Slight Stumble at Motorway Cruise Speeds-light loa

[Dermot]

Slight Stumble at Motorway Cruise Speeds

I am doing some research to see if anyone else is experiences this running condition.

Set the car up around 3500 rpm, 5th gear, boost will be almost nothing, throttle about 5% approx 85 mph. Try and keep the car at a rock steady rpm. Does the engine run completly smoothly ?

or

is there an ever so slight stumble, it is not a misfire but just a slighty lumpy ride. As soon as you accelerate it will clear and run fine. Should happen between 3000-4000 rpm under very low load.

Would be interested to hear from SE, S4, S4s, GT3 owners.

I want to know if it is one of those things that everyone experiences but just thinks that is normal ?

Dermot

[DanR]

Wish I could help. The police here are so anal I'd get locked up and lose my licence for a couple of years if I did 85mph. Our max is 110kph (about 68mph) and that's on the freeways! Other highways it's 100kph (61mph). There are good winding backroads but none really where you can maintain a constant 85mph.

Teigan if you're still coming to Oz watch out for them. No sense of humour either.

Which reminds me about a motorist who was pulled over for speeding. The copper though couldn't understand something. Asked him why he sped up when he turned on his siren and lights.

The speedster told him "Well I've had a really rotten week. Last weekend my wife left me for a highway policeman."

"So what," asked the copper, "I still can't understand why you went faster when you knew I was there. Why?"

"I was scared...I thought you were returning her."

[s4s]

Hi Dermot, mine slightly stumble at 2800-3000rpm. At 85mph or 136kph everything is fine.

[jase7678]

I do suffer something at those conditions, but I think mine is down to my throttle cable. The car is jerky when just on the throttle at the speeds you mention. It could be electronic, but I think mine is more mechanical.

[Splodge s4]

Yes Dermot, mine is exactly like that, it always has been, after a few minutes you feel you need to plant it to kind of 'clear it out'. I also have dirty black smoke if going up a steady incline at about 85mph, you can see soot (not oil) on the rear bumper after a while but only if cruising at the same speed i.e, motorway speeds, if blipping on A roads so in & out of boost theres nothing, no stumble, no soot.

I originally thought it was over boosting but Williams in Bristol said it seemed fine so not sure of the next step....

[lisalotus]

This is an interesting topic. I notice that most of you that have posted on this subject are s4 drivers, I think it must be an s4 thing because mine does it as well, more noticably in second, third gear around 2000 rpm, dont notice it all in fifth gear at any rev. Going to change plugs and leads soon to see if that makes a difference. It dosn,t bother me too much, only ever at that constant rev, in and about town. Would be interested to hear if anyone ever gets to the bottom of it.

[Dermot]

Please keep the posts coming, i want to see how widespread it is...

I working hypothesis is that may be an inherent ECU fuel control issue, as you tune/mod your engine you notice it more.

The 2800 rpm stumble was sorted by Andy Whittaker and myself and Marcus' chip now incorporate this fix.

I may have a solution to this light cruise condition.

Dermot

[fmxa]

Please keep the posts coming, i want to see how widespread it is...

I working hypothesis is that may be an inherent ECU fuel control issue,  as you tune/mod your engine you notice it more.

The 2800 rpm stumble was sorted by Andy Whittaker and myself and Marcus' chip now incorporate this fix.

I may have a solution to this light cruise condition.

Dermot

<{POST_SNAPBACK}>

Yep same issue, but I notice it just over 3000 rpm (S4s - #5 rev2 chip)

[Rick2005]

Smooth like a baby

[Dasar7]

I'm another S4 doing it, Dermot

Have just installed Marcus's #3 chip and am trying to sort out why I get intermittent stumbling at about 2,800-3,000 (in most gears).

Seems more prevelant now I have Marcus's chip. Original factory stumble gone but this new problem seems more noticable now.

Almost feels like the back pressure valve comes in for half a second.

Any assistance appreciated.

Regards

David

[dmckenna]

I've also had a similar experience in the past, I couldn't work out what it was, replaced the plugs, the leads etc. it really flumuxed me for months.

I don't get it anymore and I've only changed two things:

1) The turbo, which of course could have a bearing on this symptom, now changed the vane angle but can't be sure that this was the reason

2) I've re-routed the plug leads away from the turbo and up through the throttle bodies. IMHO these pass far too close to a red hot turbo to not be effected in some way

Like I say, no probs anymore but interested in hearing that it wasn't just my car. Try 2) above first and see.

