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My 940 (945 wagon) is a UK spec car with a B200F engine - 2 litres, non turbo.
I've got a minor issue with the idle speed. Normally starts and idles at around 750 rpm, very stable. After a run along a motorway at 70 mph or so, the idle settles back to around 1100 rpm. However if I switch off and switch back on again immediately, the idle is back down where it should be at around 750 rpm. So it seems that a high speed run is making some change to the engine which can be immediately cancelled by a re-start?
I have cleaned the throttle body and the idle air control valve, and checked that there are no air leaks in the induction system.
Any ideas please? Not sure whether it is inconsequential or an indication of something which I should be dealing with?
Many thanks
OwenF
Scotland
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Hi Owen,
I came across this lengthy thread on the topic and thought it might interest you.
https://www.brickboard.com/RWD/volvo/1409651/220/240/260/280/base_idle_goes_high_highway_speeds.html
Peter
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Thanks Peter, a lot to read!!
OwenF
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Apologies for chasing you good folk, but could anyone enlighten me on the couple of questions I've asked in relation to the idle issue on my 940?
Art - your reply was much appreciated, can you add anything more please?
Many thanks
OwenF
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Hi Owen,
You've described the exact behavior of our LH2.4 cars when they lack a signal from the speed sensor which drives the speedometer and ABS (if equipped) electronics. The on board diagnostics verifies this by posting a DTC (diagnostic trouble code) of 311.
This VSS (vehicle speed signal) needs to come from the speedometer to the fuel computer, so if your speedometer is working OK, the problem will be in the circuit wiring, most likely at the back of the instrument cluster.
--
Art Benstein near Baltimore
Ever stop to think and forget to start again?
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well that's impressive Art!! Thank you very much. My 940 is one of the early 940s with the Yazaki instrument cluster, and as you may know these fail due to capacitor leakage. I have recently come to the end of my supply of working Yazaki speedometers so have had to replace it with an aftermarket speedo to meet the UK MoT test requirements. So I have no working original speedo and no speedo signal (my 940 has no ABS).
I had believed that removing the speedo would have no impact on anything else - all other instruments appear to work fine.
I had noticed the "no speed signal" diagnostic code but thought it of no consequence.
Could I ask a couple of questions please?
1. how/why should the speed signal have any impact on the idle speed, especially as it is overcome by a restart?
2. How important is it that I try and rectify this - am I likely to be doing any damage, perhaps affecting the fuel mixture strength etc?
Many thanks again
OwenF
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"... as you may know these fail due to capacitor leakage."
Yes, I do know about this. See my thoughts in the link below.
Notes on 740/940 Volvo Speedometers
And as I mention in the above writeup, I'm not a 7/9 owner. I'm aware of the LH2.4 requirement for VSS in determining idle speed circumstance (e.g. is it in your driveway warming up, or are you at the toll plaza on the highway) as it exists in 240s with their VDO speedometers as well. Why? This is how Bosch designed it.
It frustrates those who modify their cars with rear axles that don't provide the signal, so it does come up a lot on the forums where aftermarket and drivetrain mods are discussed. Some live with it, and others are hungry to fix it. If I owned a 940 for which I could absolutely not fix the Yazaki instrumentation, I'd find a VDO circuit board and hide it behind the aftermarket replacement, run it in parallel from the VSS sender, and pipe its output to the LH2.4 VSS pin. Its an idea anyway. I doubt ignoring the trouble could hurt anything, but I don't know that for sure.
--
Art Benstein near Baltimore
Change is inevitable, except from a vending machine.
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Thanks Art. I think I will leave it as is and check plugs etc regularly to try and make sure no damage is being done. Any alternative seems a little complicated to attempt.
I have to say I can't see why this is designed in to the system. An idle speed is an idle speed, and I would think that applies whenever the engine is "off throttle".
OwenF
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Hello Owen,
My knowledge of this is limited, but I'm also curious. I had an experience with my 91 240 a few years back that left me stranded a few times. The car started normally but the gauge cluster was completely wonky. I put in a spare 87 (LF 2.2) box as a test and all lights and gauges functioned normally, but because the VSS is wired differently in that year I got a 311 and a non-op cruise control. Non abs car.
I drove the car for a year like that before repairing the original box and never noticed the high idle you describe. There may have been other factors involved or maybe I missed it, but on my everyday driving I encounter red lights on the highway and think I would notice a 1100 rpm idle, so it's probably the latter.
To test my understanding of Art's explanation I will make some assumptions which may be wrong. I would be happy to be corrected.
It seems that the ECU has a high idle option. Perhaps this is the same high idle that comes on for a few moments at start up as the one in your post. In a car with a functioning VSS, the ECU defaults to this high idle at a certain vehicle speed, and reverts back to the lower idle when below this speed. Perhaps this function is reset with key off, so that it restarts normally. To a tackless person this may not be noticeable. Without a VSS the ECU defaults to the high idle.
The above is probably not right because without a VSS, how would the ECU know that the car is rolling and hence move it into high idle ? Maybe the TPS?
As to why, a couple of things come to mind but I'm definitely shooting in the dark. Volvo warns in it's manuals to not tow an automatic over 20 mph or over 20 miles. This only applies to automatics so the issue must be the transmission. If you're running downhill at 6o mph with throttle off, does this somehow simulate towing, and the higher idle alleviate it somewhat? It's a stretch, I know.
You say "an idle is an idle", but not necessarily. The downhill trip I mention above with throttle off has the rear wheels driving the motor to some extent, and may be beyond the ECU's ability to understand what's going on. In any case it's a different matter than when the car is idling without moving.
Maybe emissions are less at the higher idle under these conditions?
Sorry if you're more confused than before you read this. I know I am.
Regards,
Peter
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Hi Peter,
Yes, idle is idle, and it is what it is. Boys will be boys. The explanation I heard before was the vacuum generated by deceleration with the throttle closed and its effect on oil pulled past the rings and valve stems was a result of the fuel economy inspired injector cut LH2.4 does on throttle down from speed. Does this make sense? The idle valve opens to keep down the blowby until the speed returns to match the pedal position. Downhill, yes!
--
Art Benstein near Baltimore
"Our A students become professors. Our B students go to law school. Our C students rule the world." - Henry Rosovsky
Henry Rosovsky was a longtime Harvard professor and administrator. An expert in Japanese economic history, the elder Rosovsky was chair of the economics department from 1969 to 1972 and served as dean of the Faculty of Arts and Sciences from 1973 to 1984 and again from 1990 to 1991.
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Thanks, Art.
After reading your explanation I seem to recall hearing 'deceleration' somewhere in a previous post, but it didn't sink in. I was unable to connect the dots between deceleration, vacuum, and blow by. It does make sense.
I can see how the LF3.1 with it's variable resistor TPS can gauge throttle or pedal position, but unclear with the 2.4. Perhaps it uses the AMM, or maybe the vacuum on the FPR enriches the mixture making the idle valve compensate.
regards,
Peter
https://www.youtube.com/watch?v=b2ccC4aULow
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