Fuel Consumption

Ian_Fearn · Post by **Ian_Fearn** » Thu Dec 14, 2006 9:18 pm

Some of you good fellows may remember I had to replace the fuel pulsation buffer on our GTi not long ago after the original rusted through.

Around this time the fuel consumption went from about 27-28mpg down to 20-21mpg. Being an auto we expect the MPG to be pretty cr4ppy but 20mpg is dire.

Since replacing the little device its made no difference.

The air filter was replaced about 2000 miles ago so that shouldnt be a problem.

I'm going to whip the plugs out and see what colour they are BUT the car passed its emissions MOT test last month with hardly even a reading.

Its also had new plug leads recently. One theory i have which is a definate long shot is that i replaced the rad and fan switch with a low temp job. The car always runs nice and cool now but i cant really see that affecting the fuel consumption to the tune of 6 or 7 mpg.

Any more ideas? The car seems to run like a swiss watch.

I dont think she's driving it any differently and her weekly drive hasnt changed either.

tom · Post by **tom** » Thu Dec 14, 2006 9:37 pm

Ok, let's start with the obvious stuff. The fuel accumulator shouldn't have affected consumption because the pressure regulation happens later in the cycle.
Making the assumption that a brake isn't dragging, then we are back to the standard stuff. It is now cold (despite being the warmest December since 1659(and how did they measure it in 1659?)) Are you only doing a short daily run? Is your thermostat stuck open?
If the car is running well, it looks (as you suspect) like a temperature issue but not the fan switch. GtI 8valve has IIRC a pressure regulator on the fuel rail. May be worth checking pipe to this from the manifold, it may have busted. Does it have the standard Bosch warm up regulator? I've known them give difficulties. Sorry nothing specific here, Plugs will tell us a lot.

Ian_Fearn · Post by **Ian_Fearn** » Thu Dec 14, 2006 9:47 pm

I replaced the thermostat as part of its cooling system overhaul. Fan switch, rad, cap, thermostat and some pipes. Could the thermostat be at fault perhaps?

The issues did begin to occur around the time of this overhaul.

I replaced all the rubber fuel pipes into the FPR earlier in the year so i know they're in good order.

I'm not aware of any warm-up regulator in the system.

tom · Post by **tom** » Thu Dec 14, 2006 9:57 pm

I reckon the stat has to be the prime suspect. Don't guess the FPR pipes, check them. If you have fitted new ones, one may have split or dropped off. Mechanical bits are like that

Kitch · Post by **Kitch** » Fri Dec 15, 2006 6:37 pm

I don't know much on the GTi's, but does it have a temp sender for the ECU? If the car was running colder, the ECU would apply more fuel.

I know the 16v does, but as I say I don't know much on the GTi.

PrutsyLJ · Post by **PrutsyLJ** » Fri Dec 15, 2006 6:41 pm

Kitch wrote:I don't know much on the GTi's, but does it have a temp sender for the ECU? If the car was running colder, the ECU would apply more fuel.

I know the 16v does, but as I say I don't know much on the GTi.

The GTi models from 1989 till the end of the prudoction have a fully electronic Bosch Motronic multipoint injectionsystem (well at least in Holland they have

) So what you are saying is true.

Mike E (uk) · Post by **Mike E (uk)** » Sat Dec 16, 2006 10:07 am

Where does the temp gauge it?

I know it is ony a guide, but every GTI I have driven has sat at about 90, and reached this temperature in only a few minutes.

Any longer and the stat is probably leaking.

Don't forget that even brand new parts can be faulty.

Mike E (uk) · Post by **Mike E (uk)** » Sat Dec 16, 2006 11:35 am

Where does the temp gauge it?

I know it is ony a guide, but every GTI I have driven has sat at about 90, and reached this temperature in only a few minutes.

Any longer and the stat is probably leaking.

Don't forget that even brand new parts can be faulty.

Ian_Fearn · Post by **Ian_Fearn** » Sat Dec 16, 2006 4:36 pm

I must admit i noticed it does take quite a long time for the temp gauge to start moving. The car does 10 mile journeys mostly but does quite a bit of knocking about locally too.

It never gets driven hard (unless i'm behind the wheel) as the missus is a bit more sedate than me....

For the pathetic cost of another thermostat i'll grab another.

When i bought the thermostat i bought a 'low temp' one. Perhaps i should have bought the normal one. I didnt think at the time it would make any difference other than keeping the car in the safety zone better.

tom · Post by **tom** » Sat Dec 16, 2006 5:55 pm

The stat, even in low temp mode should keep the water in the engine until it gets hot. Standard check is to remove it and find out if it is open. You may have dislodged the rubber seal when fitting it which would cause the engine to warm up slowly but either way, slow warm up is the reason why it is drinking.

cavmad · Post by **cavmad** » Sat Dec 16, 2006 6:03 pm

Isn`t there a way of checking the thermostat by placing it in a pan of cold water and then bringing it to the boil and seeing if it open ok?
Apologies if you already tried or thought of that but thought it might help. Incidentally Ian I`d imagine that only doing ten mile journeys will mean increased fuel costs as it won`t be long off choke? Do you give it a good run out frequently as this should help matters I think?

jeremy · Post by **jeremy** » Sat Dec 16, 2006 6:24 pm

For the cost of a thermostat its hardly worth testing it - apart from curiosity when you've done the job.

Not sure what the layout is with regard to gaskets - but when I replaced the one on the ZX (1.9D - 94) I was supplied with the correct stat but the wrong gasket. The box said it was the correct one.

Its not serious but annoying as you've got your car paprt on a Sunday afternoon and can't get it back together again!

If its any interest - both my diesels are chucking out useful heat after 1/2 mile of narrow town road with cars parked either side. I'd have said diesels dumped much less heat than petrols under those conditions.

cavmad · Post by **cavmad** » Sat Dec 16, 2006 6:56 pm

jeremy wrote:For the cost of a thermostat its hardly worth testing it - apart from curiosity when you've done the job.

Maybe, but surely he`ll at least be able to eliminate the `stat that way, it won`t cost anything to do and what if another `stat is duff?
I went through 3 reconditioned alternators on a Chevette years ago and got so pissed off with the car I sold it assuming it couldn`t have been the problem. A couple of weeks later I bumped into the new owner who got one from the scrappy for a fiver and never had a problem with it again.
I appreciate `stats may be cheap but why spend money when you don`t have to?

jeremy · Post by **jeremy** » Sat Dec 16, 2006 7:14 pm

The problem really is that you will have to find a new gasket somewhere as the chances are that if its the newer ring type, the old one will be damaged and won't seal again. Often modern ones seem to have an internal 'O' ring gasket to seal the valve to the plate and on our ZX that had in fact broken and although it looks like a smaller 'O' ring type gasket I haven't a clue where to get one on its own.

The gasket I refer to as an 'O' ring is in fact a square rubber section with a groove cut on its internal face to fit over the flange of the thermostat.

If you have the earlier diesel type thermostat (which is in fact huge and probably unique) you will probably find the securing bolts sheer off (2 out of 4 of mine did) and have to be drilled out before the housing can be removed. You then have the fun job of cleaning up the gasket faces (for a paper gasket) before the thing can be re-assembled. The next fun bit is that the thermostat is secured in the top of the housing by a large circlip which will probably have to be driven out as its corroded in place nicely. A new clip is not supplied in the kit - so care has to be taken not to break the old one.

I was actually quoted something like £14 for the gasket for my diesel - and they shouldn't be re-used. Fortunately a new one came in the Quinton Hazell kit my friendly accessory shop sole me for £8.

Having tested it - how much longer will it last? Who has an accurate thermometer?