Furthering ECT discussion

[JimBee]

Several weeks ago I posted on this when I stalled in traffic. After a couple of minutes of crank/no start, engine started and ran perfectly.

Now, twice since, it has done the same thing.

Seems to happen when the temp gauge shows fully warmed up—about 3 O'clock. It hasn't happened when I'm rolling along, just when I'm waiting in traffic—even if I'm holding the revs up a bit, around 1500-2k, it will still just quit and is very hard to start. Fortunately, I have a newish battery and it cranks long enough to evenutually get it going. When it does catch, easily revs up with no stuttering and runs normally after that, even idling as usual around 750.

Since it runs perfectly after starting, even on a 20 or more mile trip, with shutoff and restart after maybe half hour at a destination—and initially cold starts instantly, I don't suspect anything in the electrical or ignition system, or even fuel supply.

Except that, as per other current discussion, if the ECT leans out fuel delivery before engine is internally fully warmed up, then I suppose that could cause the stall. Then, to extend the theory, all those attempts to start (sometimes it almost catches) is long enough (maybe 2 to 3 minutes) for engine temp to drop a degree or 2, then maybe the ECT at that engine temp can correctly calibrate fuel supply.

So, QUESTION. If I disconnect the ECT when I have a stall, what actually does that do to fuel injection? Can the engine be run with the ECT disconnected? Or just started then reconnect?

[abscate]

I strongly suspect a wonky fuel pump relay here.

You can test out the ECT with temperature and resistance measurements

[erikv11]

...
So, QUESTION. If I disconnect the ECT when I have a stall, what actually does that do to fuel injection? Can the engine be run with the ECT disconnected? Or just started then reconnect?

Don't you have 3 P80 cars? Just try it.

But it doesn't sound like a good hypothesis for what's wrong with your car. I'd check all the straightforward suggestions in your other thread, etc.

[JimBee]

From the earlier thread discussing the ECT, I'm still not clear if disconnecting my ECT when engine is stalled after running will help a restart.

I'm usually in traffic when this happens, so not cool to pop the hood and mess with the difficult ECT connector to find out. Eventually it restarts. But does that happen because during the 2 to 3 minutes of attempts to restart, the coolant temp changes a degree or two? Something changes slightly that enables it to start and run normally after the restart—even after a half hour or so off condition at a destination. Starting back up and more travel is all normal, including idle at 750.

So this is not a random event, like it probably would be with a flaky fuel pump relay, though I do have a spare and will pop it in just to try it. It happens maybe 5 to 10 minutes after cold start, when the temp gauge indicates the engine is fully warmed up. Regardless of cold outside temp, engine performs normally on startup—until the stall.The exact amount of time in minutes varies a bit, but that it happens consistently soon after warmup seems to point to something to do with engine temperature.

So back to my original question: If I did disconnect the ECT immediately when it stalls, what would that do?

[misha]

It would choke the warm engine and still will not start.

[RickHaleParker]

So back to my original question: If I did disconnect the ECT immediately when it stalls, what would that do?

The ECM would switch to the assumed substitute value of 68℉ .
You need to look elsewhere. ECT defects cause cold start problems not stalls and warm start problems.
Cold start in reference to ECT defects means really cold cylinder walls. Not your case.

[jreed]

Only suggesting an idea here from my experience with stalling on our '97 855... the idle air control valve was sticking and causing stalling. In my case the stalling occurred during hot weather when the engine was fully warmed up and idling. Replacing it with a used IAC unit from the junkyard solved the problem.

[erikv11]

That's another good one to try - and you can clean a sticky IAC with carb cleaner, very cheap! Check the straightforward things, even the ones you don't suspect.

JimBee what I was suggesting is to warm up the/a car and disconnect the ECT. That will completely answer your question about starting a warm engine without the ECT plugged in (it won't but hey what beats real world experience?), there is no need to wait until the rare moment when the car is glitching on you.

[JimBee]

Okay, a couple of good ideas. Hadn't considered the IAC, but I think I have a spare one of those, too, (junk yard trekking became a habit for a while). Warmer days coming soon to check out a couple of things.

Though the consistent timing of the stall soon after the engine is warm does seem like a clue to something. What about the EM module (car's electronic brain?) doesn't something change in there when the engine is warmed up?

[erikv11]

Anything that involves an electrical circuit is susceptible to heat, because electrical resistance of simple conductors increases with increasing temperature. So a poor connection (which leads to low current, not enough amps) is even poorer when hot: V is constant, R goes up, this causes I (current) to goes down even more.

That's one reason why fuel pump relay and ignition are good places to look. I had a beater 850 with heavy deposits on the rotor (oops!) and it only died when hot, would restart after cooling down a bit and then run fine for a week or only a day before doing it again.

Also, when the engine warms up, the ECU (or ECM, yes the brain) switches from open loop (running on defaults) to closed loop (running using input from sensors). But closed loop comes quickly, within minutes, not after 10 or 20 minutes. The signal that the engine has warmed up or not comes from the ECT, so ECT is a suspect here, but maybe not the top suspect.