well, not entirely died. It started loosing power, than kept idling very rough (around 500rpm), and eventually stalled. I still can crank it up, and run it if I let more gas. I had to tow it home.
I'm not sure what to test, and would welcome suggestions.
More info:
- it still sort-of works, so the timing belt should be OK
- in the morning I heard something ruttling, but it went away
- I have an ammeter installed, and the needle shakes a lot (fluctuating current) at this rough and low idle.
- the tach seems to read reasonably, so I believe at least one flywheel sensor is OK.

 

Check both of the flywheel sensors. Replace them if they're at all questionalble.
The problem with my '84 was the connectors from the sensor to the harness - the male plug on the sensors literally crumbled in my hands as I took them apart.

 

Check that you're getting spark and gas to all the cylinders. Mine was running on 3 cylinders (bad plug) at one point and had the same symptoms you mentioned.

 

thanks guys:
more info: I connected a vacuum gauge, and the readings are quite erratic: the needle fluctuates rapidly between 8 - 10" when I keep it around 1200 rpm. While idling (now around only 500 rpm !) the vacuum fluctuates rapidly between 0 and 10" or so. Does this indicate an engine compression problem on one of the cylinders?, e.g. broken valve, broken piston ring, etc?

 

It sounds more like your problem is that it's only running on three cylinders rather than DME related. Check the fuel injectors (pull one connector off at a time and see if anything changes)
Also - distributor cap (one bad contact?), shorting ignition wire.
One more thing to look for is a big air leak (like the J-boot torn or even disconnected)
Does the engine run better once you get the revs up (like > 3k)

 

What you're saying is in agreement with my thinking: running on 3 cylinders.
I'm thinking of buying a compression tester.
But I'm not following your suggestion: if one cylinder is not working, how would I see a difference upon unplugging the injector?

I also hear some sort of knocking, similar to sawing machine but deeper. I can't localize it with a stethoscope. I don't think it's coming from the head.

yes, it runs smoother with an ncrease of the rpm, but definitely not at full power.

FYI: the ignition components (plugs, wires, cap/rotor) are less than 2 weeks old.

 

As you pull the injector plugs you'll hear a change in the engine. If you pull the one that's not running, nothing will happen. If you pull one of the 3 that is running, either the engine will die or run MUCH worse. It's a pretty quick way to check which cylinder isn't getting gas or fireing.

 

Here are the results of removing the injector connectors (one at a time):
cylinder #1 and #2: rpm goes clearly down upon disconnecting -> good.
cylinder #3: very little difference if any, but probably slight drop of rpm upon disconnecting
cylinder #4: runs slightly better with the injector connector disconnected !

So it looks that the cylinders #3 and #4 don't work properly?
What's the recommended next step? Compression test?

I was suspecting a clogged catalytic converter, and wanted to get a replacement. I read in one source that backpressure can melt exhause valves. Anybody heard of this?

 

You'd have to run pretty hard to melt exhaust valves due to backpressure (like extended racing conditions - a lot of wide open throttle and high rpm's)

I think I remember reading somewhere that the Bosch Motronic 1.1 fuel injector drivers were paired; i.e. the same transistor drives two fuel injectors. Let me see if I can confirm that.

How's it run if both #3 and 4 injectors are unplugged? If there's not much difference than when they're both plugged in, you might have a problem in the DME itself. It might be as simple as a bad solder connection on a transistor.

Also, a compression test isn't a bad idea.

 

update:
1. the rpm goes slightly down when 3 & 4 injectors are disconnected simultaneously, but the difference is less pronounced than upon disconnecting #1 or 2 separately. To me it looks like both #3 and #4 cylinders are not doing well.
2. I used the Noid light to check the injector electrical pulses, and it blinks similarily for each injector. I think knowing this we can rule out the DME (unless there are same phase differences between the couples of the injectors, but this is unlikely).

3. I hear quite loud knocking from the block area during the rough idling (it goes away when I open the throttle). I only hope it's the (violently shaking) engine hitting something, not some internal problem.

 

Check plug wires, cap/rotor and spark plugs?

 

man you guys are looking for big problems. Check the intake boot - not just the top side, but the underside where the idle bypass ties in. I have seen intake gaskets between the manifold and head fail if the car has been overheated before. always look inside the front belt covers(I do every oil change) and confirm the integriry of the belts. Throttle position switch could be messed up(no closed throttle reading to the dme - no idle air control) Could be idle air valve/ related plumbing. fuel filter lately? Screen in the fuel tank pick up? Anybody been playing with the idle-set screw? these cars have remarkably robust lower-end and valve train with regular maintanance and responsible use. Lots of electric sensor and soft-parts problems on older cars these days. Good luck

 

I posted the results of the compression test in:
Compression test - results