Would the big valve head cause this?

11th October 2007, 08:42

Sorry to bring this one up yet again, but I still haven't figured this out, and I'm honestly fed up!!! No kidding guys.

When you hold the accelerator pedal very lightly, the car has a tendency to jerk back and forth quite violently. IIRC, this only used to happen when the revs were quite low, 1500 - 2500. I now get the same jerking when coming off the throttle at higher speeds. A perfect example is driving on the motorway, and slowly accelerating to say 80 - 85 mph, and then when taking your foot off the accelerator pedal, the car jerks badly, and I think the exhaust burbles a bit too.

The car was mapped after the 708s were fitted, but I haven't altered the map since installing the big valve head, which in all fairness, has been considerably tweaked.

Just to add, the power delivery is smooth when accelerating, everything is fine, as long as you are accelerating... It's just when you start feathering the accelerator pedal, or come off it completely.

I have a feeling this is to do with the map, as I'm fairly sure the jerking when coming off the accelerator pedal at say 80 mph, is a hell of a lot worse than say coming off the pedal much higher in the revs. Still though, the exhaust is very loud at say 7000 rpm in 5th, and if you come off the throttle at this rpm, you can here the overrun is wierd. It's not a smooth change in the exhaust note, it's very on and off, on and off...

Well, I've tried to explain the problem as best I can, if anyone can help, please do! I'd love to get to the bottom of this!

11th October 2007, 08:49

Dont think its the map, mine does this too, i thought it was todo with my clutch, as the bearing sometimes makes a whistling noise.

I put a thread up ages ago about it. For me, it happens very noticibly in 1st & 2nd but im sure its still there through out the rest of the gears.

Accellerates fine, but if you driving at a constant velocity & back of the revs just a fraction theres a strong jerk. Originally i thought it was an engine mount too acctually.

11th October 2007, 08:53

I had a garage tell me it was the engine mounting, but I don't believe them. I think it may be something to do with the intake system, or the fuelling. I had a feeling it might be a leak in the induction, as with the dodgy idle, I could have loosened the inlet manifold from the head... I can't get underneath to tighten things up though. The other day, just out of curiosity, I checked the bolts on the exhaust manifold, and none of them were proper tight. I got nearly 360 degree turns out of most of them before they were proper tight. Again, I think this all to do with the shocking idle, causing things to come loose...

11th October 2007, 09:00

Hmm, mine idles the same as yours does, last week it cut out about 3 times aswell, which its never done before, all when coming to a halt, thus having the clutch pedal down.

Quote:

Originally Posted by Mystic

OK guys iv gotta be quick cus im about to go out but:

If im in a low gear, say, 3rd or less. Cruising at a constant speed then i back off the accelerator ever so slightly so that the engine begins to drag down my speed. Theres a fairly large jerk as this happens and if i would ease the accelerator back on again, as soon as the engine starts to drive the car again theres a jerk forward.

Anyone have any ideas whats causing it? Iv been thinking along the lines of engine mounts / gearbox mounts because it feels like there is a big of movement which causes the jerk.

As always saxp i rely on your wisdom

Cheers

That was my post from the begining of the year, suggestions were mounts, clutch, gearbox. One lad said he had this issue & the problem was his differential.

11th October 2007, 09:14

I'd checked engine mounts first

11th October 2007, 09:15

Hmm. I couldn't say for sure that it's the same issue.

I certainly don't have any problems with it cutting out in any way shape or form. Being in traffic is the worst! As soon as you cannot slowly accelerate anymore, I have to quickly disengage the clutch, or the car will violently jerk. Do you get that at all?

11th October 2007, 09:17

then again it it only started since the head went on

Either something is broke or loose

Or the car needs mapping again

11th October 2007, 09:17

Also, I recently put a different gearbox in, and the problem didn't change in the slightest.

11th October 2007, 09:21

Quote:

Originally Posted by leeroybrown

then again it it only started since the head went on

Either something is broke or loose

Or the car needs mapping again

I'll be honest with you Leeroy, when I first got the car, it had only done a few thousand miles, and I noticed the same thing, except not so bad, when driving in 1st gear. The car would be jerk back and forth when lightly touching the accelerator in 1st gear at low speeds.

This problem has been ever so slowly geting worse and worse. I don't think the head is directly responsible, though I might be wrong. I didn't used to have the problem at high speeds (coming off the the throttle) until a few months after the head was installed though.

11th October 2007, 09:23

mine did this when i rebuilt my engine on my GTI

Turned out to be the injectors were damaged!

11th October 2007, 09:25

Oh really! Are you sure the symptoms were identical? This could be a great lead mate, cheers.

11th October 2007, 09:25

The cutting out is an issue with my idling though, i think the engine was having a particularly bad day because its never cut out like that before, although iv heard stories from people with idle issues that it happens to them.

Creeping along in traffic is the same though a little accel and then clutch to the floor and coast till i need to stop, cant maintain a constant speed in first very well. Im getting particularly good at accellerting very minutely for upto a minute or something which passengers think is us just maintainting a speed, but eventually i need to put the clutch pedal in or it will jerk as the engine starts to brake us

Injectors is a new one! Might be worth some investigation.

