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Discussion Starter #1
Okay guys, the title says it all. I've got a '94 Thunderbird with a 3.8 and 185,000 miles and the latest quirk it's developed is a sputtering or roughness that occurs under very light throttle input. If flooring it equals 100% throttle input, this stuttering occurs with maybe 10% input; like if you're cruising down a road at 40 or 50 MPH and you need to hit the gas just slightly to maintain your speed. When it occurs the exhaust note becomes jumbled, there is noticeable vibration felt throughout the car and the tachometer flutters up and down within a range of about 100 RPM. The stuttering generates no CEL and the car hasn't stalled before, nor does it generate any odd smoke or smells that I can sense. The stutter is easy to reproduce simply through the light throttle input I described and can be reproduced when parked or at speeds, though it does occur less at higher speeds probably due to higher engine RPM and necessarily heavier throttle input. When the stuttering occurs, it can be stopped either by letting off on the gas or by increasing throttle input and essentially powering through the stutter.

The issue is really just an annoyance at this point and I hope it's something minor (my fear is that it's a fuel pump failure in progress) and so I don't have to justify any large repair costs. The car has served me well for more than 13 years but my daily driver, a 2012 Focus, will be paid off next year so I'm looking to replace...no, succeed the Thunderbird with something much newer. Ideally I would keep the T-bird as a third car just to have around since it isn't worth much intrinsically, but it's only getting older and I'm not as repair savvy as I used to be so I'll just have to cross that bridge when I get to it. Until then, anyone have any ideas as to this current issue? Thanks.
 

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What is its tune upstatus? Plugs, wires, rotor, cap, etc.?

Last time the IAC and MAF sensors have been cleaned?

RwP
 

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Discussion Starter #3
Spark plugs and wires have maybe 50,000 miles on them. It's a California car so it has a DIS; original coil pack and ICM. Haven't cleaned the MAF in a while. Don't think I've ever touched the IAC. How do I service that?
 

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If you have never cleaned the MAF, that's an excellent starting point. You can pull the IAC and clean it out with carb cleaner. Are those the original headgaskets @ 185k?
 

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Discussion Starter #5
Could be as simple as these sensors, eh? Like I said, I've cleaned the MAF before but not recently so I guess it's worth a try. It's not the original MAF either, so there's that. However, I've never pulled the IAC. I know where it is and everything but once it's off how do I clean it? Is it like the MAF with a large opening and the sensor hanging in the middle? Also, my headgaskets may be original. The car was already over eight years old and had 118k miles on it when I originally got it so they may have been changed for all I know.
 

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Discussion Starter #6
Okay, I take that back on the IAC. I was looking at my records and apparently I did replace the IAC almost four years and 14k miles ago. Like I said, I'm not as savvy with repairs as I used to be! I need to go get some cleaning supplies.
 

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Discussion Starter #7
I got some MAF and carb cleaners and cleaned the MAF and the IAC. On the plus side, the cleaning must have been necessary because the car felt stronger when I drove it around briefly post-cleaning, and the idle RPM is up slightly. On the down side, the stutter is still present. So, back to the drawing board.

Again, I can largely avoid the stutter if I'm more deliberate with acceleration, or if I let off on the gas altogether if the engine begins to stutter. But, if I give the throttle just a light touch this is where the stutter occurs. Also, to clarify, when parked I can maintain the stutter quite consistently if I keep pressing the accelerator at that "sweet spot" where it occurs and can keep the stutter going with a very slight increase in pressure from my foot on the pedal up to maybe 1750 RPM before it goes away. But again, if I intentionally produce the stutter when in park, I can eliminate it with a deeper stab on the gas pedal or by letting off.

Also, I feel I should mention my gas usage. As I said earlier, my daily driver is a 2012 Focus so I only drive my T-bird maybe two or three times a week, and usually only to work which is less than five miles away. As a result the miles I've been putting on the car have really dropped off in the last few years and I can usually go more than a month between fill ups. Could something be getting gummed up from the gas sitting too long? I use Chevron gas exclusively.
 

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(edit) Your miss can be produced under no load in park so it may not be like mine. I'd start with a few bucks worth of fuel injector cleaner or Seafoam in the gas tank. Like you I am due for a new fuel pump. Otherwise, my original response below...

