TPS Question - Throttle lag at idle

[rcfanatic]

Ever since I swapped to Motronic 1.3 on my eta, my engine has been hesitating when I start from a stop. It actually drops about 100 rpm when I first touch the pedal. It doesn't show any other time, such as when shifting, etc.

I finally got around to messing with it, and decided to test my DME pins as described in the Bentley manual. It turns out there is no signal coming from the TPS at WOT. I unplugged the TPS and it idles at 1000rpm now (700rpm prior). The engine no longer hesitates or stumbles when I take off from a stop.

So is it safe to say that I need to purchase a new TPS?

Also, how does the DME use the WOT feedback? Seems like at WOT, the AFM and CPS would be enough feedback to achieve maximum performance.


Thanks,
Adam

[jlevie]

If the WOT switch in the TPS isn't closing, either the TPS is bad or there's a problem with the adjustment of the TPS or throttle. Before replacing the TPS, check to see if it is really bad by testing with an ohmmeter. And verify that the adjustments are correct.

I don't know for certain what the DME does when it sees WOT from the TPS, but my understanding is that it switches to built in maps and ignores the O2 sensor data.

I've seen the idle drop on throttle opening before when the TPS was mis-adjusted with the idle switch opening late. In that case the DME still thinks the engine is idling even though the throttle plate has opened.

[BTP325ic]

were is TPS located on a bmw 1984 eta i am looking for mine b/c i having similar thing happening to me.

[nando]

when the ECU sees a WOT condition it ignores the AFM and O2 sensor (open loop) and runs off a specific full throttle fuel and timing map. When it sees idle from the switch it changes to an idle timing and fuel map, but runs in closed loop and activates the idle control. part throttle is yet another set of maps and is also closed loop.

[rcfanatic]

Well I got my AC recharged yesterday, and found out that having the TPS working is critical if the AC is running. So I pulled the throttle body and adjusted the switch every which way but could not get continuity on WOT. It could sense idle without issue. I'm buying a new one and throttle body gasket from bavarian. Hopefully this will solve the problems. I also noted that my idle does not kick up when I turn on the AC. Could this also be related to the TPS? It knows the car is idling, by proof of the test I did today. So it *seems* like it wouldn't have a bearing on the AC idle speed.


EDIT: In case anyone was wondering, with the AC on and TPS unplugged the engine stalled if I abruptly released a 3k rpm idle. Less than 3k and it would stumble and hunt pretty bad before reaching a settling point. My guess is that the engine speed drops too fast and without the TPS it cannot respond quick enough considering the extra torque that the compressor is demanding.

[rcfanatic]

Update: I installed the new TPS and gasket from Bavarian yesterday. It had no effect on the throttle lag or ac problem, but I feel like it has more power now that it knows when I'm at wide open throttle. Could just be the placebo effect though. I've read a ton of idle threads and it looks like my next steps would be to clean the ICV, check for vacuum leaks, and test the AFM. Any other suggestions are welcome. Thanks

[jlevie]

The engine will have more power with an operating WOT switch. I've seen a 10hp difference on dyno runs.

[nando]

yes, because otherwise it's running on the cruise map, with timing that's wrong and a 14.7:1 afr.

[rcfanatic]

That's cool about the TPS. I guess it really is more powerful now. So I sprayed brake cleaner all around the vacuum lines and connections but couldn't find any leaks. Cleaned the ICV and AFM with brake cleaner. They operate normally according to the Bentley tests.

I'm thinking I might just give up and live with the throttle lag. I've gotten pretty used to blipping the throttle before I start to release the clutch. About the AC issue though - does anyone know what sends the "AC on" signal to the DME? I tested it according to Bentley and the DME is not receiving signal that the AC is on. I tried changing the AC/Recirculating air switch, but it didn't make any difference. Suggestions? It would be nice if my throttle bumped up a little, not only to help cool the car better at idle, but also because it makes the throttle really lag with the existing problem. Thanks