Playing Doctor

Update by Scott Lear to the BMW 325is project car
Nov 14, 2007

Our impromptu brass T-fitting allowed us to get a fuel pressure reading.
The fuel pressure gauge indicates an insanely high amount of fuel pressure at idle.
We replaced the fuel pressure reuglator, but it did not do anything to fix the problem.
We ran hot laps at Ocala in an effort to get the brakes to seize up; it took a while, but finally the butt end of the car was dragging as it has been in competition.
The rear calipers are far from pretty, but they’re not mechanically hindered, so the problem was elsewhere.
Redline BMW’s Rennie Bryant was very helpful in tracking down our BMW’s demons.

After a few seconds, we also had the start of a nice Molotov Cocktail, so we killed the engine.

With a number of weird symptoms popping up regularly, it was time to spend a day playing doctor with our Spec E30. The rear brakes are seizing up symmetrically, the car runs rich nearly all the time, and we’re still down on power. Plus the fuel pump started making weird noises at the end of the NASA Championship race and power delivery went all wonky late in that same race.

We loaded the Spec E30 on the trailer and headed off to the Ocala Gran Prix track where we do much of our car testing. While we could have done most of the diagnostics in the garage, our rear brake issues only seem to materialize when the brake system is hot, so the track was necessary. Rennie Bryant of Redline BMW Performance came along with his years of BMW experience to help out.

Before we hit the track we wanted to try to figure out why the car is running so rich. We suspected the fuel pressure regulator, so we made a T-fitting out of some brass hardware in order to hook up a fuel pressure gauge and see what kind of pressure the fuel system was getting.

After we fired the car, the needle immediately jumped up to 90 psi. That’s more than double what it should have been reading at idle. Okay, so obviously we’ve got way too much fuel pressure. We swapped out the fuel pressure regulator, but that did nothing to fix the problem.

Rennie remembered a former customer who had a clogged return line in their E30, and he suspected that might be the case here. We unhooked the fuel return line and ran it into a large glass bottle so we could get a reading without the backed-up fuel return line sending the pressure to the clouds. We saw about 40 psi on the gauge with an unhindered fuel return route. After a few seconds, we also had the start of a nice Molotov Cocktail, so we killed the engine. The fuel return line carries a LOT of fuel in a short period of time, so make sure you’ve got a big enough container whenever you unplug it for testing purposes.

After unscrewing the gas cap, we used an air compressor to force air through the return line to see if we could blow the clog out. We were rewarded with air and some gas bubbling out of the tank, but the pressure still stuck at 90 psi after we hooked everything back up. The problem seemed to be somewhere in the lines of the tank itself, so we’ll have to remove and either clean or replace the entire fuel tank–Oh, joy. At least we’ve got a likely culprit, and a clogged return line would explain why the fuel pump is straining and the car is running rich as a pig. Fixing it might even free up some of our missing horsepower.

With the car put back together and running, we tried to get our brakes to seize up as they have been doing so Rennie could experience it and help figure out the problem. Naturally, the car drove just fine, with no indication of lockup; why would it want to provide tangible evidence of its malfunction with a certified mechanic around to help fix it? That would be too easy.

We tore into the rear calipers anyway to see if the culprit was mechanical. Although there was lots of nasty goo behind the caliper piston, they did seem to be moving as the factory intended. We bled the system and headed back out. A number of abusive ABS-engaging laps later we came back in, still lacking evidence of the problem. We chatted with Rennie about Murphy’s Law, sitting with the car at idle, and when we’d given up hope and lifted the clutch to take the car back to the trailer, the tail end sunk down as if the parking brake were on. Hooray! We ran for the tools and went right for the ABS unit, which Rennie suspected was to blame.

Sure enough, as soon as the hard line going from the ABS unit to the rear brake was disconnected, the pressure was relieved and the BMW rolled freely.

Hopefully, replacing the ABS unit and the gas tank will leave us with a much happier BMW.

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