High Idle Only In N & P
 

Hi all, got an interesting one for you (or maybe not, who knows). I have a 1990 240GL Auto with a B200F and AW70. Recently, I have noticed that when idling in Park or Neutral, the revs rise up, completely on their own after about 10-20 seconds. This is regardless of user input, or lack thereof; pressing/releasing the brakes doesn't trigger it, nor does a quick rev of the engine. Revving the engine briefly and then releasing the throttle also does not 'reset' the idle speed. Turning the engine off and back on again also does not 'reset' the idle speed.

The revs rise from about 750rpm to about 1,000rpm. This is only observable on a hot engine due to the faster idle when 'cold' - cold idle is not affected and is a perfectly normal value.

The only thing that makes the revs behave properly is engaging Drive or Reverse. When in gear, the revs stay bang on 750rpm as they should with no bouncing of the tacho whatsoever.

Staying in gear prevents the revs from rising, and shifting into gear brings the idle back to 750rpm after it has risen (again, indefinitely).

I know that the Throttle Position Switch is functional (clicks + buzzed out for continuity with DMM + verified working using the diagnostics box).

The throttle plate itself must be set correctly given that the idle is fine when in gear. Similar reasoning also leads me to believe the IAC is fine too (it's been cleaned within the last 6 weeks and clicks when the relevant diagnostic test is run).

So does anyone have any ideas on what it could be or any additional tests I could run to help isolate the fault (although 'swap in a known good one' is not an option).

I do have slight suspicion toward the Fuel Pressure Regulator: takes a few rotations of the engine before firing up, although always starts fine hot or cold, and I have on occasion had a petrol smell (no drips of fuel though) from the vacuum pipe that connects to it, although it did have a split at one end and has been replaced recently. Would the main issue be another symptom of a faulty FPR?

The only other thing I can think of is the P/N detection switch on the transmission or whatever it's actually called? Will run the diagnostic test on that soon.

I appreciate this is a long one so thanks for sitting through it.

Regards,
Adam

P.S. If it would help, I can get a video of it.

I have exactly the same issue with my 1992 B230F. I replaced the FPR and it made no difference. I switched out the IAC with a known good one and it made no difference. I'll be interested in the outcome of this thread.

Hi Adam, Is this a recent phenomenon? If so, might it not be a fault as such, but due to the high ambient temperatures coupled with the load or drag on the transmission when you engage D or R? Most of the automatics that we have owned over the last 30 years have exhibited similar behaviour to a greater or lesser extent.

Regards, John.

I would tend to agree with John, all auto transmissions have an element of drag due to the torque converter never completely disengaging, in modern (after about 1995) motor cars this is probably ameliorated by a computer adjusting the motor to suit, a 1990 Volvo won't have such a sophisticated system so I don't think a 200 RPM drop when in gear would be unusual. I certainly notice similar in the RB (a 1980 244 with even less control systems).

Is this motor car new to yourself? If not and you are sure it is new behaviour then it may be that one of the separate sensor and control systems on the motor car may have changed.

Good fortune,

Alan

PS. You say: The only thing that makes the revs behave properly is engaging Drive or Reverse ... but I suspect it will do the same in positions '1' and '2' as well - although obviously proportionately less.

PPS. You say 'The only other thing I can think of is the P/N detection switch on the transmission or whatever it's actually called?'... that would be the starter immobiliser - just test it by trying to start the motor car in any gear (D,R,1,2) - if it won't start then its working properly.

A couple of points to address:

1: I'm glad I'm not the only one, always reassuring.
2: Re: John's point, I have only noticed it in the hot weather, particularly (I believe) after replacing my thermostat so that the engine actually holds at operating temp (92°C).
3: This vehicle is my intro to automagics AND to old Volvos. Given I've only had the 240 a handful of weeks, it's hard to say what's normal. I bought it with the throttle cable tightened to the point of having 'around town cruise control' as it never quite shut, but I'm gradually bringing it up to a solid stage 0.

Most likely I'll just wait until autumn when it cools off a bit and observe the behaviour then... Any reason for the hot weather magnifying the effect?

Ah, that probably explains it Adam - this being your first auto - and your first older one. The behaviour you describe doesn't sound that abnormal for an older auto - they all creep a bit in gear and that power has to come from somewhere.

It may be worth you changing the ATF, it may be creeping a bit more that usual if that isn't in good condition. You won't really be able to change it all in one go, but you should be able to remove 2-3 litres at a time (via the drain plug if the AW70 has one, if not through the filler) and refill. Repeat that 2 or 3 times over a couple of months.

Good fortune,

Alan

I noticed that this started happening after replacing the MAF. The heater that cleans the wire failed on the old one. I could put the old one back on to test whether the idle issue in N and P goes away, but it's much more annoying driving with a dirty MAF wire (stalling, warning light on)

The ATF will be thinner and more fluid, Adam. This will result in less friction or 'drag' in the transmission, allowing it to idle faster. That is my understanding as a layman, I'm sure others will be able to explain it in more scientific terms. :regular_smile:

Regards, John.

I appreciate that this thread appears to concern only automatics, but could it in fact be nothing more than a slightly dirty Idle Air Valve, showing a tendency to stick slightly open when shutting down to Idle?
(I presume that an automatic works in the same way regarding the engine - ie does it have an IAV?)

I periodically have exactly the same fault with my manual gearbox version, and on every occasion it responds to a carefully cleaning of the shutter mechanism in the IAV using a small brush and carb cleaner.

It has been doing it a bit with the hot weather. Yesterday I decided the time had come for another clean, but then it cleared itself.

Already 8L deep on a full change (gradually in parts).

Good to know mine works then (presumably), certainly no rough running generally speaking.

A reasonable and simple explanation, appreciated :)

Re: your point on the IAC valve, Clifford: Yes autos do still have one. I guess I'll dump some more carb cleaner through mine shortly - mine was still playing up today despite the cooler weather.

If cleaning the IAC valve (again, only a 10 minute job though) doesn't have any effect, and if the park/neutral switch diagnostic test checks out okay, then I'll just accept it as part and parcel and leave it. Will post an update on the above 'fixes' sometime tomorrow evening, I expect. Thanks all so far.

As a quick extra note, being from an electronics background, it 'feels' as though when in park/neutral, the idle speed is 'dropping out of regulation', if that means anything to anyone, hence I'm still leaning towards something engine management related, rather than transmission related. I could very well be wrong though.