Shocking MAF service procedure

[Anthony]

I was watching GM's Know-How video on the 1988 3800 engine and noticed something surprising. It says the MAF sensor is calibrated to the throttle body and cannot be serviced separately. I presume they did that calibration on each individual MAF at the factory where the throttle bodies were made. I find that hard to believe, but that's what the training video says.

 

If that is the case, then I suppose cleaning the MAF rather than replacing it might be best, unless you can find an entire assembly. I do see some new MAF's available with the part of the body that includes the screen. I wonder if those are sufficiently calibrated. The video showed a picture of the entire throttle body when it made the statement. 

[2seater]

This is the MAF chart I received from GM Tuners many years ago. You can decide if it is discrete enough to require it to be pre calibrated. I can only speak personally about replacing MAF sensors, but I do not know of anyone that changes that entire assembly, despite what it says in the FSM. If you take apart the TB and MAF assemblies, there is almost no machining done after casting and the passages are as cast. I have run a couple dozen sensors on my flowbench just to get a sense of the range and if they are operational. The calibration, if there is any, consists of the sensor, the passageways, the laminar flow over the sensor and the ECM. This is the way I understand the system: The sensor has a heating element and a thermistor to read the temperature. The ECM sends a pulsed signal (probably ground) to maintain the target temperature  to infer that actual mass of the air. The frequency listed in the chart is the pulse I am referring to. I am pretty sure the air temperature sensor in the air box is needed to precondition the required heating range. The MAF reading is used in combination with other inputs for the PROM to generate the LV8, or engine load figure, which is one axis of the main spark timing chart.

 

It looks like this old chart won't open, darn it

LN3 MAF chart.pdf

Edited January 10 by 2seater

[2seater]

Hal, here is a list of Hitachi MAF sensor flow rates GM assigned the MAF sensor output frequency in a stock 3800 vin C chip...

 

MAF Vs. Output Frequency Table 1
 
  kHz   gm/sec
  2.00   3.19 
  2.13   3.53 
  2.26   3.94 
  2.39   4.38 
  2.52   4.81 
  2.66   5.31 
  2.79   5.88 
  2.92   6.41 
  3.05   6.97 
 
 
MAF Vs. Output Frequency Table 2
 
  kHz   gm/sec
  3.05   7.44 
  3.18   8.06 
  3.31   8.81 
  3.44   9.56 
  3.58  10.31 
  3.71  11.31 
  3.83  12.50 
  3.97  13.69 
  4.10  14.88 
 
 
MAF Vs. Output Frequency Table 3
 
  kHz   gm/sec
  4.10   13.9 
  4.23   15.1 
  4.36   16.5 
  4.49   17.9 
  4.63   19.3 
  4.76   20.8 
  4.89   22.4 
  5.02   23.9 
  5.15   25.4 
 
 
MAF Vs. Output Frequency Table 4
 
  kHz   gm/sec
  5.15  25.43 
  5.28  27.07 
  5.41  28.55 
  5.54  30.19 
  5.68  32.16 
  5.81  34.13 
  5.94  36.09 
  6.07  38.23 
  6.20  40.20 
 
 
MAF Vs. Output Frequency Table 5
 
  kHz   gm/sec.
  6.20   41.00 
  6.33   43.25 
  6.46   45.75 
  6.59   48.25 
  6.73   50.75 
  6.86   53.75 
  6.99   56.50 
  7.12   59.50 
  7.25   62.75 
 
 
MAF Vs. Output Frequency Table 6
 
  kHz   gm/sec
  7.25   62.5 
  7.38   65.8 
  7.51   69.4 
  7.64   72.7 
  7.78   76.3 
  7.91   79.6 
  8.04   83.2 
  8.17   87.2 
  8.30   91.2 
 
 
MAF Vs. Output Frequency Table 7
 
  kHz   gm/sec
  8.30   91.2 
  8.43   95.2 
  8.56   99.3 
  8.69  103.3 
  8.83  107.8 
  8.96  112.4 
  9.09  117.4 
  9.22  121.9 
  9.35  127.0 
 
 
MAF Vs. Output Frequency Table 8
 
  kHz   gm/sec
   9.35 126.3 
   9.48 131.0 
   9.61 136.4 
   9.74 141.8 
   9.88 147.1 
  10.00 153.2 
  10.14 158.6 
  10.27 164.6 
  10.40 170.0 

As you can see they are "close" to your test readings.  Sorry you couldn't find any 3.0" MAF sensors at your junkyard but I do have one sitting here I can send you if you want it.

