moisture droplets on it. o No:
With that kind of mileage, it could be any number of things. Any idea on the condition of the plugs, wires, cap & rotor?? Has a compression test been done lately? I'd check/replace the vacuum hoses....could be some leaks there. I'd probably pull the TBI unit off and replace the base gasket. Also, while it's off - you can pull the top part off and clean out or replace the little filters on the end of the injectors. Clean out the IAC while it's off also. Take a look under the cap & rotor and see what shape the dist. looks to be in. I've seen some of the high mileage SBC distributors really get pretty sloppy......you'll notice a fair amount of side-to-side movement. If that's the case - you could have metal to metal contact with the pickup/reluctor gears, while it's rotating. With that many miles, I would really suspect very worn cylinders, pistons, rings, valve seats & guides, etc. Guides are especially prone to wear, which of course wears out the seats too. Does it ping alot also? Or seem to burn oil? You might also rent or borrow a coolant system pressure checker. The intake gaskets could be brittle or blown......the intake itself could be cracked. I had to replace the intake on my Dad's truck (same basic engine).
Probably should add that the timing chain could be all kinds of sloppy too. The original gears could have most of the plastic teeth off by now. That might account for the fluctuation in the rpm's.
Throttle position sensors can cause this type of problem too. It's a little black thing on the passenger side of the TBI unit, with 2 or 3 wires going into it. Sometimes they don't always set off the engine light, but will set a code.
along the line of it being a possible vacuum problem, the gasket between the throttle body and intake can crack and/or shrink a little after lots of miles.. the TBI mojo that i put in my 74 vert had a cracked vacuum line that did the hesitate/surge thing ... i was real happy once i found/fixed it.
