Diesel - Cold enrichment / ZX Volcane fuel economy

PJLarge · Post by **PJLarge** » 07 Apr 2004, 02:22

I'm learning quite quickly about diesels here but I have another question (groan)
I suspect the t-stat on the ZX is stuck open, it sits at 65* all day unless I work it up a hill when it may hit 80*. On a modern petrol engine, the ECU will make corrections for fueling if it thinks the engine is cold. What happens on a diesel? I notice there is a cable that appears from behind the t-stat housing and goes to the pump... what's this then?
The reason I ask is that I'm not sure what sort of economy I should be getting, I filled it up and got 100 miles from above full on the gauge to the full indicator, then in the next 50 it dropped to less than 3/4. I'm now at 300 miles and the gauge is just below 1/2. I dont drive that hard and so far all mileage is at 80mph on the motorway. I notice the gauge moves round a bit depending on the cars orientation so I'm not sure how much I trust it yet. What are others experiences with the Volcane TD?
Thanks again
Phil.

Post by **philhoward** » 07 Apr 2004, 02:33

Fuel gauges can be very deceptive..some can drop the first 1/4 of a tank in 25 miles, then stay at 1/2 way for 350 miles...you'll only know when you fill up and brim it again. Economy depends on how you drive it, but 45-55mpg should be about right on a ZX; my Xantia TD does 47-48mpg and its a heavier car. Perhaps Jon could give us a clue, the amount of motorway miles he does!
The cable you can see is the cold idle speed cable...it shortends when the engine is cold to give you a fast idle (only another 200rpm or so), dropping off to a normal idle speed when up to temp. If your idle speed doesn't change from cold to hot, then you probably have the electric cold start function and a duff fast idle cable, like me! If it's happy ticking over first thing in the morning, then i'd be tempted to leave it.
Diesels work differently to petrols, in that (off boost) the throttle position is directly proportional to the fuel injected per revolution of the engine; there's no throttle on the air. On boost, the amount of fuel injected goes up proportional to boost pressure. If it runs "rich" (to use petrol terminology), then it just goes quicker than the same throttle position would normally. Sounds like your stat is knackered, though. There's been a thread recently on what temp shoud XUD lumps run at; look back a few pages and you'll find it.

humpy · Post by **humpy** » 07 Apr 2004, 03:15

Your experience with the fuel gauge is exactly my experience with my last two ZXs. A 1.4 and now a 2.0 volc. It takes a while to drop to full then shoots down to 1/4 then a while for the reserve light to come on. Don't worry , just get used to it!!

ScottFromNZ · Post by **ScottFromNZ** » 07 Apr 2004, 03:38

My BX TZD fuel gauge behaves erratically too. Stays at full for ages then quite a quick drop to about 1/3 then ages to the low light at which point there is about 10 litres left.

ScottFromNZ · Post by **ScottFromNZ** » 07 Apr 2004, 03:40

As far as your wax-stat is concerned I think that if it is working correctly it should be "off" well before normal operating temperature. But your coolant thermostat sounds faulty too - maybe opening at a lower temp than it should.

uhn113x · Post by **uhn113x** » 07 Apr 2004, 13:06

Phil
The fast idle is to compensate for the oil being more viscous when cold.
There is no concept of 'richness' or 'weakness' on a diesel - fuel burns as it is injected - longer injection = more power.
On a petrol engine, the air and fuel are mixed in the carb, or in the manifold (injection). When cold, much of the petrol condenses on the cold manifold walls; hence the need for enrichment. The fast idle operated by a cam on the choke, or by a fast idle device (injection) has the same purpose as on a diesel

Post by **philhoward** » 07 Apr 2004, 23:49

I'm agreeing with you on this one, Mike...I just put it a bit funny!

Stuart McB · Post by **Stuart McB** » 08 Apr 2004, 01:03

From cold you should notice a small increase in engine revs and sound. As for the cold start valve, easy check. When cold see if there's any free play at the end of the cable on top of thepump. If so slacken grub screw and push the small link arm back towards to starter motor end of the engine then do up the grub screw. Now drive untill warm then check again the cable should now be slack again by about 6mm. Leave to go stone cold (overnight) and check again. If working it should now be tight. So that's tight when cold and slack when hot. Not too much to replace with genuine part though. As for the fuel guage what can you sya it's a citroen all mine have been the same. I work on millage. Fill up every 12th day or 440 miles which ever comes first. I do a lot of town driving.

Sl4yer · Post by **Sl4yer** » 08 Apr 2004, 06:28

My petrol ZX gauge does much as described, i.e. quite a while to go down to full, then quick to around 1/4, then slow to run out.
Although I did get caught out when I'd had the car for about 3 days. The petrol light came on when I was on my way to York one Friday night. I thought nothing of it since father's Cavalier would run for about two days with the light on. On the way back (in the outside lane of the M62) the engine lost power, only to pick up again. Then it cut out altogether, only to restart on the hard shoulder. I guessed by then that it was short of gas (and not my 'new to me' car being knackered) and scraped together about £4 to get some petrol at Hartshead Moor Services.
So when the light on my 'very new to me' Volcane TD came on, I filled it up pretty soon! It only did 200 miles on the first £20 of diesel, but I suppose it was thrashed a bit! I'm sure its got quicker with the 'Italian Tuning' as well! [}:)]