Here is a quick idiot’s guide to fitting a boost gauge to an Audi A4 B5.
Mine is the 1999 UK model so others may be slightly different, but there doesn’t seem to be many decent guides online, and most are American, so I thought I’d have a go.
First, once you’ve got your gauge, pick where you are going to put it. The centre vents seem to be a popular choice. Most I’ve seen are in the centre vent of the 3 but I opted for the one closest the driver, partly because it’s nearer to the driver and partly because the driver’s side one has its own individual flap on the back that you adjust with the open/close controller wheel at the edge of the vent. The other side controller wheel controls both the left hand vent and the centre one together.
So, prize the centre vents out gently with a screwdriver. Don’t yank it out too hard because there is a wire leading into it for the vent wheel illumination. If you have this fitted to your car it’s easy to tap into this for your gauge’s lighting.
Again with your screwdriver, prize out the vent that you want to use. Remove the vertical veins by popping them out of their locator holes. They should come out quite easily with a bit of flexing.
Then, use a pair or cutters to gradually snip away at the horizontal veins until you create a hole big enough for your gauge to slide into. Don’t cut off too much at a time and keep trying the gauge in the hole over and over until it fits snugly. You can always cut more off but if you overdo it, you can’t exactly glue bits back on! The plastic is very soft and cuts quite easily. The veins are quite wide so I cut from the front and then turned it round and cut from the back also.
Slot in the gauge and make sure it’s snug and isn’t going to drop out when you go over a bump. If it’s a bit baggy, a bit of hot melt glue from the back can hold it in place or wrap the gauge housing in a few turns of black insulation tape.
Now you can splice into the wires that are attached to the vent housing. If you don’t have these, you’ll have to find power from somewhere else that switches on when the lights come on, otherwise you’ll not be able to see what you are boosting at night! Ha.
Don’t waste your time with Scotchloks. I tried but the wires on the vents are too thin and you get crappy contact, so you’ll have to solder the wires in place or just twist them together. I used a lighter to burn off the insulation in the centre if the wire then wrapped my gauge wires round the bare wire and covered with insulation tape.
Now you should have a gauge that lights up when your lights come on. So, time to fit the boost pipe.
Open the driver’s door and pull the cover off the drivers side edge of the dash where the fuse box is located. Undo the 4 bolts that hold on the undertray that fits under the steering wheel and pull the undertray out from the pedals taking care to remove the connector to the footwell light, which is inside the undertray.
Pop the bonnet and remove the rubber strip that sits at the top end of the engine bay in front of where the battery is, then slide out the plastic cover that goes over the battery. You should now see a large black plastic box in front of where the driver sits. This is the ECU box. You need to remove the lid, which is held on with 5 bolts. To do this easily you can remove the wipers and also the plastic trim that sits around them, which allows you free access to all the bolts, but if like me, you couldn’t be arsed, you can do it with this still in place, with a bit of cursing and knuckle scraping.
Now get your boost pipe. There are 2 places to “T” into to get a pressure reading from the inlet manifold. The rear one is the Fuel pressure regulator: (the short black tube going to the silver cylinder)
and the front one is the diverter (or dump) valve (the thin black tube going over the alternator)
I didn’t feel as comfortable potentially messing with the signal to the FPR in case I ever developed a leak in the system somewhere, so I opted for the one that is connected to the diverter valve. This involves considerably more faffing. While I was at it, I thought it was a good time to change the diverter valve for a new one, which I did. For this process I recommend removing the driver's headlight for light/access and also jacking the car up and putting it on an axle stand. You can slide under the car and access the dumpvalve easily. It looks like this.
There is one normal jubilee clip on it and one absolute barsteward of a press fit type clamp which is junk once removed (prize it off with a screwwy) so make sure you have a replacement new jubilee clip. I also full removed the turbo outlet pipe that the diverter valve is connected to to make things a bit easier and to gve it a good clean at the same time. It's a fairlysimple job, just fiddly.
Next, there’s a grommet on the firewall behind and to the left of the expansion tank. Puncture a hole in this and feel the boost pipe through making sure the hole is big enough to not crush the pipe. (see pic below with the grommet removed). Leave more than enough tubing in the engine bay to T off on the inlet manifold.
Now feed the boost pipe along to the ECU box and again puncture a hole in the grommet on the front of the ECU box. Behind the ECU you will see a big hole that leads down into the driver’s foot well. Feed the pipe down here then replace all the removed bits under the bonnet and pick up the boost pipe inside the car.
Drill a hole for the boost pipe in the back of the vent housing and feed the pipe up and through and connect it to the back of your gauge. Cable tie the boost pipe out of the way, being careful not to crush it and refit all the dash parts. I would cable tie the boost pipe to the back of the gauge to prevent leaks or the pipe coming off.
Now go back under the bonnet and Tpiece in the boost piping, one from the diverter valve, one from the gauge and one from the inlet manifold. As you refit the engine cover, make sure it doesn't squash or trap the boost piping.
Shove the gauge back in and check that it lights up when you switch the light on.
Now road test to make sure everything is working and there are no leaks. Mine boosted to around 13psi peak dropping down to about 10 as the revs build. Seems healthy enough.
If someone can put this in the technical section, hopefully it will help numpties like me in the future!
Thanks to all those who gave me advice with this and to Mike who sold me the gauge!