Poor Fuel Economy - Gti-6

[Joshy 0]

Another question from me.

 

The fuel economy of my GTI-6 has begun to bother me slightly. Filling it up is a similar experience as trying to fill a bottom less pit, I put fuel in, I drive it around for a bit and the fuel light comes back on, so I put more fuel in, and so on Due to not having a working speedo (more on that later), I never bothered to do a calculation on the MPG I've been achieving. Until now when I've worked out that £15 work of fuel will only get me roughly 40 miles of driving before the fuel light immulinates. That is something along the lines of 12MPG! So I think it is fair to say that there is a slight problem .

 

My joineries are mostly to work and back, 7 miles each way driving around London. Granted the engine never really comes up to operating temp until I'm parking at the other end, so my Fuel economy is never going to be great, but 12MPG.

 

Now, I have been a slightly naughty boy regarding my Speedo issue. I've got a dodgy VSS, meaning no Speedo and a sizeable 'lump' in the rev range at 1,400RPMish. I have bought a brand new replacement VSS and even attempted to fit it, but after finding the location of the part (or at least, where I think the location is, I'm still not 100% certain!), I crawled underneath my car (with my sizeable bulk, fat arms and jack stands that don't raise the car nearly high enough) I deemed that it was impossible to do at the side of the road, and decided that I'd wait until I could have a go doing it on the ramps at work. To be honest, I never got around to doing it, I've learned to with a non functioning speedo and a bit of lumpiness at low revs.

 

My this Economy issue has me worried. I understand that the VSS is really a Crank Position Sensor, so I'm beginning to suspect that a bad VSS could cause awful fuel economy?

 

My Head is also Ticking, but I don't think that wouldn't be the cause for such poor fuel consumption, would it?

 

Anything else I should start considering?

 

Thanks

 

Josh

[Galifrey 1]

With a 7 mile commute in London Traffic 12mpg doesnt sound so bad!

 

However, I would replace the VSS, check the lamda sensor, and perform and ECU reset so you are on the default map.

 

I have seen many cars that have "learning ecus" have excessive fuel consumption which was cured with a new lambda and an ecu reset.

[Miles 331 1 Cars]

First of all the VSS has nothing to do with the speedo unless it's in a 306 as the 205 is a cable drive,

So the VSS is driven from the Diff and sends a signal to the ECU on speed basically, The Dual output one which you need (Both cable and electronic) does both,

It could well be that who-ever did the install never wired this in, Common problem as a new part costs more than £50.00, Also the Lambda are not always fitted either so all worht checking without knowing the history of the car

[Joshy 0]

First of all the VSS has nothing to do with the speedo unless it's in a 306 as the 205 is a cable drive,

So the VSS is driven from the Diff and sends a signal to the ECU on speed basically, The Dual output one which you need (Both cable and electronic) does both,

It could well be that who-ever did the install never wired this in, Common problem as a new part costs more than £50.00, Also the Lambda are not always fitted either so all worht checking without knowing the history of the car

 

Yep, I have Purchased the dual output sensor I just don't have a hope in hell of fitting the thing at the side of the road (at least from what I can see)!

 

The conversion was done by Dan Taylor on here, so I'm fairly confident that the wiring wasn't lashed together

[dcc 855]

check the cam timing sensor is correctly plugged in. my gti6 went down to something like 5mpg when i forgot to plug that bad boy back in! it was insanely fast during this time though!

[taylorspug 7]

Oh yeah, i 'forgot' to wire the VSS in. What a joker! And of course it has a lambda sensor.

 

The VSS isnt 'nothing to do with the speedo', they are both driven from the same unit, hence why its called a dual output sensor. The speedo in the clocks is just driven by a cable instead of electronically like in the Gti6, so the electronic reference is used for the engine management only. There, ive even punctuated it properly.

 

Josh, as we have spoken about before, your car has the correct dual output sensor fitted onto it, but it was supplied to me by the cars previous owner with the stem section where the cable fits into it broken, which is probably why the speedo doesnt work. As long as you have the correct part (with the 3 pin plug on it and the stem section for the cable), it isnt a hard swap. In fact if you really dont fancy it bring it down the M/A23 to my workshop and il swap it over for free. Just because im nice like that.

[Joshy 0]

Thanks for the offer Dan, I think I might have to take you up on that offer.

