2001 Merc Grand Marquis with idle trouble

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  #1  
Old 02-18-04, 01:01 PM
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2001 Merc Grand Marquis with idle trouble

My 2001 Mercury Grand Marquis is giving me fits. When I start it up, it dies immediately. I have cleaned the Idle Air Bypass Valve several times and it seemed to go away for several weeks, but then reappears. I am beginning to think this cleaning and the apparent fix are coincidental. The valve can no longer be dirty.

I put a voltmeter on the connector to the IAB valve and saw 0 volts, then after trying to start the car a few times, saw just under 12 volts, then 0 volts again. So, perhaps I have a loose connection. But then, I tried starting the car this past weekend, heard a "poof" sound, kind of like a soft backfire, and now have a "check engine" light on.

The car can be driven and after it warms up it barely holds the idle, though it will stall if sitting for more than 30 seconds.

Other than changing oil every 3-4K miles, one coolant change, and one transmission fluid/filter change at 25K miles, no significant woirk has been done. Currently has 49K miles.

4.6L v-8 with dual exhaust and HD rear suspension.

What is the IAB connected to?

I have not had a chance to trace the circuit and check for short or loose wire/continuity. [Fortunately I can drive my '90 Grand Marquis wagon until I get a chance to spend some time with the '01.]

Any ideas on how to troubleshoot?

thanks
 
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  #2  
Old 02-18-04, 02:35 PM
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First, did you clean the throttle plate also. You need to scrub the backside of the throttle plate and the surrounding housing with carb cleaner and an old toothbrush. You need to scan for codes before much further and let us know what the code was. That IAC motor may need replacing. Make sure you check good for vacuum leaks. That 4.6 is notorious for sucking a hole into the large vacuum tee all the way in the back of the motor behind the throttle body elbow, especially if those codes are for lean O/2 sensor.

footnote:
If that intake housing has a sticker saying not to clean it then disregard those cleaning instructions. Some later models have a special coating on them.
 
  #3  
Old 02-18-04, 02:59 PM
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I believe Ford sells a special cleaner for those coated throttle bodies.
 
  #4  
Old 02-19-04, 11:24 AM
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Thanks, I will look at those items. Mine does not have the special coating. At least I have never seen any warning stickers. The TB looks quite clean. But I will check the back as well.

As for the codes, do I need to buy an OBD II scanner or is there another way to get the codes out? I have read about cars that will put them out using the check engine light or other arrangement. Any such luck on the Merc?
 
  #5  
Old 02-19-04, 12:10 PM
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OBDII requires a scanner to read and clear codes. The crud you are looking for will be on the back side of thr throttle plate and the housing it contacts. Have someone hold the throttle to the floor while you look inside. My money is still on that vacuum tee though.
 
  #6  
Old 02-19-04, 12:19 PM
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Ok, thanks. I'll be on that vacuum tee like a fly on you know what.
 
  #7  
Old 02-19-04, 02:51 PM
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Hahaha.....The easiest way to check it is to just touch all around it with the engine running. If there's a hole, you'll notice right away when you touch it and the sound will change immediatly.
 
  #8  
Old 03-15-04, 05:50 AM
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Well, after continuing to troubleshoot, I found no problems with the vacuum hoses. Voltage seemed to be there more consistently, though, so perhaps my probes were not quite touching the connectors the first time I tested.

Anyway, I spoke with the Merc dealer parts department and they told me they sold a lot of the newer IAC Valves, especially to taxis as the older version did not hold up. OK, so I broke down and simply swapped the part with the newer IAC valve.

Car works great. That seems to have been the problem. Though for the life of me, cannot see any external differences in the new valve. Must be inside (sure it is...)

Thanks for the posts. We'll know for sure in a couple of weeks, but I am pretty sure this is it.

Now I need to troubleshoot the burning smell when heater on and idling, but that will be another post...
 
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