Bad MAS- Rough Idle?
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Junior User
Joined: Feb 2005
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Bad MAS- Rough Idle?
Howdy fellows! According to the codes, I have a bad MAS on my '95 Aerostar 4L. Ive cleaned it (which made the CEL go out temporarily) but it never seemed to smooth out the rough idle. Should I be looking at something else in addition to the MAS? The plugs and wires are roughly a year old but installed by a shop so I'm not sure about the quality.
Once again, your help is greatly appreciated!
Once again, your help is greatly appreciated!
Former ******
Joined: Nov 2004
Posts: 4,901
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From: Salt Lake City, Utah
Yes, you should very much be looking at something else. If the plugs and wires are not OEM (I doubt they are), they can couse a rough idle. Listen at the tailpipe, and see if you can hear a defintate missfire. If it is more erratic, the look at the Idle Bypass valve located on the TB. Clean it with Throttle Body cleaner (never use carb cleaner to clean anything on a FI vehicle.) That might help, but in all likelyhood, the idle bypass is bad. Fortunately, they are not too terrible expensive to replace.
Senior User
Joined: Jul 2004
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From: Miami
I've learned where the MAF and TPS sensors are and the IAC valve, but what and where are the MAS and Idle Bypass valve?
After one day of running well, having cleaned MAF and IAC, the 93 4.0 Aero is back to idling and running rough.
After one day of running well, having cleaned MAF and IAC, the 93 4.0 Aero is back to idling and running rough.
Former ******
Joined: Nov 2004
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From: Salt Lake City, Utah
MAS = MAF on an Aerostar. The terms are often swapped or missunderstood. IAC = Idle Bypass in Ford terminology. Actually, I think IAC stands for Intake Air Charge sensor, not valve. It is the sensor located on the intake manifold. And of coarse terminology varies from person to person, manufacturer, region, etc.
You may need to adjust the idle if it is not to the specs, but I doubt that that is the issue. IF it runs really rough instead of slighty rough, and/or the roughness extends to speeds other than idle, it could be MAF sensor, but since it is an expensive part to replace, I'd look at timing, plug wires, plugs, cap, rotor, and O2 sensors before doing the MAF sensor.
You may need to adjust the idle if it is not to the specs, but I doubt that that is the issue. IF it runs really rough instead of slighty rough, and/or the roughness extends to speeds other than idle, it could be MAF sensor, but since it is an expensive part to replace, I'd look at timing, plug wires, plugs, cap, rotor, and O2 sensors before doing the MAF sensor.
Former ******
Joined: Nov 2004
Posts: 4,901
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From: Salt Lake City, Utah
If you have a code reader, you could try a cylinder balance test to isolate weak cylinders. Injectors could be at fault, and a cylinder ballance test is the easiest way to locate both bad injectors, or plugs or wires. It will shut off the injectors one at a time, and measure the performance drop. If the performance is no worse, or only slightly worse than it was before, it means that that paticular cylinder is weak. When it is all done, it will tell you the results.
Senior User
Joined: Jul 2004
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From: Miami
Thanks! Ran the codes on Sunday. 157 = MAF sensor fault, low voltage. Cleaned thoroughly, and ran better. 2 days later, back to rough idling. Tried to run cylinder balance test, without success. ?? All other things you mentioned recently changed. If I get 157 code again, perhaps this means replace part, rather than just clean.
Postmaster
Joined: Sep 2003
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From: Southern California
I got the same code repeatedly on my 4.0L until I did the following:
1) Remove the two little screws (security Torx bit required) and took the MAF elements out, spray and clean it VERY GENTLY with a soft brush. Then spray it again. Use STP throttle body cleaner for this.
2) Take out the big rubber air hose between the throttle body and the air cleaner box, and clean the heck out of its inside with soap and water.
3) Change the PCV valve.
It has been almost a year now and several long trips without that annoying code popping up daily.
Also, while you are at it, with the throttle body already opened up, unplug the SPOUT connector (gray square plug hanging from big harness a little to the left of the master cylinder), run the engine, and spray throttle body cleaner into the opening leading to the IAC. On the 4.0L, it's on the top of the throttle body, before the butterfly valve. Spray only a little at a time or the engine will quit on you, but use at least half a bottle. Also, spray some on the orifice on the butterfly valve. That ought to clean out both the IAC valve and the air passage leading to it.
After all that, if you still have bad idle, we'll try figuring out something else.
1) Remove the two little screws (security Torx bit required) and took the MAF elements out, spray and clean it VERY GENTLY with a soft brush. Then spray it again. Use STP throttle body cleaner for this.
2) Take out the big rubber air hose between the throttle body and the air cleaner box, and clean the heck out of its inside with soap and water.
3) Change the PCV valve.
It has been almost a year now and several long trips without that annoying code popping up daily.
Also, while you are at it, with the throttle body already opened up, unplug the SPOUT connector (gray square plug hanging from big harness a little to the left of the master cylinder), run the engine, and spray throttle body cleaner into the opening leading to the IAC. On the 4.0L, it's on the top of the throttle body, before the butterfly valve. Spray only a little at a time or the engine will quit on you, but use at least half a bottle. Also, spray some on the orifice on the butterfly valve. That ought to clean out both the IAC valve and the air passage leading to it.
After all that, if you still have bad idle, we'll try figuring out something else.
Last edited by copper_90680; Sep 29, 2005 at 01:36 PM.
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Former ******
Joined: Nov 2004
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From: Salt Lake City, Utah
Well, on an OBD-I vehicle, the check engine light is not very easy to trigger, so if it is giving you a code like that, that says it all. Try cleaning as mentioned above. If that doesn't work, you are just going to have to replace it. Hope it doesn't hurt you pocketbook too bad. Some people on this post actually report haveing more success useing electrical parts cleaner instead of TB cleaner.
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Postmaster
Joined: Sep 2003
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From: Southern California
Originally Posted by manasota
Would that alone actually cause a rough idle? It seems to run fine at speed and has has reasonable fuel economy.
Posting Guru
Joined: Jul 2004
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From: Mitchell, SD
Originally Posted by khantyranitar
... IAC = Idle Bypass in Ford terminology. Actually, I think IAC stands for Intake Air Charge sensor, not valve....
You may need to adjust the idle if it is not to the specs, but I doubt that that is the issue. IF it runs really rough instead of slighty rough, and/or the roughness extends to speeds other than idle, it could be MAF sensor, but since it is an expensive part to replace, I'd look at timing, plug wires, plugs, cap, rotor, and O2 sensors before doing the MAF sensor.
Senior User
Joined: Jul 2004
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From: Miami
Here's a good previous thread which details all:
https://www.ford-trucks.com/forums/s...hreadid=126093
https://www.ford-trucks.com/forums/s...hreadid=126093
Posting Guru
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From: Mitchell, SD
Originally Posted by Kanute
Just out of curiosity, where do I find the MAF on my '89 3.0?