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Discussion Starter #1
96 Tbird 4.6L. Well maintained. Ran great bit suddenly wouldn't start the next morning. Fuel pump primes (replaced fuel filter just in case), has spark, clean MAF and throttle body/plenum, intake aor temp sensor reading good voltage, coolant temperature sensor read 5V (replaced). Car starts with pedal floored. Idles up to 1800 before it feels smooth. Pull MAF connector and idle drops slightly. Pull IAC connector (on my third one in 10yrs), car dies. Ideas?
 

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96 Tbird 4.6L. Well maintained. Ran great bit suddenly wouldn't start the next morning. Fuel pump primes (replaced fuel filter just in case), has spark, clean MAF and throttle body/plenum, intake aor temp sensor reading good voltage, coolant temperature sensor read 5V (replaced). Car starts with pedal floored. Idles up to 1800 before it feels smooth. Pull MAF connector and idle drops slightly. Pull IAC connector (on my third one in 10yrs), car dies. Ideas?
Putting the pedal to the floor while cranking should disable the coil. That's how I would prime my remote filter (Fill filter with oil, fill pan, pedal to floor, crank 10sec to prime lines, then let off gas to start, oil pressure goes normal right away).

What kind of IAC are you using? If not Motorcraft, I've not had good luck with them.
 

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Autozone IAC. Has worked fine for two yrs. Never got the harmonic resonance since. I do need to check it with a multimeter to be sure its good. Just discover a 2nd coolant temperature sensor. Guess I need to check it too.
 

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Autozone IAC. Has worked fine for two yrs. Never got the harmonic resonance since. I do need to check it with a multimeter to be sure its good. Just discover a 2nd coolant temperature sensor. Guess I need to check it too.
Coolant temp sensor won't cause a crank/no start.
I'd check the IAC. Yeah, the autozone ones (and all other 3rd party) work fine, till they stop. Motorcraft are the same, but they have more longevity in them. Not super expensive, about $50, and it'll work. Not sure it's your problem, but worth looking into.
 

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Discussion Starter #5
Oddly I have seen and read where a coolant temperature sensor will go bad and read -20f when its more like 85F. Messes with other sensors. I.e. air:fuel.
IAC is reading 10.6ohms so within range.
TPS reads proper on two wires (brown w/ white=5.02V; grey w/white = 1.01 butterfly closed and 4.67V WOT). Hhowever, third wire (grey w/ red) reads 5.7ohms (suppose to be below 2ohms. Bad TPS.?.
All vacuum lines replaced 2yrs ago and look good.
 

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Oddly I have seen and read where a coolant temperature sensor will go bad and read -20f when its more like 85F. Messes with other sensors. I.e. air:fuel.
IAC is reading 10.6ohms so within range.
TPS reads proper on two wires (brown w/ white=5.02V; grey w/white = 1.01 butterfly closed and 4.67V WOT). Hhowever, third wire (grey w/ red) reads 5.7ohms (suppose to be below 2ohms. Bad TPS.?.
All vacuum lines replaced 2yrs ago and look good.
Possibly bad TPS. When mine went bad, I would get a high idle, high enough that the car would pull at lights, I had to be hard on the brake to keep it still. Similar to your issue. I replaced the TPS, that didn't fully fix it, so I replaced IAC, and hasn't been a problem since.

Give it a shot.
 

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WOT while cranking places the EEC into "clear flood mode". It disables the fuel injectors, not the coils. It sounds like you may have an injector driver hung open.
 

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WOT while cranking places the EEC into "clear flood mode". It disables the fuel injectors, not the coils. It sounds like you may have an injector driver hung open.
Thanks for the correction!
 

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Discussion Starter #9
Re-cleaned the plenum, TB, IAC, and EGR valve last night. No change. Checked the impedance on each injector this morning. All were within 15.1 - 15.2 ohms (aftermarket injectors). Started easily and scoped each injector. All were cycling. Idled down to ~800rpm. Shut it off and started immediately with no pedal touched. Idled down nicely! Will test drive it and see how it acts today and report back. Stumped on what fixed it. Dirty injector connection.?. Loose connector.?. Who knows. But for now it runs "right"!
 

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Discussion Starter #10
Well no luck. By the time I got around to test driving it, it is back to doing the same thing. Though the injectors all tested fine, I can smell gasoline on my dipstick....
 

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Well no luck. By the time I got around to test driving it, it is back to doing the same thing. Though the injectors all tested fine, I can smell gasoline on my dipstick....
Bear in mind the injector/iac/tps can all test ok, but there is something intermittent causing the issue. It's not "all the way" broken, just intermittently. My IAC and TPS would only act up after hitting a bump in the road.
 

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Check the main harness rubber feedthru grommet at the firewall, on the passenger side.

If it's loose, the wire bundle is laying on the edge of the metal firewall, shorting out random stuff.
 

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I am still thinking it's a hung injector. You could try unplugging the injectors one at a time while it is idling high to rule it out.
 

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Discussion Starter #14
First off, thank you all for the ideas/advice. I'm exploring all of them.

Wire bundle on the passenger side firewall seems fine.

Unplugged each injector and nothing changed.

Car started right up this morning but after 10min of driving, high idle returned. While standing on the brakes I got it home. Listened to each injector and was cycling fine. Unplugged the IAC and car died immediately. An indicator? Tried to jiggle the TPS and connector. Nothing changed.
 

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I would check the main harness like mentioned above, then pick up a Motorcraft IAC.

Al
 

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Discussion Starter #16
Pulled the fuel injectors yesterday and cleaned them. Car started fine and idled nicely. Test drive was smooth and a noticeable increase in power. Drove it again today and still seems fine. Good call on the stuck fuel injector. Now to get an oil change. Thanks for all the advice/suggestions. The old bird lives another day!
 

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Discussion Starter #18
Well it is doing part of it again. Ran great for a week. Sat thru a tropical storm yesterday (good ole Florida). Now it idles high and drives itself. TPS is my guess.

Cleaned the injectors via the carb cleaner and pulsing method (9V). Vids on youtube.
 

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I once had this happen. Oddly enough, was some kind of glitch on an auto zone IAC. I did a PCM reset, and it fixed it.

Get a code scanner that can see live data streams off the OBD-II connector. Look at the IAC duty cycle, commanded RPM, engine RPM, TPS Absolute, and TPS relative. Also look at coolant temp, battery voltage, maf counts/voltage. Of course this is what I would do.
 

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A stuck open injector, or a bad TPS will not cause the idle to be that high. You have too much air entering the intake, which means either a very large vacuum leak, or a sticking IAC. Since you said it intermittently fixes itself, and that you recently replaced all the vacuum lines, that tells me the IAC is the problem. Replace it with a Motorcraft part, and your problem will go away.
 
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