Check trouble codes - see the sticky at the top of the 4.x engine section for instructions and what the codes mean. The one you need to especially watch for is E39, VCC engagement problem. That's the first sign of a failing transmission.
An '88 is a 4.5 engine car - very reliable and durable engine if it's been well maintained. Get a service history if one is available.
Check for milky or lumpy oil on the dipstick, or oil slicks in the coolant. Both are signs of head or intake gasket issues. If the coolant has been changed according to the GM recommendation, and the sealant used, the 4.5 is good to go for 250k+. Other expensive items are struts, self-leveling rear suspension, A/C.
I just met a guy with a 90 Deville with the same 4.5L motor with 272,000 on the clock and he had owned it for 14 years. Motor has original top and bottom ends, changed 2 water pumps and a couple alternators. Regular maintenance is the key.
1994 Sedan DeVille with 4.9 and 72,500 miles. Has the I039 code which prompted me to short the ALDL and retrieve the codes from the module, I get two. I have a question about one.
code 23-Left front actuator error (does this mean strut? Or is there another actuator? Trying to determine if...
I Know my Mode door Actuator is making Noise since last year, However It still moves the air up and down. Now there is a new problem i think there must be another actuator for temp that controls pass side sometime it blows cold but most often hot air. the passenger control lights up and shows...