MegaSquirt '77 VW Bus Installation

Hello, my name is Ed and this little web page documents my MegaSquirt DIY EFI controller installation on my ’77 VW bus.


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Info regarding MegaSquirt can be found at:






This is a stock 2 liter type IV VW aircooled engine – set up for a nearly stock compression ratio of 7.2:1


I have utilized the following stock FI components:

        Cylinder head temp sensor

        Intake manifold / air distributor / throttle body

        Fuel pump

        Fuel pressure regulator

        Aux air valve



My MegaSquirt setup replaces these stock FI components:



        Thermo-time switch

        Cold start valve

        Double relay

        Ballast resistors (for injectors)

        Air filter


MegaSquirt Hardware:


I am running version 1.01 hardware with the following modifications:


1.      I have pulled up both the ground and signal pins on the opto isolator. I have run separate wires to the external connector (pins 24 and 25). I cut the trace from 24 that used to carry the ignition signal. All components used to condition the ignition input are now in an external ‘blob’. See comments for Figure 4 below.

2.      I tagged a 6.5 k ohm resistor in PARALLEL with R7 to bring my head temp sensor into range with the standard sensor tables.

3.      I tagged on a voltage divider to effectively clamp my throttle position input at 2.5 volts. (10 k ohm to +5v and 10 k ohm to gnd). (I don’t use any TPS).


MegaSquirt Software:


I am currently running the completely standard version of the 2.0 code. This includes the standard sensor lookup tables.


Air Cleaner:


I removed the stock air cleaner along with the mass air flow sensor. I fabricated an air filter holder from 2 PVC toilet flanges, a 22.5 degree 2” PVC elbow, and a 2” PVC cap. The cap plugs the flange used on the end of the air filter. The elbow mates the other flange to the rubber intake boot. Note that the elbow and cap mate to the flanges using their OUTSIDE diameter. This is not the intended surface to mate on these parts, but it just happens to provide a snug fit to the flanges. I used standard PVC cement to make these joints solid. Long bolts that fit in the flange holes snugly and wing nuts hold the whole thing together. I am using a filter element that I found at NAPA and it just fits inside the diameter outlined by the 4 mounting holes of the toilet flanges.


Air Intake Sensor:


I am using the standard GM temp sensor. I have this mounted on my air filter ‘holder’. I suppose it is best to mount this in the air stream as close to the head intakes as possible. It seems to work fine for me near my air filter. See the ‘top view’ photo of my engine bay. I simply drilled a hole slightly smaller than the sensor threads in the PVC elbow on my air filter holder. Then I simply screwed the sensor in by hand. It is nice and snug.



Oxy Sensor:


I mounted my oxy sensor on the ‘J” tube that connects my exhaust manifold to my head exchanger. See picture below. Other year busses may not have this nice place for mounting – many have the head exchanger bolt directly to the head. This spot that I have it mounted is close enough to the head to marginally keep the sensor hot enough to control mixture even at idle. If you mount one after the heat exchangers (for instance) you may not get adequate output from the sensor at idle.


I have a bung that I will weld onto the J tube the next time I am under the bus. For now I simply drilled a hole just the size of the sensor part of the OXY sensor and twisted it into the hole – then wired it secure.


I found a cheap source for Bosch 11027 single wire universal sensors:


 … only 16.84 each! Currently I am using one from NAPA (part #OS101) – about $25.


Coolant Sensor:


I am using the stock Cylinder Head Temp sensor in place of a coolant sensor. I am using the standard coolant lookup tables in the code and have simply modified the bias resistor to bring my sensors A/D reading into the range of the standard sensor. See comments in the section ‘Hardware:’ above.


Throttle Position Sensor.


I am NOT using any TPS. The stock FI did not have any acceleration enrichment either. Seems to work fine. I do have a bit of hesitation when not fully warmed up. See comments in the section ‘Hardware:’ above.




Performance is great. Probably very much like a properly tuned stock FI engine. On my last trip (500 miles) I averaged 21.4 MPG.




1)     An earlier version of the SW had a bug in the math routines that would cause an occasional 25.4 ms injector pulse. This has since been fixed in the current software.

2)     I had problems with my processor resetting. This was when my MS box was mounted in the engine bay. The reset problem would sometimes erase my settings in flash. This was a pain in the butt. This problem went away when I rewired and relocated the MS box to the location shown in the pictures.

3)     Some folks have suggested that the engine bay is too hot to run the MS – but the hottest that I measured was about 130 degrees after a long run and after the vehicle sat for a while. I cooked a running MS box in the oven and did not see any problems even at 180 degrees F. This is no longer even a potential problem since I have relocated the box.

4)     The ignition input signal conditioning circuit is something that needs to be customized for many installations. Since I am running the stock points/coil ignition and since this type of ignition is noisy, I had to tweak the input circuit – see further info in section ‘Hardware’ above and Figure 4 below.




Here is an estimate of my total cost. Some of the items are guesses from memory:


MegaSquirt Unit – 1st group buy


Stimulator (Device for exercising the MegaSquirt)


Oxygen Sensor


Air sensor + connector


Injector Connectors


Air filter hardware


Air filter element




Wiring/Crimp Connectors/Tubing/Etc










The setup of my MS controller can be obtained by downloading my MegaTune file:


MegaTune MSQ File


(If the above file won’t download, try this one – it has it’s extension changed to .bin – so you will want to rename it back to *.msq after you download it)

Alternate MSQ File


Or by looking at the following MegaTune screen captures.


 I don’t have it tuned perfectly, but it is tuned well enough to run absolutely great.  I actually have attempted to tune it about %5 rich. The oxy sensor feed back then tweeks the mixture back. I have done this simply to provide more room for error in terms of making sure I don’t run lean.