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Old 08-26-2001, 06:37 PM
TSL TSL is offline
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Join Date: Jul 2001
Posts: 81
72 Slc 3.5 ?

Can it be explained why, when my choke is hooked up, that when on cold start, the engine revs up and down in a consistant manner until warm? Almost like you are sitting there and doing it yourself to warm up. Of course you disconnect the choke and it will not do that but does fast idle until warm.

1972 3.5 in a SLC
116.982 engine code

Other than this is run wonderfully and very strong. 86,000 original miles.

Thank you,
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Old 08-26-2001, 10:04 PM
cth350's Avatar
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Join Date: Dec 2000
Location: Long Island, NY
Posts: 3,673
Choke? the 3.5 EFI doesn't have a choke. If you are disconnecting an electrical connection, you are either:

- Disconnecting a faulty part, making the system behave.


- Disconnecting a critical part, making some other behavior mask the real problem.

The system is incredibly sensitive to vacuum issues. If you're disconnecting a vacuum line, then I would look under the master cylinder, where you will find a vacuum sensor that has a thick hose running back to the manifold.

There are extensive posts in the archives of various EFI and Jetronic issues (your system). It is identical to the other pre-1975 3.5 liter motor cars, ie 300sel 3.5 and the 280se 3.5. It is almost identical to the 4.5 systems, except for how the throttle position sensor is wired in (which means the brain boxes aren't identical either).

Worse case, you need a new brain box. Rebuilt, they are 400-500$. Used, if you can find it, is about 100$.

I have a .zip file of diagnostic info if you want it, send me an email (you can't send attachments any other way).

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Old 08-26-2001, 10:06 PM
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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