So what does a clogged fuel filter feel like?

[Scurrie]

OK gang,

Help me diagnose a problem that seems to getting worse on my 99’ R1100RT. I started noticing increased vibration in the engine about 8K miles ago, mostly centered around 3K and again at 4K RPM. Subtle at first, but growing more noticeable. I have done all the standard checks, new plugs, valves are right on, TBS is right on, no vac. leaks. I took a few relatively long rides this summer, and in fact, just did about 350 miles on Monday. Things seem much worse this week. Around 4K really now feels like it is hesitating/missing, and roll on in 5th starting at 4K is rather lack luster and the vibration at 5K and above is now very strong. Also, backfire on deceleration is worse than usual. I think it acts like a fuel/air problem, not ignition, but I’m no Boxer expert.

 

I am the second owner of this bike, first owner put all of 6K miles on it in 5 years, it did a lot of garage time. I have put on 14K in the past 14 months. I don’t think the fuel filter has ever been changed. Ideas?

 

Thanks,

[Haynes]

Scurrie, As you,ve already recognised, there is a wide range of possible problems, or combinations of problems, that will produce these symptoms. If all else has been checked within reason and you think it could be fuel system related, the fuel filter and hoses within the tank are a good place to start. It only needs a small leak in one of the fuel hoses within the tank to either cause a leak in pressure or to introduce air into the fuel line.

It would be prudent to have the tank serviced first and then move onto the possibility of dirty injectors.

[ShovelStrokeEd]

Just change it, its not that hard and only requires a few bucks in parts.

Go to the FAQ and you'll find the part #'s. You will probably be able to search out the procedure.

 

The recommended filter change interval is 24K miles but, at 6 years old, it won't cost you to change it now.

 

You could also have a coil going bad, a wiring issue with the HES or the HES itself going although that is ususally a more sudden thing.

 

At any rate, pull the tupperware, remove the tank, do a cannisterectomy while you are at it and change the fuel filter. You can inspect the ignition coils while you are under there.

[jimswms]

do a cannisterectomy while you are at it

 

I've heard this mentioned before. What's the purpose of the canister, and what is to be gained by removing?

 

thanks,

Jim

[Ken H.]

What's the purpose of the canister,

It is an emission control device. Captures evaporating fuel from the tank. They've been on cars for decades.

what is to be gained by removing?

There is a lot of speculation (mostly hype IMHO) about the horrible things that can happen if it becomes clogged/contaminated. I've only ever seen one with an actual problem and that was because hoses had gotten reversed in a service error and water from the fuel filler neck was going into it instead of fuel vapors. Hardly the systems fault.

[skinny_tom (aka boney)]

If your fuel filter is clogged, your fuel pump will not be able to provide enough fuel to the injectors, first at higher flow rates, to support proper combustion. So, like you said, you'd start noticing hesitation and power issues typically at higher RPM's and when you "whack" open the throttle. Unless it's really plugged, you shouldn't be experiencing symptoms at lower RPM's.

 

Like Ed says, change it. But before you put the bodywork back on, g on a test ride to see if you fixed it.

 

There is a lot of speculation (mostly hype IMHO) about the horrible things that can happen if it becomes clogged/contaminated.

 

Mine had dumped carbon down the hose, all the way to the servo, and hopefully not into my intake system.

[RFW]

I can't recall if your R1100RT is fuel injected or not. I expect it is.

 

With any fuel injected engine, an increasingly clogged fuel filter will make NO DIFFERENCE WHATSOEVER to the bike's performance, until (of course) it gets completely blocked.

 

What happens (and it happened to my good old K100 last May) is that an increasingly clogged filter simply causes the fuel pump to increase its output pressure in order to get the fuel pushed through the filter. Downstream of the filter (i.e. at the injectors), the pressure remains unchanged. Remember that the so-called "roller cell" pump is a constant VOLUME pump. It's output pressure is determined only by the backpressure it encounters. In other words, the pump generates whatever pressure is needed (up to the limits is is capable of) in order to push that constant colume of fuel past the filter.

 

In my case, I started hearing a whining noise from inside the tank. Upon investigating, I measured the pump output pressure at well over 120 psi (normal, is about 30 PSI!!!). What had happened was that the filter became plugged, and the poor pump was was straining like hell to get the required volume of fuel through the filter.

 

But in spite of the whining noise and the absurdly high pressure needed to overcome the filter backpressure, the bike ran perfectly well. The only thing that alerted me to the problem was the pump noise.

