Interesting diesel behaviour

[Ferret]

Just discovered something interesting on my way home from work...

If you absoloutely hammer the car from a standing start, it winds all the way around to redline, but then wont change up.

At first I thought this might be a gearbox failure... but then it kinda hit me. The car only does this when it's tank is less than a quarter full, and while it's doing it, the engine speed oscilates like it's losing power.

What I *think* is happening is that the fuel line is cavitating because the upper pump is pulling so hard - and causing engine power to machine gun drop out. The result is that the gearbox hasnt a clue what to do and just stays in gear...

I'd like some of the other diesel guys thoughts on this, I'm gonna scoot off to the gas station now and fill it to the top - see if it stops doing it.

The one thing that is /highly/ amusing is the amount of diesel smoke it spews when this happens, you cant see the car behind you any more

[winfred]

loads of black smoke in a diesel is unburnt fuel due to too much fuel or not enough air to burn it, i have no idea how the e34 diesels are set up injection pump wise so i can't really offer advice

[whiskychaser]

[QUOTE=Ferret]If you absoloutely hammer the car from a standing start, it winds all the way around to redline, but then wont change up.

At first I thought this might be a gearbox failure... but then it kinda hit me. The car only does this when it's tank is less than a quarter full, and while it's doing it, the engine speed oscilates like it's losing power.

Most diesels will kick out SOME smoke under load. Watch trucks downshift on a hill and you will see what I mean. But if you cant see the car behind you, you have something serious going on. Auto boxes have all sorts of sensors that decide if you have enough torque for the next gear etc. Kicking that amount of **** out you probably dont. Unless you are into diesels you could do worse than asking some outfit that knows (like Feather) for an opinion

[skr]

does it pull good? excesive smoke under full load is strange for your car coz it has an AFM. so if the turbo is not pumping enough air, your car won't pull that strong but should'n put out too much smoke. unless you hand-tuned your IP for overfuel. i don't think smoke and fuel lines have much to do with each other , because the smoke means it's getting more fuel than it can burn. maybe your wastegate lets go too early? or you got a boost leak ?

[Ferret]

does it pull good? excesive smoke under full load is strange for your car coz it has an AFM. so if the turbo is not pumping enough air, your car won't pull that strong but should'n put out too much smoke. unless you hand-tuned your IP for overfuel. i don't think smoke and fuel lines have much to do with each other , because the smoke means it's getting more fuel than it can burn. maybe your wastegate lets go too early? or you got a boost leak ?

Yeah it pulls damn good still - I've owned a few diesels before this one, they're quite common in the UK as the fuel prices here are rediculous, so people really count MPG here, diesels are <3 for fuel economy. (Hence why black smoke out the exhaust on random occasions doesnt worry me too much, it makes me laugh when someone really stands on the gas pedal and disappears a car following it.)

I've since filled the tank and this behaviour immediately stopped...

I've a sneaking suspicion that the smoke has something to do with the engine oscillation, and the turbo spooling down. When in the past I've run the engine without the airbox, if you tan the engine then take your foot off you get a whopping *pshhht* noise like there's a dump valve somewhere...

EDIT (To add relevance): This would cause the turbo to spool down /very/ quickly under a minor power loss, and would infact cause the turbo to run at very low boost constantly if the engine was oscillating...
End Edit

I'm wondering if the smoke's to do with the sudden loss of power (and hence exhaust pressure,) causing the turbo to spin down slightly, followed by sudden power restore and full fuelling because the input air flow never changed...

Tbh, I'm not overly concerned, because it only does it when the tank's less than a quarter full.

I know I'm having the fuel pump problem because it almost point blank refuses to start under a quarter tank - you can sit there cranking for a good 30-45 seconds before it catches. Does anyone have a link to a site about repairing this problem/thread about it?

Where the heck is the AFM though? I've not spotted it, unless its lurking down by the intercooler...

I'm going to have a go at tweaking the wastegate slightly some time later this month, to notch the boost pressure up .2 to .3 bar - done it on all the diesels I've owned and it makes a noticable pulling difference for little change in the MPG/engine temperature.

[Ferret]

loads of black smoke in a diesel is unburnt fuel due to too much fuel or not enough air to burn it, i have no idea how the e34 diesels are set up injection pump wise so i can't really offer advice

Yeah I think the ECU is getting 'confused' due to turbo RPM and therefore boost pressure changes, due to the power oscillation problem - filling the tank has totally cleared this problem straight away...

[Barney Paull-Edwards]

Must be a uk problem! The only difference between our cars now is gearbox.I posted something like this some time ago and drew a blank.If I tighten the filter normally(fuel) it smokes on full chat and stutters below half tank with air in the line filter to pump.If I gorilla the filter it is a bitch to start cold but runs ok and to the last drop.I have a spare filter head which i will get round to soon.Somebody online suggested the in-tank pump is on the way out.Dont know about you but whatever fixes it is worth paying a stealer for but cannot be electronics as I have swapped all, no difference,turbo, no diff,fuel feed pipe to filter, no diff,Whoever finds it first buys the other a beer!

[winfred]

imho computers have no place on a diesel, i like my truck it's one of the last mechanical diesels dodge made




here it is with just shy of the weight of a e34 on the back 3100 pounds of cement and slate, the amusing thing is a toyota truck thought he was going to beat me up the ramp to the interstate, he was wrong

the bastard is only just getting on the helper springs so it had plenty of ass left

[Barney Paull-Edwards]

that looks like a VM italian diesel with separate heads? Or is it a cummins big inch designed by a scot.The only diesels worth it are painted yellow,my `work truck `has two 3412TA`S and would tow that old box up Niagara!

[duby55]

I have a similar problem with a Chevy pickup. Above 1/4 tank its great, below that and it will die out. I was told I have a cracked pickup hose on the base of the in tank fuel pump. You may want to look that up for a possibility.