Sorry for the long post, but I wanted to be clear.

I'm having problems with hot idle. My car is a 73 1.7. Same old story, it runs fine before it warms up, but then it won't idle. The weird part is that if I pull the plugs on the injectors one at a time, pulling 1 or 3 will kill the engine before I can plug it back in, but pulling 2 or 4 will just lower the idle to almost dead levels. And even then, 4 seems to have less of an effect than 2.

1. Trigger Points
My trigger points seem to be working okay. I checked them with my oscilloscope and got a nice square wave pattern on both contacts.

2. Air Temp Sensor
I usually run with the air temp sensor unplugged, since plugging it back in gives worse performance while the engine is warming up.

3. Head Temp Sensor
Replaced with a new unit from Napa Auto Parts. Measures at 70~80 ohms hot. Does this seem low?

4. Hoses
I have unhooked the vacuum retard and the decel valve. Both are plugged off. I wouldn't expect this type of behavior from leaky hoses, because it is my understanding that leaking hoses cause a high idle and not a lean/rich condition.

5. Fuel Pressure
Checked to be at about 28 psi. I was reading something about kinked lines causing problems. Could a kinked line give correct pressure, while lowering delivery rate? Or would you notice a pressure drop if you drove with your guage connected so you could see it? If there's a problem, could fuel pressure change under load, and look fine at idle?

6. Ground Connections
I spent about 1.5 hours cleaning the engine ground connections about 3 weeks ago, so I hope there isn't a problem here. Has anyone used dielectric grease on ground connections to stop corrotion and give a better ground?

7. ECU
Part number: 0 280 000 037
Also, 022 906 021E

8. MPS
Calibrated to '73 1.7 standards by our own Jeff Bowlsby. Tested to be in acceptable condition.

9. Valve Adjustment
I adjusted the valves last week. All the valves measured to be in very good adjustment before I changed anything and the car ran about the same before and after.

10. TPS
Recently replaced witha new unit from Pelican. Seems to be adjusted correctly, idle circuit is on when throttle closed and breaks when openening the throttle only a tiny bit.

Well, that's all I can think of right now. Any ideas?

Won't idle hot hmm, I would supect you are running rich due to the unplugged air temp sensor. Where is the idle adjustment screw set on the ECU?
Have you checked the timing?-27degrees BTDC@3500rpms, hoses off & plugged
Sometimes its a combination of several things....you have the retard vacuum hose unplugged, why?
Why not get a OEM CHT instead of that NAPA one?
I have fought the idle bug on several cars now, and everything must be factory pretty much otherwise you will have problems.
What is the exact spec's on the MPS? The inductance readings...I am curious to see what the reading is at 15hg.(idle)


Geoff

Do this:

Swap the #1 and #2 injectors. Make no other changes at this time!

Repeat your 'pull injector wire' test, and see if the problem follows the injector.

Report back the results.

If the problem follows the injector, increase the fuel pressure to 32 PSI. Move idle back down to 800 RPM, and repeat 'pull injector wire' test.

The fuel demand at idle is real low, it's probably not a pump/kinked hose problem. A kinked supply hose shows up as loss of power at high RPMs. A kinked return hose shows up as too high a fuel pressure.

I do have a question about what you are calling the trigger points. The trigger points are in the distributor, and throttle position has -nothing- to do with them. Are you sure you're not looking at the 'throttle position switch'?

The trigger points can do what you describe. One set of points controls injector 1 and 3, the other controls injectors 2 and 4.

Gary.

garyh,
I just replaced my #4 injector with a spare one I had lying around that I know has a decent spray pattern. Unfortunatly, it didn't help. I did mix up the TPS and trigger points. I'll go back and fix that. Trigger points seem good, TPS is new and adjusted.

Mark,

The reason for the particular swap (1<->2) was to help isolate the problem.

I'm thinking that it's one of two things:

1) Bad Injector(s)
2) Bad injector driver.

There are two injector driver circuits in the 'system'. They start at the injection points (in the dizzy) and end with a transistor in the ECU.

An injector that is electrically bad can make the other injector of the pair look bad.

Hey, you've got an o'scope. What do the pulses at the injectors look like? There's sample waveworms on the PBAnders site. Look at all four....

Hiho Mark,

one question : What's the idle speed, when the car is cold ? It should run @ 1300 RPMs when cold. After warming up the
AAR is closed and the idle speed drops slowly down to 800 RPMs. My guess is that your setup for the idle speed under warm condition is not correctly adjusted. Try to raise the idle speed with the idle speed screw at the throttle body, when the car is cold. After warming up the engine should run ...

best wishes

Benno