milage and timing

[kevin77]

I confused :confused: I have a 94' suburban 4 wheel drive with almost 190,000 miles on it. Last week, I drove it gently, rarely taking it over 55 mph, no jackrabbit starts, and my average trip was 10 miles one way. I averaged 14 mpg.

So I fill up at the same place I had been filling up (High volume station) and this time I push it. 70 to 75 mph down the expressway, WOT starts, Etc. I filled up again to check the milage and I got 16 mpg.

Is this an early indication of a timing chain starting to strech a bit too far? is there a way of checking without pulling the water pump and the front cover off? Or am going up the wrong tree on this one?

Kevin

[Hubert]

I would not wrench on it yet just from that post.

I get a little sporatic milage like that sometimes. Depends on so many things. Ambient temperature, humidity, fuel quality (have noticed subtle tank to tank difference even from same station), hills driven over, load, traffic flow, wind, and some error filling up from the diesel foaming and me not wanting to let it "degass" and completely fill it to the exact same level. You'll need to average several tank fills to check milage accurately.

As I see it the three main influences to milage are RPM, throttle postion, and cruise vs stop start averaged over the tank usage.

If on one tank avg trip was only 10 miles the engine was too cold most of the trip to be effecient and a harsh comparison to a hot steady state operation reguardless of driving conditions.

[Marty Lau]

Kevin;
I see you all ready have done the cooling upgrade. I think it makes sense when doing the cooling upgrade to do the timing set if you have any appreciatable of miles on the truck. (125-150,000 miles plus) Since the timing set resideds behind the water pump about 85% of the labor to do the timing set is done. Another thing that could be affecting MPG's is your injectors. If you have the orginal injectors they could stand to be changed. In filling the fuel tank the only way to be sure of consistent fills is to fill all the way to the top. (PIA)

[rjwest]

Fuel quality

45 cetane is great for milage,

Southern truck stop fuel ( 40 cetane ) can be real cr*p

Also I had a fuel stop where I put in 37 gals in a 34 Gal tank ( I was real low but still running )
Complained to manager, he said pump just calibrated,
Shure it was,

Really though, I see a big differance in
Northern Summer Diesel ( good milage )
vs southern Diesel,( Poor milage )