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Discussion Starter #1
Hi I have a 91 3.8 Base Thunderbird that has an odd issue of either the idle dropping too low when reversing or stalling in reverse.

I noticed it feels like it has either some roughness or a random miss at times while at a cruising speed and other times it runs smooth? The odd thing is it will run smooth at a normal idle as well so you can't tell something is visibly wrong under the hood. Gas mileage seems inconsistent too, it was doing nearly 19-20mpg in town, now it'll barely do 15mpg in the same driving conditions, highway is better though.

Here's some stats for it:
  • 80,500 miles currently, 4,500 miles since head/headgasket replacement
  • Replaced both the cylinder heads (with fully rebuilt heads, originals were not re-used)
  • Replaced the head gaskets with the Felpro gasket kit
  • ARP head studs
  • Air Silencer delete
  • 70MM maf housing (original maf sensor though)
  • K&N filter (it had the same issue with the paper filter)
  • New plugs & plug wires
  • Cleaned the IAC valve
  • Ran fuel injector cleaner through it
  • Cleaned the MAF sensor with MAF cleaner
The engine doesn't overheat at all and is in the correct temp range (usually right between the "M" and "A" in NORMAL or slightly lower). No coolant loss or other fluid losses, I am checking all fluids at least once a week. No check engine light either, it does work though.

I noticed that after cleaning the MAF sensor, it either sometimes runs slightly better or has a greater tendency to stall for a day then it normalizes. Should I just replace the sensor or does this sound like another issue? I do know their is a quick "wind-up" sound that happens sometimes after a long drive (my guess is PS Pump) but the stalling/almost stalling happens even with a cold engine and no other noises present.

Thanks for your help and your time.
 

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Have you cleaned the throttle body? Also check your new wires and make sure they are as far away from the exhaust manifolds as possible. I had an issue like yours with my 4.6, I went out at night and started it up to allow me to see the arc from any bad plug wires. When troubleshooting I try to start with the common issues because they are easily overlooked most of the time.
 

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Discussion Starter #3
Have you cleaned the throttle body? Also check your new wires and make sure they are as far away from the exhaust manifolds as possible. I had an issue like yours with my 4.6, I went out at night and started it up to allow me to see the arc from any bad plug wires. When troubleshooting I try to start with the common issues because they are easily overlooked most of the time.
I haven't cleaned the throttle body but I did check the plug wires and replaced them all last week. Replacing the plug wires and re-positioning them helped as at idle it runs smooth now most of the time, it used to have a constant slight miss before eventhough the wires weren't old.

Still feels a bit rough at cruising speeds below 60mph and it has the stalling/low idle in reverse issue as well. I'll clean the throttle body though, it might be the cause of the random roughness in the idle and while cruising.
 

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What is the purpose of the larger MAF housing? I would think that could be part of the problem. Check for vacuum leaks.

Describe the 'wind-up' sound. Where does it come from? A bad IAC will make a 'honking' sound. Power steering will whine when running (thanks Ford :rolleyes:) and will get louder if under load or low on fluid.
 

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Discussion Starter #6
What is the purpose of the larger MAF housing? I would think that could be part of the problem. Check for vacuum leaks.

Describe the 'wind-up' sound. Where does it come from? A bad IAC will make a 'honking' sound. Power steering will whine when running (thanks Ford :rolleyes:) and will get louder if under load or low on fluid.
The larger 70 MAF housing was standard on the later years of the 3.8 and 4.6 IIRC (94-97). The 91-93 3.8 mn12s IIRC use a smaller 55mm MAF housing. It runs a bit better with the 70mm housing and doesn't stumble/hesitate when cold anymore in drive. It is a bit better in reverse now as well, it doesn't stumble in reverse or stall as easily as it did before but the RPMS still drop low in reverse causing a stall once in a while.

Although the MAF housing now that I think of it might explain the variance in gas mileage. I'll have to see how both compare. I figured it wouldn't make much of a difference since it is an early mass air system but maybe that combined with no air silencer (stock airbox though) is causing a problem?

