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Someone Please Explain SD and MAF systems to me.

5456 Views 13 Replies 5 Participants Last post by  miesk5
OK, here's my Bronco. 92 300I6.

I'm pretty sure it's Federal Emissions compliant and not California. Then again, I don't even know if they still had 2 separate standards in 92 or not. OK, so reading these threads, I hear alot about Speed Density and Mass Air Flow systems. What does one look like compared to the other? How is one different from the other? I'm still working through, trying to figure out the issues with my rich exhaust and finding any leaks that need repair. I think maybe if I knew for certain what I was working on, I wouldn't waste my time chasing things that aren't typical for one system or the other.
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Some good reading here at Ford Fuel Injection: Four main styles of EFI programming
I'd recommend picking up a copy of Charles O. Probst's book on Ford Electronic Engine Control.
Thanks for the info. I'll look for the book.
Speed Density EFI; "...In 1986, Ford switched to EFI on 5.0 Bronco engines. 1987 brought the first EFI 5.8. The EFI Ford used for almost all Broncos is what is called speed density. Speed density EFI uses a manifold absolute pressure sensor (MAP), a throttle position sensor (TPS), an air intake temperature sender (ACT) , and the knowledge of the factory intake/exhaust tract which gives your engine certain volumetric efficiencies over it's RPM range. This allows the EEC-IV (Electronic Engine Control IV) to determine the mass (different than volume) of air coming into your engine, and hence the amount of fuel needed to go with this air. Speed density EFI also uses what is called bank fire injection. 4 injector are fired at one time on each side of the engine (only 2 wires for firing injectors from EEC-IV)...."
Source: by James O at

Mass Air EFI; "...A mass air sensor directly reads the mass of air (or number of molecules of air). By doing this, it can detect the changes in the volume of air, in addition to it's pressure and temperature. Mass air EFI is usually what is called SEFI (Sequential Electronic Fuel Injection), which has one separate wire for each injector and fires them twice for every power stroke of the engine. This makes the injector timing more accurate and will help emissions along with power..."
Source: by James O at
John, you have a Manifold Absolute Pressure (MAP) Sensor in that 4.9;

I looked at your yr/4.9 VECI Label using the 1st Cal # (2-51E-R00) in the list and it shows the MAP; here is an image;

See where the "HDG (49 States) Decal" above the label on the on-line version?
And at bottom inside the label, "This vehicle conforms to U.S. EPA regulations..."..etc.
but for Cal # 2-51R-R00, I see "HDT (California) Decal"

So look at your CAL # and see what it the online version to what is on top of your radiator, passenger side usually.
And at bottom inside the label, "This vehicle conforms to U.S. EPA and California regulations...."..etc.

Vehicle Emission Control Information (VECI) Decal;
Vacuum Diagrams & Calibration Parts List for 88 & UP.
On-Line for Free at Ford.
Click "Quick Guides" in left panel;
Scroll to & CLICK VECI Labels "Provides Vehicle Emission Control Information (VECI) and a related calibration parts list."
Enter applicable info (need to know your Calibration number from your B-pillar sticker).
Vacuum Diagram is the same as the one on the core support or hood or air filter cover.
Source: by Ford @
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Thanks. Much appreciated. I looked up the info on the Ford site. My cal is 2-51H-R00 so I'm Federal but I guess having the MAP, I'm SD and not MAF. I don't guess there's a way to convert a 4.9 over to MAF. So, from some things I've read in comparison between SD and MAF, the SD doesn't have the real ability to compensate for changes to engine over time due to wear or if improvements/significant modifications are made. I wonder if that's why I cannot seem to get the exhaust to burn cleaner. I've gone through and tested all the vacuum lines and repaired all the leaks. It still smells like it's running rich.
What's the manifold vacuum at idle?
this probably will not help but i think i'll share.. when i changed out the sensor that detects engine temp, (ECT maybe?) the old one was rotten and black and had a little diode about to fall off. the new one had a brass bottom to it. at any rate my truck ran wayy better than before..i had that rich smell to it too before the switch
I don't remember the last time I checked it. I want to say it was 17-20lbs at idle.
yo J
Did you ever get those DTCs cleared-up?
here is your thread;
you had 553 (O) AIRD solenoid/circuit failure
& DTC 311 - Thermactor air system inoperable
The 2ndry air system is known to fail in a wide variety of ways. The check valves that prevent hot exhaust from entering the rubber hoses age, rust, leak, & crack open melting the plastic TAB & TAD valves, creating exhaust leaks that can damage other components, raising exhaust oxygen levels (setting lean codes or rich adaptive limit codes), and making rattling noises. The hard steel tubing between the exhaust & the check valve can rust or crack (especially the infamous "crossover tube" on the backs of V8 heads). The vacuum controls leak (including the "coffee can" reservoir on the R wheelwell), get misrouted during other repairs, or the diaphragms rupture. The electronics that control the vacuum controls can fail electrically or mechanically, or the wires can be damaged. But all of these failures are either A) relatively cheap & easy to repair, or B) cheap & easy to prevent with normal inspection & maintenance. see

