MoTeC Global Forum

I have a Motec 48 system with a TWM ITB setup.
with a
D-Drive white throttle position sensor, LH #56002

This weekend I was exhibiting some odd (and dangerous) behavior.

My normal idle is about 1000rpms. On track my throttle seemed to stick at 4K rpms.

Came off track and if I shut down the car. Some times I'd fire back up at 1K rpms,
sometimes at 4K (or slightly higher) RPMS. Never touching the throttle.

If when I did fire up at 1K rpms, I could modulate the throttle and return to 1K rpms
as long as I kept it below about 3000 rpms (non-scientific testing). But if I went above
about 3000 RPM or so I would stick at 4000 RPMs (or higher if I adjusted the throttle
accordingly) -- It would hold there even as the linkage was returned.

Most times if I killed the ignition and restarted, it would fire back up at normal idle.

This suggests to me the linkage is not the issue. As firing up there is no mechanical change relative to
shutting down at high RPM and starting again.

Note after packing up and returning home -- I could not get things to repeat -- I think there
might have also been an engine temperature or sensor temperature relationship.

What suggestions might one have for testing the TPS?
measurements -- etc?

thanks,

Mike

For idle RPM to increase from 1000 to 4000RPM you would not only need more fuel but also more air to enter the engine. Since a faulty TPS signal is not going to influence how much air is allowed into the engine the suggestion is you have a mechanical issue - Most likely either a big vacuum leak or the throttle blade is not closing properly.

I am assuming that the ITB are not drive by wire... As I could picture a case where the TPS in a DBW system would give you the same behaviour.

Some heat (warpage) induced change, causing it bind up, seems like a probable mechanism.
Heat gets transferred by radiation, conduction and convection.
So if you knew the temp of the air box, and the mechanism, it may be possible to see if the condition repeats.
(Assuming it is the ITB that are warping, and not from the head flange)
It may be hot and no airflow, or only hotter than X.

Or it is not heat...
Many of these ITBs have brass, bronze, or special oil laden bronze bushings where the butterfly shafts run through.
Fuel is a pretty good cleaning agent, so if fuel hosing onto them from the throat - or fuel standoff, then they usually are pretty clean... With "clean" being defined as a "lacking the oil that is required"...(for them not to gall the shaft with the bushing).

Or one could have a brake pressure sensor, and if the brake pressure was greater than X, and the RPM is much above idle... then cut fuel. (You then could wire the throttle WFO, and just keep on and off the brake ).