2.8 runs better without MAF sensor?

[Mur]

Hi, gotta 88 ciera with 2.8 MPFI, lately runinng a little rough, unplug mass air sensor, runs smooth again, tried a new MAF sensor, still runs rough....latley i just run without maf sensor...anyone know why it runs better without??

[CamoDeafie]

its posible you've got a Speed Desnisty upgrade which uses a MAP (manifold Absolute air pressure) sensor..?

[Mur]

maybe, i converted the cable speedo to electronic, but i got everything from an 88 pontiac 6000, i noticed it had a sticker that said it was upgraded to speed density, but im pretty sure that motor had a MAF and MAP sensor, just as my 88 ciera has both sensors....what exactly is that speed density upgrade, just for the electronic speedo? maybe i got a wire mixed up or something

[CamoDeafie]

well go over to the ciera AC dash thing in the interior section and read pages 1 or 2, Turbokinetic explained basics of the MAP vs MAF systems better, as his 88 XC has a 2.8 as well, but has a speed density upgrade...

[CamoDeafie]

ohell to save ya the time, here is what he had to say

The Ciera XC has MAP only, but there is a dead, de-activated MAF meter still in the air filter housing. It was part of a factory update.

None of our cars have BOTH systems active at once. Only the newer (OBDII) stuff has both. They use the 2 calculations to verify all the sensors are working right. It is confusing because the cars like my XC have both, but the MAF is NOT HOOKED UP because they installed the MAP and reporgrammed the ECM.

Now for some pretty heavy EFI theory......

The ECM controls 2 things, fuel and spark advance. All it's fancy calculations start with 2 main factors. For all systems, the most important factor is engine RPM. It can run (half-assed) with nothing but an RPM signal, guessing at the other values. In very simplified terms, I will try to explain it.

For speed density, the other main factor is MANIFOLD ABSOLTE AIR PRESSURE (MAP). The units used are "kPa" with 0 kPa being perfect vacuum and 102 kPa being ambient atmospheric pressure. This measurement is taken from in the manifold plenum, after the throttle and before the intake valves.

For metered air, the other main factor is AIR MASS FLOW. It is measured in grams (of air mass) per second.

For a speed density system, the ECM uses MAP pressure to calculate air density and therfore how much air mass is in each liter of air. For each speed pulse, ECM starts with the constant that represents the engine and injector combination (the base pulsewidth constant) and scales that amount by how dense the air is in the manifold. Less pressure = less fuel; more pressure = more fuel.
There are "VE tables" that tweak the result of the speed-density calculation up or down (slightly) to compensate for the engine's power curve. For all given engine conditions, the ECM looks up the engine's VE (volumetric efficiency; "breathing efficiency") and adjusts each shot of fuel.

The ignition spark advance is very simple. The engine's timing advance is a simple table that has engine speed and MAP pressure. It simply looks up the appropriate cells and uses that spark advance value. It's the same idea as a mechanical distriubutor with a vacuum advance (manifold pressure) and centrifugal advance (speed).

For metered air, the fuel delivery is simple. The ECM reads directly the airflow signal, and at each speed pulse it injects the amount of fuel to burn with the amount of air measured. There are no VE tables, the airflow meter measures the actual flow and compensates for the engine's power curve.

Metered-air spark advance is more complicated. The ECM does a sort of "reverse speed-density" calculation. It compares airflow and engine size to engine speed. The more airflow for any given speed; the HIGHER the manifold pressure will be. This is called "Engine Load Factor" or LV8. The LV8 variable is used in the same way MAP pressure is used. The ignition advance table is comparing LV8 and engine RPM to look up the advance value for those engine operating conditions.

In a nutshell:
Speed-density Fuel: MAP, RPM, Volumetric Efficiency, Base Pulse Constant.
Speed-density Spark: MAP, RPM
The crank position sensor and MAP sensor are the 2 most critical inputs to the ECM.

Metered air Fuel: Airflow, RPM.
Metered air Spark: RPM, LV8
The crank position sensor and MAF meter are the 2 most critical inputs to the ECM.

For a factory-stock car they are both fine and dandy. For a modified car, they each have distinct factors that have to be taken into consideration.

GENERALLY SPEAKING, the speed-density systems take more time to tune and calibrate the fuel. BUT they have virtually no limit on what you can do with the system. MAP sensors are available that support 30 PSI turbo boost. The fuel and spark tables are not inter-related and you can adjust one without messing up the other.

Metered air systems are almost "self-tuning" on fuel; because the MAF measures the air and compensates for improved airflow. The downside is, any changes to the fuelling usually also affect the spark advance. It is hard to correlate the LV8 spark tables into real-world units of measure that can be monitored and tuned with. On the hardware side, a mass airflow meter is a very delicate instrument and has a limited range of flow that it can measure. Once you get near the limit of the MAF, it gets very unpredictable. Early-style (think LG3 and early 2.8) MAF's can also be unreliable.

The speed-desity MAP sensor is almost failure-proof. Most cars in the junkyard still have their original one and it still is good. On a downside, you can NOT run a turbo without changing to a higher rating sensor, such as 2-bar or 3-bar. The metered-air system can be turbo-tuned keeping all the stock components and changing only the programming.

Hope this didn't bore or confuse you, it is hard to explain any part of this without explaining all of it.

Thanks,
David

[Mur]

thanks!

[turbokinetic]

Steven, thanks for copying that over here!

Mur, when you swapped over the electronic speedometer, you did install the wires into the ECM for the magnetic speed sensor, and the output wire from the ECM to the Cruise and Speedometer? How did you get the ECM to work with that? I'm just curious. I had to go the other way (magnetic to cable) on the my car's ECM programming. It required changing settings in the chip.

The only reason the dead MAF is left in the system is for the temperature sensor it contains. The ECM makes some minor timing and fuel changes based on air temp.

Thanks!
David

[Mur]

I took the ECM from the 88 6000 as well as the speed sensor. my olds is an 88 as well and i thought would GM really make 2 different ecms in 88 just cause one had a speed sensor and one was cable driven? both ecms had same number of pins, so after re wiring i plugged olds ecm in, speed was way off, 10mph actual driving showed 50mph on speedo, plugged pontiac ecm in, worked perfectly, go figure. you're right, two wires from speed sensor to ecm, one to speedo...on my 88 ciera i don't think cruise needs input from speed sensor, it still works.....but there was an extra wire left over when i rewired dash, but everything works fine