Engine 1989 5.0 running rough - MAF

79pace

something stupid will fall out of my mouth
Founding Member
Jul 21, 2000
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www.carbdford.com
Ok, Good deal. The AFU55 part confirms you have the 55mm version so it should be reading correct airflow numbers and its a 1991 unit.
 
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Mustang5L5

i'm familiar with penetration
Mod Dude
Feb 18, 2001
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For the sake of trying to eliminate potential sources of the issue, do you have a stock 55mm MAF housing and a set of 19# injectors?

That combo should still run with 19's. 24's give you some room to grow and won't be at nearly 100% duty cycle, but for the sale of troubleshooting you should be able to get the engine running on a stock 55mm MAF and 19# injectors. That will at least confirm that the issue is, or is not related to the MAF and/or injectors.
 

SFR

New Member
Apr 12, 2021
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55
Sudbury, MA
Unfortunatel, I don’t have 19# injectors or original MAF housing. I asked around on the local car group that I’m part of and no response yet.
 

KRUISR

Well-Known Member
Apr 16, 2015
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More details on how to do it or more details on why I think a 9/16" outlet hole will work?
I will start with the why I think it will work. If you look at the C&L chart that Mustang5L5 shared, you will see that the recommended tube for 24 lb injectors is the Blue Tube (0.65 IN, 0.555 OUT). It is the recommended tube for both the 73mm and 76mm. This usually means it is a compromise for both MAFs. Using my "crude" calculator I get the Blue Tube on a 73mm is about 7% richer (would be the equivalent to using a 20.3 lb injector on an 100% stock set up) and the Blue Tube on a 76mm is about 3% lean (equivalent to a 18.7 lb injector on 100% stock set up). In both cases not an exact match to 24 lb injectors.

When I plug in my modified sample tube (0.65 IN, 0.5625 OUT) in a 76mm MAF and 24 lb injectors I get an equivalent injector of 18.93 lb injector. Still not exact but the closest so far. I chose the 0.5625 outlet dimension because it is a drill bit readily available. If I pick a drill 1/64" bigger (37/64ths) I get an equivalent injector of 19.38 lbs. Still pretty close, a little rich vs little lean but not quite as close, but would work just fine.

So to clarify, when I say an equivalent injector this is what the 24 lb injector will behave like because of the "tricking" of the ECU. You want it to be as close to 19 as you can (no matter what size injector you use). The ECU has no way of knowing what injector has been installed. I has no way of knowing what size MAF is installed. The C&L MAF/calibration tube combo lowers the signal voltage back to the ECU which causes the computer to pulse the injector less which makes the injector behave like a smaller unit.


So to try and further explain this lets use this fictitious scenario...

A 302 running at 2000 rpm pulls 175 cfm through the intake, TB and MAF. With the stock MAF the computer reads a voltage and pulses the 19s and delivers say 4 lbs of fuel (made up number).

Now change MAF to a C&L 76mm and Blue Tube and 24 lb injectors. Everything else the same. Now computer reads a lower voltage and pulses the 24s less (than the 19s) and delivers 3.88 lbs of fuel (3% less than stock 19s - as determined above), which is lean, but still good because we still have the same amount of air to mix with, so should ideally have same amount of fuel.

Your Red Tube in this scenario with the 24s will give an equivalent injector of 17.2 lb or about 10% too lean. Applying this to our fictitious scenario would have the computer deliver 3.6 lbs of fuel (remember 4 lbs would be ideal).

The Clear Tube you tried would be an equivalent injector of 21.8 or about 20% too rich or about 4.8 lbs of fuel in our fictitious scenario.

The balance in sizing between MAF diameter and calibration tube size determines how close the factory computer will get with a larger MAF and injectors.


So after all that some will question the exact accuracy of my calculations and I will say I am not 100% in taking into account and calculating all possible variables. I am calculating at a very basic level, assuming things like low air speed or high air speed effects and others are the same between two different size MAFs and tubes and therefore assuming they balance out.




Now for the second part of the question... how to modify the tube. This is the easy part.

Remove tube from MAF. You will need to remove the sensor to get at the tube mounting bolts. Be careful with the sensor so as to not damage the wires.

Once tube is out, install a 9/16" drill bit on your favourite drill. Clamp tube in a vise and drill out the discharge hole. You will find you will go in approximately 3/4" and then be through. Clean out any metal shavings. Re-install tube and sensor on MAF and hook it all up in the car. Try and see if we helped.


I hope this long winded explanation helps. If it raises more questions, I will do my best to answer.
 
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SFR

New Member
Apr 12, 2021
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Sudbury, MA
Thanks for the write up Kruisr. I will try it.

I have a BBK FPR and I bumped the pressure to 45#’s (vacuum off) and it seems to run better (not lean) - no backfiring from intake; is this pressure OK?

Thanks
 

KRUISR

Well-Known Member
Apr 16, 2015
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I have a BBK FPR and I bumped the pressure to 45#’s (vacuum off) and it seems to run better (not lean) - no backfiring from intake; is this pressure OK?
Which calibration tube do you have in?
 

KRUISR

Well-Known Member
Apr 16, 2015
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You could also put the clear tube in and lower the pressure down to low 30's. Probably still a little rich, but should stop the backfiring.