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Discussion Starter #1
I've had a problem with my 1994 Mark VIII for about 8 months now, and have yet to find a solution. Here is your chance to be a hero and look like a genius :D

The problem is that while cruising down the road, the car will hiccup, or stutter for a split second. This happens about every 15-20 seconds or so. It kinda feels like when you are driving and get hit by a huge wind gust. The car feels like it slows for a split second. Everytime it happens you can actually see the instant MPG drop, and then rise back up. It is definetly not "normal" for a car. Here are the conditions that have to be present for it to happen:

Speed of 60mph+
Ambient temp. of 40+
Cruise on or off

I have ruled out the possibilty of an ignition or fuel problem. I will explian that more in detail later. To insure that it wasn't, I have replaced the following items:

Plugs
Wires
Coil Packs
O2 Sensors
Fuel Filter
Air Filter
Cleaned MAF
Replaced MLPS

The thing that actually told me that this wasn't related to spark or fuel is this. In addition to the requirements listed above, it will only do it when the Climate Control is in the MAX AC, PNLFLR, FLR/DEF, DEF, or Auto position. I have no problems when I switch it over to VENT or FLOOR, or simply turn it off. So, I am stuck on thinking that the problem is directly related to the HVAC System. I have replaced the climate control unit and it didn't change anything. I also had the freon level checked and it was perfectly full. This leaves me thinking of three things:

1. Bad vac line in system
2. Bad sensor in system
3. Physical problem with the compressor

While doing some other work on the car recently, I didn notice that the red vac line coming from the climate control unit appears to have been kinked at one point in time. I inspected it and it doesn't appear that it has been broken, but it is still a concern to me. The car also doesn't spit out any codes, so that would seem to rule out the bad sensor theory. So I'm left with thinking either a vac line or the compressor itself is bad. Anyone have any ideas on what might be going on here, or where to check? Thanks so much for reading this very long post. This has been a very frustrating problem for me and has been ongoing. Any help will be much appreciated!

Thomas
 

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Discussion Starter #3
I don't know why the TPS would be directly effected by the HVAC system??

Thomas
 

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Try this... Unplug the A/C compressor and go drive it under the conditions you described. It could be that your compressor is on the verge of failing and when it engages, it puts a bigger load on the engine than normal. You could also have a restriction in the A/C system that's making the compressor have to work that much harder.

If unplugging the compressor makes the problem go away, start by having a shop change out the filter or "sock" in the receiver/dryer (I think it's in there) and see if that cures it. If it doesn't cure it, I'm not sure how you could test / have tested the compressor itself but that would be my next suspect. Good luck! :D
 

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He's right. Sounds like you AC is cycling. The factory makes it do that so it don't freeze up from non use in the winter.
 

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Discussion Starter #6
I never even thought of unpluging it to test it. Thanks for the replies. I will do that and see what happens!

Thomas
 

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Refrigerator Raider Hater
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damn, late...I was going to say the Ac clutch as well.

BTW, Throttle position does have something to do with the HVAC, and that's because the HVAC is controlled via vacuum.
 

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High-Mileage 4.6L Thrasher
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Stock EEC programming has the TCC unlock when the AC clutch cycles.Tjhis si to keep the passengers from feeling the small "thud" when it cycles.
That's why you're feeling a slowing of the car,and mpg drops on the economy gauge.
Next time it does this..check to see if the rpms rise easily with a slight blip of the throttle.If the TCC is locked,then it won't,and you'll see the rpms shoot up quick when the tcc is commanded to unlock from the TP increasing.If it's unlocked,then it'll rev higher with little or no effort on the the throttle.
JL
 

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Warped said:
. It could be that your compressor is on the verge of failing and when it engages, it puts a bigger load on the engine than normal. You could also have a restriction in the A/C system that's making the compressor have to work that much harder.
ya i had my a/c pump lock up.. nothin like seeing smoke commin from ur hood when ur driving.. turn the car off. hear a sizzle. then SNAP goes ur belt......thank god for custom length belts to by pass that pos... locked that sucker up!
 

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I would check ur wires again. The shop installed wires on the '94 and after a few thousand miles it did the exact thing do described. Ended up being ****ty new wires.
 
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