Revs sticking

[ian974]

Car is an S1, and at the moment the engine continues to rev when lifting off the throttle changing up gears or slowing down.
I have checked the throttle linkage and the IACV was replaced not that long ago, and neither of these seem to have been the issue.
I have also tried swapping the throttle body, but the one I have is the plastic one, and I've only been able to swap it for another plastic one, with no results unsuprisingly!
When this happens, it seems to let off the power a bit, but takes maybe 1-2 seconds to drop back to idle.
I have looked for air leaks but can't find anything. I'm a bit used to how it is now, but would obviously like to sort it out!
The next thing I was wanting to try would be a metal throttle body, but would there be any other suggestions of what to check or try before forking out?

[s29ttc]

I would say you have got most of what I would have thought of. Just take the throttle body out and make sure it opens and shuts without rubbing or catching. I think the problem is when it heats it can expand and make it rev high . I would also say take out the IACV and give it a quick spray and clean just to make sure incase theres dirt and grim in there. Apart from that I can't think of much

[robin]

If the revs return to normal after a few seconds, especially if that return to normal is predictable and repeatable, then it probably is nothing to do the throttle body or IACV. If the revs hold above normal for prolonged periods then the throttle body is most likely cause.

You probably have an air leak - either in the inlet manifold or perhaps the vacuum pipe that runs from the drivers' side end of the manifold to ECU in the boot or the shorter pipe that runs to the fuel pressure regulator at the same end of the manifold. Also check that all the breather hoses are fitted and not frayed.

Finally, if you have hotter cams fitted against standard ECU, this behaviour is normal and can be improved by (a) tweaking the cam timing a bit to reduce overlap and/or (b) reducing the amount of air that can flow through the IACV by placing some sort of constriction in the short pipe that runs from the throttle to the IACV. The latter being cheaper than the former unless you already have verniers fitted.

Cheers,
Robin

[Rich H]

Mine does that.
I ignore it as it otherwise runs just fine.

Rich

[ian974]

If the revs return to normal after a few seconds, especially if that return to normal is predictable and repeatable, then it probably is nothing to do the throttle body or IACV. If the revs hold above normal for prolonged periods then the throttle body is most likely cause.

You probably have an air leak - either in the inlet manifold or perhaps the vacuum pipe that runs from the drivers' side end of the manifold to ECU in the boot or the shorter pipe that runs to the fuel pressure regulator at the same end of the manifold. Also check that all the breather hoses are fitted and not frayed.

It is pretty predictable the way it happens. I just had a quick of the vacuum pipe, and the small length in particular (1") that runs straight off the manifold to a plastic T join looks pretty cracked up, wouldn't be suprised if it was bad enough to allow an air leak. I'll see if I can get a piece to replace it, can't do any harm.
Will report on any improvements or lack thereof!
Cheers!

[ian974]

Well, replaced the small piece of hose, and check a few bolts, but no difference still.
The car had a 135 hp kit, but I am unsure what all would have been included in this, I assume cams would have, but not too sure on engine remapping.
Also, I have fitted a pipercross viper kit as the ducting to the original airbox had been breaking off the original airbox.
It wasn't sticking after I fitted it, but started a while afterwards, though I wouldn't think this would cause it to act this way?
Might get the old airbox back on over the weekend, see what difference that makes.
Is there any easy ways of checking for an air leak?

[mainsy]

I'm no mechanic but I read somewhere that if you over tighten the jubilee clip that attaches the filter to the tb it can stop it closing properly, but I may be talking complete nonsense

[ian974]

I'm no mechanic but I read somewhere that if you over tighten the jubilee clip that attaches the filter to the tb it can stop it closing properly, but I may be talking complete nonsense

I had heard of that, but have been using the original pipe from the throttle body to the filter with the locking clip thing that was on before, so don't think that's it.

It seems to happen more noticeably above 3k rpm as well if that means anything.

[philthy]

my guess would be an airleak somewhere, get a length of garden hose and listen to certain areas of the inlet while the engine is running, see if you can hear any leaks.

I fixed this in my old exige by replacing the vacuum hoses then again when the inlet manifold gasket was poorly fitted.

[robin]

135 kit can be done either by cams or by headwork - if you do both headwork and fit cams, you'll get more than 135, so I think it's either/or.

As an experiment, allow the car to warm up so that it's at 85+ on the dash. Now remove the 5-wire plug on the IACV. Now go for a drive - note it will have a tendency to stall on the overrun, but what you're looking for is whether or not the revs still stick. If the problem "goes away" then you might need to look at restricting the airway to the IACV.

If the revs stick for long enough when you just blip the throttle in neutral you should be able to just grab hold of the throttle quadrant and rotate it anti-clockwise back to the end stop to see if it moves at all, and if so whether the revs immediately drop. If they do, either the throttle cable isn't slack enough (try slackening it off a turn or two on the adjuster) or the butterfly is sticking. In the latter case this can be caused by the throttle body being distorted (but not in your case if you're not using a jubilee clip) or by the plastic absorbing moisture (allegedly). The metal TB will cure this, but not worth doing until you're sure that's the problem.

Cheers,
Robin

[ian974]

Well, I've tried it with the IACV unplugged, which does seem to sort things out, feels like it's on the brakes lifting off though!
I'll get the rest of the vacuum hose replaced over the weekend and will check it then before restricting the IACV.

As the vacuum hose runs to the ECU, i'm guessing it reads the pressure in the manifold to adjust how far the IACV should be open? Therefore if there is still an air leak in it it might not be working the valve correctly?
Happy to be told otherwise, just trying to figure out how it's all related!

[robin]

The interaction between the ECU, IACV and cams (if you have cams) is that when you fit hotter camshafts you get a bit more pulsing back into the inlet manifold on idle (pulsing happens because the piston is rising, the inlet valve is still open and some of the air/fuel that was drawn into the cylinder is pushed back out. The ECU monitors inlet manifold pressure to decide how much fuel to use/what ignition timing to use. Unfortunately the ECU assumes standard cams and doesn't always cope well on idle with the hotter cams. Depending on the timing of the pulsing and the ECU's sampling point for inlet manifold pressure, you can either get oscillations (where the revs climb and fall) or continuous fast idle.

If you restrict the inlet to the IACV (you just need some sort of plug with a smaller bore hole in it) then it might lead to other problems (stalling on overrun), but might also cure the fast idle.

I've always been tempted to add in a chamber of some sort inline in the vacuum pipe to the ECU (something like the vacuum reservoir used on later S2's for the exhaust flap actuator) - this might have the effect of dampening the input to the ECU and thus removing the pulsing. It ought not to effect throttle response because the ECU uses the throttle position sensor for that.

Cheers,
Robin

[Rich H]

On some megasquirt installs it's suggested to put a big ali drum style fuel filter in line with the MAP sender to damp the input a bit.

[s333fee]

hi,

just a thought but have you looked behind the accelerator pedal, on our s1 we had a similar problem and it was the black sheath over the cable was not located in the hole at the back of the pedal and it caused the cable to jam. just a thought before you keep taking your car apart anymore.