Need help with MAF/IAT Sensor measuring

[thezet]

Hey guys,

I've done a lot of reading in this forum and must admit that this is the best forum ever

I have a problem that has been discussed in a couple threads here; unfortunately none of the proposed solutions work for me.

Quick description:
Engine stalls at low rpm when warmed up. When I start it up in the cold, everything is fine. Once the engine reaches temperature, when in idle, the rpm keeps dropping to 500 and then back to 1000, back and forth. After a while, the rpms drop to below 500 and the engine stalls. This happens in idle or when driving and coming to a stop. I checked the TPS and it is okay. No error codes popped up so far and the problem began slowly and got increasingly worse in the span of two days. Above rpm car sounds and drives just fine, as if there was no issue at all. Checked for vacuum leaks and found none.

I cleaned the MAF and it looks like new, however I don't know if I get the appropriate reading on the terminals of the MAF/IAT.
I finally got codes P0113 and P0122 today, but I think the P0122 is from unplugging the TPS with the engine running.

I've done some resistance measurements on the TPS and the MAF/IAT. the TPS turned out ok, which was a consistent resistance increase/decrease while working the throttle.

According to my docs from autozone members area, the wiring to the MAF/IAT should be as follows

and in the test instructions for the IAF, it says to check the resistance between IAT and the SIGRTN, which are according to the diagram pin 1&2.

Am I correct about this? I've read the table and know I should be at room temperature at 3-4 KOhms.
Here is the plug from my MAF/IAT

And here are the readings I've got on those pins.
As I understand
1 - IAT
2 - SIG RTN
3 - MAF
4 - MAF RTN
5 - PWRGRND
6 - VPWR

My reading in resistance are;

1 - 2 UNLIM OHMS (OL)
1 - 6 78.2 kOHM
3 - 4 0OHM
2 - 5, 3 - 5, 4 - 5, 22.7 kOHM (I guess thats to ground)

Could someone confirm these readings, or does my open/unlimited OHM between 1 - 2 indicate a faulty IAT? or a faulty connection?

I took the MAF out and measured directly on the IAT metal pins right below the thermic resistor and got a 33 kOhm reading that reduced while i blew hot air on it. So I am confused whether I am trying to measure the wrong pins on the connector or whether the wires inside the MAF are messed up, or maybe if I am going crazy...


I would appreciate any help


Alexander Gwosdz


P.S.: It's a 2003 Mountaineer V8 4.6L with 120k and might need a new fuel filter

 

[2000StreetRod]

Incorrect pin diagram

I suspect that the AutoZone wiring diagram is incorrect. On my stock 55mm MAF sensor the outside pins are associated with the IAT sensor. In the early years the MAF sensor and the IAT sensor were not integrated. Later the 4 wire connector became a 6 wire connector. That's why the labeling on the electrical connecter is EABCDF. E and F are for the IAT sensor.

According to the table the reading for 50 degrees F should be 58.75 Kohms and for 68 degrees 37.3 Kohms. I don't know your definition of "room temperature" but the reading from pin 1 to 6 of 78.2 Kohms seems reasonable. I have simulated the IAT sensor with a potentiometer and found that it has only a minor impact on the fuel mixture. I doubt it's the source of your problem.

I would test the engine coolant temperature (ECT) sensor which has a significant effect on the mixture. If it's telling the PCM the engine is cold when the engine is warm then your fuel mixture will be rich and cause poor idle.

 

[thezet]

Thanks

I suspect that the AutoZone wiring diagram is incorrect. On my stock 55mm MAF sensor the outside pins are associated with the IAT sensor. In the early years the MAF sensor and the IAT sensor were not integrated. Later the 4 wire connector became a 6 wire connector. That's why the labeling on the electrical connecter is EABCDF. E and F are for the IAT sensor.

According to the table the reading for 50 degrees F should be 58.75 Kohms and for 68 degrees 37.3 Kohms. I don't know your definition of "room temperature" but the reading from pin 1 to 6 of 78.2 Kohms seems reasonable. I have simulated the IAT sensor with a potentiometer and found that it has only a minor impact on the fuel mixture. I doubt it's the source of your problem.