plus... some that were caused by self test procedures...
common reasons for a rich running condition are a faulty fuel injector(s), bad Fuel Pressure Regulator (FPR) or a problem in the ignition system (plugs, wires, cap, distributor, etc.).
These problems will not throw codes directly
How does the exhaust look? black smoky?

Pull the vacuum line off the FPR; any hint of gas aroma or gas is a sign it's shot.

maybe the PCV valve has worked it's way out of the grommet?
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I pulled vacuums on the TAD and TAB right after I bought my mighty vac and I do remember it did change how the engine ran for the moment but it's been so long ago that I cannot remember if it ran rough or just increased idle. Next weekend, if I get the time, I'll actually take the units off and inspect them. I remember going through most of the vacuum system looking for leaks and everything seemed to take and hold a vacuum or when running produced a steady vacuum at idle. I will have to go back through everything and this time make notes for you. I'll also check those other items you listed.
ok, because of the possibility that (following is an excerpt of a condensed discussion of how a bad TAB/TAD/vac line DTC 311 ... and so-on problem could cause the rich aroma;
"...Your smog pump... blows fresh clean air up through a hose to the diverter valve. The solenoid next to the diverter valve which has a .... pink little hose plugged into ... it, creates a vacuum (sucking) that opens up the valve, and allows the fresh air to pass through the valve and make it's way to the exhaust, where the cats burn the unburnt gas more efficiently since this all happens during "open loop mode". Open loop mode happens when you turn on your car and it revs at 1200rpms for the first 15-20 seconds. During this process, your ECU feeds more gas to the engine to warm up the car quickly before driving. (also known as choke on older cars). During this open loop procedure, the extra unburnt gasoline will usually cause your exhaust to smell very rich, and the air that this diverter valve sends to the cats, causes it to burn the extra unburnt gas more efficiently like I mentoined above...thus eliminating the rich gasoline smell that us older mustangs sometimes suffer from..."
by 006 (Speedconcepts)
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Thanks for that info. I definitely have a better idea now of what to look for than I did a couple of days ago. I read through both the old Chilton and the newer Haynes that I have and it did cover some of it but not to as good of a detail.

So if I need to replace any of these, would Ford still have them in stock? I looked awhile back at NAPA, Advance and the Zone and nobody listed these items. I just don't want to get taken too badly if/when I have to replace them.
VALVE ASSY., (Thermactor Air Control Solenoid Vacuum) E7TZ 9H465-A avail @ Troy Ford via Amazon (great source! save $100.00 EACH on both DPFE & IAC sensors last year plus free shipping on many parts and no sales tax, so my savings over 4 local Ford dealers was over $225.00 total

$37.49 + $12.00 shipping
VALVE ASSY., (Thermactor Air Control Solenoid Vacuum) E7TZ 9H465-B
Same price at Troy

These are same PN for many Broncos and Fords..incl back to at least an 89 Bronco up to my 96 5.0
So yard parts are readily avail.

and as our pal SigEPBlue wrote here recently; It's an emissions part, so you should be able to get 'em still. They were made by Rochester for Ford, IIRC. Should be service numbers E7TZ-9H465-A and E7TZ-9H465-B..."
Ford MC parts used to have an interchange search but since they "improved" their web site, all the good stuff is gone incl these Air Sys emission parts.
now, when entering my 96's VIN; the message come up as "We're sorry, the VIN you entered is not supported by the catalog at this time."

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