Hum from transmission tunnel area?

[Daniel81DMC]

Heya guys - long-time reader, seldom poster - mostly because my DMC is usually running quite well apart from easy-to-fix age-related repairs. As an aside, I've owned the car for nearly ten years, now (I'm 30, it was great taking the DMC to university!).

I noticed just the other day a low, constant humming coming from my transmission tunnel area (I have the 5-speed in my '81). I *barely* heard anything in the cockpit of the car, but as I am sure you guys know, we all get very used to how our D's sound. Putting my head down near the gear-shifter, I could hear the noise more clearly - no grinding, nothing really nasty, just a constant humming, and I don't remember it ever being there.

The car is low mileage; there's only 53,000 miles on the Odometer, it's been babied its whole life (I'm the second owner, but have every nut and bolt receipt right back to '81 from the original owner), and the only transmission work it ever had was a new clutch about five years back which can't have more than five thousand miles on it.

Any thoughts as to what this could be? Could it be something other than the tranny, just something in the area?

I should mention that my general understanding of cars is quite good, but I'm no expert, I'm a hobbyist, and the DeLorean isn't exactly the same as the Corvettes and Trans AM's I am used to.

I apologize for the rather vague description - I'd get sound-byte if I could, but I don't think the recorder on my IPhone is sensitive enough.

[Spittybug]

Maybe a fuel pump humm?

[Bitsyncmaster]

What's your VIN?

There is a problem with a nut backing off that mostly occurs on early VINs.

[Daniel81DMC]

Interesting; I replaced the fuel pump about four years ago - the sound *is* similar to a fuel pump noise, but it doesn't seem to be coming from the tank area, it's coming more from the tranny-tunnel area.

By they way, my VIN is 2991 - built in August of 1981.

[tgraham]

You didn't share a few important pieces of information to help narrow things down: is the ignition ON, accessory electrics ON, car moving/rolling, and/or engine running when this noise is being made?

I'm guessing it's probably not moving since you mention putting your ear near the shifter to hear it... but...

Travis

[Daniel81DMC]

Car on, ignition on, everything running; the hum is *barely* audible when driving.

[Soundkillr]

Could be fuel pump accumulator. My bracket was slightly loose and allowed it to barely rub inside of frame. Made a high pitch whine. Moved the bracket some, tightened it back down, now no more noise. Btw it is located in the transmission tunnel.

[Daniel81DMC]

Interesting - would that be the accumulator "going", or just it randomly coming loose?

How would one check either way? Easy to get at?

[Bitsyncmaster]

Could be fuel pump accumulator. My bracket was slightly loose and allowed it to barely rub inside of frame. Made a high pitch whine. Moved the bracket some, tightened it back down, now no more noise. Btw it is located in the transmission tunnel.

That is a good guess. One of the funtions of the accumulator is to absorb the pulses from the fuel pump. So it the accumulator was "shot" you may get the pump noise at it. Of course it could also be the bracket or foam rubber on the accumulator.

[David T]

Check the motor and transmission mounts. One can be cracked. Check all of the rubber bushings in the pivot points for the shift linkage. Make sure the rubber cover is in place over the shift mechanism in the console. As long as you are looking check the fuel pump and the boots to make sure everything is where it should be.
David Teitelbaum

[Daniel81DMC]

Thanks for the suggestions, guys; the car is going back to my mechanic next week (I have a really good shop out here that's brave enough to work on the car), and it's going in to have the passenger-side emergency brake cable installed (old one snapped). The car was in last week for a full coolant flush, and I couldn't get it done at the time due to lacking the part.

So - check transmission mounts, check accumulator (is that easy to get access to?), and... I have no idea what is being referred to by the rubber cover on the shifter - I've never really had even seen a pannel off on the DeLorean before, and I assume we're talking something visible from underneath the car, only.

I take it the most likely issue is something related to the accumulator, though - either the part itself or the way it is mounted? If the accumulator actually "dies", is it much the same as a dead fuel pump - i.e. the car won't even start?

Any other suggestions? Thanks for the help, I appreciate it - this problem is rather weird, I know.

[jackb]

I have no idea what is being referred to by the rubber cover on the shifter - I've never really had even seen a pannel off on the DeLorean before, and I assume we're talking something visible from underneath the car, only.

Lift up the leather boot and you will see a rubber "thing" that fills the square hole in the body that the shift lever comes through. It lifts out of the body easily and if the shift know is off will come all the way off.

