1979-1993 & 94 Conv [Subscribe to Daily Digest] |
What's happening is that the CPU is sending a wildly fluctuating signal to the modulation valve. I'm guessing that the car starts and idles fine for maybe 30 or so seconds, then begins the roller coaster ride idling. Considering how the cpu box has a limited range of temperature and oxygen content (via O2 sensor) settings, your engine has reached a point where it has gone outside the fixed CPU parameters. If you disconnect the O2 sensor for at least the first few minutes of the warm-up, you'll probably notice a nice smooth idle.
Couple of reasons why this is. Could be a bad O2 sensor. With my old 82 base car, that wasn't the case. The next obvious weak link is vacuum leaks from any connection or component along the chain. I had a serious idle hunt problem that I helped to minimize by tightening* up all vacuum line connections, including the ignition advance unit on my distributor, which had died, causing a vac line leak. All those fixes helped somewhat, but to entirely solve the problem I simply installed an O2 sensor kill switch that I could control from my dash. Only when the car reached normal operating temps would I switch it on. Shortly thereafter I parted the car out to help rebuild my current 900T, so I didn't look any further as to why the idle fluctuated so much.
You also mentioned the fuel injectors. If they are partially clogged, they could be spitting fuel at the low idle settings instead of spraying. That will definitely cause a misreading by the O2 sensor. Cleaning them isn't always effective with commercial fuel additive cleaners. Oftentimes you have to physically clean them out by forcing fuel through them, a proceedure you can do yourself if mechanically inclined, or done by a shop. Or buy new ones.
One theory I have about the idle hunt with these older cars is that as these cars age the compression and oil rings lose their integrity and it's only when the engine reaches operating temps do they function within a normal range. Until that time, there's quite a bit of compression loss, which may effect how the fuel burns and thus what the O2 sensor reads.
Two other reasons why: might need a new air filter. And the air fuel mix may need adjusted.
My current 82T, with all systems checked and tweaked to optimal level, I still get a small amount of idle hunt during warm-up, but not enough to where it's a problem.
* even if the vac lines are connected to their respective port doesn't mean they are tight. If the connection to the intake manifold or valve cover is loose or floppy in any way, you have a leak. The rubber hosing may have stretched out around the connections. Cut a half inch off the end of the hose and reattach. Or get new vac hoses to be sure.
posted by 32.100....
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