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Old 11-25-2017, 02:42 PM   #1
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Exclamation 91 3.0 MAF Sensor mod issue, lean

Okay, so I have a 91 3.0. I just did some intake work on it, new tubing, filter and swapped the MAF sensor housing from a 91 Crown Vic 5.0. I kept the original sensor, not the 5.0's. I was thinking that more air in would equal more power. Now my truck struggles at start up and full throttle. Do I need to get my computer tuned or is there any mechanical fix I can do to remedy?
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Old 11-25-2017, 03:59 PM   #2
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It will be interesting to see what the experts tell you. My understanding is that the sensor is heated and the air passing by it changes it's temperature. So, by using a larger diameter housing, you have reduced the velocity of the air passing by the sensor. Therefore, even though it is easier for more air to flow, the computer is "seeing" less air because the air is moving slower and affecting the sensor less than expected.

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Old 11-25-2017, 06:42 PM   #3
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If you restrict engine air it will have less power, but if engine is getting as much air as it can use then extra air won't help.

The MAF sensor is designed to sample 10% of the air flowing thru the MAF opening
If you increase the size of the opening the 10% is no longer correct and computer calculations will be off, so Lean codes
Put the 5.0l sensor in, it will sample the 10%

Or better yet put 3.0l housing and sensor back, as far as I know stock 3.0l never had restricted air flow.
If you were to stroke or change the cam then may be more air flow would be needed
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Old 11-25-2017, 06:51 PM   #4
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Quote:
Originally Posted by RonD View Post
Put the 5.0l sensor in, it will sample the 10%
Well, I tried that first. It ran like crap. I put the 3.0 sensor in and it ran better, but still bad.

So, say I put a small funnel in front of the MAF sensor, could that push more air through the sensor? Helping it read more?
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Old 11-26-2017, 01:00 PM   #5
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Not sure you will be able to "fine tune" it enough.

Most earlier years can run without MAF sensor connected, but they run in Safe mode/Limp mode.

See if your model will start and run without MAF plugged in

Each computer has software for the engine size it is running, so it knows the air flow based on RPM and throttle position, without MAF sensor data
Air/fuel mix is WEIGHT based
14.7/1 is the ratio used for air/gasoline engines
So 14.7 POUNDS of air to 1 POUND of gasoline
14.7grams of air to 1 gram of gasoline
(this is why pre-vaporizers are hooey, lol, volume doesn't matter, WEIGHT matters, just FYI)

Fords MAF sensor is a heated wire, heavier air(sea level) cools it more than lighter air, colder air cools it more than warmer air.
Heavier and colder air has more oxygen so computer needs to add more fuel to stay at 14.7/1
So MAF sensors main function is to tell computer the WEIGHT of the air coming into the engine so it can adjust fuel WEIGHT added accordingly.
There is an IAT(intake air temp) sensor also used.

If computer adds too much fuel you get a flooded engine
Not enough fuel you get pinging/knocking and damaged engine
So pretty important to get the WEIGHTS right
On a carb you could manually adjust the ratio, and get it wrong often, lol, not "good ol' days".
Computer fuel injection is better, in oh so many ways, but it does require the correct information to do the correct calculations, just like carb needed correct adjustment to do correct fuel mix.

You want to modify the air flow into the engine, which is fine but you also need to give computer the correct info for that air WEIGHT.
Not sure you can with 5.0l MAF/housing, but never tried

Found this page: http://www.rogueperformance.com/ModifiedMAF.html

Never tried this modification but it has a line I found interesting
Quote:
Note: This is just the MAF BODY. Ford used 3-6 different calibrations on their MAF's each year,
so a complete replacement is not practical.
So not sure you will get where you want to go with the 5.0l housing

Last edited by RonD; 11-26-2017 at 01:27 PM.
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Old 11-26-2017, 05:31 PM   #6
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I briefly tried a 4.0l MAF and housing on my 3.l a while back. It didn't work. I also tried the 3.0l MAF in the 4.0l housing. That didn't work.

I quickly went back to the original 3.0l setup. My intake piping was restrictive between the MAF housing and throttle body. Replacing that seems to have helped the engine breathe better.

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Old 11-26-2017, 09:05 PM   #7
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Thanks for the link and ejumacation. Next chance I get I'll clean the MAF sensor and see if that helps.
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Old 11-27-2017, 10:45 AM   #8
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You can test for restricted air flow using a Vacuum gauge mounted in the cab and a long stretch of "private" road where you can "let the big dog eat", lol, run at Wide Open Throttle(WOT)

Restricting air flow causes vacuum, i.e. throttle plate
Unless you are forcing air in, i.e. turbo/super chargers, some vacuum is good/needed even at WOT.

So put a temporary, or permanent, vacuum gauge in the cab where you can easily see it, "T" it to intake vacuum
At full speed and WOT you should have 1" to 2" of vacuum in intake manifold
More than 2" would indicate restriction in air flow
Less than 1" would indicate floating valves most likely

Vacuum means air is being pulled into the cylinders, so fuel is also being pulled in
0" means no air movement
+ number psi is Boost pressure, turbo/super charger
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