 

Am I right in thinking that the VSS is the white sensor on the bulkhead side of the gearbox/bellhousing, best viewed from underneath? I tend to be an awful mechanic without any diagrams to fall back on!

[Anthony 1,003]

Am I right in thinking that the VSS is the white sensor on the bulkhead side of the gearbox/bellhousing, best viewed from underneath? I tend to be an awful mechanic without any diagrams to fall back on!

Yes, pretty much - towards the rear of the gearbox near where the drivers side driveshaft enters the diff. Normally the speedo cable goes in the top, held in place with a rubber pin, and then there's a 3 pin connector next to it that the corresponding plug on the ECU loom connect to. The VSS is held in place by a single bolt, and I personally find it easier to lean over the wing and down the back of the engine/gearbox to reach it.

[Galifrey 1]

Oh yeah, i 'forgot' to wire the VSS in. What a joker! And of course it has a lambda sensor.

 

In fairness to Miles, I doubt he realised you had done the job, and there are a fair few bodge ups out there

[Joshy 0]

Yes, pretty much - towards the rear of the gearbox near where the drivers side driveshaft enters the diff. Normally the speedo cable goes in the top, held in place with a rubber pin, and then there's a 3 pin connector next to it that the corresponding plug on the ECU loom connect to. The VSS is held in place by a single bolt, and I personally find it easier to lean over the wing and down the back of the engine/gearbox to reach it.

 

Really, Thank you for that. I had another go and getting at it from the top really wasn't nearly as bad as I thought it was going to be. I got it out, well, most of it! I tried over coming the sealing ring of the sensor by giving the body a good yank, the plastic broke leaving a sizeable chuck of plastic sensor to remain lodged in my gearbox. Weather permitting, I will resume tomorrow!

 

One thing I have noticed is that the new sensor and the old sensor are rather different in the body, and where the cable sits there is a metal 'sawblade' in the recess of the New sensor, but not such thing on the old one. Both the cable and the plug fit perfectly though.

[Joshy 0]

Right, almost there, just having some worries regarding getting the new sensor in.

 

The gear at the end of the spline that travels through the VSS into the gearbox - Is it supposed to sit flush with the bottom of the sensor? Mine has a 0.5 to 1 inch gap between the gear and the bottom of the sensor.

 

What am I supposed to do with the positioning of the gear? I'm really struggling to push the sensor right down into the gearbox. It was my worry that maybe the gear may not be meshing with that it is supposed to over come those last few MM.

 

Overcoming the rubber seal. How much force is realistically required to do this? I believe I'm putting in ample enough force onto the sensor to overcome the seal, but it just isn't going down.

 

Here's a thought, is there a possibility that the gear shouldn't actually have come out with the old sensor, and that I need to position it in the gearbox first and then place the sensor onto it?

 

Is there anything I've missed?

[taylorspug 7]

The gear is meant to come out with the sensor, and it just slides into place in the new sensor. Yes the top of the gear should be basically flush with the bottom of the sensor housing. They can be a right tight fit into the gearboxes at times, a bit of WD40 and a technical wiggle usually sees them in though.

 

I suppose the one thing that has been so far overlooked is there could be some damage to the corresponding plastic gear in the gearbox itself. This would also cause the speedo not to work. This can also be changed but requires the driveshaft to be removed. Its also worth noting that the plastic gears come in pairs (ie sensor and gearbox ones), mismatching them will cause the speedo to be innacurate.

 

When i did the conversion the car came to me broken, so i didnt have a chance to drive it and see if the speedo worked originally.

[Joshy 0]

Dan, I'm a bit worried that you say that the gears come in pairs, as 3 - 4 of the teeth on the gear are slightly damaged. I did put it all together and the speedo does work although it seems to under read a bit. On the tyres I'm running 1800RPM in 4th should be around 30MPH, but the speedo reads about 25MPH!

 

It was well worth the struggle though, it is It is SO much nicer to drive without the low RPM flatspot and is apparently a LOT quicker than I ever imagined - It was quite something to see what the speedo was reading after a few second of WOT, I think I'll have to behave myself more .

[Miles 331 1 Cars]

Which box is it? All bar the 1.9/1.6 will give different readings as the gears are different ratio's on most others

[Joshy 0]

1.6 box.