Engine stutter on uphill with low fuel level.

[Tbone]

Hey guys my 3.1 mu seems to have develpoed a weird quirk, its only happened a couple of times, but when the vehicle is facing uphill the idle speed drops about 200rpm and if the fuel tank is low (fuel light on or close to coming on) then the engine starts to splutter. also if i am facing downhill with the same amount of fuel the idle speed goes up about 200.

The last time it was at an intersection at the top of a hill and as i stopped, it gave a big shudder, the battery light (charging system light?) on the dash flashed on then off, then it shuddered again, light flashed on then off and did it about half a dozen times before i pulled out and then stopped on the side of the road (on the level) by which time it was running fine again...... (!?)

i don't know much about engines or fuel pumps or anything but i drained the fuel pump in case it had any water bubbles in it, which there were none at all (i even tipped the fluid into some water to make sure it was actually fuel that was coming out and not just fuel flavoured water).

One of my mates suggested perhaps the fuel pump is stuffed/leaking so its fighting gravity when facing uphill or low fuel pressure from the tank and when downhill its got more pressure behind it so the pump doesn't have to work so hard, but he said he doesn't know how they work either so its just a pluck.

Any ideas?
- dunno if its related but it seems to take about 2-3 mins from startup to reach idle speed (720) when cold, and before it gets there it chugs up slowely from about 5-600rpm.

[geeves]

How steep a hill?
Ive had the same thing once with 1/4 tank on a very steep hill. Had to use full throttle for over 5 seconds before I got usable revs and even then useless power till I got onto smoother ground. Thought I was going to need to be towed which would of been fun as we were in thick bush and I was in front.
My guess is it was sucking air from the tank. Ive done 10000km since then with no related problems. It is possible that a leaking internal seal in the pump could cause this but hope its not as that would be a pump rebuild at $1000. (in my case it would be a tow to pick a part and wreath)

[Tbone]

$1000. ouch.

no it wasn't that steep a hill (it was in in town on a moderately steep road) not even one you'd need to use the handbrake.

If that does turn out to be the problem, is that a common fault on those pumps? i mean would it be an option to get a secondhand one from the wreckers?

[geeves]

It may still of picked up air in the line. This depends on your driving before you stopped there and whether it sloshed the fuel. First and best advice is dont run the fuel that low. Running the fuel low also stirs up the rubbish on the bottom of the tank which can block the filter (easy fix) or the strainer on the outlet from the tank (typical isuzu fix)
There have been a couple of pumps that have sucked air but this normally shows up as hard to start after sitting a few days. Its not common but these pumps are 16 years old in your case which is a lot to ask of rubber seals. (mines 21).

[wbski007]

Your fuel guage is probubly giving you a higher reading then what is actully in the tank, happened to me once, vehicle ran out of fuel at a 1/4 tank, drove me crazy because I thought I had gas so I didn't look for that problem, guy I bought it from forgot to mention it to me and I left his place to go home with my new purchace with a little over a 1/4 tank but only made it half way, called him and then he says OH I forgot to tell you about the gas gauge problem, thanks alot, a#####e, Its not good to run it to low anyway. I'm pretty sure that's your problem.

[BalkaRoo]

Your fuel guage is probubly giving you a higher reading then what is actully in the tank, happened to me once, vehicle ran out of fuel at a 1/4 tank, drove me crazy because I thought I had gas so I didn't look for that problem, guy I bought it from forgot to mention it to me and I left his place to go home with my new purchace with a little over a 1/4 tank but only made it half way, called him and then he says OH I forgot to tell you about the gas gauge problem, thanks alot, a#####e, Its not good to run it to low anyway. I'm pretty sure that's your problem.

This is a likely possibility.
My brother has just fixed a simmilar thing on his Nissan Pootrol that he uses for work, I had to tow the thing 50 miles to his depot even though it had 1/3 of a tank showing on the guage and it turned out to be a faulty guage sender unit. It had been playing up for several weeks and he thought it was something broken in the pickup in the tank leaking air into the system and always carried a jerry can with him. He put that in 100 miles before it started mucking around again and didn't have any more to put in and being way out in the country we only had 2 choices, either drive 50 miles to get fuel or tow him, so I towed him with my old hilux. Turned out all that was causing it was a faulty sender unit letting him think his tank was still 1/3 full.

[Tbone]

i don't think its reading low, it seems quite accurate, ie says half and it takes just over 35 - 40 litres to fill (81 litre tank), i've had it around 750km with the fuel light on from a full tank and no problems (although thats not something i would normally do), this would have been about 650ks from full so i'd be more inclined to think that as Geeves suggests, some crud may be sitting in the pipes or filter i'll try draining the tank and checking the filter and the strainer if its easy to get to when i get a chance.
Is that strainer accessable without any major work? and if theres a way to test the fuel sender easily i'll give that a go too

[geeves]

The sender and fuel inlet are one unit in the front of the tank. Easily visable when lieing underneath. From memory its a simple screwdriver job to remove. Dont mess up the 2 hoses. The other is a fuel return which wont go to the bottom of the tank. Make sure its empty first

[turnturn]

Tbone, regarding "dunno if its related but it seems to take about 2-3 mins from startup to reach idle speed (720) when cold, and before it gets there it chugs up slowely from about 5-600rpm".
Your engine will always idle slower when cold except if the QWS (Quick Warm-up System) is activated. When the QWS is active the idle speed is increased by a vacuum operated fast idle system as part of the injector pump (this is also activated with the air conditioning on).
If the slow idle is a problem there is a manual fast idle speed knob on the dash. It looks a bit like a choke knob but works by rotating rather than pulling out. Turn it clockwise to increase the idle speed and anti-clockwise later to return to normal. I use mine every morning to hold a decent idle until the engine warms up.

If your wondering about the QWS it is a system fitted to your vehicle that restricts inlet and exhaust flow to help things warm up quicker. There is a butterfly type valve fitted to your intake system (between the turbo and the intake manifold) and a similar one fitted to your exhaust (just about under your drivers seat). It is activated by a button on your dash that has what looks like a small engine on it (mine is beside my rear glass release button). This whole system will only activate at ambient air temperatures below about 5deg C (approx - maybe 3degC).
I see you live in Auckland so you will probably never need this system. I live in Dunners and don't need it.

About your initial problem, I try and never go below about a quarter of a tank to try and avoid fuel tank problems.

[Tbone]

thanks geeves, i'll have a look at the sender too.
turnturn - thanks for that, i don't seem to remember it idling so low, but i'll make sure i use the fast idle to keep it about right until its warm, and see how it goes.

[geeves]

Quick warm up on the 3.1 is more complicated on the 3.1 than our engine and more nessecsary.