low rev stutters, p38 4.0

[woz]

I have a 99 p38 4.0 ex police rangie. Its in good nick and runs well. BUT......
when on tickover, you can hear a Ticking noise, not the head gasket, but something like the manifold or a leak on the exhaust. When running it goes fine, but when in top gear, at 35mph or 40 mph, add a little gas to speed up, ( not kickdown)
the engine seems to stutter, and sometimes a small backfire can be heard. Once down a gear, or press further to kickdown, the stutter goes and the engine powers on as normal.
I am an amateur in these areas, i thought the manifold had a leak or the exhaust. but on further research could be the air flow meter or map sensor??? (not sure what that is).
Anyway, just wondered if anyone out there had the same problem or any ideas.
Any advice is greatly appreciated.
Cheers

[Range Rover Blues]

Ticking noise amongst other things could be exhaust or head gasket leak, either of these would get worse when under load.

Not sure but a ticking noise could be a slipped liner, could also be a worn valve lifter.

Does it sound mechanical or like a high pressure leak (AIR JET)?

[woz]

The ticking noise, sounds almost like 'tappets', i know that sounds crap, but once moving the noise fades.
i've briefly looked around the manifold gaskets, can't find a leak. I don't think its the head gasket, again no leak, i've seen a video on you tube with some guy whose 4.6 has a leaking HG, it doesn't sound like that at all.
There is a slight leak on the exhaust on the join just after the cats, but that shouldn't cause any problems??
I recently took the beauty down to cornwall on holiday and it went like a dream, but the slight ticking was always there.
Someone has looked after this car very well, the engine bay is very clean, no signs of anything leaking there.
Sorry if i'm a bit vague.
Thanks for replying.
Cheers

[Range Rover Blues]

No No that's fine.  Tappets is another possiblity.  With hogh mileage camshaft the profile of the cam goes "off" because theyr are tappered slightly front to back.  thsi means the tappet runs on it;s edge against the back edge of the cam lobe so that it spins.  When the lobe wears it runs on the centre and doesn't spin, so it doesn't scavenge oil and therefore lifts slowly.

Does it drink a bit? high fuel consumption is a symptom.

Also could be a weka or broken valve spring, almost certainly a head off to fix job.

Now if it's present on over run as well as under load then it's truely mechanical, if it goes away when you lift off then it's something else possibly.

Loads of things it could be, hard to say without listening.

[woz]

 To be honest its pretty good on the fuel (if thats possible!!!) when you lift off it doesn't stutter.
Starts ok after being left for a couple of days, and runs along ok.
Ill keep looking,i'll find out what it is eventually.
Thanks for the advice, much appreciated

[Range Rover Blues]

Starts ok, runs fine, economical, doesn't use water,

turn the radio up and forget about it ;)

[thermidorthelobster]

The air flow meter doesn't explain the noise, but it could feasibly lead to the other symptoms - it sounds a bit like what my 4.6 did when there were intermittent faults with the MAF and the crank sensor.

[woz]

 I have read some online stuff about the MAF sensor, that was my next course of action giving that a good clean.
Keep you posted.
Failing that, keep the stereo on full blast!! :dance:
Thanks for the advice tho, much appreciated.
Woz

[Range Rover Blues]

I've been doing a bit of reading.

I'm not sure just how accurate this is with your engine management but:

At start up the engine will run on a default fuel map, running rich, to start
at idle the engine will look at a MAF signal and runn slightly rich depending on engine temp
at higher revs/under load the engine will look at the lambda signal.  it may also adjust the base idle setting depending on lambda feedback, THE RRC does this if lambdas are fitted
At wide open throttle the engine runs rich to giev more power and protect the engine from excessive pre-ignition etc, this is based on a fuel map too.

So the engine runs closed loop between about 10% and 90% throttle (varies depending on manufacturer and emissions regs).

So a faulty MAF would affect the idle more than the running mixture.

[thermidorthelobster]

On the 4.6 I had problems with both the idle and the mid-range under load.  I don't however know which was to do with the MAF and which was to do with the crank sensor.

[woz]

Checked the maf sensor at the weekend, it was fine, it looked brand new to be honest.
but still, when you add a few revs the engine stutters then runs fine, even when stationary this occurs. Could the fuel injection do with a clean??
I'm stumped, being the amateur mechanic that i am. Although, the manifold has no leaks, the headgasket seems intact, something isnt running quite right.
I shall keep going!!!
Once again thanks for the help chaps!!!

[Range Rover Blues]

If the MAF is new it might need setting up?

[woz]

The MAF appears (on looks alone) to be new. How would one set this up??
Something has changed, few months ago it was fine, but now runs rough at low revs.
 

[Skibum346]

I'd fire some engine cleaner additive into the fuel tank next time you fill up and run it till the tank empties... then check it... if it's injectors just dirty... that should fix it.

Course.. as other have said... lotsa other things it could be!

Skibum

[thermidorthelobster]

A new MAF may not mean it's reading correctly.  You'll probably need to get it on a diagnostics system like Rovacom and look for correct outputs.  From memory it should read something like 6 on tickover to 50+ when you rev it, but I may be confusing this with a totally different vehicle, so don't quote me on that.