Rough Idle in 1981 Cadillac Fleetwood Brougham with 368 ci engine...

[tradow1]

I've also posted a message as to a good Dallas, TX source to help a fellow CLC member diagnose his 1981 Fleetwood Brougham with rough idle.

Car has 40K miles.

Fully original and always maintained.

Modulated displacement feature disconnected.

Upon start up which occurs instantly even when sitting long periods, car will idle down and then run rough at idle.  At times when driving, check engine light comes on.  Diagnostic codes point to Throttle Position Sensor as a possible root cause and the car also has an issue with the Temperature light coming on soon after start up, which might also mean a new Temperature Sensor should be installed.  There is no sign of actual overheating.

Car drives fine, doesn't hesitate when driving, but simply has a rough idle/Check Engine Light and now a temperature light to address.

Thanks for the help!

Travis Dowell
Carolina Region CLC
Charlotte, North Carolina
#21371

[TJ Hopland]

I remember reading that they had a lot of issues with the computer and sensor connectors with the early DEFI systems.  I think they changed the type of connectors for 82 to solve the issue which is also what makes 80 and 81 a lot of 1 year only sensors and bits.   Maybe at least un plugging and plugging all the connectors you can find would be a good start to see if the issues clear up.   

Temp sensor is easy to check with an ohm meter.   I believe there are charts showing ohms vs. temps so its easy to test.

How did they disable the displacement system?   I seen a lot of them done wrong which causes all sorts of other computer issues.  The proper way to do it is at the transmission.  There are 2 wires, one is the 'kickdown' which you still want.  The other is the 3rd gear. System can only engage if it thinks the transmission is in 3rd gear.  Disconnect the 3rd gear sensor and it will never think its in 3rd so it will never deactivate.  Many people just install a manual switch on that line and flip it on when they are on the highway.  One I saw recently had it on a relay tied to the cruise control so it would only work if the cruise is on.  The other methods I have seen attempted the computer still thinks its shutting down cylinders so it changes fuel and timing for what it thinks is going on.  People then mess up other settings trying to compensate for the computer thinking its running on less cylinders.   I have seen fuel pressure boosted and all sorts of other hacks trying to make them run with various other parts disconnected.   

[r2law]

I have a 81 Fleetwood that had an erratic idle speed where the check engine light would go on/off.  It turned out to be the vacuum hose.  It was cracked.