Overfuelling? - 30Mpg

[Grim.Badger 15]

I've had my current GTi since 2009 and it has never really given the MPG I came to expect from the two before it (a 1.9 and a 1.6), it always seems to have done around 28-32mpg depending on my driving. I do a lot of urban miles, but motorway miles only get a little better MPG, still not around 40mpg like my last two.

It has failed two MOTs for emmissions, the first was fixed by changing the AFM, and also a cleanup of the air system and plugs - the second was fixed by changing the spring tension on the AFM, however this caused quite bad underfuelling under load and so I changed the AFM again afterwards for one that is meant to be in good working order (still sealed, gave the last owner good MPG) but I have yet to check the emmissions - it does however run better on this AFM.

 

The plugs get what appears to be a "normal" beige dusting, except one which is a little black (I assume the valve seals on that chamber leak a little). The car smells quite a bit of fuel, or possibly oil, when starting. The coolant temp sensor has a teal cap, whereas my old GTis had the proper blue cap; I have changed the sensor, but for another teal coloured sensor. I have a persistent coolant leak that I cannot trace, but is probably from the plastic multi-block on the back of the engine block, but I'm not sure if that is relevent. I'm fairly certain that I have no air leaks, or at least no major air leaks.

 

Despite the fuelling problem, or possibly because of it, the car seems to be the most reactive to the throttle of all three of my GTis, and once warm drives very well.

 

I don't know much about the fuelling and spark systems, but I suspect that this is coming from the fuelling end given that the MPG is stable and the car drives well. I suspect that it may be a wiring fault in between the AFM and ECU, the earth points, or that someone may have modified the fuelling system at some point (although I have not noticed anything added to it). I do have a spare ECU to test with, although I'm not sure if it works.

 

Any ideas? Is there even a problem or am I expecting too much?

[Anthony 1,000]

I'm assuming that this is a Jetronic (non-CAT) car rather than a Motronic (CAT equipped) one?

 

Is the vacuum advance working and does the TPS work properly?

 

Both of those being faulty won't show obvious symptoms but will noticeably lower fuel economy.

 

To give you an idea, replacing the faulty vacuum advance alone increased the average economy by 4mpg.

 

Worth checking that the ignition hasn't been retarded to run 95RON fuel too, as 1.9's do suffer quite noticeably.

[Grim.Badger 15]

I'll check those, but I'm fairly certain the TPS works. It is Jetronic, although it was first registered after the compulsory CAT introduction (must have spent time on the forecourt).

 

I have checked the advance and think I have adjusted it correctly to 97-99ron but I am starting to think that I need to get an engine specialist to check it for me.

[GLPoomobile 958]

I'd hate to cloud things over with unnecessary suggestions, but I'll throw this out for consideration - Do you know the history of the ECU? Have you got access to a known standard ECU you could test?

 

Back in the day when the likes of Superchips and Pug Performance used to offer "chipped" ECU, I believe I'm right in saying that the one and only modification was to snip a diode (number 11 rings a bell) and this would both remove the rev limiter and also remove the fuel cut off on the over-run. Removing the fuel cut off also meant that on-off throttle response was less jerky so kagarooing in traffic was greatly reduced.

 

Obviously if the fuel cut off is disabled, it'll use more fuel.

[dcc 855]

Its hard to say as we cant know your driving style, 2 people doing the same journey will use different amounts of fuel. It is worth fitting a new ECU Temp sensor for the cost.

 

Other than whats been said by the others, the main thing will be how healthy your engine is on comparison. I.e. An un healthy engine will work harder to perform at the same level.

 

Have you done a compression check? And you say this engine is a bit more eager, does it have an aftermarket cam fitted? Or maybe a fse power boost valve thingy

[Weser 30]

I would second replacing the ECU Temp Sensor. I fitted a new one and was getting 26-28mpg. Just replaced it because I thought it was a bit suspect and straight away I was getting 32-34mpg

[jackherer 543]

If you know the coolant temperature you can test the coolant temp sensor with a multimeter, just do a google for the table of temperature vs resistance for your sensors part number.

 

And the same applies for the throttle switch, test it with a multimeter so you can be sure it works otherwise you are just guessing and throwing money at parts you might not need.

[dcc 855]

Not strictly true. If it shows a good reading at normal temp, say correct ohms for 15degrees celcius, it is not garuanteed to show the right ohms ar 80/90 celcius. Or maybe it does, but nowlt in between, etc...

 

Also could be a increased resistance in the wiring causing fault... these cars are 25 +\- years old...

[welshpug 1,627]

I think I have only ever come across one duff CTS, so save yourself £15 and get the Multimeter on it

[jackherer 543]

I've seen quite a few knackered temp sensors and they tend to fail throughout the range but yes to test it thoroughly you need to test it at various temperatures. The point about wiring is valid, you should test the sensor and then test the corresponding pins on the ECU plug to ensure the reading is the same.

 

However step one is to put a multimeter on the sensors and switches in the engine bay.

[Stu 18]

As above, for me the first thing i check if i suspect any fuelling issues is the CTS; its dead easy to check too, couple of small crimp tags onto a small length of wire straight onto your multimeter leads, get a pan of water on the hob with the sensor submerged and 'map' the temperature of the water vs. resistance of the sensor as it rises up at say 10degree intervals.