It's might not be a bad IAC, thanks for mentioning that sound as I remember now what it sounds like as I had a Grand Marquis that had the IAC fail on it which made that sound. Not sure about the whine, it happens so quickly that by the time I open the hood, it's often already gone. It might be just normal power steering whine under load (it happens when it's being reversed to park on a hill) if that's typical of Fords.
 

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The larger 70 MAF housing was standard on the later years of the 3.8 and 4.6 IIRC (94-97). The 91-93 3.8 mn12s IIRC use a smaller 55mm MAF housing. It runs a bit better with the 70mm housing and doesn't stumble/hesitate when cold anymore in drive. It is a bit better in reverse now as well, it doesn't stumble in reverse or stall as easily as it did before but the RPMS still drop low in reverse causing a stall once in a while.

Although the MAF housing now that I think of it might explain the variance in gas mileage. I'll have to see how both compare. I figured it wouldn't make much of a difference since it is an early mass air system but maybe that combined with no air silencer (stock airbox though) is causing a problem?

It's might not be a bad IAC, thanks for mentioning that sound as I remember now what it sounds like as I had a Grand Marquis that had the IAC fail on it which made that sound. Not sure about the whine, it happens so quickly that by the time I open the hood, it's often already gone. It might be just normal power steering whine under load (it happens when it's being reversed to park on a hill) if that's typical of Fords.
I'm not sure why you believe using the original sensor in a larger housing is supposed to help things without tuning for the difference in air.

A MAF sensor reports a difference in voltage based on how much air flow cools the heated wire and changes the voltage. The MAF transfer function in the ECU is calibrated because they know that at a certain CFM, XXX cu-ft of air is let in.

By changing the diameter of the housing, you are in effect changing the amount of air being let into the engine for a given reported voltage. That transfer function is now out of sync with reality so hillarity ensues. Had you switched ECUs and the entire MAF at the same time, you probably wouldn't have these drivability issues (but you would need to confirm that no other functions are being left out and/or not accounted for by comparing pinouts. Ex: 95-97 ECUs also controlled the transmission and won't be happy if that's missing).

I suspect you are seeing a hesitation at idle because your idle air control valve (IACV) is gunked up and/or not working. When cold and at idle, the IACV is supposed to open up to let more air into the engine and keep it rev'd up more to warm things up.

Now, my theory doesn't completely explain why you claim the engine is better at idle when cold and you have the frankenMAF hooked up because when the throttle body is closed, I wouldn't expect that much more air to flow through the pinholes in the throttle plate.

As far as drivability is concerned from your original post, my first suspect would be the O2 sensors. Have they ever been replaced? O2 sensors degrade over time and that's often a key reason higher mileage cars start seeing a drop off in fuel economy.
-g
 

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Discussion Starter #8
I'm not sure why you believe using the original sensor in a larger housing is supposed to help things without tuning for the difference in air.

A MAF sensor reports a difference in voltage based on how much air flow cools the heated wire and changes the voltage. The MAF transfer function in the ECU is calibrated because they know that at a certain CFM, XXX cu-ft of air is let in.

By changing the diameter of the housing, you are in effect changing the amount of air being let into the engine for a given reported voltage. That transfer function is now out of sync with reality so hillarity ensues. Had you switched ECUs and the entire MAF at the same time, you probably wouldn't have these drivability issues (but you would need to confirm that no other functions are being left out and/or not accounted for by comparing pinouts. Ex: 95-97 ECUs also controlled the transmission and won't be happy if that's missing).
I found a couple of threads on here saying it's ok to use a 70mm MAF housing in an older 3.8 such as mine and some said it's ok to run without a tune. Here's two I found below but if you do a search a couple others will come up too.

http://forums.tccoa.com/showthread.php?t=82972&highlight=3.8+70mm+maf
http://forums.tccoa.com/showthread.php?t=44159&highlight=3.8+MAF+housing

I had a 94 Crown Vic MAF already so I figured it might be worth a shot on what I read. I appreciate the info though, I'm used to dealing with Speed Density and MAP sensors so I haven't fully understood how a Mass Air system works.