This sounds a lot like the light miss I have under load. Mine was noticed after installing shorty headers rebuilt injectors and Cobra intake on a 5.0 engine. Ideas are its a lean cylinder maybe from partially plugged injector or fractured spark plug. It's weird how you can stomp on it and you don't feel a miss anymore.

When I installed 3.73 gears later the car doesn't get in a lugging situation as often where this miss presents itself.

There are two spark plugs on my right side I have not been able to get to yet so I cannot rule out a fractured plug yet. I will need to remove my shorty headers and the smog hose to reach them easily.
 

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Discussion Starter #9
I neglected to mention that I tried Seafoam last week. No effect, but probably not a bad idea. I hadn't used it in long time.

I'm still skeptical, but I may have had a breakthrough with the stutter issue. The references to the MAF and IAC got me thinking about the intake in general and I thought to check my throttle position sensor since it's right next to the IAC. Long story short, it's in a more awkward position to remove it than it looks and I was having trouble removing the screws that hold it on, so I skipped it because I was short on time. However, I thought to check the throttle body itself while I was in the area. I removed the intake hose and sprayed the inside of the throttle body a few times with carb cleaner. Saw a good amount of dirt come off. I started the car and tried to reproduce the stutter and I couldn't! But I wanted to reserve judgement until I could drive the car some distance. I went to work and on the drives to and from I intentionally tried to reproduce the stutter. I was able to get it to happen again but it was much more difficult and the stutter itself more subdued. Even in park when it was previously very easy to cause the stutter, it was now almost impossible to cause it. Part of me still thinks it's a fluke, but nothing else that I've tried has had as much of an impact as spraying carb cleaner into the throttle body. I'm going to give the throttle body a more thorough cleaning and see if I can eliminate the stutter completely.
 

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I had a shudder just like that once, and it turned out to be loose spark plugs.

After rebuilding my transmission 3 years ago, the car developed a similar shudder in the following months. It was my first rebuild, and I feared the worst. Feeling defeated, I took it to the local dealer for a proper diagnosis. I was fully willing to pay shop rate and let them hook up their computer, test pressures, whatever they do.

They fixed it while I was in the waiting room. I paid for one measly hour of labor.

The technician told me that some of the spark plugs had come loose, initiating the shudder. I protested that the CEL never flashed, but he replied that the misfire was so minor that the OBDII wouldn't trigger the light. He said he had seen this happen on other engines that also had Autolite plugs installed, presumably due to a slightly different chamfer on the plugs. At his recommendation I bought some Motorcraft plugs to install later.
 

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Discussion Starter #11
Well, I thank everyone for your input on this issue, but I think I'm going to call it: the stutter was throttle body related. I disconnected the intake hose and removed the IAC again and gave the throttle body a more thorough cleaning. I sprayed carb cleaner into the throttle body inlet and into the holes where the IAC goes, used about a dozen q-tips on all the nooks and crannies as best as I could and even used a toothbrush to scrub the inside walls of the throttle body and the butterfly valve.

I drove to and from work and tried my best with the feather-touch pressure on the gas pedal that previously would generate this stutter and....it still happened, but only once for a split second. So, not perfect, but it's worlds better than it was where I could maintain the stutter at will. When I got home I had the car in park and tried to reproduce the stutter and I really could not. I could feel and see in the tach a slight hesitation of sorts when I would feather the gas pedal, but it was nearly imperceptible; sort of like you notice it only because you knew that's where it was happening before. And this is when something actually happened. Most of the time now nothing happens and the engine revs up smoothly as you feather the accelerator. So unless there's a more detailed and worthwhile way of cleaning the throttle body more than I have, I have to say this problem is fixed.
 

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Discussion Starter #12
Or not fixed...

It's been a few days and, to my chagrin, the stutter mystery continues. I don't know if the cleaning of the throttle body temporarily masked the stutter problem somehow, but ever since I did it I've been intentionally trying to reproduce the stutter to see how effective the cleaning was. On the road the car drives well enough with the hiccups being almost nonexistent. However, in park the stutter is about as easy to reproduce as it was before. Really annoying.

