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Revs rising & falling at idle - never happened before

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Old Jun 12th, 2019, 16:45   #1
CharlesStevenson
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Default Revs rising & falling at idle - never happened before

Our 1.6 manual X reg has, sometimes, started to surge or drop it's revs when at low speed.

We've owned it for around 7 months and it's never happened before.

Symptoms are:

1. revs rising & falling without pressure on the accelerator, and

2. revs dropping when pulling away in a low gear, as if I've slipped my foot off the accelerator or selected too high a gear when pulling away.

Our previous S40 1.8 had a noisy VVT (so I know the sound to listen for) but I can't hear any noise from this one so I don't know if that's causing this issue.

When this happened for my wife, and again today for me the electric fan stayed on for a few minutes once the car was parked up, but normally the fan never stays on when we've parked, even after a long motorway journey.

I've checked the water level & that appears correct, so I don't believe it's overheating that's causing the fan to stay on.

The only additional observation I can make is that the CEL is on, but that's been on for months & as it's a mid-phase car, when I plug a fault reader in there are no codes to be read.

I appreciate that the two might be related, but as this issue's never occurred until the last 2-3 days it seems perhaps unlikely.

Mileage is c. 145k.

I've searched on here but can't find any suggestions, other than " ... carb cleaner for the throttle body and ICV ..." so I'd appreciate any suggestions as to what might be causing this.

Thanks
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Old Jun 12th, 2019, 18:10   #2
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The VVT will NOT create a surging idle,-the Idle Air Control Valve will, as will a vacuum leak. Remove the IACV and clean it with carb cleaner etc. Lubricate it with WD40. Is it functioning electrically? Does it get the 12 volts it's supposed to get?
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Old Jun 13th, 2019, 14:21   #3
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thx emtor.

I managed to find quite a few more posts on here after putting mine up & they also mentioned the -the Idle Air Control Valve.

Will check things out over the weekend, assuming it stops raining long enough for me to have a go!
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Old Jun 14th, 2019, 12:50   #4
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The IACV may have a leaking gasket, it may be stuck mechanically due to oil residue and dirt, or the solenoid may be open circuit or shorted, or it all works but does not get any voltage from the harness. If all checks out OK, then check for vacuum leaks somewhere/anywhere.
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Old Jun 14th, 2019, 18:27   #5
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Thx.

Looks like the rain's finally stopped so I'll check under the bonnet this weekend.
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Old Jun 14th, 2019, 18:54   #6
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Quote:
Originally Posted by emtor View Post
The IACV may have a leaking gasket, it may be stuck mechanically due to oil residue and dirt, or the solenoid may be open circuit or shorted, or it all works but does not get any voltage from the harness. If all checks out OK, then check for vacuum leaks somewhere/anywhere.
Quick question, if I take the IACV off to check it's condition inside I'm assuming the gasket will be u/s afterwards, so do you know whether I might be able to source one from a motor factors or would I need to make do with some in-a-tube gasket paste?

A quick look on ebay showed the only gaskets were being sold out from the US for $20 + postage!

https://www.ebay.co.uk/sch/i.html?_o...volvo&_sacat=0

I don't fancy leaving my wife car-less for the school run on Monday!
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Last edited by CharlesStevenson; Jun 14th, 2019 at 18:57.
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Old Jun 14th, 2019, 20:55   #7
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Quote:
Originally Posted by CharlesStevenson View Post
Quick question, if I take the IACV off to check it's condition inside I'm assuming the gasket will be u/s afterwards, so do you know whether I might be able to source one from a motor factors or would I need to make do with some in-a-tube gasket paste?

A quick look on ebay showed the only gaskets were being sold out from the US for $20 + postage!

https://www.ebay.co.uk/sch/i.html?_o...volvo&_sacat=0

I don't fancy leaving my wife car-less for the school run on Monday!
If it comes off in one piece I wouldn't hesitate to use it again. But if it's super old, it will likely crack and require replacement. Creating liquid gasket will be too messy. I would not bother unless it's an emergency. Get the proper gasket.
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Old Jun 16th, 2019, 09:23   #8
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Quote:
Originally Posted by CharlesStevenson View Post
Quick question, if I take the IACV off to check it's condition inside I'm assuming the gasket will be u/s afterwards, so do you know whether I might be able to source one from a motor factors or would I need to make do with some in-a-tube gasket paste?

A quick look on ebay showed the only gaskets were being sold out from the US for $20 + postage!

https://www.ebay.co.uk/sch/i.html?_o...volvo&_sacat=0

I don't fancy leaving my wife car-less for the school run on Monday!
I re-used the old gasket since it came off in one piece and I applied dish washer soap on both sides before re-mounting. The soap will harden into a hard but still flexible goo much better than any gasket paste I've tried. Here, (Norway) milk is sold in containers made from thin cardboard coated with wax. I use this material in combination with dish washer soap as gaskets where the pressure and temps are moderate. I've even re-used copper cylinder head gaskets (inboard marine engines) with dishwasher soap with great success.
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Old Jun 16th, 2019, 10:12   #9
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That's a neat idea re the dishwasher soap and the packing.

I imagine you mean the Tetra Pak style aseptic packaging container material.

It'd be good to be able to use some of those packs rather than simply throw them away.
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Old Jun 16th, 2019, 12:30   #10
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Originally Posted by CharlesStevenson View Post
That's a neat idea re the dishwasher soap and the packing.

I imagine you mean the Tetra Pak style aseptic packaging container material.

It'd be good to be able to use some of those packs rather than simply throw them away.
Yes, tetra pak.

Regarding dish washer soap: -I once changed piston rings on a one cylinder marine gas engine and could not for the life of me get the cylinder head to hold pressure. A couple of revs and the gasket blew. I tried all the gasket cements that existed in town but to no avail.
In fact the cements only made it worse. An old man came by, halted and said: "Dish washer soap!"
A thin coat on both sides and the engine ran happily and still does.

Where there is not very high temps and pressures tetra pak and soap seems to work just fine.
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