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Hi guys, it's been a while since I've had to seek MN12 expertise but, nevertheless, my car has developed a new issue. In short, like the title says, I've been getting a sort of stuttering after the car sits for a while with the engine at idle speeds. When it occurs there is a slight shudder/vibration and the tachometer bounces up and down about 50-100 RPM from idle. I first noticed it a couple days ago when sitting in a drive-thru and I was afraid the engine was going to stall. The stuttering stops with even slight throttle input, so things seem normal driving around. The car appears to start normally as well and I haven't gotten any CELs. I hooked up my code scanner to the car and did a KOEO test anyway just in case and it turned up nothing.

The last bit of maintenance I've done was an oil change almost two weeks ago. I replaced my O2 sensors almost two months ago to resolve a lean code. Prior to that it's been no more than regular oil changes and air filter cleaning for the past two years. I replaced the camshaft and crankshat position sensors early in 2009 (resolved CEL/code I had back then). Current MAF sensor is seven years old with about 45k miles on it. Current fuel filter is eight years old with about 50k miles on it. Spark plugs and cables were replaced about six years and 43k miles ago. The car itself of course is a 1994 model and has about 171,500 miles on it.

Any ideas? I fear something's wrong with the fuel system (dying pump?). I searched around the forums a bit and nothing obvious has jumped out at me. I'm kind of hoping it's something minor like bad gas but this issue started in the middle of my current tank as far as I can tell.

Thanks in advance.
 

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Well if you wanted to check the fuel pressure you'll need a gauge. 171k is a lot of miles for a FP but many have lasted longer.

Other common things to check would be the IAC, TPS. Check for vacuum leaks. Look for old/cracked vacuum hoses.

Dang, you only put 50k miles on it in 8 years? I wish my mileage was like that lol.
 

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Discussion Starter #3
Thanks for the response. I haven't really had time to do much maintenance since my original post and now so, needless to say, the situation hasn't changed much. A couple people I've talked to suggested that I might have dirty fuel injectors so I ran some Seafoam in the gas tank a few days after my original post. The stuttering hasn't gotten any worse but I must say that there have been days where it doesn't appear to happen. Probably coincidence. Today I decided to change the fuel filter for the heck of it but there was no change. Afterward I put in some Lucasoil, which seemed to smooth out the stuttering a bit but I don't know how long that'll last.

As for the IAC and TPS, by "check" do you mean replace or is there some way to clean them or otherwise isolate them as a problem before resorting to replacement?
 

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Discussion Starter #4
I just had an idea about vacuum hoses. Are there any hoses that are more prone to failure? My dad recently had a vacuum leak issue with his '01 Ranger (4.0L OHC V6; 100k miles) that was causing a CEL and lean A/F ratio code. Turned out to be an easy fix through replacing a hose that fails more often than others (according to what I found on Google). Do you know part numbers for T-bird hoses?
 

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Success?

I decided to go to O'Reilly and see the prices for a TPS and IAC valve and they were about the same; around $60 each. They would have needed a day to get a TPS but they had a IAC valve in stock so I decided to get it. I was pleased too because from what I've seen this part is easier to install. I brought it home and installed it and it appears to have eliminated the stutter. I drove the car around a bit and the engine seems to be as smooth at idle as it was before the problem started. I'll see how it does over the next few days but it looks like I won't have to bother with the TPS or any other parts. Thanks again for the suggestion, BlueEyes. The 3.8 lives on.
 

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One thought: Try cleaning out your old IAC with air intake or carb cleaner. It's dead anyways so if you fail to loosen it so it's stuck open/close, you haven't lost anything more than a few minutes putting it on and trying it out.

If it works, you can save yourself a few bucks.
-g
 

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Vacuum Leak. Try disconnecting the IAC valve with it running. The engine should idle VERY LOW or die. If it doesn't, it's sucking air somewhere = vacuum leak.
 

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If I remember right there is a vacuum line T connector around the firewall on the drivers side and that tends to be the spot where lines leak. There are no part numbers for vacuum lines that I am aware of just match up the size and cut it yourself.
 
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