Weird D-Jet FI stumble

[bowlsby]

Strange problem maybe some of the FI gurus can give a second opinion on.

New throttle position switch (TPS) believed to be properly calibrated. Engine revs strong and smooth through all rpms, but there is one teency-weency minor dilemma that consistently occurs.

It idles beautifully. When I just barely, every so slightly put peddle pressure on the accelerator, the engine momentarily drops idle speed abruptly and might even stall, and then takes off.

Does this happen to anyone else? I am thinking that maybe the TPS is just slightly miscalibrated so that the idle switch cuts off and the accelerator switches have not yet engaged. What do you think, any other suspects before I go ripping into it?

I have tried several pressure sensors and this condition occurs with all of them. New vacuum and fuel lines, proper fuel pressure, all intake gaskets and seals (every one of them) and all FI components test good.

What with the imprecise calibration procedure ("rotate switch until the switch closes then rotate one more notch"), I wonder if there is some tolerance we are missing.

[JP Noonan]

Check PB Anders site and make sure you have the TPS that matches the rest of your FI.

Does it do it with the TPS totally disconnected?

[pbanders]

Hmm. Well, it's either going abruptly lean or abruptly rich. My non-EPA approved method of checking this would be to have someone else crack the throttle open while I smelled the exhaust, if there's a big rich dump of fuel, it should be easy to smell. Lean isn't easy to smell.

What is the position of the ECU knob? If it's really rich, maybe opening the throttle gives you a sudden lean condition.

Brad Anders

[retro74]

Same problem here, I was too rich on the idle mixture. Signs were no power off the line when fully warmed up and bad response on throttle blip. Turned my ECU idle knob to the left (leaner) and it is better now. I'm still going to replace my air temp sensor on the top of the air distribution box due to sleightly non-standard ohm readings.

Prior to all of this stuff, I replaced all vacuum hoses, injector seals, set the timing, etc. My fuel pressure was set at 38psi and I reset it to a standard 29-30 psi. This also helped to lean-out the idle condition and then I reset my ECU idle setting.


Hope this helps.

Joe

[This message has been edited by retro74 (edited 10-04-2001).]

[bowlsby]

I love this forum.

You guys were right on. A few weeks ago, don't ask me why, I richened the ECU mixture knob without looking at its position.

Turns out I was 10 clicks rich above the factory notch. So I dialed it down to the 3 clicks rich position, adjusted the idle air screw to compensate and it works fine now. I guess in retrospect it was coming off the idle contact on the TPS and going massively rich which caused my abrupt loss of rpm immediately off-idle.

Thanks for saving my saturday chasing the wrong suspect. Now maybe I can get to other 914 projects I've been putting off.



[This message has been edited by bowlsby (edited 10-05-2001).]

[pbanders]

Originally posted by retro74:
I'm still going to replace my air temp sensor on the top of the air distribution box due to sleightly non-standard ohm readings.

Joe, what readings did you get? The air temp sensor has a fairly small effect on mixture, so a small variation in the readings probably will not have an effect. See the PL analysis section in my ECU web page for a description of how much the resistance value of the TS1 sensor affects the mixture:

http://members.rennlist.com/pbanders/ecu.htm

The TS1 analysis is below the PL waveform diagram.

Brad Anders

PS: Jeff, I totally spaced this week on checking out your MPS's. I'll get to it this weekend, sorry!



[This message has been edited by pbanders (edited 10-05-2001).]