D2 2002 V8 4.0 rough. Misfire? Help...

Hello
I have a 2002 Disco 2 V8 with LPG conversion and need to sort out a rough running problem.
Driving steadily at, say, 30mph there will be a slight jerkines or hesitation when I accelerate, a bit like running over a rough road, less obvious at much higher speeds.

It does it on Petrol as well as on LPG.

I have Hawkey Diagnostics and carry it with me all the time!

When I start the car from cold in the morning it sounds quite rough as though it is missing on one cylinder and the engine management light will then come on with misfire on 4, cat damage etc. After a couple of minutes it then runs smoothly, I reset the fault and it will be ok for a day or two.

The jerky, hesitant acceleration on both LPG and petrol is my biggest concern.
Hawkeye never gives any misfire readings looking at live data for any cylinder.
Hawkey does show numbers against "rough running" especially at idle but the numbers fluctuate and are quite low.

I have magnacore leads and am fairly confident that all plugs are ok.
Lambda sensors (I replaced one 6 months ago) are reading ok.
MAF has been cleaned and as far as I can tell is ok.
New air filter.

I did breakdown and had to be towed in a while back and the mechanic changed the crankshaft sensor and fitted the magnacore leads.

Reading Rave it implies that it could be related to crankshaft sensor or camshaft sensor or MAF or any number of other things.

I really need your help here......

Coil packs?
Had a similar thing with mine , more pronounced on lpg than petrol , only happened once the torque converter locked (50mph ish)
as the engine is under load any weakness in the ignition system is magnified and it showed as hesitation / shudder in mine.
replaced coil packs and put new leads on and it was fine.

to be honest the magnecore leads have the quality fiting of a coop shirt loose baggy and somtimes fall off!
as i have always said gen leads 1/4 of the price change every two years.
as diags go the hawkeye does have it's limitations for the home mechanic compared to testbook and others.
faults could be maf, coil pack, head gasket, porus block, inlet manifold gasket

I am having a similar problem with my P38. see my thread "misfire on cylinder 3". I am going to do a compression test next to see if it is the exhaust valve....

Yesterday I replaced the MAF (£145 inc vat original LR part) and am hoping that it may have solved the problem, it has certainly done something.

Using my Hawkeye diagnostic gadget I had monitored the old MAF (live readings) and assumed that it was working ok because the readings seemed fairly sensible and increased with throttle opening as you would expect.

A lot of people report MAF problems and it is easy to replace even though it seemed ok.

I picked up a new one from Marshalls but did not have a screwdiver with me so could not fit it on the spot, instead I left the old one in place with the lead to it disconnected to see what happened.

With the old MAF disconnected Hawkey was still showing the same range of readings!!

The engine management light did not come on but the readings rose and fell with throttle opening.
When I got home and re-checked for codes, Hawkey did show error codes for No MAF and No Temp, obviously, but I had not expected to see it display any MAF air flow readings while driving, I did.

Presumably the ECU has gone into a "limp home" mode and is simulating air flow readings based on throttle opening and it is these that are being read by Hawkey.

Now I have fitted a new MAF the air flow readings are much smoother and linear where before they were more erratic, but without experience and knowledge of what the readings should have looked like the old unit did seem to be ok, don't be fooled into thinking your MAF is working properly when maybe it isn't.

I have only made one journey and the car was much smoother, it settled down after a couple of miles and no longer misfires.
Presumably, in an ideal world, you would use Testbook to do a full system re-set (adaptive re-set?) but I cannot do that so it will take a few miles for the ECU to learn new fuel trims.

Fingers crossed. I will put a few more miles on the car and report if that really has properly resolved the issue.

I was getting 11mpg on LPG probably a bit less if you consider that a couple of miles of each journey are on petrol. It will be interesting to see if that has improved.

A bit depressing.... Took the car out to fill up with autogas and it was as lumpy as ever, lacking power and missing when over about 35-40mph.
Same on lpg and petrol.
In 5 miles I stopped twice to reset the engine warning light, same fault codes as always, multiple misfires, cat damage, cyl 4 misfire.
Got to petrol station, filled up with gas, reset codes (again) then started up and drove home without any problems, nice and smooth - until the next time.
This intermittent fault is really annoying, there does not seem to be much logic to it.
Next steps will probably be to replace Coolant temp sensor, crankshaft sensor and camshaft sensor, simply because they are there and reasonably easy to do. I can't help think that it is throwing good money after bad though but what else do you do?
Anyone know a good independent in Royston / Cambridge area? (North Herts, South Cambs).

I think you need to have aprofessional test the sensors guessing will cost a small fortune just clearing the faults will make diagnosysis harder as you clear the faults leave the faults and let it be tested

Have you cleaned & checked the plugs carefully, mine was doing exactly the same with my Faultmate indicating a persistant misfire cyl 1, plug looked ok at first but when cleaned up with some electrical solvent & a toothbrush a hairline crack was revealed, fitting a new plug & doing an adaptive reset cured the problem.

dadioles said:

When I got home and re-checked for codes, Hawkey did show error codes

I have only made one journey and the car was much smoother, it settled down after a couple of miles and no longer misfires.
Presumably, in an ideal world, you would use Testbook to do a full system re-set (adaptive re-set?) but I cannot do that so it will take a few miles for the ECU to learn new fuel trims.


I was getting 11mpg on LPG probably a bit less if you consider that a couple of miles of each journey are on petrol. It will be interesting to see if that has improved.

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If you have a hawkeye then get the (free) V5 upgrade and you can reset the adaptions. I reckon your fuel trims are out because the ecu is trying to make up for incorrect maf readings.
11mpg is not a good figure.

I was getting this sort of mpg and when I got a hawkey both lambda sensors were not working so it defaulted to open loop.
new sensors and an adaption reset and I'm getting closer to 15mpg

Andy

dadioles said:

A bit depressing.... Took the car out to fill up with autogas and it was as lumpy as ever, lacking power and missing when over about 35-40mph.
Same on lpg and petrol.
In 5 miles I stopped twice to reset the engine warning light, same fault codes as always, multiple misfires, cat damage, cyl 4 misfire.
Got to petrol station, filled up with gas, reset codes (again) then started up and drove home without any problems, nice and smooth - until the next time.
This intermittent fault is really annoying, there does not seem to be much logic to it.
Next steps will probably be to replace Coolant temp sensor, crankshaft sensor and camshaft sensor, simply because they are there and reasonably easy to do. I can't help think that it is throwing good money after bad though but what else do you do?
Anyone know a good independent in Royston / Cambridge area? (North Herts, South Cambs).

Click to expand...

Just a thought , is it under load (TC locked) when you get the misfire, worse on gas than petrol? If so then try coilpacks. my were changed and the difference was amazing
Andy