Unplugging MAF Improves Performance. Rough Idle / Hard Shifting

[mbruffey]

1997, 5.0, 75K.

Today I disconnected MAF, but first:

I have had a problem with rough idle and somewhat rough/erratic shifting for about a year now, I suppose. Idles rough--noticeably but not severely--most of the time, cold and hot. Improves some days, especially damp days. I drive gently, but upshifts roughly even on mild acceleration. Tends to "drop" up into 3rd gear abruptly if near shift speed and decelerate at stop sign, say 20 mph. Used to upshift religiously around 2000 RPM, but now holds until say 2100-2200. Sometimes start-stall when hot, but not very often. Light pressure on accelerator usually starts it. Tailpipe is nice and brown (not black) inside. Exhaust seems to have a deeper, throatier thrumming than it used to, but this varies with performance. Downshifts earlier than it used to, when I call for power or climb a hill at moderate speed (30mph). More recently, noticeably downshifts 3-2 when descending a hill and slowing/braking. NO CODES. NO CODES.

Actions taken (none resulted in noticeable/permanent improvement):

*Replace Plugs and Wires
*Replace Fuel Filter
*Replace Fuel Pressure Regulator
*Clean IAC
*Clean MAF
*Measure TPS Resistance / Travel
*Deoxit TPS and ream holes reinstalling slightly CW in case flat/dead spot.
*Fresh Silicone Grease on Sensor Connectors mentioned above
*Check Battery Voltage (12.5 off; 13.8 running)
*Clean Air Filter

Today:

Disconnected MAF and drove home (6 miles):
*Idle smoother, but not perfect. Roughness of idle different in character; hard to explain.
*More power accelerating from stop, but I did not push it.
*No more downshift climbing hill at 30-35mph.
*Shifting overall exquisite, except a little rough 1-2.


Conclusions [from today's MAF experiment] (correct? you tell me!):

NOT EVEN POSSIBLY the Fuel Pump, since fuel pump clearly can deliver.
NOT LIKELY A Vacuum Leak, since character of rough idle changed and acceleration/power returned.
NOT NECESSARILY MAF, since disconnecting MAF "disables" many other sensor to computer signals.
NOT LIKELY Tranny, since shifts fine (except 1-2) w/MAF disconnected.
NOT LIKELY Bad Gas. Disconnecting MAF unlikely to improve.
NOT Ignition. Tailpipe would be black; would run worse damp days.

What's happening? My working hypothesis: Computer "thinks" engine is running rich and is leaning it out when MAF is connected. NO CODES.

Am I right? Wrong? Talk to me! I need a strategy so I can poke at the most logical spots in the most logical order.

What is it? Why no codes? Possibilities:

*Knock Sensor? Where's that located?
*Temperature Sensor? Where's that one, too?
*Bad MAF? Noticed ends of one wire inside are browned. Backprobe? Expected Voltages?
*O2 Sensors?

What?

Thanks, LMB

 

[gtyates]

I think you are on to something with the MAF. When you unplug it the computer goes into a pre-programmed mode to replace the missing signal.

Do you have hard start issues when the engine is hot? My OHV engine did. Replacing the Coolant Temp Sensor solved that.

 

[my98nnj]

Replace the MAF.

 

[mbruffey]

I think you are on to something with the MAF. When you unplug it the computer goes into a pre-programmed mode to replace the missing signal.

Do you have hard start issues when the engine is hot? My OHV engine did. Replacing the Coolant Temp Sensor solved that.

Thanks GT:

Hard start hot, occasionally. Starts-stalls. Have to give it a little gas to prevent immediate stall.

Is the Temp Sensor the one you can get at without removing the coil pak mount, or the one you can't get at without removing the coil pak mount? This is a '97 5.0.

I'm thinking replace temp sensor, as it's the cheapest part that is associated with the start-stall hot. I plan to backprobe the MAF, first, however, since I can actually get at it easily.

Anyone ever noted browning of ends as normal on either of the MAF elements? Do those elements get hot enough to glow in the dark (like nichrome wires) when working properly? I suppose I could check it that way if so.

LMB

 

[Dan Whitaker]

The temp sensor is under the coil packs. The other one to the left is the temp sending unit for the temp gauge.