[sdg3205]

Hey Daniel,

You're always welcome to drop by my shop with your car if you ever feel like working on it or if you want a hand with a repair.

I'm in Surrey now.

Where is your mechanic?

Cheers

[Daniel81DMC]

Hey Daniel,

You're always welcome to drop by my shop with your car if you ever feel like working on it or if you want a hand with a repair.

I'm in Surrey now.

Where is your mechanic?

Cheers

Hey there - do I dectect a fellow DeLorean owner in Vancouver? Ye gods, I thought there were only one or two in the whole city, of which mine was one.

I live in Coquitlam, and my mechanic is in Maple Ridge - a place called Scandia Motors. More or less they're Volvo specialists, but they do amazing work, and have never minded taking the time to figure out how to do a repair on my car in the eight years that I have been going to them - they don't even charge me shop time for the "figuring out" part, only the labour, which in my experience is rare (I used to be a mechanic's assistant at a dealership, back in my university days).

This problem is annoying me - the noise isn't loud enough that I'm panicking that something is about to horribly go wrong, but it conerns me that a part of my fuel system may be wonky, which could (theoretically) mean that the car could die at any time.

[Daniel81DMC]

Audio recording of noise - http://www.youtube.com/watch?v=-pknNE7x13I

Last update - took an audio recording of the hum by hodling the audio received up to the transmission tunnel area from inside the car (I basically held the speaker against the padded center console/shifter area).

The noise is separate from the cooling fan noise up front, and you can hear the slight valve/exhaust issue in the back end I'm needing to deal with, but the hum is pretty audible.

Sounds sort of like a fuel pump, so I'm leaning towards the fuel accumulator suggestion I read, but if anyone else wants to take a guess...

[Ron]

I've never tried it to know how long you might have to listen, but you might try killing the fuel pump while the engine is running and see if the sound goes away/changes before it dies...?

[Ryan King]

I would be worried that the transmission nut is backing off...I would suggest you take a look at that to be sure. The early VIN cars were very prone to this happening...

[Daniel81DMC]

Interesting about the transmission nut - where is that and how would one check it?

I actually drove over to a local DeLorean owner's place (there's all of three in our rather large city), and his best guess (and one I'm inclined to agree with) is that it's the fuel pump - I opened the trunk, and the humming was definitely audible in there. The thing that ticks me off is that it's a new part (well, new-ish), and was installed only four years ago, replacing the original which lasted over 25 years.

I'm not 100% certain, of course, but this really is looking like the pump. Thanks for the previous suggestions, guys (and any others that might come up) - most kind.

[Bitsyncmaster]

Interesting about the transmission nut - where is that and how would one check it?

I actually drove over to a local DeLorean owner's place (there's all of three in our rather large city), and his best guess (and one I'm inclined to agree with) is that it's the fuel pump - I opened the trunk, and the humming was definitely audible in there. The thing that ticks me off is that it's a new part (well, new-ish), and was installed only four years ago, replacing the original which lasted over 25 years.

I'm not 100% certain, of course, but this really is looking like the pump. Thanks for the previous suggestions, guys (and any others that might come up) - most kind.

I was going to suggest the pump may just be getting louder. After thinking about the talk of the accumulator, they don't really fail compleatly so it would still be absorbing the pump pulses.

[DCUK Martin]

Lift up the leather boot and you will see a rubber "thing" that fills the square hole in the body that the shift lever comes through.

Given your vin# this may be missing and the fuel pump buzz sounds as though it's coming from under the gear stick.

[Spittybug]

While fuel pump gets my vote (see post #2), a couple of posts in this thread have suggested the secondary shaft nut in the transmission since this is an early VIN. While I DO AGREE that you should separately address that, I don't believe it is the source of your noise.

Inside the transmission case lie 2 parallel shafts loaded up with gears that get meshed together to give the necessary combinations of speeds. These two shafts run end to end and and each is held in place with a big nut on a threaded end. Well, someone in the Renault(?) plant got lazy and on a bunch of the early VINs didn't do the final step when tightening the nut down. He was supposed to hammer the nut's collar against a small flat on the shaft, thus preventing the nut from ever backing off. By not doing so, he has allowed many of these nuts to ever so slowly start spinning off with normal use of the transmission. Soon the shaft gets a little bit of play, for and aft. I believe this contributes to the symptoms of making it harder to shift into second gear as the little bit of play misaligns things a bit and the weak roll pin gets extra stress on it. Ultimately what happens is the nut starts carving a groove in the back part of the relatively soft aluminum cover and one good shift later, the shaft moves enough to crack the case open. Whether related or not (I'm sure it is), also check springs that may get chewed up as a result of things not quite being aligned correctly any more. Ask me how I know this..
:vomit_2:

So, concentrate on the fuel pump/rubber sound boot/accumulator, but pay very close attention to any change in your shifter feel. Whether you go in proactively just for that or take the opportunity to change the clutch, perhaps add drive shaft sleeves or whatever else needs doing under there, I would plan on doing that for peace of mind.

