fuel pump noise

[suddha]

Hello,

I have searched the forum but didn't find an exact match to my issue. I have a 1990 Spider (Motronic). Lately, I have been hearing a high-pitched hum from behind the passenger seat when the car is warm and the gas tank is below half full. The hum is only when I push the accelerator and then lasts for a few seconds until I am well underway. Happens at every stop/start. When I top off the tank, no sound. Needless to say, I suspect the fuel pump.

Funny, but the "low fuel" dash light flickers when I accelerate when the tank is below half full as well. Unrelated, I am sure but worth mentioning.

Am I in imminent danger of getting stranded with a dead fuel pump? How difficult is replacement for a pretty unsophisticated shade tree mechanic?

Cheers.

[Tifosi]

Sounds like mabe you're getting a bit of starvation in the main pump.

Have you ever pulled the float and pump assembly to check the little hose that ties the in-tank pump to the fuel system?

If that in tank hose is whupped, it'll push air to the main pump instead of fuel, which in turn can make it yowl pretty handily.

The other exrteme would be a partially clogged filter, either in tank or the external one, making the main pump work harder than it should.

Your float may be mis-orientated in the tank as it should pivot to the side, not for and aft, causing the light to come on under acceleration. (the float is only pertinant to fuel level, not the other issue you're having)

[ghnl]

If it is not the fuel pump, might it be the manifold pressure (vacuum) sensor?

Does the Motronic have this item? I know the L-jet does. It hides behind the vertical panel behind the right side door. If it begins to leak it can make a humming &/or whistling sound.

I cannot figure out how to correlate it with the level of fuel in the tank, though. So fuel pump seems more likely.

[Gubi]

No manifold sensor on the Motronic.

My money is on a dead in-tank pump. You can run 12V to it in-situ and see if it hums to indicate it's working (it's hard to hear when the engine is running). I forget which color wire at the gas tank is +12 though.

If that's not it, either your little hose is busted (as Tifosi said) or you've got a vent system problem. If cracking the gas cap temporarily fixes the issue then think about the vent system.

[Vintre]

Hello suddha,

This problem just started with me tonight! Exactly as you described it, even the low fuel light.

In fact, the car started hesitating as if it was running out of gas even though the level showed 1/2 full. I pulled into the closest gas station and filled up.

While the hesitating stopped, the high pitch whistling continued.

Did you ever figure out what was wrong with yours and if so, how did you fix it?

Thanks,

Vin

[suddha]

Nope, never did solve this issue, nor did it get any worse. I just always keep the tank above half-full now. And now it's time to mothball the Spider for winter, so it will have to wait til next spring!

[Vintre]

Quote:

Originally Posted by suddha Nope, never did solve this issue, nor did it get any worse. I just always keep the tank above half-full now. And now it's time to mothball the Spider for winter, so it will have to wait til next spring!

[Vintre]

Quote:

Originally Posted by Vintre That's too bad. OK, at least we have a partial solution and we will never run out of gas!

[dghii]

I think the board has solved this one for you.
1. Your main pump is whining due to lack of fuel input. It may be on its way out at this point. I purchased a pump from Autozone for $160 in August that is quieter than the Bosch pump and so far, is working flawless.

2. Replace both filters.

3. Verify in tank pump is working.

4. Replace short fuel line that is part of that connects the in tank pump to the fuel sending unit.

5. How old are your fuel supply and vapor hoses? If original, its time to replace them.

[Vintre]

Well,

What else would I do on a Friday night but try to figure this noise thing out? !!

I followed the noise behind the passenger seat and found that it was coming from the Vacuum Sensor and not the fuel pump at all.

As you can see in the picture there is a little black hose at the bottom of the sensor. When I pull that off, the noise stops. While the engine is running that hose is pulling a vacuum so I can assume that there isnt a problem there and must be the vacuum sensor.

I will do a search now, but question is, how long can I drive with that hose unplugged?

Vin

[ghnl]

The ECU (computer) uses the info from that sensor to adjust the timing (ignition). I assume it advances the timing under cruise conditions (light load/moderate to high rpm's). Without that sensor's input, the timing will not advance and you'll probably loose some economy (mpg). Otherwise it shouldn't hurt anything (except your wallet).

Be sure you are sitting down when you look up the price of a new unit...

There is a thread here where a BB member opened up one of those sensors. I don't think it is (easily) repairable however. Try a search and see if you can find that thread.

[Spike]

I've owned my '84 spider for ten years, vacuum has been disconnected all this time and runs fine. The vacuum hose is plugged with a bolt, not sure if this is necessary.

[Tifosi]

Quote:

Originally Posted by Spike I've owned my '84 spider for ten years, vacuum has been disconnected all this time and runs fine.

[Spike]

Thanks for the input Tifosi ! I'll install a new unit in the spring, but hard to believe I'm giving up power, she pulls real good,(shrug), mileage definitely.

[Tifosi]

You'll prolly find it'll have some more grunt up high, say 3200-3500 and on up when it gets up into the higher advance ranges. Rolling smoothly up into the 3900-4000 then giving the throttle a good whack to activate both the advance and the VVT will likely give you the most noticable/detectable punch. (in proper tune and once you hit around 4k, these engines really come to life)

Down low it's not going to be a super difference in the seat of the pants calibration unit, but as the timing will be accurately controlled (Bosch's word, take it for what it is) your mpg should be a bit happier. (likely you'll discover you don't have to press the gas quite as far/hard to get things scooting along as the ignition will be responsive to the engines needs, and if you do give it a bit of welly, it'll get a wee bit more snotty than what you likely feel now)

You may even find that it starts a bit quicker too.