85.5 NA Subtle surging?

[new_2_944]

Hi!

My 85.5 NA seems to be afflicted with a subtle surging at part throttle (fixed throttle opening, say 30%) in lower gears.

I'd say the frequency is about 1Hz, just a momentary rise in RPM ( maybe 50-100RPM ) and then it goes back down and the cycle repeats.

It's quite subtle, but noticeable.


Is this a "normal" oddity?

Thanks.

[Kaos]

This is at idle, I assume.

The surge may be caused by a vacuum leak, but it could also be a number of other things.

Try unplugging your TPS sensor underneath the throttle body. This will disable your idle air motor. If the problem goes away, then you might have a bad idle air motor or a bad TPS switch.

Other potential problems:
-Bad O2 sensor (faulty readings cause large swings in Air/Fuel Ratio, and consequently torque and idle speed)
-A/C switching on/off intermittently due to over/undercharge of A/C system
-Mass air meter out of range at idle (causing erroneous fueling which the O2 sensor cannot fully compensate for)
-Bad TPS switch.

For a first stab at it, I would look for any obvious vacuum leaks, then check your TPS sensor. Unplug it with your car off. Check continuity between the pins on the TPS. If you have continuity between two pins, open the throttle halfway by hand and see if the continuity goes away. On my TPS, I can actually hear a click when the TPS closed throttle switch is engaged.

Just some ramblings, hope they help

Kaos 1985/2 Black

[new_2_944]

No, it's not at idle...

This is while driving. (in lower gears the surging is more noticeable due to less torque reduction)


It occurs only while gently accelerating away (30% throttle). Full, or near-full, throttle operation is not affected.

Ideas?

[dmac]

I'm guessing that the air flow sensor is sticky. Try some Gumout spray.

Could it be your A/C kinging on and off?

[Kaos]

Mine does the same thing. Kind of feels like something is wrong, but I haven't been able to find the problem.

I thought it was engine mounts, so I replaced them. When I removed the old ones, they were clearly bad as the rubber had cracked and all of the coolant leaked out. Unfortunately, the surging is still there.

I have also changed my shocks and rebuilt my transmission and C/V joints. None of these seem to help.

I am just assuming it is a chassis tuning thing (known in the industry as "bucking"). The only thing that I haven't checked is the clutch. Seems to work fine though.

I used to have a throttle response cam on the car, which made the problem even worse. I could occaisionally get the car into a self feeding chassis bucking. The car would buck which would cause my foot to move on the throttle and feed the oscillation. The stock throttle cam is much smoother.

So, I have the same problem but don't know what it is.