Engine Performance problem
1990 Oldsmobile Ciera 6 cyl Front Wheel Drive Automatic 73000 miles
This car was purchased w/40k miles, had excellent maintenance and had a new computer installed, as well as an intake manifold gasket - service performed by GM dealership. I have kept the oil changed regularly and other tune up as well.
About a month ago I started noticing a slight change in RPM while in town - about 25-30mph in high gear and when I stopped, some slight surging. I replaced the O2 sensor 6 mos before because it broke (blew out of the hole).
The codes are 12 only. Not showing any other trouble codes.
I thought about the iac and pulled it, cleaned it. Didn't look bad or very dirty. Also sprayed the throttle body with injector spray. No change. Have checked plugs - NGKs look great after 10k, new plug wires, replaced fuel pressure regulator and yesterday (unrelated maybe?) A KNOCKING serpentine tensioner roller/bearing. I now notice that when the engine is started and cold, it runs perfectly smooth. Within 2 - 3 minutes, as the engine warms and I hold the accelerator @ about 1/4 throttle, it begins missing - just a bit at first, then progressively worse until within 5 minutes it is very noticeable. On acceleration on the road it feels good, tho some loss of power over previous good running condition is noticeable.
I have not taken any voltage readings (I have a digital VOM) or values, tho I'm suspecting something that is affected by the temp sensor - which I think would be the throttle pos sensor. But can't afford to just start throwing $ away on things without some verification. The replacement of the fuel press reg was because I'd had a Ford truck that acted very similarly. Your help is appreciated and I have no objection to making a donation. After reading a few posts here, you guys deserve the support! Thanks!
It is not uncommon for the throttle position sensor to fail on these vehicles. GM had released a bad batch of these sensors for the late '80s to '90 model years. Simple way to check for one of these bad sensors is to check to see if there is a yellow dot on the side of the TPS housing. It takes a lab scope to test for faulty TPS and usually it will not throw a diagnostic trouble code.