I would test the engine coolant temperature (ECT) sensor which has a significant effect on the mixture. If it's telling the PCM the engine is cold when the engine is warm then your fuel mixture will be rich and cause poor idle.

Thanks for the answer. Would that match the symptoms? That's the first time I heard that ECT could cause that. I'll check the connections again tomorrow and see if 1 - 6 gives me the same reading as directly on the iat resistor...

Another question: Could the fuel filter be responsible for that? Or even the alternator?


Thanks again

Alex

 

[2000StreetRod]

Fuel filter & alternator

Thanks for the answer. Would that match the symptoms? That's the first time I heard that ECT could cause that. I'll check the connections again tomorrow and see if 1 - 6 gives me the same reading as directly on the iat resistor...

Another question: Could the fuel filter be responsible for that? Or even the alternator?
Thanks again
Alex

A clogged fuel filter can cause all sorts of weird symptoms. If it's been more than 30,000 miles since it's last replacement I suggest that you change it.

It's not likely an alternator problem but it's easily checked with a voltmeter.

It may be difficult to get to your ECT sensor. Make sure that you know the difference between the ECT sensor and the engine temperature sender that drives the temperature gauge in the instrument cluster.

 

[thezet]

I will look into that...

A clogged fuel filter can cause all sorts of weird symptoms. If it's been more than 30,000 miles since it's last replacement I suggest that you change it.

It's not likely an alternator problem but it's easily checked with a voltmeter.

It may be difficult to get to your ECT sensor. Make sure that you know the difference between the ECT sensor and the engine temperature sender that drives the temperature gauge in the instrument cluster.

Would it help to mention that even with 24F outside the car warms up normal and the heater in the cabin comes on within 5-10mins
(did not time it, but ride to College takes about 15 mins and after I got there I Had to open the window since I was hot...)
And normal operation above 700 RPM is excellent, no performance issues, nothing noticeable anyways

Do you know the easiest way to find it in the car? I know it should be somewhere neat the thermostat, or at least in that area...


Alex

 

[skeeter123]

Why not check your Idle Air Controller? The IAC is how the computer maintains proper idle rpms. They can get dirty with carbon build-up. Take it off and clean it with throttle-body cleaner; clean mating side of throttle-body. Be carefull not to press on or mess with the end of the IAC, but clean any carbon off. It may take a few "drive-cycles" for the PCM to complete its adjustments and get back to smooth idle control.
Hope this helps.

 

[thezet]

Why not check your Idle Air Controller? The IAC is how the computer maintains proper idle rpms. They can get dirty with carbon build-up. Take it off and clean it with throttle-body cleaner; clean mating side of throttle-body. Be carefull not to press on or mess with the end of the IAC, but clean any carbon off. It may take a few "drive-cycles" for the PCM to complete its adjustments and get back to smooth idle control.
Hope this helps.

Already considered that and cleaned it with MAF cleaner, it looked pretty clean inside... Is there any way to test the motor in it? The other thing is that the engine is adjusting idle speed pretty good when cold... On a cold engine the rpm are up to warm up the engine and slowly return to 1000 rpm after the warm up cycle... Since it does what it does, my first guess was the TPS, but I bought a new one from autozone and put it in and the problem was still there.


Thanks

Alex

 

[skeeter123]

Have the IAT and TPS codes gone away? Though you had this prob before the codes showed up, you could attack them and get back to where you were (idle prob w/ no codes). P0113 = "IAT signal too high". If you can get a code reader/scanner that will show you live data, see if PCM is getting valid IAT and TPS readings. It could be open/short wiring from the sensors.

 

[thezet]

Thanks

Have the IAT and TPS codes gone away? Though you had this prob before the codes showed up, you could attack them and get back to where you were (idle prob w/ no codes). P0113 = "IAT signal too high". If you can get a code reader/scanner that will show you live data, see if PCM is getting valid IAT and TPS readings. It could be open/short wiring from the sensors.

Thanks for the help; I tried every possible thing I could think of and all the sensors turn out fine and throw no code.