I still have no check engine lights amid all of this, but now that the stutter is easy to produce again just above idle I'm concerned about passing my smog test next year. The high speed test would probably be okay, but the low speed test (25 MPH, I think) would involve light throttle input and I fear the stutter will manifest and cause an anomaly.

So, other ideas?
 

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Discussion Starter #13
I was looking around the website and, since this issue may still be intake related, I saw that Seafoam can be used to clean the engine intake through using vacuum lines? I'll have to get some more since I already used what I had left in the gas tank, but I was reading that there is also a Seafoam spray. Would this be just as effective? I was reading on their website that you spray it directly into the throttle body.
 

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Still could be a MAF issue. See what happens with it unplugged entirely. A:F or spark is off when it's missing/stuttering/bucking. The MAF sensor is the single most important player that influences all of these. If the electrics are going bad, it could be giving bogus output only in a certain airflow range. Running with the MAF unplugged will help rule that out as the car relies on separate tables to compute fuel/spark with a failed MAF.

Also unhook the electrical leads to the EVR solenoid to disable EGR (do this NOT at the same time you try running with the MAF unplugged) to see if that has an effect. Too much EGR that isn't compensated for by extra spark will slow the combustion process down significantly enough to cause a slight misfire.

California car, right? EDIS (no distributor)? Crank sensor controls spark; if it's gone screwy it can do weird things too (although usually when it goes you get a no-spark condition). There's a separate EDIS module (I believe near the air box) that could also be doing strange things.
 

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Like any old car and especially those that have lived in areas of higher than normal heat it's really a good idea to have a close look at ALL the vacuum lines. High underwood temps eventually cook and hasten dry rot of the rubber hoses. This could be a reletively minor vacuum leak that's masked at larger throttle openings. This wouldn't be the first car that idled rough from a vacuum not leak and ran fine at higher RPMs.
 

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Discussion Starter #16 (Edited)
Still could be a MAF issue. See what happens with it unplugged entirely. A:F or spark is off when it's missing/stuttering/bucking. The MAF sensor is the single most important player that influences all of these. If the electrics are going bad, it could be giving bogus output only in a certain airflow range. Running with the MAF unplugged will help rule that out as the car relies on separate tables to compute fuel/spark with a failed MAF.

Also unhook the electrical leads to the EVR solenoid to disable EGR (do this NOT at the same time you try running with the MAF unplugged) to see if that has an effect. Too much EGR that isn't compensated for by extra spark will slow the combustion process down significantly enough to cause a slight misfire.

California car, right? EDIS (no distributor)? Crank sensor controls spark; if it's gone screwy it can do weird things too (although usually when it goes you get a no-spark condition). There's a separate EDIS module (I believe near the air box) that could also be doing strange things.
Funny thing about having a car so long is that the repairs you make start to get old after a time and it's like starting over. My MAF is not original. According to my records the original went bad 11 years and almost 60k miles ago. It was causing bad stuttering, even stalled once, and was generating CELs. Replaced it with a new one (remanufactured). So, could be issue, but it's not original at least.

EGR's all original so if there's an issue with it then that could make sense. Do the EGR components have any sensors that would generate a CEL? And with disconnecting the MAF or EGR, what sort of behavior am I looking for? The car will run poorly with either disconnected anyway, right?

And the car is Californian: distributorless. I had a different stutter some time ago (would randomly manifest with consistent throttle while on the road and sometimes generate CELs) and fixed it by replacing both the cam and crank position sensors. That was about 30k miles ago. The EDIS module is precisely where you described it and is original. They're not all that expensive. I'll take a look at all these parts on my next days off.

Like any old car and especially those that have lived in areas of higher than normal heat it's really a good idea to have a close look at ALL the vacuum lines. High underwood temps eventually cook and hasten dry rot of the rubber hoses. This could be a reletively minor vacuum leak that's masked at larger throttle openings. This wouldn't be the first car that idled rough from a vacuum not leak and ran fine at higher RPMs.
Very true. I've been lucky and as much as I'd like to see the car run forever it's so old and has so many miles that it has the right to break down. But if I can keep it going without spending too much, I will. To be specific about my current issue, if it makes a difference, the engine idle is smooth. It's when you feather the throttle, particularly when the car's in park, that the stutter occurs, starting at a little over 1000 RPM.

