My 1987 Cougar, 252K miles, V8, 5.0 liter, runs okay and gets 26 MPG (mostly highway miles). Emissions test failed due to excessive hydrocarbons (HC was nearly 400, while the limit is 160) on low and high speed test. The computer reports a code 94 (left thermactor air system problem).
I've checked the smog pump, air bypass valve, air diverter valve, TAB and TAD solenoids, vacuum lines, and the air check valve leading to the exhaust manifold on the left side. The check valve was very tough to remove, and had some internal corrosion but only allowed air to be blown in the proper direction.
So I thought perhaps that the code 94 could be caused by the oxygen sensors going bad, although they were replaced last year.
I read some spec from the internet or somewhere that the sensor should read 2.5-5 Ohms when cold, and 5-20 Ohms hot. My right sensor measured
5.4 cold, the left one by the exhaust manifold measured 6.0. One that was replaced last year measured 7.0. A new one at the auto parts store measured 4.2. When hot, my left sensor measured 8.7 and within a few minutes dropped to 8.2.My question is should I replace the O2 sensor(s) to try to alleviate the code 94 and pass emissions test? Seems like that's the last component in the thermactor air system (other than the catalytic converters, one of which was also replaced last year). If the sensors are bad, I imagine that I've got a slight head gasket leak allowing silicon from the coolant to contaminate them. I don't notice any obvious coolant level problems though.
I think I can get at the left O2 sensor, but the right one may be near impossible. What special tools are needed here?