 

-ryan

 

Edited January 10 by 2seater
Did a copy and paste instead

[Ronnie]

I wonder how much it matters. RockAuto sells the MAF both ways. You can buy the complete assembly or just the sensor. I have changed just the sensor a couple of times without problems.  My experience has been that they start out going bad by causing an erratic idle when you come to a stop with the transmission in gear. Cleaning didn't help but replacing it with a known good used one cured the problem. I never replaced the sensor and the throttle body as a unit.

 

I have a friend who owns an auto parts store. He owned a big Buick with a 3800 back in the '90s. His MAF went bad often for some reason. When it went bad the motor would hardly run at all. We didn't know that the engine would run better if you unplugged the bad MAF.  He carried a complete MAF unit and tools in the trunk. He would swap it out on the side of the road. He couldn't buy just he sensor without the throttle body from the warehouse he got his parts from. He was told that they were only sold as a calibrated unit like you mentioned

[DAVES89]

As an owner of numerous Reattas for over 15 years and close to 300000 combined miles I have replaced numerous MAF sensors and not the throttle body. Like Ronnie says, not worth cleaning the MAF just replace. 

 Also on my recently purchased 199 Buick Regal I replaced the MAF sensor on that one as well without replacing the throttle body. No issues and also after replacing Ignition Module, plugs and wires car runs like new.

[2seater]

To be fair, it is possible to alter the airflow past the sensor by doing some polishing, drilling, grinding or other operations to change the flow path. It is possible there is some form of calibration available, but in the overall scheme of the system, it doesn't seem to matter as long as it is close.

 

The vin C engine uses a separate casting that bolts onto the throttle body. The later 90-91 vin L engines, the tuned port style, have an integrated MAF and throttle body, but the frequency response is the exact same as the earlier model. Both of these types use an indirect flow path with a separate passage for the sensor. The sensor is not exposed to the main airflow like used in some other styles of MAF's. In the course of more than 25 years, I have used four different MAF configurations and even mounted a stock Vin C MAF on the inlet to a turbocharger, and they have functioned acceptably with the stock ECM. Yes some have involved other tuning in the Prom, but that is because the operating parameters of the engines has been substantially different, usually due to some form of boost.

[Anthony]

On 1/10/2024 at 8:24 AM, 2seater said:

Hal, here is a list of Hitachi MAF sensor flow rates GM assigned the MAF sensor output frequency in a stock 3800 vin 

 

Thanks for providing those tables and the extra information. I've never seen those before, and they aren't in the FSM, which is very vague on how it works. The only thing it says is the reading should be 4-7 at idle. In another section it says 4-12 🤷.

 

The previous version of this engine, the LG3 for '87, had the MAF in a removable section of the air duct, closer to the air cleaner housing. I've replaced those many times by themselves because, well that was pretty much the only way, since it wasn't even near the throttle body. 

 

I guess there must be some advantage in precision of the readings by moving it closer for the LN3 in '88. 

 

19 hours ago, 2seater said:

The vin C engine uses a separate casting that bolts onto the throttle body.

It's possible that there was some miscommunication in the making of the training video, because after some searching in the FSM, I found this:

It says to not remove the throttle body itself, just the aluminum MAF housing.

 

I wonder if they do some kind of test during manufacture, on a flow bench, as you did, then make some kind of adjustment by pairing them up with a different housing or MAF, or even simpler--just discard assemblies that don't meet specs. I have read that Porsche suppliers do that, and the ones that are slightly off get sold to aftermarket supply houses 🤭.

[2seater]

I wondered about the terminology used initially because the throttle assembly removal is a whole different process. The only thing that could possibly be calibrated in some fashion is the housing itself. The casting has a brass ring pressed into the throttle body side which leaves an open slot that the incoming air passes by. The passage for the sensor  is offset from main air inlet and is less than 1/2” diameter. The other end of the passage terminates in the slot mentioned above. It is a high speed air flow area so pressure would be low to draw the air past the sensor.