Revs dropping, car juddering - MK4 Golf 1.6

JustAnotherSaver

My wife has told me about this a time or two. Experienced it myself tonight. Not really sure what could be the cause.

Coming back on the motorway the revs were between 3k-4k. Then all of a sudden the rev needle just drops way down - say 1k-2k, the car judders & then it carries on as nothing ever happened. The whole thing lasts no more than say a couple seconds.

Then continue on with the drive & again the revs drop, this time between 2k-3k, the car judders, a second or two & then back to normal.

I didn't notice the headlights dimming during this. Not to say they didn't but i just didn't notice it.

If it helps then it was recently in for the MOT & there was talk of maybe needing a new cat to do with the emissions, but the mechanic just chucked in some (fuel?) additive & it was enough to pass the test.

Other than that there was nothing else on there that'd have me think it could be linked.


Any ideas as it's my wifes car & she's losing confidence in it. It wouldn't bother me because it's not actually cut out on her/me before, but she isn't as confident. I can see us looking at having to replace it if we can't solve this, which would be a shame as other than this it's in fine order.

facade

If the rev counter drops, assuming it didn't immediately slow to 30, then there is something wrong with the signal informing the ecu of the engine rotation- so crankshaft sensor..

These rarely if ever put the warning light on.

Browntoa

Or the lambda sensor mis detecting oxygen flow , I've had that

Definitely a sensor issue

Strider590

JustAnotherSaver wrote: »

If it helps then it was recently in for the MOT & there was talk of maybe needing a new cat to do with the emissions, but the mechanic just chucked in some (fuel?) additive & it was enough to pass the test.

Yeah, that was probably a con to squeeze a few extra £ out of the customer without it showing up on the MOT records.

What's happened is the engine has cut out and then the still moving car has bump started itself again. There could be many causes, EGR MAP/MAF sensors, lambda sensor, idle valve, ignition coils, bad fuel, fuel pump.

You haven't really specified how it's normally driven or the type of mileage. If it usually does low miles and short journeys OR it's driven very "carefully", then the first thing i'd do is take it for an "italian tune up" down the nearest NSL zone.

BeenThroughItAll

Browntoa wrote: »

Or the lambda sensor mis detecting oxygen flow , I've had that

Definitely a sensor issue

The lambda sensor doesn't measure oxygen flow or any rate of flow for that matter It measures the proportion of oxygen in the exhaust gas by continuous sampling. It's unlikely it would cause the symptoms described, and if it were reading erroneously to the point of drivability issues, would certainly put the EML on.

My money is on the MAF.

facade

Depends on the exact symptom

If the ignition is cutting out- rev counter drops more than the car is slowing, then crank sensor.
If the engine is losing power- rev counter falls gradually as speed decreases despite foot on the floor then could be MAF.

baz8790

Join a local Facebook group for car sales and put up a post asking if anyone can read your car with VCDS (formally known as vagcom)

Or you might find someone advertising on gumtree or eBay.

They should be able to scan the car and see what fault codes are stored in the ECU

If they have the genuine Ross tech cable then it will read any fault codes shown and will be a lot cheaper than paying VW to diagnose it.

Might cost you £10 or £20 depending how greedy the person is but will be better than buying sensors hoping it fixes the problem

JustAnotherSaver

The actual speed of the car didn't change. Not enough that i noticed anyway & the 2nd time i was actually watching the needles, but then how much would it really change in the 1-2 seconds that this happens? In fact 2 seconds is probably overkill. More like just 1 second tops.

The rev needle didn't slowly drop like it would if you let your foot off the accelerator nor did it instantly drop as though it was like a light switch - on/off but it went pretty rapid, probably in line with as you'd turn the engine off i'd guess.


As for how it's driven - mainly it's just short journeys (yeah i know, get a bike - heard that one on the internet before but moving on....) & it's done 80k. It's an X-plated so 2000?


As for the mechanic trying to con us, i understand that's the internet way of thinking, but just no. That's as much as i'm even going to bother with that one ... just no. If it was Kwik Fit or a guy i didn't know i'd entertain your idea, but no.

JustAnotherSaver

baz8790 wrote: »

Join a local Facebook group for car sales and put up a post asking if anyone can read your car with VCDS (formally known as vagcom)

Or you might find someone advertising on gumtree or eBay.

They should be able to scan the car and see what fault codes are stored in the ECU

If they have the genuine Ross tech cable then it will read any fault codes shown and will be a lot cheaper than paying VW to diagnose it.

Might cost you £10 or £20 depending how greedy the person is but will be better than buying sensors hoping it fixes the problem

I actually have genuine VagCom. I'll connect it up & post up the readout.


EDIT: I know others may doubt how genuine it is, but since it's a Ross Tech one with genuine download from genuine seller it's pretty genuine

facade

JustAnotherSaver wrote: »

The rev needle didn't slowly drop like it would if you let your foot off the accelerator nor did it instantly drop as though it was like a light switch - on/off but it went pretty rapid, probably in line with as you'd turn the engine off i'd guess.

Crank sensor. It won't show an error code as the ecu thinks the engine has stopped rotating- which isn't a fault.

JustAnotherSaver

What's involved in replacing that sensor then (& i don't mean taking old sensor out & putting new one in)?

I've replaced a few sensors before but they were all fairly easy to locate & change. Never done one of these before though.

What sort of cost would she be looking at for this - both the part itself & also the fitting?