Running lumpy at low revs ..

Mate (no really) has a '96 300Tdi Manual Disco 5 door, which is de-egr'd, de-catted and which usually runs spot-on. Recently it's started to run 'lumpy' at low revs. When he's trickling along at tickover, in heavy traffic say, or on a green lane, the engine 'hunts' ... so it looks like he's putting throttle on and taking it off continually. Revs don't rise much, but it's basically speeding up and slowing down. If he increases revs ever so slightly it runs nice and cleanly. If it's left at tickover out of gear it sounds a little lumpy and also hunts a little, revs rise and fall very slightly. Tickover looks about spot-on at ~750 rpm.

He also says that when he is coming off throttle, to turn a corner say, he comes down from 40 mph to whatever, off throttle, into second gear puts throttle on again straight away to pull through and away and for a few seconds it kangaroos slightly. I'm sure he's getting fuel 'cos it flies at higher revs, and he has no trouble anywhere else in the rev range other than at tickover/ish revs.

It never stops, doesn't backfire and he doesn't see it as a big problem but compared to mine (which runs cleanly and sweetly at low revs) his sounds and feels like an old bus! If it was a petrol carb I'd say float or needle heights need checking, but I dunno on a diesel ... So, I suggested clean all airpipes out, oil air and fuel filters and sedimenter, he did all that and marginal if any at all change so I then suggested he check timing etc and he says it's all been done, and that he had a cambelt change a few thousand miles ago and had all the injectors/pump/timing etc setup by the local independant (RCV Doncaster) who he says did a good job and that the lumpiness has only developed over the last couple of months .. it's definitely not an overnight development though, and has steadily got a bit worse. It also recently passsed an MOT with clean exhaust emissions

Any suggestions to get his low revs running cleanly again?

Cheers ..

Bang some injector cleaner through, might help but not if you've got a drippy injector... whip em out and get em tested for what it takes!

Filter?

Could be a valve starting to go .

nathan5346 said:

Filter?

Click to expand...

Es dun em...:doh:

I have a feeling he had the injectors either re-furbished or at least fully cleaned and checked ...

I'm thinking it's likely to be timing .. cam or injector pump ... but that ought to also screw it up for higher revs, I think.

Any more thoughts?

Heh, stuck some Forte injector cleaner in the tank and went for a 'normal' drive with him. An hour driving round the area, motorways, side roads, main roads, country roads, built-up & crawling, fast-moving etc etc. Didn't do a thing ... BUT ... his 'normal' driving is driving like a baby, barely touches throttle to set-off, never boots it, never gets above 3k revs ... indeed when we stopped he said it'd never done even 70, he sticks to 65 max and he thinks he's never actually floored the throttle!!

So I got in the driving seat ... floored it down the road, trying to get it to pull and rev .. coughing and farting after 3k, onto the motorway and kept it floored, wouldn't get above 3250 ish .. all of a sudden, engine barked, clouds of black smoke, revs rise rapidly and we shot off, smoking badly for about 50 yards or so.

Jobs a good 'un .. the thing was just coked up, a few miles blasting it for a bit, foot flat down seems to have cured it completely .. it now ticks over nicely, he doesn't have to have any throttle on to set off and it runs at tickover purring sweetly even in third gear .. I've suggested to him that at least once a week he runs between two local motorway junctions and floors it ...

We'll see how long it lasts ...

Bah... sunday drivers!

Good to hear it's sorted, remember - if at first you don't succeed, try a bigger hammer