BMW E36 Fuel System Maintenance and Diagnosis


fuel injector rail

BMWs are not known for premature fuel system failures. However, these vehicles are growing older by the day, and fuel components eventually fail just like any other part.

In this article I will discuss common fuel system maintenance that any owner should be performing, and tips on diagnosing the most common fuel problems.

Components

BMW fuel systems are not that complicated, pretty much every vehicle is made with the same components:
Fuel Tank
Fuel Pump (Usually underneath the backseat in a BMW, but they vary in location with other cars).
Fuel Filter (usually an inline filter underneath the vehicle, but if you’re a classic car junky you know that many older vehicle have fuel filters in the engine bay).
Fuel Injectors
Fuel Filter
Fuel Pressure Valve (Located on drivers side, close to the filter)

Maintenance

By far the most common maintenance item on any vehicles fuel system is replacement of the inline fuel filter. Fuel filters get gunked up overtime, and there capacity to clean fuel declines as a result. Luckily, fuel filters are cheap and relatively quick to replace. E36 fuel filters sit underneath the drivers side, and are mounted to the frame. They can typically be picked up for as low as $20 at a local auto parts store.

Fuel Filters should be replaced every 20,000 – 25,000 miles, although I see vehicles every day that haven’t had a fuel filter replaced in more than 60,000 miles. Unfortunately, neglecting fuel filter replacement can overtime cause additional fuel system problems, particularly concerning the fuel pump and fuel injectors.

When you do change the fuel filter, flip it over a bucket and take a look at the fuel color. You will most likely notice that the fuel is very dark and has an “old fuel” smell to it.

There are also rubber hoses on either end of the fuel filter. They degrade & crack overtime, so take care to examine them when changing the fuel filter!

Want to check out a video tutorial on changing a BMW E36 Fuel Filter? Click Here!

Fuel System Treatment

fuel injector cleaner

Lucas Fuel Treatment & Seafoam are God’s gift to man in my opinion. I pour in a bottle once every two weeks. The benefits of fuel system cleaner have been well documented, and they certainly help extend the life of your fuel system. I highly recommend you throw in a bottle at least once a month.

Different people will tell you different things about what is the best fuel system treatment, but my personal favorite is Lucas. Other notable treatments are STP, Royal Purple & Red Line.

What do they actually do?

Fuel injectors work by spraying a mist of fuel through the nozzle of the fuel injector. Without an injector, the engine would take on the fuel directly & would not last nearly as long, among other issues. The problem here is that the holes on the nozzle of a fuel injector are tiny, and overtime can get clogged up. It doesn’t take much of a blockage for a fuel injector to stop operating correctly. On top of that, there are a number of ways for junk to be introduced into the fuel system. A well operating fuel filter will catch most of it, but it doesn’t work 100% of the time.
Fuel Injector Cleaners help keep the fuel injector ports cleaned, helps maintain your vehicle’s gas mileage & performance.

I will advise however that fuel injector cleaner is not a snake-oil fix all of your fuel system problems. If your fuel injector is already clogged or your fuel pump has gone bad, a little fuel injector cleaner isn’t going to cut it.

Diagnosing Fuel Issues

Like many other parts on a vehicle, fuel system maintenance is often ignored. My brother called me one day, informing me that he was stuck in a bad neighborhood at a cheap DVD rental store (yes amazingly some areas still have DVD rental stores). I had no idea what the problem could be, especially since I knew that he drove his car as if it was a discardable toy.
When I arrived to check out what was wrong the car would turn over, but that was it.
Possible battery issue?
I checked the fuse panel & what do you know, the fuel pump fuse was blown. 99% of the time when the fuel pump fuse is blown it’s a sign that the fuel pump is going bad & blowing the fuse.
I went ahead and changed the fuse & it started right up.

At that point I decided to put the vehicle under a load and revved up the rpms, a few seconds later it died once again. At that point I had the car towed to the house, changed the fuel pump, & the problem was solved!

Later on, my brother informed me that the car had been having performance problems for several days & he thought it could last “a little longer” 😔.

Fuel Injectors

fuel injector rail

Diagnosing a clogged injector can be somewhat time consuming, because the symptoms are pretty much the same as a bad spark plug or bad ignition coil. Usually a well clogged fuel injector will give you a check engine code for a cylinder misfire.

Old fuel, carbon & residue will cause an injector to clog up. Old fuel is in bold because I have seen several times where a car owner decided to try and start his car after many months or even a year or two of the car sitting. When a car has been sitting for a lengthy period of time the fuel can very easily cause a fuel pump or an injector to clog up. I have seen numerous fuel pumps clog up as a result of starting a car with old fuel.

Symptoms of a clogged fuel injector include starting issues, reduced gas mileage, poor performance & misfiring. Once you have a severely clogged fuel injector no amount of cleaner will save it’s life until you take the injector out of the rail.

A simple way to test the injectors is by performing a “flow balance test”. You need a fuel pressure gauge and a scan tool to “pulse” the injector solenoid. Your checking the fuel pressure drop between the cylinders, a drop is good whereas no drop indicates a clogged injector.

There are numerous services where you can ship out injectors to have them professionally cleaned. It typically costs around $18 an injector, however you will have to remove the injectors yourself. You will probably be able to find a local mechanics shop as well that can perform professional injector cleaning services.

My advice if you decided to have them professionally cleaned is to go ahead and do all six, instead of just having the one or two on the misfiring cylinders cleaned.

Fuel Pressure Regulator

fuel pressure regulator

This component rarely goes out on E36’s as compared to many other vehicles, but when they do it renders the car nearly inoperable. BMW is one of the only vehicles I have seen that doesn’t put the FPR on the fuel pressure rail. Instead, the FPR is underneath the car on the drivers side. If you have changed the fuel filter before you’ve probably noticed it sitting there on the fuel line.

Most E36’s have a Schrader valve on the end of the fuel rail which allows you to test the fuel pressure. Once you have the fuel pressure gauge attached;
Record the fuel pressure at idle, and then have someone rev up the rpms a bit.
The pressure should increase a few psi & then drop back down to normal.
Afterwards, pull the vacuum line off of the front of the fuel pressure rail.
Did the pressure go up around 10 psi & stay there?
If it didn’t then you most likely have a stuck fuel pressure regulator & it will need replacement.

An additional note to make E36 FPRs is that when I have seen them go, the fuel pressure sits much higher than it should be.

Fuel Pump

I have a separate post just on the symptoms, diagnosis and replacement of the fuel pump. That article can be found here.

Hope this article helped with your car’s fuel system maintenance! I also have an additional video on fuel system maintenance here.

Stephen Metellus

I am a BMW enthusiast and owner of abetterbmw.com! I have been repairing, flipping, and parting out BMWs for nearly ten years. I love these vehicles and I hope you will find my articles and YouTube channel helpful for whatever BMW project you have in store!

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