Where about is the MAP sensor, i gather its on the throttle body?

11th October 2007, 09:28

Sounds similar problem to me mate. Do you have any issues when coming off the throttle at higher speeds, like I have described? It's like the brakes are being applied sometimes, it's quite a fierce snatch.

11th October 2007, 09:28

I'd drive along and it was fine

Would let of the trottle would jerk abit and as soon as my foot was back on the juce pedal it would jerk like hell

In the end it ran on 3 cyclinders

11th October 2007, 09:32

Hmm, that may be a different issue. I don't think the problem is quite the same, as at WOT I have no issues. Power is good. If the injectors were playing up, I wouldn't be able to get a smooth delivery of power, surely?

11th October 2007, 09:35

suppose so matey

11th October 2007, 09:36

Havent driven at anything over 40 for a couple months Toad, so i dont know. It still jerks in higher gears but not as violently as it does in 1st gear. Its tough to describe but as you come off the revs there would be quite a solid jerk back & then when you put your foot back on its the same jerk but forward this time. Almost feels like the engine leans back and then forwards, kinda like something really heavy in the car is moving back & forward when the jerk happens. Hmm awkward as hell to describe lol.

How easy are injectors to change?

11th October 2007, 09:39

Mine does this really baddly in first gear at low revs...

11th October 2007, 09:39

easier enough i'd say just a few torques then pull them out

11th October 2007, 09:44

I have a spare set i could try swapping them if i get a chance.

11th October 2007, 10:25

Hmmm, the mystery continues...

11th October 2007, 18:30

my mate had this in his astra sri found out he had fucked the gearbox and diff

11th October 2007, 21:21

Extensive headwork and changing the size of the valves could upset the way the gas flows through the head at different engine speeds? Like it could be fine when accelerating but when you lift off it disrupts the flow if you know what I mean? Causing it to stutter?

Could it be that the bigger valves are making it more responsive? And you’re just feeling the throttle inputs more through the car?

Are you using standard engine mounts and clutch? The rubber mounts and the springs on the clutch plate dampen the throttle inputs… could be that something’s up with the spring on the clutch? Or something’s been put on wrong when the head was fitted back on so that the engine doesn’t twist as much?

12th October 2007, 08:51

Interesting points Joe, thanks for the comments.

A few answers:

The clutch is only a month or two old, it's the stock Valeo one. This problem was no different with my previous (stock) gearbox and clutch, so I guess that probably rules that out.

The engine mounts are standard, and an engine building garage were fairly positive the problem is caused by the mounts. Personally, I don't believe them!

When you are slowly accelerating in 5th to around 80 and you take your foot off the gas, the transition is so rough, but SOMETIMES it's not as bad as others, which is confusing as hell! They best thing I can think of to describe the problem and or pinpoint the issue, is this, but it's a guess:

The problem all circulates around the actual amount you are holding down the throttle. As I get the jerking in low'ish revs when featheing the throttle, and I also have the issue just described, when coming off the throttle. So it's all there or there abouts with regard to the accelerator pedal. I'm thinking that there's some confusion on the car's behalf, with regard to (possibly) either the throttle postition, or the fuelling, ECU map.

Something I forgot to mention... When the engine is stone cold, (last night being a perfect example) sometimes the revs stay up, even though the clutch is not engaged. For example, I started the car last night, let it tick over for about 2 mins, as it's a bit slow on the old cold start business... and I took off in 1st gear very slowly. When I disengaged the clutch to pop it into 2nd gear, the revs stayed at 2000 rpm, and didn't fall back to usual 850. I'm wondering if this is all related, I.e., there's some confusion as to how open the throttle is. What do you think??

12th October 2007, 10:00

my car sometimes jerks on light throttle in first gear

as if its kangarooin

12th October 2007, 10:02

Quote:

Originally Posted by samcook

my car sometimes jerks on light throttle in first gear

as if its kangarooin

same,
its only done it since ive had a straight through exhaust,
perhaps my manifold at the weekend will sort it out..

12th October 2007, 10:11

Quote:

I'm thinking that there's some confusion on the car's behalf, with regard to (possibly) either the throttle postition, or the fuelling, ECU map.

Try changing the throttle position easy enough to do Toad

12th October 2007, 10:11

Quote:

Originally Posted by Soulless

same,
its only done it since ive had a straight through exhaust,
perhaps my manifold at the weekend will sort it out..

doubt it mate manifold exhaust and decat on mine and it does it! only done it a few times but that was quite a while ago, has been fine for agess

12th October 2007, 10:45

Looks like the "kangaroo" effect in 1st is very common with these cars.

Leeroy, are you refering to changing the TPS sensor? I know what you mean, but I don't want to start replacing sensors when I'm not 100% sure if they are defective. I'll probably end up replacing everything and still be no further forward.

12th October 2007, 10:52

Yeah The One On The Throttle Body