Rough idle after MAF clean

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JamesBB

JamesBB

Active Member
Posts
761
Location
Sandwich. Kent.
Hi

P38 4.6 - 1996


I was getting a bit of a flat spot in acceleration and slightly sooty tail pipe to match, so from what I have read up it sounded like time to clean the MAF sensor. Which I did with proper electronic contact cleaner, then let it dry. It looks like the original MAF sensor on there, a Lucas one.

A lot of dirt was in the air induction pipe, so I wiped all that out and also cleaned out the metal throat near the throttle. Looked like the breather hose has been spitting a bit of oil up, but nothing much, just a fine film.

Well all seems ok, except for idle speed. It is sub 500rpm and waves up and down, to the point of near stall. :(

Kick down is now far more responsive, and it generally feels far smoother, so just an idle problem.

Is there a way to adjust the idle?
Or should it be reset in some way?

Many thanks in advance.
James
 
hi there.if u read threads here,u will notice that in most of them if any sensor is replaced,the adaptive values must be resetted.messing with the MAF will throw something back at u,i suggest u reset the adaptive values,or wait for the prob to cure itself,the ECM will sort things out after few miles unless u screwed ur MAF.
 
Thanks for the info.

Is it worth re-cleaning with carburator cleaner spray?

If I drive it for 100-200 miles will the fuel trims reset in the ECM?
 
they will be resetted.about re cleaning it again?when it comes to me (that is just me),i wouldn't touch the MAF,i would replace it.dunno if u clean it again it will work!!!if u want my advice i suggest resetting the values and checking for faults while ur at it.
 
I unpluged my MAF to make sure it was working and re-connected it as it was working ok, but the same happened to me as it has to you, I drove only about 2-3 miles and it re-corrected it's self so no worries there. :)
 
try to give it a varied run. a fifty mile blast down the motorway will set the trims only for motorway. and don't worry, there is nothing wrong with cleaning your maf. grime and dirt can and do foul the sensor without necessarily damaging it. only when its dead does it need replacing.
 
Well probably done about 20-30 miles around town, also let it idle for a while outside the house.

No change, rough and low idling. In fact it was low idling before, but not rough.

I have the motorway trip to do tomorrow, 30-40 mins each way.
 
JamesBB said:
Well probably done about 20-30 miles around town, also let it idle for a while outside the house.

No change, rough and low idling. In fact it was low idling before, but not rough.

I have the motorway trip to do tomorrow, 30-40 mins each way.
Click to expand...

Clean out the idle air control valve for the idling problem. In cleaning the MAF you will have changed the resistances of the hot wire and IAT sensors. The ECU will take time to adjust itself. If you have permanently damaged anything in there you should have warning messages.
 
I've cleaned the MAF on mine twice now, I used carb cleaner. I would agree that a good clean of the throttle body/ idle valve will be beneficial. The result of cleaning mine was succesfull in changing the plug colour from black to light brown. However, I had to drive 500 miles of varying roads until I was happy with the 'repair' so i'd say it will take your ecu at least that long to 'adjust'.

Also, if you have an lpg conversion, don't run on gas until you're 100% witht the petrol running!
 
Thanks again guys. No LPG here, too expensive ;-)

Idle air control valve is in the throttle housing what what I can see on Rave. That was all oily on mine, although I cleaned it off, at least from the surface of it.

Also page 351 suggests there should be 1.4 volts at idle on the MAF sensor input pin 16 of connector C507 on the ECM. Wonder how easy that is to measure with a DVM.

On page 360 I have found a full explanation of the IACV.
I need to check, but from what rangerovers.net says I reckon it could be the culprit, or at least the suspision is on the IACV. My reasoning behind this is that is where there was a lot of oil that has spattered all over the throat of the throttle housing by the plenum intake. The souce must have been from the breather/vent hose. I wonder if the hose is gunked up, and also if the IACV is too.

Inside the throat of the throttle housing, there are 2 small holes to the lower left, these had oil in them, cleaned the surface as I said, but did not dismantle. Will check soon and report back.

Is it worth taking the plenum cover off and cleaning all around the throttle and air induction areas? It was oily, I would not be suprised if there is a film of oil inside too.

Cheers again all for the pointers. Excellent stuff.
 
I wouldn't worry too much about removing the plenum completely, just get the IAC valve clean and the throttle body.

Definately whip a plug out on either side to check it's colour. Although modern petrol injection systems have come a long way (is GEMS modern?!), nothing gives better clues of the state of you're fueling than plug colour (in the absence of a 3-gas analyser).

Also, it's worth remembering, these systems run very rich at start up, so you'll want to check the plugs after a run, with minimal idling in traffic. I keep the right plug spanner/ratchet in the car with me and regularly check my plugs!! Fuel is too expensive to throw away.. :(
 
I did a motorway run today, it probably added up to 40 miles each way, then back through town, it is very responsive since cleaning the MAF sensor. Idle still rubbish.

This weekend = check plug colour and try to clean the IACV.

I have heard it is best to remove it to clean it, but a new gasket is required. What would you suggest to clean it with? Wynn's Carb cleaner?

Cheers again
 
Yes carb cleaner is the best thing to use. It's pretty agressive stuff, will shift the varnish that causes the sticking, but won't damage anything, even the super-delicate MAF sensor.

You may be able to salvage the IAC valve gasket..
 
Still not cleaned the IAC valve, but got some carb spray.

Drove from Wilts to Kent and averaged 21mpg, not bad. I find that pretty amazing bearing in mind I was sitting at about 70mph all the way.

Idle still rough, so may aim at the valve clean tomorrow or Monday.
 
Right, cleaned the IAC valve, it was very dirty and oily. Used carb cleaner.
Cleaned (best I could) the hole it fits in, also very dirty and oily.
It idles better, but still too low 400-500rpm, it then dips down again (200rpm and less) and near stalls. Also the revs drop off in traffic when coming to an idle.

I think I need to clean it again as I am not sure I got all the gunk out, also the throttle body hole again.

Also thinking about cleaning the MAF again, this time with carb cleaner.
Wondering if it is worth cleaning inside the plenum, I can see it is pretty mucky in there when shining a torch into the throttle. :confused:

Finally, after reading up on:
Range Rover P38/4.0/4.6 Common Problems and Fixes
I am wondering if there is an alternator fault. Twice in the last 2 months an alternator fault has appeared on the display, however I ignored it as it disappeared and ran as normal afterwards.

Other symptoms: :(
From cold start, the idle drops to near stall, so pressing the accelerator and keeping it about 1500rpm for 10-20 seconds then the revs rise up to near 2500rpm without depressing the pedal at all. Coming off then it idles ok, but still low.
Around 1500rpm (or so) it feels very occasionally lumpy when coming off the gas.
A blip on the gas pedal will occasionally result in a possible misfire as the revs drop back down, or is it back pressure in the exhaust?

Thanks again in advance. :)
 
did you say what the plugs look like? that behavior with the rising revs sounds like an air leak to me.
[but I've no familiarity with the setup you have - I could be completely wrong]
 
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