I'll try swapping in the old housing though, but I have no idea why it runs better either when cold either? My only guess is maybe the MAF sensor was cleaned well enough to work properly as I cleaned it again before putting it in the 70mm housing.

The 02 sensors are likely old as I never replaced them, even after replacing the heads/headgaskets and I know coolant can contaminate them. I guess they are only working well enough to not throw a check engine light or regular driving finally caught up with them, hurting gas mileage.

I'll check over the IAC valve and the throttle body as well. Cleaning the IAC stopped a random high-idle start it had after first getting it back on the road but I noticed it has a slight surging idle that may appear at random. Most of the time it runs fine even at startup though.
 

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Swapping the MAF for a different size one without getting a tune is definitely going to mess with the air/fuel ratios and cause issues. Swap the stock one back first. Also, I don't see new O2 sensors anywhere on your list of things replaced. Coolant kills those things extremely quickly, so if the car was blowing white smoke when the head gaskets failed, the O2 sensors are shot as well. Next, as S4gunn pointed out, if the problem is only at idle, a sticking IAC could be the culprit. Pull it out, and clean it real good with carb cleaner, even filling it up with the carb cleaner and letting it soak for a few hours to loosen up all the carbon buildup inside. Do all that, and also do another thorough once-over of the top of the engine looking for vacuum leaks, and report back. I'm betting though that doing all that will solve your problem.
 

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Discussion Starter #10
Swapping the MAF for a different size one without getting a tune is definitely going to mess with the air/fuel ratios and cause issues. Swap the stock one back first. Also, I don't see new O2 sensors anywhere on your list of things replaced. Coolant kills those things extremely quickly, so if the car was blowing white smoke when the head gaskets failed, the O2 sensors are shot as well. Next, as S4gunn pointed out, if the problem is only at idle, a sticking IAC could be the culprit. Pull it out, and clean it real good with carb cleaner, even filling it up with the carb cleaner and letting it soak for a few hours to loosen up all the carbon buildup inside. Do all that, and also do another thorough once-over of the top of the engine looking for vacuum leaks, and report back. I'm betting though that doing all that will solve your problem.
It has the stock 1991 3.8 MAF sensor but in the larger 94-97 70mm housing (used on both the 4.6 crown vic and thunderbird). I'll swap back in the 55mm stock housing though, I was hoping for an easy mod based on those threads but I guess without a tune that's not going to work.

All the vacuum lines are good, I just checked that and I replaced alot when I did the headgasket job. For the 02 sensors, would I need a specific 02 sensor socket to get them out? It looks like I could get away without needed one but I don't want to risk if it's better to buy the specific socket.

Also, for the IAC, that is good to know that carb cleaner is ok to use, Ill have to try that as I'm sure it's still a bit gummed up with carbon.

Thanks for all the help!
 

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The O2 sensors use a 7/8" flange. Don't use the open end to try to get the old ones out cause they will be rusted in there pretty good, and you'll end up stripping it. If you cut the connector off the old one, you can slip the closed box end of the wrench over it, and that should work to get the old sensor out. For the new one, tighten it with the open end wrench, but don't go nuts on it, just a little tighter than snug is tight enough.
 

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Discussion Starter #12
It's fixed! The main two problems were the idler pulley and belt tensioner were both bad, it got worse two days ago and both were making a bit of noise. Once they were fixed, that cleared up most of the roughness and random "miss" at cruise issues. It seemed like it fixed the reverse stalling issue too, the idle stays where it should now. I also installed a ASP underdrive crank pulley and alternator pulley at the same time but didn't notice too much of a difference, it seems a little bit quicker off the line and at higher rpms.

Then the next day I installed the 55mm MAF housing and reset the computer. That cleared up any remaining roughness at cruise or idle issues and it idles and drives perfectly in drive and reverse now, basically as nice as a new car. It accelerates a bit smoother as well, surprising that a small mm difference between the housings can impact the engine so much. The 70mm housing did make it a bit more fun at WOT (it was more responsive and shifted a bit more aggressively) but definitely impacted normal driving conditions too much.

