‘74 1.8L L-Jetronic Cold Start Low Idle, Mystery solved! Options

[Van B]

Ok fellas, here’s the run down Aux Air Regulator (AAR) and Cold Start Valve (CSV)/injector both work fine. Many of you have also been keeping up with my high idle issue that we figured out.

But yet, when the car is cold, i.e. room temp, I don’t get the high idle I should on start-up. Instead she cranks a bit and then lumbers to life. Idle lopes around 700-800rpm and then smooths out as it warms.

In all other aspects of operation, the engine seems to be pretty happy.

Thoughts? Experience? (IMG:style_emoticons/default/confused24.gif)

-Van

See post #419 on page 14 for a synopsis of the outcome.

[Van B]

Emery, AFM is attached to the air filter housing. It's the sunset box on that diagram.

Subset* stupid autocorrect

[emerygt350]

Emery, AFM is attached to the air filter housing. It's the sunset box on that diagram.

Wow! That is nuts! I had looked at the layout in the Chilton's but it never hit me it was pre everything else. It seems insane to measure total air entering and then try to use a part of that total air to modify the afr. In fact it still doesn't make sense. Cake and eat it too. I must read....

[emerygt350]

Wow, so basically a pot on the throttle and a TPS on the flap, and the aar cheats the TPS on the throttle by sneaking air without moving the throttle plate. Interesting, and explains the extreme sensitivity to vacuum leaks in ljets. If it is all based on ratios of signals between those two pots...

Yeah, squeeze away or get yourself a good hydroponic valve like mine....

[ClayPerrine]

Wow, so basically a pot on the throttle and a TPS on the flap, and the aar cheats the TPS on the throttle by sneaking air without moving the throttle plate. Interesting, and explains the extreme sensitivity to vacuum leaks in ljets. If it is all based on ratios of signals between those two pots...

Yeah, squeeze away or get yourself a good hydroponic valve like mine....

Actually, there is no "TPS" on an L-Jet system. There is only a idle switch and a wide open throttle switch, so the ECU only knows those two states of throttle movement. And the 912-E didn't even have the WOT switch.

It was that way on the evolution of L-Jet called DME until 1995. The 964 has a switch like the 1.8L L-jet that only shows WOT or idle. The first DME to get a real TPS that actually measured throttle position was the 993.

Clay

[emerygt350]

Crazy. I am surprised just the flapper can tell the engine everything it needs, very cool. Just thinking of all the potential combinations like part throttle decel (high volume moving past a small throttle opening) vs very open throttle chugging at low RPM.

[ClayPerrine]

Crazy. I am surprised just the flapper can tell the engine everything it needs, very cool.

With the L-Jet system, there is more than just the flapper. There is the cylinder temp sender, the air temp sender (located in the air flow meter), and the engine RPM. The throttle position is extrapolated from the RPM and the air flow meter information.


Clay