Thread: 72 Slc 3.5 ?
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Old 08-26-2001, 10:06 PM
stevebfl stevebfl is offline
Join Date: Mar 2000
Location: Gainesville FL
Posts: 6,844
Yes, I can explain that very common 3.5 occurance.

You are experiencing decel cutoff, inappropriately. Decel cut-off is the fuell injection shutting down when you are decelerating. This occurs to lower emissions and to get better economy.

The conditions for it to occur are something like this. Engine speed above 1600 rpm and the throttle closed (as signalled by the throttle switch). Decel cut-off cancels once the rpm drops to below 1200rpm ( the numbers are unimportant and from memory).

The early EFI 3.5s were very sensitive in their idle set-up. The 4.5s either changed the amount of vacuum or centrifugal advance deviations at these rpms (I think) or they increased the rpm differential in the program. I am really not sure why this is so much more common to 3.5s. Be very sure that your vacuum retard is working. Unfortunately these cars like timing but if you are timed ahead it will cause this problem.

To verify the condition you can lower the idle, till it goes away (It will then be too slow when hot) or you can disconnect the throttle switch. I have had some cars that liked timing so much and couldn't be idled cold to avoid this condition. I have on such an occasion entered the throttle switch and deactivated the closed throttle position.

This is not advised, as this also deactivates the idle mixture compensation adjustment knob on the computor.

The auxilary air valve also causes this problem by causing the idle to be too high all the time. The problem would happen just the same hot if your idle speed was as high then. Try it by introducing an air leak (these systems will correct the fuel and run faster unlike most vacuum leaks). If you can get the speed above a certain level (with the throttle closed), the event will occur.
Steve Brotherton
Continental Imports
Gainesville FL
Bosch Master, ASE Master, L1
33 years MB technician
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