 

You'll soon see if its within tolerance

[Grim.Badger 15]

I'll get a multimeter when I'm paid next week, I've been putting off getting one for too long!

 

I still have the Dizzy and Vac Advance from my old 1.9, so that would be a quick test - obviously if nothing improves I accept that it could be both advance units are knackerd.

The ECU still has it's limiter in place, so I don't think it has had any of the obvious modifications made. As I said, I have a spare but I don't know if it works.

I'm not sure about FSE valves, I haven't noticed anything unusual, where would one be fitted?

I don't think it has an aftermarket cam but will check the limited reciepts that I have from the last owner - I'd rather not disturb the cam cover seal if I don't have to!

I haven't checked the compression yet, but will try and get it done in a minute.

 

Another question - would faulty earth wires cause this, or would they cause more inconsistent running?

[Anthony 1,000]

Easy enough to check the vac advance in situ - remove the vacuum hose from the underside of the throttle body and blow/suck through it. If the vac advance is working it should be impossible to blow/suck and with enough effort you should hear the vacuum advance moving inside the dizzy housing. If you can suck/blow through it (you'll likely hear a faint hiss from the vacuum advance capsule on the dizzy) then it's knackered.

 

I'd guess that the majority of vacuum advance units these days are probably dead.

 

Whenever I've seen knackered coolant temp sensors they've usually given terrible MPG (typically low 20's) but it's worth double checking anyway like others have said.

[Grim.Badger 15]

Quick update:

 

With the dizzy off the Vac advance can be blown through easily - I'm not sure if doing this with the dizzy off will change the result? I have changed to my other dizzy and advance, and will check that later now that I know how to do it properly, and tried to set the timing as best as I can at the moment.

 

Compression (tested three times each):

Cylinder 1 - 175, 170, 170

Cylinder 2 - 185, 185, 185

Cylinder 3 - 180, 175, 180

Cylinder 4 - 190, 190, 190

 

Seems good

[welshpug 1,627]

if air passes though then it certainly is shot, though whilst the initial timing should be set with it disconnected, it would be a minor air leak upstream of the afm, and of course you wouldn't get the small changes in timing on part throttle.

[Stu 18]

Like Mei says, if your blowing through the actual advance unit with ease then its buggered. As you'd expect, you should be able to suck it and it hold whatever vacuum you can generate with your mouth

[Grim.Badger 15]

I forgot to add, the TPS clicks at closed throttle but not at WOT, is that right?

[Anthony 1,000]

That's correct, but you need to check it with a multimeter - the "click" doesn't mean that it's actually making an electrical connection internally.

[Grim.Badger 15]

I will be doing, thanks, I just wanted to check that the lack of click at WOT wasn't a sign of a major problem

[Grim.Badger 15]

Finally got around to checking things, although I forgot to get a thermometer so can't check the CTS with any accuracy.

 

The replacement vacuum advance is also knackered, so I will have to order a new one - there only seems to be one retailer online, any suggestions for others?

 

The TPS reads 0ohms off the purple wire pin at WOT, 0ohms off the red wire pin when closed, and 1ohms at all other points. I assume this is correct?

 

The CTS reads 3.97ohms at around 15 degrees celsius, which I know is only an innacurate snapshot but is it about right?

Edited April 18, 2014 by Grim.Badger

[jackherer 543]

What scale was your meter on when you tested the CTS? If you mean 3.97 K or 3970 ohms I think that's about right.

 

To test the TPS you should unplug the loom and measure continuity on the switch itself.

[iPlod999 143 1 Cars]

These guys refub VA's.

 

http://www.h-h-ignitionsolutions.co.uk/

 

Quick postage turnaround £20.

[Grim.Badger 15]

What scale was your meter on when you tested the CTS? If you mean 3.97 K or 3970 ohms I think that's about right.

 

To test the TPS you should unplug the loom and measure continuity on the switch itself.

 

Err, yes I think it must be 3970 ohms - I forgot to turn it down to the lowest setting (can you guess that this is the first time I've used a multimeter?).

 

I did check the TPS itself, after unplugging it from the loom, it's just easier to say "red wire pin" than "the male pin on the TPS that the red wire connects to". I used the central pin on the TPS for the earth/negative.

 

 

These guys refub VA's.

 

http://www.h-h-ignitionsolutions.co.uk/

 

Quick postage turnaround £20.

 

Cheers, I'll have a look

[jackherer 543]

That all sounds correct then, obviously to be sure you need to re-check the CTS at a range of different temps but in my experience if it has a sane resistance at ambient temps it's likely to be OK through its range.

I've just had a quick re-read of the thread and you didn't sound 100% about your ignition advance;

I have checked the advance and think I have adjusted it correctly to 97-99ron but I am starting to think that I need to get an engine specialist to check it for me.

To get to the correct advance you really need to push it until it starts to pink then back it off a tiny bit. If you haven't heard it pink while setting it up you haven't optimised it.

[johnhenry 20]

can recommend H&H wholeheartedly, proper good blokes! went up and dropped by dizzy off as head office is 10 mins from them. great turnaround, really knowledgeable guys.