940 LPT Idle Woes... Help!

[LPTJoe]

As per the title chaps.

The car is fairly new to me, a 1996 940 LPT (B230fk) on 86k.

Idle is slightly wayward and seems to hover for the most part between 800-900 rpm although it does seem to fluctuate slightly, sometimes dropping slightly lower at which point there is a slight judder and the car seems to catch itself before stalling. Also, the most worrying issue is that every now and then the revs will jump to circa 2000rpm without the throttle being pressed and stay there for 4-5 seconds or so before dropping back down. Weird right?

The car will do the jump up to circa 2k trick very occasionally, perhaps once every half an our or so. It happens when cold or hot, although it does seem much more prevalent when cold and it happens much less frequently once warmed up.

I can't see any vacuum leaks and have examined pipes carefully, other than that I'm not sure where to start. I've been spoiled by OBD2 cars in the past and whilst my car does have the port, it's not OBD2 compliant I'm told and none of my readers will speak to it...

EDIT to say, this may or may not be relevant, but the car turns over for 2-3 seconds before firing whether warm or cold, it's always been like this and has never failed to fire up, just far slower to fire than any cars I've owned in the past. Battery is all good. As mentioned I'm not sure if this is relevant but thought it might be worth mentioning.

Any common issues/wisdom/suggestions/past experiences would be very welcome and much appreciated.

TIA

[Jimsiss]

Most likely dirty idle control valve under the inlet manifold. Take it out and clean it with something like carb cleaner to remove oily build ups.

[ThePurplePanther]

I'd definitly 2nd the above.

The Idle Control Valve (known as an ICV in a lot of pics/literature) is just under the inlet manifold - there is a hose going to it from the intake and one from it into the manifold, held in with a circular bracket and 1 plug at the rear. It is about the size of one of those kiddie/'fun' sized drinks cans you can get.

Give it a good blast with carb cleaner and see how freely it moves with your finger.

If you need a new one get a decent brand one, the cheapy ones have a tendancy to either not work out of the box or fail quickly.

But hopefully a clean will fix it.

PP

[TonyS9]

A few people have (I believe) had interference problems with transplants (ie the wiring loom is transplanted to another car). I think I had this with my B230FT on my 360, the cause being some or all of the below;

AC on input detecting noise
Auto drive select input detecting noise
Bad throttle body adjustment

In my case the car had neither so the inputs where open and only cut near the engine, cutting right back to the ECU and grounding them seemed to help.

The throttle body issue was resolved mostly by a slight rotation/movement within the tolerance of the stud holes. It still rises a little occasionally, but usually a tap on the throttle helps settle it. Some day I should look up how to adjust it precisely.

The problem was definitely the ECU had decided to do deliberately this for some reason, resetting by turning the ignition on/off usually resolved it.

[Laird Scooby]

Any other clues Joe such as engine management light (EML) on?

Other than the good advice already offered (cleaning ICV and throttle body), check the throttle butterfly moves easily, also check to see if it has one of those silly poppet valves on it. Usually associated with carbs to prevent rich running on the over-run, some injected cars have them and if the spring goes weak, manifold vacuum can cause them to "flutter" altering the idle speed.

Also check the wiring to the crank position sensor, this is mounted on top of the bellhousing and is subject to lots of heat and also oil from leaks etc so the wring can deteriorate considerably.

If this is giving strange signals back to the ECU as a result it could throw the idling speed out, as could faulty wiring to the Lambda sensor and possibly a failing Lambda although that probably won't show up yet.

[LPTJoe]

Thanks chaps

Cracking responses with loads of stuff to check. Just what the doctor ordered!

I have a weapons grade 'to do' list on my car at present but I shall investigate and report back.

[Delta66]

List of suggestions here;

https://www.volvoclub.org.uk/faq/Eng...#Idle_Problems

I started as recommended;

"First Clean the Fuel System. Before you take apart the throttle body, add a bottle of Techron or equivalent fuel system cleaner to your tank. The symptoms above can be caused by deposits on your fuel injector nozzles. Drive the car through the tank and then decide if you also need to clean the throttle body."

and added STP Petrol Treatment and my variable idle settled back to normal. If that does not work, the ICV is easy to take off and clean.

[lnparry]

Hi Joe,

Try the thread below and especially my post on it (the last one) which explains how I got a cheap fix for these symptoms.

https://www.volvoforums.org.uk/showthread.php?t=248168

Might be worth trying before exploring more complex possibilities.

Good luck,
LeeP

[LPTJoe]

Thanks for the additional responses

@Delta66: I shall try some STP/Redex or similar, cheers.

@Inparry: Thanks, is this the pipe you're referring to...?

SAM_6739 by [url=https://www.flickr.com/photos/152293344@N05/]

If so, mine looks to be fine but appearances can be deceptive. I shall have a good look at it, thanks.

In other news, I've been driving the car all week and whilst the idle isn't up to modern standards the slight wobble is acceptable and could be down to lack of use, the occasional surge to high revs (1.5-2k rpm or so) only seems to happen in the first ten minutes of running (Cold engine)…

I'm now thinking coolant temp sensor (The sensor, not the 'sender') could be the issue, apparently it's a bit of a bugger to get to though with lot's of folk suggesting manifold removal is required...

[Laird Scooby]

Originally Posted by LPTJoe

@Inparry: Thanks, is this the pipe you're referring to...?

SAM_6739 by [url=https://www.flickr.com/photos/152293344@N05/]

If so, mine looks to be fine but appearances can be deceptive. I shall have a good look at it, thanks.

That is the hose, yes, and it is the vacuum/pressure feed to the fuel pressure regulator.

I can't remember the exact figure but it's something like 36psi and the fuel pressure regulator maintains fuel pressure 36psi ABOVE manifold absolute pressure.
To explain that, when the engine is off with no vacuum or boost, fuel pressure will be 36psi. If you now start the engine and have 8 psi of boost, the fuel pressure will rise to 44psi.
If you rev the engine and snap the throttle shut to create maximum vacuum in the inlet manifold, let#s assume it's a perfect vacuum so you've lost atmospheric pressure from the manifold (call it 15psi to make the sums easy) so the fuel pressure will be 21psi.

I might have that 36psi figure wrong but you can see what it does from these figures.

It has been known for the FPR to go faulty - if you pull the vacuum hose off it and plug the vacuum hose (either a well-placed thumb or small bolt) and observe the stub on the FPR for about 30-40 seconds with the engine running.

If any fuel comes out of the stub then the FPR is fault and needs replacing.