 

[2000StreetRod]

Test before replacing

Why not test the MAF sensor output before replacing it? See My Helpful Thread MAF Sensor Test Procedure

Your voltages will be different for your V8 from my SOHC V6 but you still should be able to tell if your MAF sensor is working.

 

[mbruffey]

The temp sensor is under the coil packs. The other one to the left is the temp sending unit for the temp gauge.

I was afraid of that!

 

[mbruffey]

Why not test the MAF sensor output before replacing it? See My Helpful Thread MAF Sensor Test Procedure

Your voltages will be different for your V8 from my SOHC V6 but you still should be able to tell if your MAF sensor is working.

Thanks, 2000. I just had just discovered and saved the link to your procedure late last night (or early this morning!). Thanks! LMB

 

[Dan Whitaker]

I was afraid of that!

It's not that hard. Just remove the three bolts and move the tower back and you can access it easily.

 

[gtyates]

Another temp related sensor is the air intake temp sensor. On my OHV engine it is located in the air intake tube near the throttle body. On mine the red "sensor" wire part was black and corroded. It is inexpensive and very easy to replace. You can try cleaning it with the MAF sensor cleaner. Cleaning did not help mine.

 

[Dan Whitaker]

I've been chasing a problem similar to yours. I rulled out the MAF & IAT as I have a 97 Cougar with a 4.6 that uses the same IAT & MAF sensor. When I put the MAF & IAT from my Mountaineer in my Cougar I had zero issues and no codes.

Last night I unpluged the TPS and my surging/rough idle, stalling at idle & stumbling/stalling when I give it gas was gone.

You might give it a try and see.

 

[mbruffey]

I've been chasing a problem similar to yours. I rulled out the MAF & IAT as I have a 97 Cougar with a 4.6 that uses the same IAT & MAF sensor. When I put the MAF & IAT from my Mountaineer in my Cougar I had zero issues and no codes.

Last night I unpluged the TPS and my surging/rough idle, stalling at idle & stumbling/stalling when I give it gas was gone.

You might give it a try and see.

Yes, that TPS is a peculiar little device. I have had them go bad with no codes on previous occasions. Although I do have "no-starts" once in a while, I do not have stumbling, stalling, etc. Just rough idle and goofy shifting.

FWIW, yesterday I reset the computer, but instead of driving gently, as is my custom, I drove like a madman--full throttles from dead stop, etc. I hate to treat a vehicle that way! Interestingly now, though the idle is still a little rough, many performance issues have improved. If anything, it has more power from dead stop and shifts a little early rather than late (driving gently). I did, however, have a no-start one time during the day and shifting is still a little bit erratic at times. Nothing extreme.

Still need to backprobe the MAF before replacing anything.

LMB

P.S. I notice a lot of threads on this forum seem to die prematurely. I wonder if this is the case simply because we never know for sure when we actually have the problem eliminated . . . . With these EEC systems, the problems often seem to come back about the time we think we have them pinned down.

 

[greatdane041]

yes, a little browning is normal. my v6 sohc browns them pretty fast but not bad. I've been chasing almost the same issue. for me, idk exactly which it was, but it was either my maf, iac, lower intake plenum gaskets, pcv valve, or my intake because I replaced all at once and it (mostly) fixed it. it hasn't been running as well as it was 6 mnths ago and yesterday unhooked batt for 30 min and ran through and cleaned all my intake parts and sensors, hooked batt back and boom. horrible idle at first crank, (sounds like its camed with my exhaust) but a HUGE power gain and it smooths out.. I cant remember the last time it whistled and squalled tires on accident. lol

 

[ncrockett]

Ok the maf sensor has been cleaned, no vacuum leaks found, however when I unplug my maf sensor my v-6 explorer runs better. When I plug it back up it idles really rough almost or sometimes dies. It has been stalling while at stop light/signs unless I power break it (so dangerous). Can someone please help as I had several engine codes run and they all say 02 sensor maf sensor running lean on bank one. Course I was told these are not always the fix these are just merely probable causes.

 

[craigssvt]

Was this ever resolved? I'm having the same problem after engine rebuild. Not the MAF but idle drastically improves when unplugged.