Idle loping and stalling under load

regarrr

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I was driving and out of the blue the engine quit running. I placed it in park and turned the key. I could hear the fuel pump and started it. It loped in idle for about 10~15 seconds and then ran normal. I put it in gear, hit the gas and it stalls. Started in neutral, gave it gas and dropped it into to gear. This got it rolling and again when I hit the gas it stalled. So I did the same thing, got it rolling again, and gave it just enough gas to keep it moving at about 5mph. Made a U turn, and drove it down a small hill and into a parking spot.

I checked for vacuum leaks and didn't notice anything obviuos. I don't have a stethoscope or a diagram of the vacuum lines but I checked all the ones I could find and didn't see any disconnected and didn't see any cracks. After talking to a few guys at work we came up with the idea that it could be the idle air bypass valve (IAC valve). I changed the valve and didn't have much better luck. It seems to be runnng better in park, still lopes a little but not as bad. But when I throttle it, it dies out just like before. It seems like the problem is a lack of air. Any ides?

 

Broncobill78

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Well, for the engine to run you need air, fuel & spark.

Generally speaking the engine don't have *too* much trouble getting air. Fuel would be what I'd look at. Your truck has 2 fuel pumps, a low-pressure pump in the tank & a high-pressure pump on the framerail. If the HP pump goes it will get enough fuel to fire & idle but not much else. The fuel filter can also cause similar problems if it's been awhile since it's been changed.

Any of the Ford's with the distributor mounted TFI module are prone to suddenly dying when it gets hot & then restarting just fine when they cool off ( http://broncozone.com/forums/index.php?showtopic=17293 ) but that doesn't sound like what you're dealing with.

If the IAC valve is new & the air filter isn't clogged solid then I'd consider a fuel problem. Can you put a gauge on the fuel rail & see what you've got for pressure ?

 

regarrr

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I don't have a guage to check fuel pressure. I went out and bought a new fuel filter. I've never changed it and I've owned the truck for about a year, so that's probably my problem. It runs the same wether the engine is warm or cold but I have heard about that problem too. I don't think I'll be able to change the filter today. We're under another thunderstorm watch and I don't want to get into it and get rained out. I had to leave it in a parking lot so I'd be open to the elements. If these storms are anything like last week, I would be in a world of trouble if I got caught outside. Last week we got 3" of rain in 15 minutes. So I'll probably head back over tomorrow. If the filter doesn't fix the problem, I'll get a guage and check the pressure. Do you know what the pressure should read?

 

miesk5

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regarrr

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Alright. So if the new fuel filter doesn't solve the problem, I will look for a place with a smoke machine. Thanks for the info. Of course it might be a good idea to have the smoke test done anyway eh?

 

miesk5

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Yes, I'd do it esp due to the age of your Bronco.

or just replace all vac hoses as a DIYer. But if it's a leaky intake maifold gasket, you'll have new hoses

How long has it been since the fuel filter was last changed? Same for air cleaner; one more thing, if you have a K&N type filter, is it "over-oiled"?

 
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miesk5

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Sorry for replying one-at-a-time but I have a few calls coming in here. Can you hear both Fuel Pumps running while idling?

 

regarrr

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I have never changed the fuel filter and I have no idea when the previous owner last changed it. I'm gonna do that this morning right after breakfast. I've owned the truck for about a year and have only put about 5,000mi on her. I just use it mainly for a beater trail truck and campouts and stuff like that. The air filter is stock, but I would like to install a K&N. And the fuel pumps sound to be kickin strong.

 

regarrr

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Fuel filter changed easy. The old one was pretty dirty, but unfortunately it didn't fix my problem. Just do be sure, I checked to make sure my TPS wasn't gunked up. It was fine. I wonder if it needs to be recalibrated. Anyway, I got it home by doing about 10mph. It has almost no power and if I try to give it any more than a light touch on the throttle, it stalls. for some reason I'm still thinking TPS, but I have no check engine light. :huh:

 

regarrr

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Fuel. Alright, I'll have to get a pressure guage then. The hobby shop on base has one, but they're closed Monday and Tuesday, so I'll probably get back at it Wednesday after work. Money's tight so I can't go out and buy one. Gotta love that Navy pay.

Just to clarify, It only stalls when I give it gas while in gear. If I give it gas in park or neutral it will run, but only if I do it slowly. If I hammer down the throttle it will stall everytime. If I slowly apply the throttle in park, as if I were taking off from a stop, it will run up to full throttle.

Good links by the way. I'm sure they will come in handy.

 

miesk5

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See if the MAP's vac line is kinked.

If the base shop has a DVOM w/frequency measuring & a hand operated Vacuum Pump try a test first; have to save you that very hard-earned $;

Testing w/digital volt-ohm meter & Overview :...The most accurate way to test this sensor is to measure output using a DVOM (digital volt-ohm meter). In this case the meter must have a frequency measuring ability..."

http://web.archive.org/web/20040720101102/...s/EEC+codes.pdf

Notes:

"...Vacuum gauge testing and DVOM tests should be used to verify scanner readings and engine manifold vacuum..."

"...use a hand-held vacuum pump to check the sensor itself for leaks. The sensor should hold vacuum. Any leakage calls for replacement"

"...Testing requires two tools: a vacuum pump (with gauge) a 5V DC supply and a Digital multi-meter with a frequency (Hz) setting

"Warning: Do NOT use an ordinary voltmeter to check a Ford BP/MAP sensor because doing so can damage the electronics inside the sensor. This type of sensor can only be diagnosed with a DVOM that displays frequency, or a scope or scan tool"

"If a vehicle is more than five years old, the vacuum hose that connects the MAP sensor to the engine should also be replaced"

EDIT: added following:

More MAP info @ http://www.aa1car.com/library/map_sensors.htm

esp. "...use a hand-held vacuum pump to check the sensor itself for leaks. The sensor should hold vacuum. Any leakage calls for replacement..."------------

Thanks Steve!

 
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miesk5

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Bad Ford MAP Sensor Waveform by omitec.c

"...This bad ford digital map sensor was captured using the digital MAF/MAP autometer. This vehicle was experiencing an extreme rough running condition as well as stalling out at times.

Notice the autometer shows the current frequency reading plus the minimum and maximum reading recorded, a histograph of the frequency, and a labscope display of the waveform."



.

 
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miesk5

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ok, I'm driving you nots w/all these posts. I found another test;

Testing, Symptoms & Overview; "...multimeter that can read frequency is normally required to check the sensor

 

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