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Excessive Stumble at low rpm


CarlC

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My car has never really had a problem with the report 3k stumble but for some reason very recently its developed a really bad low rpm stumble.

Symptoms

For example, cold or warm, approach a roundabout and slow down to second from higher gears. Revs drop to say 2500 rpm and cars still rolling. Roundabout clear so you push the throttle to say 40%. The car obviously knows the throttle is depressed as you can hear the exhaust go quiet is the only way I can explain it. It different to if your of the throttle basically. On average after about two or three seconds you get one quick splutter and it picks ups.

There is no problem above about 3200k and the problem happens about 6times out of ten.

So far I've tried;

ECU reset

Checked the spark plugs, new in April and done no more than 1k.

Contact cleaned inside both ends of HT leads

Wire brushed and contact cleaned the Ignition coils contacts to HT Leads.

She had a new fuel filter at the same time as the spark plugs.

Always runs on Premium Fuel

The only other clues as to when this start that I havent tried yet are that it started after an ECU reset and cleaning the one way check valves running to the crank case breather, if thats of any help.

I've posted up a The Freescan log from last night if thats of any use. I've had a good look through but I cant really see anything to give me any clues.

EDIT ->> Freescan Log Added

cc_stumble1.zip

Edited by CarlC
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Dermot has come up trumps again!

From the logs the MAP was showing unusual reading so Dermot suggested checking the Map sensor line and the line from the fuel pressure regulator that runs to the check valve and crank case breather/oil air seperator. I checked these last night and they all looked properly connected and no obvious sign of splits or damadge. I knew they should be in very good condition as they had all be checked about two months ago when I put the throttle bodies back on the block after a gasket leak. However after a further suggestion from Dermot to blow down the map sensor line I found air was blowing out some where. I followed the line back to the MAP sensor which is located behind the quater window tank covers and found that the rubber connecting boot was ina very bad way with a large split right along one side. I must have put this line under strain during the fitting of my electric CC pump.

So theres another beer on Dermots tab ! :)

... and no more wierd looks from people as I try to join a round about at very low speeds ! :)

Edited by CarlC
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What is the 3K stumble I keep hearing about?  How do I know if my car has this problem?

Thanks,

Chandra

91SE

<{POST_SNAPBACK}>

I think what Carl was experiencing was some hesitation when the revs hit 3K.

It feels as if the engine is going to die as the revs drop, but then suddenly return.

Graham.

Wing Commander Dibble DFC<br /><br />
North Midlands Esprit Group<br /><br />
NMEG "the formidable squadron"<br /><br />
"probably the most active Esprit group in the world" Andy Betts, Castle Combe May 2007

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The 3k stumble is apparently an issue with *many* of the s4 just before 3k rpm which is a slight hesitation in acceleration as the revs pass though this zone. The problem is to do with the stock s4 chip and has been addressed in the update chips that are available.

However, I would have to say that in my 12months of ownership I have never really experienced this problem first hand. But then I do have quite a few core mods done on my car that may reduce the effect of this problem.

The issue I started this thread with really relates to a very serious stumble at low RPM. The issue is simply caused becuase of a vacuum leak from the manifold to the MAP sensor. In most cases this is caused by a split or fractured rubber connector that attaches to the engine just south of the chargecooler. You cant really miss it. It also makes the car idle a little lumpy but it depends on how bad the leak is. My car idle wasnt too bad but putting you foot down at low rpm 1-3k would result in no acceleration at all for between 1 and 4 seconds. :rolleyes:

Fully fixed now though but I also needed to do an EDU reset to it could relearn the correct valves. B)

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