2000 Metro cold start problem: missing, backfiring, stalling

[MetroMPG]

I'm getting an intermittent cold start problem, and am not sure how to proceed.

Circumstances:

- "Cold" start (ambient ~75 - 85F / 27 C), after not driving for several days.

Symptoms:

- It starts first try, but runs very rough, bucking and missing, and fairly quickly stalls out.

- Simply pushing the throttle causes it to bog and immediately stall out.

- However, pumping the throttle constantly, I can keep it running, but:

(A) the tach is bouncing wildly (much more than actual engine RPM),
(B) it's backfiring occasionally through the intake, and
(C) it's missing and bucking and not generating any power, even at high RPM -- ie. almost impossible to engage the clutch and move the car without stalling out.

- The last 2 times this happened, after multiple attempted starts (like 20+), pumping the throttle, etc., the idle eventually smooths out and everything goes back to normal.

Codes:

- First time it happened, I nursed the car along to where I needed to go (about 1.5 km, mostly downhill), and got a code for a cyl #3 misfire. Pulled the #3 plug wire, and the plug end connector FELL OFF.

What I've tried so far:

Replaced all 3 wires, cap & rotor with good parts from the Firefly. No problems for several more cold start cycles after that.

- But it's happened 2 times since then with NO codes.

Any suggestions on how to diagnose this or what parts to systematically change to find the culprit? I have the Firefly to pull good parts from to ID the problem.


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EDIT - Comparison to no spark on one cyl:

So the cold start problem happened today. After 20+ starts/stalls/playing the throttle, the engine eventually went back to normal, and I used the car for a short trip. It worked perfectly.

Just for fun, I pulled a plug wire when I got back to see how the cold start problem compares to missing spark on one cylinder. It's not the same. The car is MUCH more driveable with one spark wire disconnected than it is when it's doing this bucking/missing/backfiring routine.

[arcosine]

Check the map sensor. Sound like an open loop problem. What does your scan gauge say about f.a ratio?

[MetroMPG]

Good idea.

I once had a cracked vacuum line to the MAP sensor on the Firefly and it ran similarly badly (though I don't remember it backfiring, and it was still easier to drive down the road than this).

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EDIT -- just did this test:

1) Key on, engine off, OBD-II MAP value shows atmospheric ~14.7

2) Cold start (engine running poorly), MAP value drops to mid single-digits with throttle closed/pumping a bit.

So the sensor at least appears to registering manifold vacuum.

3) I went and got the MAP from the Firefly (same part #) and plugged it in.

4) Car started & ran normally. Yay!

5) Immediately plugged the original Metro MAP sensor back in... Car started and ran normally. Confused! Maybe just a bad electrical connection?

I'll keep the Firefly sensor in the car to see if it solves the problem again next time it won't start & run right.

[me and my metro]

I suspect your distributor is wearing out and the air gap is changing. Swap with the poor Blackfly and see if that helps.

[MetroMPG]

Would either of these problems explain the occasional backfiring when I'm nursing the engine along by pumping the throttle?

Bad MAP / open loop could cause dumping excess fuel in the TB.

Bad distributor would mean no spark, accumulating unburnt fuel in the affected cylinder/s.

[jjackstone]

Intermittent ignition coil? Sometimes they'll develop small cracks in the housing which can cause spark to the frame. Usually you can see a spark jump in the dark.
JJ

[MetroMPG]

Well, it's not the MAP sensor! After 2 trouble-free cold starts built up my confidence, today's cold start reverted to crap - and that was with the Firefly's MAP sensor in play.


So I immediately shut it down, fetched the Firefly's ignition coil and hooked that up.


And it fired up perfectly! So I immediately shut it down and reinstalled the Metro's coil. And it fired up perfectly again! So much for A-B-A testing... I hate intermittent problems.


I'll leave the Firefly's coil in play. If it turns out not to be the solution, next up will be the distributor or just the pickup sensor inside.

[California98Civic]

What is happening with fuel trim? If there was a spark problem from a worn dstributor the fuel would not gather in the combustion chamber so muchas blow throw the exhaust ports to the cat and O2 sensor. The sensor would detect less oxygen, the ECU would react. Is a Metro EFI? Does it have like one injector or something? I forget.

[California98Civic]

Wait... is this only in open loop?

[MetroMPG]

Correct: one injector, throttle body ignition.

It's a cold start issue, so it must be open loop to begin with anyway. I'm not sure what coolant temp it needs to see to go to closed loop.