Engine misfire under load 3500 - 4000rpm

[waynef]

Just want a second opinion on this guys.

Yesterday my S4 started to stumble slightly as soon as she crept into 3500 - 4000rpm range as soon as she passes this she is fine.

She idles smoothly will go all the way round to 7400rpm in neatral smoothly with no stumble and appears to do this only under load. Note she does not throw up any error codes nor does the Engine check light illuminate.

Possibilities:

1. Its really been flooded where I live so potential water/dampness in HT Circuit

2. Spark plugs - due to go in for B service in two weeks (Plugs are almost at 12000miles) This is my gut feel!!!

3. HT Leads - these were replaced 18000 miles ago and are race spec

4. Coil packs - Possibly, I guess could be water ingress. Where are they situated in the car e.g. are they low down or in the rear sealed off area behind drivers seat?

My inclination is Spark Plugs. If I remove them tonight will I be able to see any difference between them bear in mind the misfire only happens momentarily?

Also is there any easy way of testing the HT Leads is there a specific resistance check I can perform?

Not too worried that its anything else as only happens under load and in any gear apart from 1st. I guess first does not exert as much engine load

any advice or second opinions would be welcome. So depending on weather I can sort this will depend on my attendance at Brighton meet on Sunday.

I've got new plugs ordered for Saturday none the less.

Waynef

Ok just stumbled upon Dermots post

Click here!

In the last two days have been driving the car Miss Daisy! due to weather so might be due to the fact i'm in this rev range. Did Dermot ever find out the cause of this and is it inherent in ALL modified S4's

Waynef

[andydclements]

Coil packs, are in the engine compartment near to the petrol tank, right hand side. They can fail, but you'd expect it to either be at low RPM, high RPM, hogh load or something, not a specific rev range.

I'm tempted to suggest it's a mapping/ fuel ratio/ timing issue. As the map shouldn't have changed without a major ECU failure, ditto timing (unless the ECU doesn't hand over to the ignition module correctly), I'd say air to fuel mixture.

Although it's controlled by the ECU it can be thrown out in a few ways.

Air filter- is is (and the air ducts) clean & clear of obstriuction?

Throttle Position Sensor- this is quite likely, but abit more expensive than the above. Does it correctly report to the ECU how far the butterfly flaps are open?

Fuel- is the fuel pump and fuel filter OK (clean & clear of obstruction) and reaching the right pressure.

Sensors- other than TPS, temperature, MAP etc.

Andy

[waynef]

Andy thanks its sorted now....

I reset the ECU and took her out for a quick spin. Pulls in 1, 2 and third right through to redline really smoothly. For a short burst in 4th under load held her at 3500 - 4000revs and no stutter hoorah!

Ok is this theory possible:

Car has been stood up outside in freezing wet conditions. Is it possible the IAC could have frozen and when the car was started it was slightly out. I'd imagine in this weather there would be lots of condensation inside the engine as you get in dipstick etc.

Anyhow the problem has gone with a quick ECU reset so i'm happy. However would be nice to know what caused it.

Waynef

[mr.oogieboogie]

they tend to have a stumble around the 3000 rpm mark, mine appears better since i took the cat off

[waynef]

Ok the engine stumble is back.

Weird thing is that when the ECU is reset it goes away until the car has relearnt then will come back.

It threw error code 44 which is Lambda sensor, I have since replaced this and its back.

I have removed all spark plugs and checked gaps etc. All HT Leads read 7 - 8K Ohms resistance.

In theory if its HT then this would happen all the time so would still persist on ECU Reset strange thing is it does not.

This has all occured since filling up at the BP in Petersfield last week. The pump ran out as I was mid fill, the forecourt attendant told me to use a pump on the other side of the forecourt. I continued to refill and that pump to ran out. I paid and left within four miles is when my troubles started. The engine was stuttering and missing at 3.5 - 4K revs.

Last night I ran the car as low as possible and refueled with Optimax. it still has a stumble but not as severe but seems to be getting better. Is it possible its stumbling due to dregs left in the fuel from BP or is this a myth?

Am still unsure what the problem is but it does appear to be slowly clearing with the Optimax.

Waynef

[Splodge s4]

Wayne, have you noticed dusty soot on or around the rear bumper? You may if you go for a steady drive for half hour on a motorway (not up & down the gears like driving on an A road for example).

I

Edited January 29, 2009 by Splodge s4

[waynef]

Well I've flushed the tanks and put through two tanks of Optimax since.

So far all is better with just a slight stumble when holding it at 80ish and then booting it. Its livable but its also annoying.

Glad to hear its not just me then.

Waynef