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LT35 Mk1 'Misfire'

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saljo View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote saljo Quote  Post ReplyReply Direct Link To This Post Topic: LT35 Mk1 'Misfire'
    Posted: 16 Aug 19 at 12:08

Hi,

Apologies, I posted in the wrong section; try again!

Here's a real head scratcher.

The LT35 Mk1, we converted to a camper, has the D24 2.4 6 cyl non turbo diesel. 184,000 miles and used with care over the past 24 years of our
ownership. Annual oil/filter changes and fuel filter changes. No veg
oil ever used. Slow warm ups before using higher revs.

Symptoms: Engine ran smoothly at low revs, up to around 35mph in top &
from 40 + also smooth. Runs rough between 35mph & 40mph, just
like a misfire on a petrol engine. Same symptoms for similar revs
in the lower gears, so its rev rather than speed related pointing to
an engine issue. Particularly noticeable when you hit the 'bad' spot going up hill.
No smoke when warm. Symptoms came on over a two day period around 6 months ago but haven't got any worse in the last 2,000 miles.

Performance, such as it is, is about the same as ever and MPG again
pretty much the same at 27mpg. Engine sounds ok when running smooth.
ie, no nasty noises. Same when 'misfiring'.

What have we done so far?

- Injector pump changed for recon unit with 12 month guarantee.
- Injector nozzles professionally cleaned.
- Injection pump timing checked as ok.
- Fuel filter changed.
- Fuel system bled several times, no sign of bubbles in the
supply pipe to the injection pump.
- Vehicle run from separate fuel supply to eliminate any fuel
pipe issues.

Local diesel expert (well thought of) is mystified and we were getting
poorer by the day!

Now for the really wierd bit. Took it in for an MoT last week and it passed with no emission probs, went for a 20 mile run and all the misfire/vibration had gone!!! Engine completely smooth right through the rev range. First time I've ever known an engine problem put itself right - or has it?

If anyone has come across similar symptoms and has any ideas as to
the cause, I'd love to hear.

saljo - newby.
saljo
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saljo
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tellytubby View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote tellytubby Quote  Post ReplyReply Direct Link To This Post Posted: 18 Aug 19 at 00:05
Interesting one as you seem to have covered all the obvious.  Fuel lines then pump would have been my first ports of call as you say it is restricted to a particular rev range.

Did the symptoms change at all after any of the various components were changed?

Only other thing I would check would be the cam tining as you have already confirmed the pump timing -  how long is it since the belts were changed?

TT
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saljo View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote saljo Quote  Post ReplyReply Direct Link To This Post Posted: 18 Aug 19 at 11:35
Hi tellytubby,

Thanks for your response.

I should have added to my post that the timing belt, pump etc, were also changed at the same time - so I guess we know what's NOT causing the problem! One other thing I should have mentioned is a compression check
we took showed compressions 15 - 17 bar on all but cylinder 6 which was down to 13. There was no difference in the symptoms after we made the changes. The improvement came immediately after the MoT.

I know as part of the MoT process the garage runs the engine up to max rpm maybe five or six times before they take the emission readings. So my thinking is that as this engine has hydraulic valve lifters that can stick/jam, the process of running up to max revs has freed up a lifter, or maybe even cleared a piece of carbon stuck under a valve. The low compression on one cylinder also seems to point to this.

We have done a further 700 miles since and all is completely smooth, no vibration or surging at all.

I'll do another oil change, check the compressions again and if no. 6 cylinder has regained compression, that maybe the answer. If so, fingers crossed.

Definately a wierd one.

Thanks for your interest.

Saljo.
saljo
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