Surging engine whilst coasting

[axel_sv21]

Gday everyone.
I have a problem with my '89 camry where there will be on and off surging whilst slowing down for traffic lights usually somewhere between 45-55 km/h. Doesn't always happen but will happen at least a couple of times per drive. When the speed does slow to 45 or less the surging stops. I can see the tacho needle jump 100 rpm before my eyes.
I will be actioning some suggestions from the previous thread I found how do I fix a surging idle?
I believe it most likely is vacuum related. Braking however doesn't seem to make a difference.

[lexusmech]

I'm not familiar with the I4 motors, but your idle air control motor could be acting erratic or the passage for the idle air is dirty. I've had the same problem on my 2vz, but it doesn't happen enough to worry about. Even at the higher rpm's if the idle air motor is cycling, it's letting air in that isn't needed. Plus you probably have an automatic with I'm not exactly sure if/how it ties into the idle air. Tough to diag. without being there...

[axel_sv21]

Thanks for the reply.
This motor is a typical 3s-fe.
The only device I found in the manuals here that might be what your talking about is the Idle Speed Control Valve?
There are some resistance checks/air leak checks I can do on this to make sure it is operating correctly.
The actual idle of the vehicle seems to be OK.

[TBayToyotaBoy]

Yeah, my auto 3s-fe does that too--at exactly the same speeds you mentioned (between approx 40kmh and 55kmh). Only happens while coasting or braking slightly--hard or moderate braking doesn't cause it.

It doesn't actually affect anything, so I'm going to leave it be...

[Doctor J]

Try to unplug the speedometer cable and see if the problem goes away. On one car the engine had “revs at pulses’ at regular intervals every second when coasting down around the 40 mph. The condition was evident when engine was warmed up, with lights on.
The problem disappeared after I cut the speed sensor wire near the engine ECU.
This action raised fuel cut RPM.
While car is in motion, THE IDLE SPEED CONTROL VALVE IS NOT OPERATING. Once car is still (the SPEED sensor has no pulses), the ISC valve takes over. With the throttle closed, car slows down, (no braking) the ECU controls engine rpm by cutting/resuming injection pulses, using the inputs from engine temperature, vehicle speed, gear position, engine rpm (distributor).

[axel_sv21]

Make sense to me.
Cutting speed sensor wire = raised fuel cut RPM. This concerns me.
I'll disconnect the speedometer cable and see what happens.
Thanks!

[scribby]

Try lowering your idle speed, I had the same problem and this fixed it.

Its something to do with when braking the fuel stops but as this happens the car idle becomes too low and so it tries to bump it up by letting some fuel in and so you get this random fluctuation of revs.

[axel_sv21]

Easy enough to try. Might play with the idle up too. Sometimes when selecting reverse when stationary the revs drop too low and the same fluctuation occurs, interesting point.