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Matt, Engineer
Category: Car
Satisfied Customers: 21714
Experience:  Honors degree in Mechanical Engineering, worked 8 years as a Formula 1 engine engineer.
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BMW: 1996 rangerover p38 manual..6cyl..filters..surging..flicker

Customer Question

Hi, I have a 1996 Rangerover P38 manual with the BMW 6cyl 2.5ltr diesel. After changing a failed intank fuel pump ( tank washed out,pipes purged, filters changed) i developed a a constant "misfire" it happens when i try to drive at a constant speed, it develops at 1500 rpm and continues upto around 1800 rpm, The engine starts fine, but there is very slight surging/pulsing of revs at tick over ( maybe as little as 20/30 rpm, very small flicker of the rev counter needle). Moving in trafic at speeds below 30mph means using 3rd geat and 2000 rpm. I have had the error codes read and it showed nothing, only message i got was cruse control switch faulty, which im told is normal for a car without C/Control. Thank you, Tom
Submitted: 5 years ago.
Category: Car
Expert:  Matt replied 5 years ago.

This could be an airleak after the airflow meter, any air dragged in here isn't 'seen' by the ECU and so not compensated for and can lean the engine out and can also allow the engine to rev up when not desired causing rough running.
As its a mechanical fault it tends not to turn on the fault light and you can sometimes hear a 'hissing' noise with the engine running.

Check the hose clips for tightness and inspect the trunking for any cracks or splits and also all the vacuum system, the small bore pipes and fittings for cracks and missing parts.

The best way to locate a leak is to have the engine running and warm and then spray lighter gas /propane around each joint in turn. If the engine rev's up you've found your leak.
Work your way through each possible joint one at a time and you should find it. I use a slightly flattened piece of brake pipe and some rubber hose from the can of lighter gas to provide a spraying 'wand' and allow a direct blast of gas into each area, especially those difficult to reach with large implements.

Might also be worth checking the wiring and connector to the airflow meter for any signs of corrosion or damage. you can do a quick fault find if you unplug the meter and run the engine without it.
if the engine condition is the same then chances are the meter or the connection to it is faulty

Air leaks are very temperature dependent as gaps can open or close up as things expand with heat, so the weather and engine temperature can effect them.

This leads them to be quite intermittent in the case of mild leaks

its also worth cleaning the airflow meter by removing it from the car and spraying the exposed sensor wires inside the tube with a brake or switch cleaner - ensure that the cleaner is one of the old fashioned, non Eco type that does not leave a residue

On no account touch the sensor wires with anything physical as they are extremely fragile
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Customer: replied 5 years ago.
Hello Matt, Tried all the above no change, only thing i forgot to say is that it only misfires under load,,, free reving its fine. Cheers Tom
Expert:  Matt replied 5 years ago.
Hello Tom

If it only misfires under load then this does point to an airflow meter issue - so its worth trying the 'unplug test' for the airflow meter as I suggest above

If OK then also check the connections to the EGR valve, if either the vacuum pipe of electrical connection are damaged / corroded then the EGR can be on all time which certainly will hurt performance. Its also possible that its a fault with the EGR valve so its worth removing the valve and cleaning it out with brake cleaner, if it looks particularly clogged then replace it.

If this is OK then check the small vacuum lines to the turbo and its connected control solenoid on the bulkhead, any cracks or leaks can give turbo issues and its best to have the boost pressure measured actually measured with a boost gauge to check that the turbo is healthy and that the sensor is reading correctly

If its a variable vane turbo (they’ll be an actuator on the turbo body - but not a wastegate) then check its vacuum pipework as above and check the connections to the diaphragm / solenoid valve

Also worth checking that the glow-plug relay is switching off as they can stick on and leave the glow plugs also on - on some cars this can force it into limp home

Also worth considering a bottle of injector cleaner into the tank as a clogged injector nozzle will reduce power and give poor combustion - the next stage on from this is to remove all the injectors and have them ultrasonically cleaned and flow checked
Customer: replied 5 years ago.

Hello again Matt, Well ive gone over all the hoses and conections,,, what i found was this....... 1. no EGR...... sensor in manifold. 2. with air flow sensor disconected it improved . 3. with heat? sensor disconected it improved about the same as in (2). 4. with both disconected it got worse.

I can only think its a combination of both, i shall replace both in turn, ill let you know how its goes.........

cheers Tom.

ps, Oh Lordy give a 4 pot perky and big hammer any day


Expert:  Matt replied 5 years ago.
Good luck Tom

just get back to me on this post and I'll help further if I can
Customer: replied 5 years ago.

Hi Matt, Just an update.

Run all day yesterday with the MAP disconected, misfire was much improved but not totally gone, power/torque was about the same upto about 2500, then went flat,,( no turbo boost as expected).. Knowing i had a long run on this morning i needed the top end more than no misfire so reconected MAP,, The misfire reappeared as normal but the whole thing was running rough, after about 20 mins i made a stop for diesel/fags and the loo. Well in all it stood for about half an hour before restarting, it was now running nice but still had misfire,,BUT, power/torque was noticeably improved ( by how much, well it pulled in every gear as if it was a gear lower if that makes sense ).

Infact its never run so well or pulled so strong as this morning, so im still thinking the MAP is shot but wondering if i i have some unseen fault lurking in the shadows ?. Tom

Expert:  Matt replied 5 years ago.
Hello Tom

well done for persevering with it

before just replacing the MAP sensor check over the wiring and connector for any damage or corrosion as its possible by unplugging / re-plugging it you've just cleaned up contacts or re-made a dodgy connection

a good squirt of contact cleaner on the connector points certainly won't hurt

if this then works well you know that the sensor itself is fine and its just the wiring issue