 

This also helps explain the incidences of "exploded" fuel filters that have been occasionally reported. Roller cell fuel pumps are capable of delivering extremely high pressure if they encounter enough backpressure. The extreme pressure resulting from a clogged filter, is sometimes enough to rupture the filter. Kind of like closing your mouth and nose when you sneeze (OUCH!).

 

Bob.

[skinny_tom (aka boney)]

Re: Roller Cell Fuel Pumps

 

Within the pump is a pressure relief valve that lifts off its seat at 8 bar to arrest the pressure if a blockage in the filter or fuel lines or elsewhere causes it to become obstructed.

 

http://www.picotech.com/auto/tutorials/fuel-injection.html

 

8 Bar=120 psi or so.

 

A dirty fuel filter causes a decrease in volume. As the pressure increases behind the filter, more fuel will be forced through until the pressure relief valve (built into the roller cell fuel pump) activates. This prevents the fuel filter from providing any more pressure or volume (it is being recirculated back into the intake of the pump.) A dirty fuel filter will then be only able pass the volume of fuel which can be forced through at 120 lbs. If indeed, a filter is contaminated beyond this point, it is very realistic that the engine could exhibit fuel starvation issues as it consumes more volume than can be forced past the filter.

[Scurrie]

Well, I replaced the fuel filter and can now confirm the proceedure is a real PITA!

 

Didn't improve the way the engine runs though, on to other things I guess. Maybe 0=0 TBS?

[brinkjm]

Back in the good ol' days, we (we being those that played mechanics on TV) would either scope an engine to check the ignition system or simply check the resistance of the spark plug wires. Whenever I come across a missing or lack of power issue, once I've ruled out the obvious, I'd check the resistance of the ignition wires. First, check them against the spec. (sorry, I don't know what the spec. is for the BMW leads but I would be suspect of anything more than 20k ohms) and, while still checking the wires, twist them around and see if the resistance changes at all, indicating a broken lead. SSEd-Ster, what says you?

[RFW]

If your fuel filter is clogged, your fuel pump will not be able to provide enough fuel to the injectors, first at higher flow rates, to support proper combustion. So, like you said, you'd start noticing hesitation and power issues typically at higher RPM's and when you "whack" open the throttle. Unless it's really plugged, you shouldn't be experiencing symptoms at lower RPM's.

 

That not how injection systems work. The fuel pump pumps is a "Positive Displacement Constant Volume" pump. It develops whatever pressure needed to push that constant volume (liters per minute) through the fuel ring, past the pressure regulator and back to the tank. This volume of fuel is much more than the injectors can use even at full power. All that happens when you "whack the throttle" is that the injectors "steal" some of the fuel that is being pumped around the fuel ring, and so there is a little less that makes its way back to the tank. The pump and the filter cannot tell whether all the fuel is being recirculated back to the tank, or some of it is being diverted to the injectors.

 

What happens with a partially (or even nearly TOTALLY) clogged filter, is that the pump pressure automatically increases in order to overcome the increased filter backpressure. That is how ALL positive Displacement pumps work. But once the fuel has made it past the filter, as far as the injectors are concerned, everything appears normal. Fuel ring and injector pressures are still normal, since the regulator still does its job.

 

As I posted earlier on this subject, my K100RT had a plugged filter. The only thing that alerted me to this was that the pump started making an increasingly annoying whining noise. Upon checking, I measured the pressure at the pump to be WAY over 100psi!!! The pressure in the fuel ring was absolutely normal (30-some psi, as I recall), and the bike ran perfectly normally. The difference was that the filter, that usually only has a couple psi back pressure, had about 100psi back pressure as a result of the clogging. But the pump pressure simply increases automatically to overcome this, and downstream of the filter, everything is perfectly normal.

 

One thing I should add. There is a pressure relief valve in the pump, but it is ONLY there to save the pump from damage, and opens only way above the normal pump operating pressure.

 

It is also impossible to have "low injector pressure" as a result of a nearly plugged filter. The pressure regulator will only open to allow fuel to flow in the fuel ring, when the ring/injector pressure reaches the desired value. Below this pressure, NO fuel will flow in the ring. So if there is fuel flowing in the ring, and recirculating back to the the tank (which is easily visible by looking in the tank when the bike is running), then (baring a defective regulator) the ring and injector pressure is normal and the injectors cannot "know" that the poor pump is busting a gut trying to push fuel past the clogged filter.

 

Bob.

[skinny_tom (aka boney)]

With respect to your opinion, my disagreement stems from my experience with EXACTLY the problem I described on a vehicle I recently owned. Every time the fuel filter plugged it behavied as described by OP'er and was fixed by replacing it- until it plugged again.