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Discussion Starter · #1 ·
I am having a problem with the engine RPM's not coming back down in a reasonable time when I lightly touch the throttle. I have a 92 Bronco with 302 V8 engine.
The engine idles pretty smoothly at approx 700 rpm (have to guess--don't have a tachometer). Might have a little surge between 700 and 750 RPM (again, just a guess on RPM's) Whenever I give it gas, it jumps to the higher RPM and stays there for approx 4 seconds. Then, over the next 5 seconds, it drops slowly back down to idle.
Engine drives fine with no misfiring, etc. Am getting ready for a long trip and had it tuned up (new plugs, cap, rotor, and wires) and fuel injectors cleaned. Had new fuel filter installed. O2 sensor was replaced because it caused check engine light to go on and was running rough when driving. Runs fine when driving now, but RPM's won't return to idle in less than approx 9 seconds.
Checked for vacuum leaks with water spray. Didn't cause a change in RPM's. Installed a new IACV and can't say that it made any difference.
Sure hope someone has an idea. Thank you in advance.
 

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It is either the idle air control valve or the throttle positioning sensor but my money is on the IAC Valve. Unplug the sensor going to it and see if it helps with the problem. It will idle kind of low and rough but it if it fixs the problem try cleaning it with carb cleaner and if that doesnt work go buy a new one.
 

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Discussion Starter · #3 ·
95_bronco_limited said:
It is either the idle air control valve or the throttle positioning sensor but my money is on the IAC Valve. Unplug the sensor going to it and see if it helps with the problem. It will idle kind of low and rough but it if it fixs the problem try cleaning it with carb cleaner and if that doesnt work go buy a new one.
You and I are thinking the same things. Actually, I installed a new IACV and it had no effect. I'm really beginning to think it's the TPS. Think I'll remove the TB tomorrow, clean it, and install a new TPS.
 

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try that and keep me posted the results. ill keep thinking on anything else that it could be.
 

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The throttle body shaft is worn out and needs replacing. That is the complete TB needs replaced.
 

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Discussion Starter · #8 ·
I think the problem is resolved. Thank y'all for your help. Last weekend when I replaced the Idle Air Control Valve, I didn't disconnect the negative battery cable. The thought occured to me that now I had a new sensor installed but the computer might be using improper driveability settings.

So.........I took off the negative battery cable for 10 minutes, re-installed it and ran the Bronco for 10 miles to get new driveability settings. Turned it off and restarted and everything works great! No problem with the RPM's not returning to idle anymore. I've driven it around town for an hour, starting and stopping the engine and it works like it should.

I still think I will clean the throttle body with throttle body cleaner, but at least I can do this with it on the Bronco........not sure I have the skills to remove the whole thing and replace the TPS unit without screwing something up.

Any thoughts..........could just resetting the driveability codes to match the new IACV really be the final solution?
 

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The EEC would have adapted to the new IAC in time - you just forced it to do it immediately.

The IAC is an actuator - not a sensor.

Yes, that MIGHT have been the fix, but I doubt it. More likely: the EEC has now adapted to the sticking throttle. :shrug
 

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Discussion Starter · #10 ·
Steve83 said:
The EEC would have adapted to the new IAC in time - you just forced it to do it immediately.

The IAC is an actuator - not a sensor.

Yes, that MIGHT have been the fix, but I doubt it. More likely: the EEC has now adapted to the sticking throttle. :shrug
Well Steve83........you got it right. The problem is still there........seems like it comes and goes. Bronco runs perfectly for a couple trips, then back to the throttle sticking at high RPM's and taking long time (still approx 9 seconds) to come down to normal idle.

So..............thanks to Kris on the FSB technical repairs forum and his great photographs.........I decided to remove and clean the TB and replace the TPS. It took about 4 hours and two cans of TB cleaner from AutoZone but I got it all cleaned, new TPS installed, and everything hooked back up. Throttle cable popped off during check out run, put it on firmly and it seems to stay. The high RPM problem hasn't gone away, but it seems slightly better..........only takes 5 seconds to come down to normal idle.

This one has me stumped at the moment. I'm beginning to wonder if the throttle return springs are strong enough or binding or something. I soaked them with WD 40 and scrubbed them with a toothbrush to clean them when I had the TB off the engine. I didn't notice any binding of the actual throttles in the bores when I manually worked the accelerator and cleaned the TB. Throttle plates seemed a good fit, tight, and smooth opening and closing operation.

I guess I'll call it quits for today and wait and see if anyone has an idea.
 
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