Hi all. Getting VERY frustrated. Truck-95 F-150, 5.8liter, E40d. Problem-Upon any SLIGHTLY warm to hot startup, the engine will start fine. After about 10 seconds it will start to stumble around. Pull out into traffic, it falls on it's face, but doesn't die. After about 30 seconds of this, like flipping a switch, it goes away. This will NOT happen after sitting all night on a cold night. I feel dumb, because I had access to a SUPER STAR NGS tester, and STILL cannot find the problem. Here's what I've checked (long list):
1) Verified HEGO heater operation (batt voltage at hego heater, and 5 OHMS resistance on the HEGO heater itself.

2) Cleaned all grounds
3) No vacuum leaks
4) No EGR problems (plugged off egr for test. NO help) Egr valve OK;putting vacuum to the EGR causes the engine to stall at idle)
5) No thermactor problems(tried with the thermactor disconnected) Thought maybe the upstream air was washing the HEGO signal out.
6) Fuel pressure 34 psi it idle goes to 42 at wide open throttle. Pulled the regulator vacuum line, no gas, and pressure goes to 42psi
7) Using the NGS tester, I looked at (open loop/closed loop), HEGO voltage, ECT, ACT, MAP, , and idle speed duty cycle.
8) Good gas and new fuel filter.
9) New cap, rotor, and wires.
10) Tried with the canister purge plugged off. No help.
11) KOEO pass. KOER -Code 177 (HEGO is rich) if stumbling during the test. Otherwise, pass.
Found the following (average values): ECT 184 deg., ACT 140, MAP- 12 HZ.,
Now here's the kicker: 15 secondes after a warm (or slightly warm) startup, the HEGO voltage goes to zero (lean). The processor then begins to dump fuel to the engine (watched the fuel pulsewidth go up). NOTHING else changes. The engine then stumbles all around. Then the HEGO starts to switch, and all is well again!! Then as long as the engine isn't shut off, it runs FINE for minutes or hours. I can repeat this test over and over. The reason that I don't see this in the morning is that the engine stays in open loop longer.

What am I missing????? Sorry for the long post. HEGO -(Ford's term for heated O2 sensor)
Thanks,

Tim in Detroit

 

I failed to add that I already replaced the HEGO.....twice.

 

One thought that I have for you is that the HEGO doesn't actually indicate rich vs. lean. It merely indicates the presense or absence of oxygen in the exhaust stream.

If one or more cylinders is misfiring, the air/fuel mixture in that cylinder will not react, and the oxygen will flow out into the exhaust. This will happen even if the nominal air/fuel ratio is sloppy rich. One indication of this would be that the increased pulsewidth commanded by the computer does not cause the oxygen sensor to move off its "full lean" condition.

According to my book, code 177 is a continuous memory code, and not a KOER code. I could imagine code 177 comes about if the computer trims the mixture rich because of a misfire, then the misfire goes away, and the computer doesn't lean it out fast enough to avoid code 177.

The TFI module and/or hall effect pickup can give flaky, temperature dependent symptoms like this. An ignition oscilloscope, if you have access to one, could tell the tale.

An excessively rich mixture, due to a confused computer or sensor, should not cause the engine to stumble. It may get poor gas mileage and stink of unburned fuel, but it should run smoothly enough.

If the problem is fairly repeatable, you might try running for a while with the oxygen sensor deliberately disconnected. I suspect the symptoms will not change, indicating the problem lies somewhere other than in the closed-loop feedback system.