Here is what I did to make the problem go away or maybe even fix it.

While I was checking for vacuum leaks, I needed somebody to keep the foot on the gas, just a bit, to prevent the engine from stalling. Well, since my two year old cannot really be trusted with handling a running car or spraying starter fluid around hoses , I shimmed the throttle at the intake manifold...
And that's when it hit me: Why don't I just turn the idle speed adjusting screw out? I grabbed a set of vice grips and turn the threaded rod a couple turn, and voila problem went away... No stalling in idle, running at a smooth 700-800 rpm no matter how warmed up the engine is.
I figured out, after running around between three shops to see which one can try to attack that issue, that even if it is a crack in the intake manifold, I will see that in a couple weeks/months. Assuming it is a crack, and that it will get bigger with time, then my little adjustment won't be enough to compensate for the extra air and the problem will return. Or if it is a sensor issue, it will eventually become bad enough to throw a code.

Or maybe that was it. Instead of having the dealer flash a new software to the PCM to adjust the idle speed, as suggested in some of the posts I read, I did it the old way. The measured Gas mileage is the same as before and is pretty good for that gas guzzling V8, so I don't think there is a major issue with it.

If anybody could tell me if my solution has some negative consequences, that would be great... I sprayed a whole can of Starter Fluid around every line, bent, and air related tubes I could think of. And then I sprayed all around the intake manifold and especially the plastic part that most likely is to crack; no difference in RPM or no stalling, so I assume that thing is tight.

Maybe that was all it needed after 120.000 miles


Thanks all for the suggestions, I'll keep you guys updated if anything comes up


Alex

 

[2000StreetRod]

Idle adjustment procedure

You may be interested in reading my Idle adjustment procedure. Click on My Helpful Threads in my signature for a link to the procedure.

 

[thezet]

...

You may be interested in reading my Idle adjustment procedure. Click on My Helpful Threads in my signature for a link to the procedure.

Oh,

I did not do all the steps mentioned in your idle adjustment procedure since I don't have the tools. I adjusted everything with the engine running until it "sounded" smooth and no jumps in rpm and gave it 2 further turns with the vice (about 1/8 turn of the screw). The dash RPM reads 700-800 at idle with heat on or off in Park and in gear. In addition to that, it shifts easily without any jerks or noise into 1st gear from park or neutral... I know it is very subjective and inaccurate.. I'll probably take your readings to the shop and let them read the specified values and readjust as stated in your post.

The very interesting and somewhat peculiar thing is that the whole problem started the moment I crossed 120,000 miles


Thanks for the great procedure, will definitely check out the idle speeds in the pcm

Alex

 

[akjm]

I suspect that the AutoZone wiring diagram is incorrect. On my stock 55mm MAF sensor the outside pins are associated with the IAT sensor. In the early years the MAF sensor and the IAT sensor were not integrated. Later the 4 wire connector became a 6 wire connector. That's why the labeling on the electrical connecter is EABCDF. E and F are for the IAT sensor.

According to the table the reading for 50 degrees F should be 58.75 Kohms and for 68 degrees 37.3 Kohms. I don't know your definition of "room temperature" but the reading from pin 1 to 6 of 78.2 Kohms seems reasonable. I have simulated the IAT sensor with a potentiometer and found that it has only a minor impact on the fuel mixture. I doubt it's the source of your problem.

I would test the engine coolant temperature (ECT) sensor which has a significant effect on the mixture. If it's telling the PCM the engine is cold when the engine is warm then your fuel mixture will be rich and cause poor idle.

Thanks for the info on the sensor, installing a GForce chip and been looking for the pin out E&F got it.
AKJM

 

[matt0248]

I too hasd hot idle problems. Turns out the IAC from autozone was Chit. Replaced it with a new motorcraft one I got off Amazon (for 1/2 the price) and cleaned the Throttle Body and the idle is much better. You should never have to mess with the idle stop screw. By changing this you are only masking the real problem.

 

[rocco123]

Did you check for a split elbow at throttle body? This is from a 5/8 vac line leading to the right valve cover. VERY common to be cracked. Many posts on this.