My buddy just purchased a 92 Deville (182k miles) 4.9 300 and the caddy is having problems running, I think its running to rich since he only gets like 7-9 mpg on the display. My buddy's caddy is throwing DTC (diagnostic trouble codes)

E30 ISC RPM out of range
E55 Closed Throttle Angle Out of Range [TPS]
E91 Transaxle Range switch problem
E70 Intermittent TPS Signal

What he replaced within the last 2 weeks
Alternator
Battery
Water Pump
Radiator
Thermostat
ISC (Idle speed Control)
TPS (Throttle Posistion sensor)
4 Bosch plugs of the 8 plugs and 4 Bosch wires (says other 4 are to hard so I'll probably give him a hand)

Unfortunately I dont know alot about the Caddy's since I own a Grand Prix GTP (Have To Love that Supercharger Wine :)). I believe possibly his timing might be off, I've always had bad luck with Bosch plugs/caused multiple misfires (however never tested them on a Caddy). All parts were bought brand new from Murrays Auto Parts Store. Things I'm going to reccomend to him: Finish plugs and wires, check compression, make sure he's running premium fuel, Check all va***m lines.

I'm open to any ideas.

Were the codes there when he purchased the car and if they were was the car sitting? If so have him clear the computer by disconnecting and then reconnecting the battery. If these problems are really problems it will shoot the code back up, If not don't worry about them. I just got my grandfather 92 deville back on the road and was replacing stuff left and right but the light was still on until I done this. The longer the light stays on the more Trouble codes will be detected and they are not necessarily truly troubling the engine. Thanx

When he put a new battery in, he left it unplugged for a while so, these codes need to be addressed since they came back up.

Thanks

Start by checking for any vacuum leaks under the hood and connect a vacuum gauge to it and to make sure you don’t have any mechanical problem like bad valves.

and then if you have a digital read out in side your car put in to diagnostic mode and see if you can red the o2 sensor cross count and if not try to get a diagnostic code reader to read the cross count and if its low after the car heats up like 5 or lower you may have a bad o2 and not know it.