Unless you guys have any advice against it, I think my next move is going to be to try the Seafoam spray into the throttle body. When I cleaned the throttle body and IAC before, I had the engine off so I wasn't really getting deep into the intake. After that, we'll see.
 

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The DPFE monitors EGR flow and the PCM compares feedback from it to what it expects to see based on its commands. If there's a mismatch (either too much or too little) it will throw a code (at least in OBD-II, not sure about OBD-I).

If the MAF is indeed bad, it should run better with it unplugged than plugged in.

If there is an issue with EGR monitoring, you will run better with the EGR vacuum regulator solenoid unplugged, which disables the PCM from actuating vacuum to permit flow through the EGR valve.

One other thing you can try is unhooking the battery for a few minutes to clear out the PCM's keep-alive memory, just in case you have a bogus pair of O2 sensors or something that may have the PCM delivering a weird fuel trim or something else like that being off when you're having issues.
 

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Discussion Starter #18
The DPFE monitors EGR flow and the PCM compares feedback from it to what it expects to see based on its commands. If there's a mismatch (either too much or too little) it will throw a code (at least in OBD-II, not sure about OBD-I).

If the MAF is indeed bad, it should run better with it unplugged than plugged in.

If there is an issue with EGR monitoring, you will run better with the EGR vacuum regulator solenoid unplugged, which disables the PCM from actuating vacuum to permit flow through the EGR valve.

One other thing you can try is unhooking the battery for a few minutes to clear out the PCM's keep-alive memory, just in case you have a bogus pair of O2 sensors or something that may have the PCM delivering a weird fuel trim or something else like that being off when you're having issues.
Okay, I had some time today and was able to try a couple things. I was able to drive the car around a bit to warm it up and try that Seafoam spray. I used it as instructed, got a little bit of smokey exhaust, but far from the "smoke show" that some have described when using the regular fluid. Drove it around afterward to blow out any deposits that may be left, as instructed, but, unfortunately, no improvement on the stutter, though the engine felt stronger during regular driving. So the stuff didn't hurt the engine in any noticeable way at least. The spray was kind of awkward to use though so I may not have got it in as well as I could have. I bought a second can so I can try again since a good intake cleaning probably isn't a bad idea.

Now, as for the MAF, after the intake cleaning with Seafoam, I tried disconnecting the sensor as you recommended. My results were mixed. With it disconnected I started the engine back up and, right away, my idle was a little uneven. Almost like the stutter I've been having but more subtle, but nonetheless present right at idle. I revved the engine up a little and, no surprise, the CEL came on, but I wasn't able to produce the stutter. So, good news on maybe isolating the MAF as the stutter culprit. However, when I shut the engine off, reconnected the MAF and restarted the engine, I tried reproducing the stutter and I couldn't. So what does this mean? Does the MAF have some sort of learning process with the PCM?
 

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I'm not sure about the MAF specifically, but there are certainly other things the PCM keeps track of and adjusts (like IAC and fuel trims, for example) Having the MAF unplugged may have had the PCM fall back on something closer to a stock value for one of those. Drive the car a while and see if it comes back; if it does, try unplugging the battery for a couple minutes to reset all the things the PCM's learned and test again.
 

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Discussion Starter #20
I'm not sure about the MAF specifically, but there are certainly other things the PCM keeps track of and adjusts (like IAC and fuel trims, for example) Having the MAF unplugged may have had the PCM fall back on something closer to a stock value for one of those. Drive the car a while and see if it comes back; if it does, try unplugging the battery for a couple minutes to reset all the things the PCM's learned and test again.
Done. Unplugged the battery for a few minutes, reconnected and drove around for a bit. I had driven the car for a while a couple hours prior so it was already warm. I was able to sense a slight stutter while driving around, post-battery disconnect, when I feathered the throttle as before. Got back home and feathered the throttle in park and the stutter presented itself as before.

So, next step? Should I bother with the EGR test or go for a MAF replacement?
 
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