[guss23]

Humming? Is it a tune you recognize?

[Daniel81DMC]

Yikes! I never imagined that it might be the gears; thanks for the tip on that one. My mechanical expertise sort of precludes me from getting in there and checking this, but my mechanic may be brave enough to have a look down the road.

I really am leaning towards fuel pump, though - the noise is particularly noticeable up front, under the hood, nearest to the windshield, which is almost exactly where the fuel pump is located - I'm pissed off because I replaced the thing no longer than four years ago, after the original of a quarter-century died.

[sdg3205]

With regards to the 5th gear nut, if you pressed the clutch all the way in would it stop?

Also, would it change in speed/pitch if you revved the engine?

[Daniel81DMC]

With regards to the 5th gear nut, if you pressed the clutch all the way in would it stop?

Also, would it change in speed/pitch if you revved the engine?

Good question - I don't think it changes, and heck, you were there the other night - I'd find it odd that the transmission would make audible noises (strong ones at that) as far forward as the trunk area.

I'll see if I can have a good listen tommorrow; car is going to my mechanic's to have the e-brake cable replaced (one snapped).

[DMC5180]

With regards to the 5th gear nut, if you pressed the clutch all the way in would it stop?

Also, would it change in speed/pitch if you revved the engine?

Yes, eventually the rotating parts would stop turning. Even better if you also put it in gear after depressing the clutch this is guaranteed to stop internal rotation.

I'm 99.9% sure it's his fuel pump. They don't build them like they used too. He also did not say what level the fuel was at in the tank. A Low fuel condition with hot fuel (and possibly a collapsing pickup hose) can cause cavitation of the pump which will get very noisy. (transmitted up the tunnel area) This is usually temporary, if the car is allowed to sit over night then restarted and the sound is greatly reduced this is likely is a contributing factor.

What was the reason the pump was changed 4 years ago? OEM pump failure or just because you were afraid it might and figured NEW would be better. If the old one still worked, did you save it as a back up spare?

[Daniel81DMC]

Heya!

Fuel level in the tank is low-ish - a little over 1/4 tank. The sound is definitely quieter when the car is cool and gets louder when the car is warm and has been running awhile.

OEM pump failure is the reason I swapped the last one - it was REALLY loud, particularly on hot summer days. Replacing it made the sound go away, of course, and I was worried that the pump was just going to up and die if I didn't replace it. I'm not one to replace parts unless they really need it. This is not because I am cheap, but rather out of having no desire to waste money on things that are perfectly functional!

[Delorean Industries]

Heya!

Fuel level in the tank is low-ish - a little over 1/4 tank. The sound is definitely quieter when the car is cool and gets louder when the car is warm and has been running awhile.

OEM pump failure is the reason I swapped the last one - it was REALLY loud, particularly on hot summer days. Replacing it made the sound go away, of course, and I was worried that the pump was just going to up and die if I didn't replace it. I'm not one to replace parts unless they really need it. This is not because I am cheap, but rather out of having no desire to waste money on things that are perfectly functional!

The aftermarket pumps available from vendors including myself have been known to be rather noisy during operation from time to time. It is strange because I can pull one brand new and install another one and its completely quite. I have very rarely seen a loud aftermarket pump fail even under heavy use. My daily driver makes noise but hasn't quit after 30k in almost two years of continuous use. The weak link now is electrical connections and if you actually have 12v + a the pump. Low voltage will cause early pump failure and a loud pump. Even though it is proven and I believe even by Bosch that the pump can run at a sub 12v status I think it kills the life of the unit. Also having it completely submerged in fuel matters too. I think keeping the fuel level up is important during extended use to transfer heat from the pump.

[Daniel81DMC]

I was looking at SpecialTAuto for fuel pumps, and they seem to have... several different kinds, some with extra bits (i.e. lines, clamps, etc.) and some without.

I really have no idea which pump I would want. Anyone have a suggestion? By the way, thank you to all of you for being so helpful, it's a real pleasure to be around such a great community.