What might a pulsing vacuum (off throttle) indicate??

[genphreak]

Every now and then, probably dependent on ambient and operational temperatures, my 535's econometer (vacuum gauge) fluctuates about 4 psi as if it is trying to run the engine for a second at one vaccuum and then the next moment trying things at another. I guess this is the ECUhunting around with fuel flow, but does it generally indicate a bad AFM signal, or poor calibration, or maybe something else? I think my AFM's adjustments and physical condition are good as I have taken the settings back to stock after the motor rebuild. The ICV is not new, but its clean, does not leak and from what I can tell works cleanly. Perhaps it is this sticking (and I don't notice it when its off the car). The AFM track delivers a gradual change in resistance as you increase throttle, so perhaps it's just my idle mixture. I'm planing on using a home-made exhaust sensor (3 LEDs one for lean, good and rich) to set the idle air screw if I can, just wondering if any of those experienced members might have any tips?

The M30 engine is rebuilt, injectors, cam, plugs, everything is new. It's not very often that this pulsing thing happens, but I suspect fuel economy is still not up to scratch. Car goes hard though, much better than ever...

Incidentally it happens in my gf's mum's car too (much worse)- it has an AFM too and is an old '91 Toyota Camry... I have all the new bits for it, so it's going in for a major fixup as well.

Many thx in advance! :) Nick

[winfred]

the guzzle-o-meter is electronic and is a real time reading on what the computer thinks mileage is, as to why it's pulsing i don't know

[zygoteer]

I assumed from the characteristics, that its reading will swing to maximum with a 'foot-off' throttle and to minimum ( zero !!! ) if 'wide-open' ?

Can anyone else confirm this ?

[Robin-535im]

I think your car is mad because you never drive it at WOT! :) :) Maybe it's Morse code for "Come on man, floor me!!"

[Kalevera]

Nick, Fritz used to do this when coasting at ~ 40mph. It was visible in the econogauge and (more blatantly) the tach. I called it pendulum shifting (and it did feel like the torque converter was locking/unlocking, but the behavior existed across two different transmissions). I never determined why it happened -- didn't appear to be EGS or intake related. Installing the EAT chip eliminated it (I suspect the chip uses more fuel while coasting under certain speeds); when I went back to the stock chip for the last ~ month before winterizing, the behavior had completely disappeared. Prior to the EAT being installed, the intake had been cleaned with the BG system (post AFM) and I'd replaced the injectors. When the car gets going again, it won't have a vane type AFM, so we'll see what happens...

best, whit

[zygoteer]

I think your car is mad because you never drive it at WOT! ...

oh yes i do .. short periods of course

[genphreak]

Nick, Fritz used to do this when coasting at ~ 40mph. It was visible in the econogauge and (more blatantly) the tach. I called it pendulum shifting (and it did feel like the torque converter was locking/unlocking, but the behavior existed across two different transmissions). I never determined why it happened -- didn't appear to be EGS or intake related. Installing the EAT chip eliminated it (I suspect the chip uses more fuel while coasting under certain speeds); when I went back to the stock chip for the last ~ month before winterizing, the behavior had completely disappeared. Prior to the EAT being installed, the intake had been cleaned with the BG system (post AFM) and I'd replaced the injectors. When the car gets going again, it won't have a vane type AFM, so we'll see what happens...

best, whitThanks Whit, I dare say you are right- I should have described the symptoms better- it happens at 40mph-ish and it snot at zero or wide over on the vaccum scale, and (why didn't I say this) yes it shows on the tacho very clearly. A friend of mine that is a real car-boffin said it sounds like the ECU is hunting for a setting, so perhaps (given Motronic 1.3) it is comparing O2 sensory outputs in order to more accurately set the fuel mix, so after really carefully checking my ICV and setting my CO, I will report again, hopefully interestingly :)

[genphreak]

I assumed from the characteristics, that its reading will swing to maximum with a 'foot-off' throttle and to minimum ( zero !!! ) if 'wide-open' ? Can anyone else confirm this ?Yo sorry zygo, winfred- it is pulsing about 4 L/100 on the 'guzzleometer' :D and about 200rpm on the tacho, as Whit said around coming off acceleration once one is doing about 40mph. Its a weird thing, but the Toyota does it very noticeably, jerking the car a little as it does it... maybe I will isolate the problem on the Toyota (it might work out simpler, it usually is :D ) : ) Nick