I'll tackle the 02 sensors a bit later and the IAC valve once time/money allows again, for now it's at least in good working order.
 

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It's fixed! The main two problems were the idler pulley and belt tensioner were both bad, it got worse two days ago and both were making a bit of noise. Once they were fixed, that cleared up most of the roughness and random "miss" at cruise issues. It seemed like it fixed the reverse stalling issue too, the idle stays where it should now. I also installed a ASP underdrive crank pulley and alternator pulley at the same time but didn't notice too much of a difference, it seems a little bit quicker off the line and at higher rpms.
Noise sure but I'm trying to logically understand why the accessory belt not moving would cause miss/cruise issues.

The accessory belt drives your water pump, PS pump, alternator, and AC.
There's no smog pump or something that could choke the engine on a 3.8L that I'm aware of.

Therefore, the only thing that could cause this issue would be if at higher RPM, the alternator wasn't spinning enough to supply the necessary ignition sparks and the battery couldn't make up the difference.

Do you buy that MadMikey?

Obviously, it's good that you fixed it but keep in mind that cleaning the IACV is a free, 15 minute job (two bolts, spray it, soak it, spray it out later, and install it).

-g
 

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Noise sure but I'm trying to logically understand why the accessory belt not moving would cause miss/cruise issues.

The accessory belt drives your water pump, PS pump, alternator, and AC.
There's no smog pump or something that could choke the engine on a 3.8L that I'm aware of.

Therefore, the only thing that could cause this issue would be if at higher RPM, the alternator wasn't spinning enough to supply the necessary ignition sparks and the battery couldn't make up the difference.

Do you buy that MadMikey?

Obviously, it's good that you fixed it but keep in mind that cleaning the IACV is a free, 15 minute job (two bolts, spray it, soak it, spray it out later, and install it).

-g
A mechanic friend of mine who has worked on SC's pointed out that they were bad. We changed them and viola, I could reverse and the RPMs stayed where they needed to be instead of at the stalling point. We also replaced both the crank and alternator pullies at the same time (ASP underdrive kit).

It stopped the random annoying vibration/roughness it would have when driving in traffic (normal, light acceleration) and idling too. Installing the 55mm housing stopped the miss at cruise and any remaining roughness altogether. The computer wasn't reset until after the 55mm housing was installed as well.

Either way, I'm glad it works now and it wasn't too hard to fix.
 

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Discussion Starter #15
Well I spoke too soon, apparently I have a minor head gasket leak. I used ARP heads studs with the Felpro kit and torqued them to SC specs too (75ft/lbs final with an expensive torque wrench). The head studs probably saved it though, the head gaskets haven't blown and it still runs/drives pretty normal in the correct temp range.

I was driving it to work on 476 at 70mph when it started mis-firing on and then the CEL kept coming off/on. It eventually smoothed out after 20 minutes and I left work early to take it to a mechanic, one that actually serviced the car most of his life. Apparently it's burning coolant and some coolant is getting on the 02 sensors, causing the check engine light and random roughness/mis-firing. It isn't putting out white smoke but it does dump a small bit of water out of the exhaust if left running long enough apparently.

The mechanic said try re-torquing the heads, it might be fine after I do according to him and it runs fine otherwise. Does this sound like a solid fix or should I be prepared to cut my losses? Headgasket job was done 5,000 miles ago, rebuilt cylinder heads, fel-pro kit, etc. Followed the chilton's, hayes, and threads on here to a T, the car ran well until recently for the most part.
 

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Something doesn't sound right. First off, if you are burning coolant, and coolant is contaminating the O2 sensors, then you should also be seeing white smoke and smelling coolant in the exhaust. If there is no smoke, then there isn't enough coolant in the exhaust to kill the O2 sensors. Also you said you were not losing any coolant; is this still true? Based on what I am reading here, I really doubt you have bad head gaskets again. I have seen head gaskets re-blow when people either don't get the heads checked/resurfaced, or re-use the original exhaust bolts, but you did the job right, so that shouldn't be an issue. I will say that I have seen on SCs where new head gaskets were killed by running lean, which you definitely could have been running lean with the wrong MAF in there, but with no boost, I don't think it would be enough to burn up the gasket. Just trying to think what else your problem could be, and I see you replaced the plugs and wires. How old are the cap and rotor?
 