D

[doct93]

Hi there,

Used to have a very similar problem on the GT3, after MUCH investigation, cured it by double earthing and double wiring all the injectors! To be fair, it was a bit more than a slight stumble - used to be quite a misfire... Tried (in no particular order) plugs, leads, ECU, new injectors, coils - all to no avail!

Now no problems, running the upgraded chip and recirc BOV.

Tim

[dmckenna]

Could you throw any light on how to double wire and double earth the injectors please ?

D

[DAVES_GT3]

Hi there,

Used to have a very similar problem on the GT3, after MUCH investigation, cured it by double earthing and double wiring all the injectors! To be fair, it was a bit more than a slight stumble - used to be quite a misfire... Tried (in no particular order) plugs, leads, ECU, new injectors, coils - all to no avail!

Now no problems, running the upgraded chip and recirc BOV.

Tim

<{POST_SNAPBACK}>

Thats interesting tim as i have a stumble on my gt3 and have tried exactly the same things no no avail. so are you saying you basically re-wired the injectors with thicker better quality cable?

what year is your car?

my stumble comes in around 2000 rpm. it depends on the load really but it is a lot worse with more load.

dave

[docf]

I think this has something to do with the transition from boost/vac and since I have replaced or improved almost everything else and I still get this same stumble. The only thing that ever improved it was when I had the wstegate adjustment almost completely turned up due to a faulty spring in the wastegate capsule. After I installed my eletronic boost controller the I had to turn the adjustment back 4 full turns and the stumble magically reappeared. I will do some more testing with the boost controller to see if I can tune the stumble out but my guess is that its a mechanical issue related to degree of opening on the wastegate flap and how it interacts with map readings and fuel delivery. As a matter of fact I can reliably reproduce this stumble in 3rd - 5th gears by creating the conditions you described.

[Paul C]

Doh!

Meant to check for this at the weekend and forgot. Don't think i was off

boost at that speed at any point , will make an effort to do it this weekend

to see if it's an SE thing too.

[Dermot]

Keep the posts coming,

In the ECU code there is big change in the way it controls the engine above and below 4000 rpm. There are separate VE tables etc...

I am trying to find if modified cars are the ones that suffer or do stock ones show it as well.

Dermot

[Dasar7]

Dermot

I have been testing a bit over the weekend. If I disconnect the battery, and thereby reset the ECM my stumble goes away for approx 2 days, then slowly starts to reappear. Like was mentioned above from one of the GT3's it feels almost like a small misfire.

My mechanic took it for a test drive and although scratching his head a bit, suggested charging my battery to the max, as apparently S4S's can suffer from a similar problem if there is insufficient charge to the alternator.

Anyway, just thought this might help. My stumble seems to definitely 'occur' when ECM goes from open loop to closed loop at around 2,800 - 3,000 rpm range.

Hope this is helpful.

Regards

David

Esprit S4

[doct93]

Hello all, sorry for the delay,

I should be more clear - the injector wiring is the same but the earths are all double wired and a few more have been added - I think there are now 3 or 4 visible- seems like one on every bolt!

I can confirm that my misfire also seemed to go away on resetting the ECU, then got steadily worse over a day or two.

Touch wood, all has been well for over a year now...

Certainly easy to try - just earth everything you can see, then add a few more for good measure!

Interestingly, SW lotus put it on their (presumably) tech 1 scanner and found what looked like an electrical "spike" going through the injector circuit, which seemed to correspond with when it was misfiring. Sorry to be vague, it was all a while ago now, but did take about 9 months to sort out!

Tim B

[x6gas]

Gave this a go on the way home from Stoneleigh at the weekend.

My 1993 S4 has the stock chip and is bog standard engine wise apart from a K&N filter.

Couldn't detect anything and she was running smooth as can be.

Cheers, Ian

[Paula&Marcus]

... Anyway, just thought this might help.  My stumble seems to definitely 'occur' when ECM goes from open loop to closed loop at around 2,800 - 3,000 rpm range.

Hope this is helpful.

Regards

David

Esprit S4

<{POST_SNAPBACK}>

David,

Open Loop Mode only is applied right after startup (when the O2-sensor is cold and gives no data) and when the engine is warmed up fully only with throttle openings > 95%

Marcus