Engine surges and stalls at cold idle


MMB

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There are two vacuum hoses going into the top of the air box as well; one from the egr valve and one from the intake so it makes sense that it needs to be sealed, at least on a motor like mine with the egr setup.
 


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PetroleumJunkie412

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There are two vacuum hoses going into the top of the air box as well; one from the egr valve and one from the intake so it makes sense that it needs to be sealed, at least on a motor like mine with the egr setup.
May have been if you still had egr.

When I relocated my tfi, I saw MAJOR improvements.
 

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Fingers crossed but I think mine is OK now. Thermostat was definitely stuck open - temp gauge reads right and the heater works but! I still have a slight hesitantion right at a cold start but the idle speed is very consistent after a few seconds.
 

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I got a chance to check voltage and ohms on my ECT sensor, and I found out that my sensor is only getting 4.6 volts. My understanding is that the reference voltage should be 5. Is that a sufficient drop to make the ECU get bad readings all around? I'd imagine all my sensors are giving goofy readings, since the reference voltage for all sensors comes from one wire on the ECU.
Would it be worth it now to chase down a faulty wire in the harness?
 

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I got a chance to check voltage and ohms on my ECT sensor, and I found out that my sensor is only getting 4.6 volts. My understanding is that the reference voltage should be 5. Is that a sufficient drop to make the ECU get bad readings all around? I'd imagine all my sensors are giving goofy readings, since the reference voltage for all sensors comes from one wire on the ECU.
Would it be worth it now to chase down a faulty wire in the harness?
That's odd. And AFAIK, yes, it's critical that they get 4.98-5.02 or something like that as a reference voltage. I very easily could be wrong, though.
 

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I just chased the wire to the ECU. Absolutely no wiring issues. I believe the only thing I may be up against is a bad ECU. Any suggestions before I commit to buying a new one?
 

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I just chased the wire to the ECU. Absolutely no wiring issues. I believe the only thing I may be up against is a bad ECU. Any suggestions before I commit to buying a new one?
Before buying a new one, may want to do some homework on repairing yours. If you can solder/desolder they're fairly easy to repair. Usually the capacitors just take a crap.
 

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Before buying a new one, may want to do some homework on repairing yours. If you can solder/desolder they're fairly easy to repair. Usually the capacitors just take a crap.
That would be good... Im a little time pressed, since this is my work truck/daily driver, and gas is getting prohibitively expensive here in CA. I ordered a new one. Will update with results when it's in.
 

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Ok... Back to the drawing board. The new ecu didn't fix it. I have yet to double check that it's putting the correct voltage out to the coolant temp sensor. If it's correct with this new ecu, awesome, I'll keep it, and consider that another "bug" worked out.
I think I have to revisit the O2 sensor... I'm leaning towards that now, because I've noticed that my truck will start and run fine for a minute, and then start surging something fierce (and stalling). From my understanding, these engines use a heated O2 sensor, so maybe that minute of smooth running is just the amount of time it takes to warm up the O2 sensor, and when the computer decides it's ready for sensor input, it gets a stupidly lean reading...
I guess I can think about it till I'm blue in the face... Time to get under the hood... again.
 

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Only O2 my factory ECM liked was Bosch. YMMV
 

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Check your ECU power relay to be sure you've got good voltage feeding the ECU. How are you checking voltage at the ECT? Battery negative to the LG/Y wire or the BL/W wire? The ECT is basically a thermistor, so voltage on the LG/Y wire should be close to 5vdc (if the ECU is getting good voltage), and voltage on the BL/W wire is after the thermistor. I think that's right :). Here's a sensor voltage chart.

20190206_192738a.jpg
 

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Please let us know when you solve this, looking forward to hearing what was wrong.
 

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Have you tried a different MAP sensor? Those will cause an extreme rich condition sometimes if they fail.

Might be worth getting a fuel pressure reading too... see if it's way out of spec or bleeds off when the pump isn't running.
 

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If this all started after lifting the engine to change the oil pump, you must have damaged some wiring in the process. Have you gone back and checked to determine if any of the wiring harness' were likely to have been stretched, damaged, etc? It's unlikely that a sensor or component would go out just when you replace the oil pump. Figure out how much the engine was raised, and if any wiring looks like it would not have responded well to be raised that high. Maybe the upper intake was pinching the wiring where it goes across the firewall near the intake. It does sound like the issue could be with the MAP or O2 sensors.
 

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Ok. Update. To answer some questions... I did test the map sensor early on in this problem. Mine was testing bad, so I got a new Motocraft one. No change. I checked my wires, and double checked them. Today I checked the O2 sensor wiring and a new sensor is installed. No change.

However, I did remember a detail from when I did the work. I disconnected the catalytic converter to pull the engine up, and in the process snapped two of the three bolts holding it on. I got a nice exhaust leak right there where my sensor is. Is it at all possible that this is letting air in and giving it a false lean reading?
 


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