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I didn't get a chance to really check the coolant levels (left the car behind in PA, traveling) but the reservoir appeared full on a quick inspection Friday when the engine was hot (been running for about 2 hrs). I'll have to check it throughly once I'm back in PA next week. It's not smoking visibly but it's definitely burning something, even the mechanic showed me with it idling in the garage and I noticed it on the drive home too with the windows down at low speeds. I couldn't tell if it was specifically coolant though that was the burning smell. It wasn't smoking in the garage either, just some water dumped out of the tailpipe.

The weird thing is it was fine until I changed the oil last week and put a little bit of seafoam in the gas tank. I first used cheap walmart 5w30 that I had with a motorcraft filter (FL 400s) but it read way over on the dipstick (almost to L in the word full on the dipstick). I then changed it to Pennzoil 5w30 high mileage with a Motorcraft filter. With 5 quarts in it read slightly over the crosshatch section so I figured it was just a fluke still (the dipstick has a hard time going into the dipstick tube) or gunk in the pan.

I think the oil thing might be bad news though, IIRC that could mean coolant is getting into the crankcase or something else. The oil was clean though when they checked it and even a few days after I checked it. The old oil looked fine too, no signs of coolant.

The cap/rotor has around 5k on it (did it when I did the gaskets) but they are the cheapest cap/rotor set you can buy. Possibly they are causing an issue, it was rainy/humid and it ran poorly before the CEL came on while on the highway.

The only other thing I can think of is a bad intake manifold gasket, I did have to replace one bolt with another from a hardware store as it snapped removing the manifold originally. It was short about 1-2mm but the closet match, torqued down ok but maybe it loosened up? It never leaked visibly externally but the car has been run on hard highway runs lately (bad traffic/construction) and maybe it was enough to cause an issue.
 

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Intake gasket is a possibility. I know the intake bolts can loosen up from heat cycling, and whenever I do an intake on a 3.8, I make sure to go back around 4 or 5 times re-torquing them until they hold the same torque value.
 

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Discussion Starter #19
Thanks for the help Mike! I followed the head gasket job directions to a T so I'm a bit puzzled about the shop's findings, especially as I checked over it today and it's not showing signs of a blown headgasket or burning coolant.

I came back and checked the car today after it's been sitting since Friday afternoon. The radiator is full and the coolant reservoir is still full. The oil is clean, still looks fresh from the oil change last week and it's at the same level. No signs of coolant leaking from the engine either. It ran ok too, there was some water in the exhaust but it dumped maybe a 6-10 inch spot on the ground and nothing more after about 5 minutes or so, no smoke as I think that was just condensation in the exhaust. No signs of sweet-smelling exhaust either, and the water seemed normal to me as my 5.0 87 grand marquis coupe would do the same, especially after installing dual exhaust with open-chamber mufflers on it.

The idle is rough but not a harsh roughness to cause a stalling or driving issue, just not smooth as it should be, seems like it's slightly missing until it warms up then it smooths out steadily. I'll check the torque on the bolts for the intake manifold and then replace the cap/rotor along with the 02 sensors. The only other thing I can think of is the heads need re-torquing as the shop mentioned or it's a problem with the catalytic converters (coolant did get in there before while replacing the headgaskets and likely when they blew).

Also, it could have been burning oil off the exhaust/cylinder head, I forgot that I spilled a bit onto the driver's side valve cover when changing the oil last week and it wasn't driven as much since I drove my Jeep for a few days. That would cover the burning smells the shop noticed and I did driving it later on Friday afternoon.
 

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With no loss of coolant, I am wondering if your intake bolts loosened up and you have a vacuum leak when cold? I would first check to see if any of the lower intake bolts are loose. If they are, try tightening them all up, and if that doesn't work, try replacing the intake gaskets.
 
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