V8 hesitating

Hey,

Well, I thought I had sorted my problem, but my disco's engine started running rough again today... No stalling at idle any more, but now under load it's hesitating and producing a bumpy and jumpy acceleration, which feels like it's down on power. I've given up trying to do anything myself and have booked it into the local garage, but I suspect there's either another leak somewhere in the plenum intake system, or a blockage in the fuel system.

On top of that my radiator has started to leak :frusty:

Oh well, at least I got three or four days of it running like a dream

check your CO trim on the airflow meter.

someone had turned mine right down to like 0.21 volts or something. id had jumpy/hesitant acceleration for ages and it got noticeably worse after fitting my tubular headers and big exhaust.

i was searching for a substantial misfire fault and decided to check the AFM which i had never done in the 4 years of ownership. misfire turned out to be plugs and a bad lead, but setting up the AFM correctly has made her smooth as silk, much better than it was before in all aspects. also did unblock that little T piece at the same time which i think we have already discussed...

anyway, its an idea cos mine doesnt do it anymore and thats one of the major things i adjusted, voltage should read between 1.0 and 1.2 volts ideally, lower the voltage to lean off the mixture basically...

Ah ok, will do.

Cheers Noisy, helpful as ever!

does it only do it when fully up to temp? if so it may well be the tps playing up

fett said:

does it only do it when fully up to temp? if so it may well be the tps playing up

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No it did it today while cold, too. It's etting really annoying.

Got the car back from the garage today... They could only think that maybe the spark plugs or HT leads are bad. I haven't tried your suggestion yet Noisy.

But... The problem seems to be getting slowly better by itself. It's certainly not as bad as it was when it first started. Which got me thinking: since it first started not long after I had replaced my fuel filter, is it possible I accidentally got some crud in one of the hoses? Could that have caused that sort of lumpy bumpy acceleration under load?

It could have done but a fuel blockage usually leads to worse than lumpy acceleration.

A teeny fuel blockage makes a big difference to the mixture in the cylinders and a teeny mixture difference is what makes acceleration slightly lumpy.

A fuel blockage would make it splutter and miss, lumpy bumpy is mixture.

I really would check that MAF mate, mine was lumpy bumpy jerky jerky all the time on part throttle (it was fine on WOT once it powered through the lumpy range) until I adjusted the MAF!

Do check the MAF but also check ignition timing and VAC advance.

^^^ agreed

Could be any of those. Tbh I thought I had a jumpy advance problem that was giving me the bumps, certainly felt like it could have been that.

Basically check everything, in the old days it was called a 'tune up', something that modern car owners don't need to do anymore!

Mr Noisy said:

^^^ agreed

Could be any of those. Tbh I thought I had a jumpy advance problem that was giving me the bumps, certainly felt like it could have been that.

Basically check everything, in the old days it was called a 'tune up', something that modern car owners don't need to do anymore!

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haha okay, gramps... I'll get on it

Tubes63 said:

haha okay, gramps... I'll get on it

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I'm 24

Mr Noisy said:

I'm 24

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4 years wiser than me then haha

Mr Noisy said:

I'm 24

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but mentally your 70

fett said:

but mentally your 70

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24 years of experience, 46 years of guesswork

:hysterically_laughi

Update: just went down to try the checks you mentioned but before I did I decided to have a look at the HT leads again...

Turned the car on, and wiggled all the leads on the boots where they attach to the spark plugs. All but one were fine. When I wiggled one of them (passenger side, closest to the front) the engine stalled. I turned it back on fine, went back and wiggled it again and it stalled again. Tried it a third time but nothing happened.

So that could be the cause of my trouble right? A dodgy HT lead connection (caused, no doubt, by my ham-fisted technique of DIY mechanics) which "wiggles" when the engine is under load?

I didn't try the MAF check or anything because I got too excited at my revelation

What do you think?

Tubes

I can't see how a car would stall from 1 bad lead.

It would either fire on cylinder 1 or it would misfire. Should still run on the other 7 though.

Doesn't make much sense this one unless there's something you're not telling us!!

Mr Noisy said:

I can't see how a car would stall from 1 bad lead.

It would either fire on cylinder 1 or it would misfire. Should still run on the other 7 though.

Doesn't make much sense this one unless there's something you're not telling us!!

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If I'm not telling you something it's because I don't know about it haha.

I'll have another check today

Ok, from the top.

You have a standard? 3.5efi that runs on petrol? and is a manual? without cruise control?

It has new/recent good quality dizzy cap and rotor arm, quality (you'd be suprised how many are ****) HT leads and a new set of NGK BPR6ES plugs?

Now please explain in detail exactly what happens when starting from cold, when driving from cold, if the driving changes at all as the engine warms up, and then idle and restart from hot.

Things to check are MAF as above, and pretty much every other sensor with quoted values on this website Rover 14CUX Hot Wire Mass Flow EFI: Service and Troubleshooting

You will need a multimeter and a brain!

Have a beer and get stuck in, then report back

Mr Noisy said:

I can't see how a car would stall from 1 bad lead.

It would either fire on cylinder 1 or it would misfire. Should still run on the other 7 though.

Doesn't make much sense this one unless there's something you're not telling us!!

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Land Rover Discovery V8i bad idle/misfire under load - YouTube

It's definitely cutting out because of that lead... When it first cuts out I'm just wobbling the boot of the No. 1 lead gently back and forth. After restarting I'm poking the lead I was previously wobbling and the idle plays up. I think the connection between lead and plug must therefore be bad. The explanation might be more complicated but that's certainly at least a symptom.

As for your questions:

Standard 3.5 efi on Petrol, yes. No LPG here. It is a manual, it does not have cruise control.

It's dizzy cap is not "new" but it's not terribly old. Probably about five years, but probably only about 20 - 25,000 miles old. HT leads are about 1 year old, and the plugs are brand new, yes.

Starting from cold and hot it's fine, idles fine (now, unless I play with that lead as in the video) runs fine in 1st and 2nd gear. Depending on the day and the mood of the car it'll usually pull fine in 3rd as well. Any time I apply pressure to the noise pedal in 4th-5th gear though it starts bumping back and forth as if it is missing on a cylinder. After about 30-40 minutes of driving that symptom effectively disappears and the car runs fine (usually).

Temperature does not seem to effect it; the engine gets warm long before the problem starts to disappear.

As for the readings, I'll get a multimeter as soon as I can, life's getting a bit hectic at the moment. When it rains it pours

Cheers

Geoff