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940 classic LPT estate missing under heavy throttle

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Old Feb 18th, 2018, 00:45   #1
Hollick
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Default 940 classic LPT estate missing under heavy throttle

I have a 1997 940 Classic LPT estate, 246.000 miles and counting, engine never given a problem like I have had the last month or so.
If driving around 50-60mph in 4th or 5th flooring the accelerator to overtake or on a short slip road the engine misses quite suddenly at exactly 3,100 rpm - always exactly the same. If the throttle is treated more gently, then it will accelerate smoothly right up the revs, no problem.
I am lucky in having a local main dealer with excellent techies - one with 35 years with Volvo, and always very open and helpful. So far we have replaced plugs, high tension leads, distributor cap and rotor, coil, air filter, and still a problem. The MAF sensor was replaced two years ago, and is sometime flagging up on the diagnostic, but not putting the engine in limp mode. The flywheel sensor less than a year old. The car generally has always been maintained to A1 standard, and I have had it for 12 years, with a complete Volvo history from new.
Any ideas welcome - lamda sensor maybe? MAF perhaps? but what is puzzling is that this fault always occurs at exactly the same rpm, and otherwise the engine showing no signs of any problem - cold starting, idling normal.
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Old Feb 18th, 2018, 05:40   #2
aardvarkash10
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not lambda - that goes open loop under acceleration.

Loads of threads addressing similar problems on this forum - a quick search will save repeating what's out there already.

A good starting point would be to check the diagnostics and see if they throw up a code.
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Old Feb 18th, 2018, 10:50   #3
Laird Scooby
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First thing i would check is your plug gaps, should be 0.65 - 0.7mm.

Also check your wastegate actuator, it may be jammed or there may be a leak causing overboost. If this is the case, it might not be so lively once fixed but more driveable.

Another good one to check is your ignition amplifier module. This is mounted on the L/H inner wig behind the headlamp, hidden by the air filter box.
Remove the filter box and you should be able to see the ignition amp. Unplug it, remove it, clean and polish the back of it (large aluminium heatsink) with something non-abrasive - treat where it mounts the same then smear heatsink compound on the back of it and refit. Pay particular attention to the earth connection that if memory serves is close by. While you're checking this, also check the coil LT connections, particularly the mounting bolts, coil body to clamp etc making sure they are all clean and corrosion free.

Check those things, see what you've got. If they don't fix the problem they will all be helpful anyway and eliminate them as possibilities.
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Old Feb 19th, 2018, 00:26   #4
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Thanks Dave - Plugs unlikely to be a problem - just fitted new Bosch high performance (four prong electrode), same as the engine has run on for years. Not clear what the wastegate actuator is?
Ignition amp is one thing the Volvo guys want to check, and I have a replacement second hand from Lakes Volvo at St Neots. Good advice about cleaning the back of the heat sink etc, so thank you for that as well. LT coil connections cleaned when I fitted the new coil, and the clamp etc clean. I also have the Bosch component near the ignition amp, a metal casing with four small cylindrical elements inside - I think a fuel system relay or? I have the old MAF sensor which was still apparently OK when replaced after flagging up a code - which the new one has now also done, but seems to be working OK - can also try changing that over. There is another sensor? or something on the T of the hose from the MAF sensor to the turbo, and as I was advised that this large hose can fail when old, I have a new replacement, which includes the T and the sensor. So another thing we can try, one at a time!
Will let you know what we find, as it may help others.
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Old Feb 19th, 2018, 11:02   #5
Laird Scooby
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You didn't inadvertently swap the LT connections when you fitted the new coil did you? The Blue/Red wire should be on the +ve terminal - worth double-checking as reversing a coils polarity can cause bad starting, poor running and speed-specific misfires.

Intrigued by the mystery component, have you got a photo? Could be the dim-dip resistors at a guess.

I'm making a guess that the other sensor on the hose between turbo and MAF is a pressure sensor possibly to bring on the overboost warning light (or was that only on diesels?) or tell the ECU it's overboosting. Leaks in this hose could cause an overboost condition without warning lights and often with overboost, detonation/pinking/knocking occurs so the knock sensor could be retarding the ignition to save the engine.

Have you tried altering your driving style to see if you can recreate the misfire at a different engine speed?
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Old Feb 19th, 2018, 20:36   #6
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Thanks again Dave! - the coil LT connections definitely correct - I did this myself just outside the Volvo workshop as it's such a quick easy replacement to do. I haven't got a photo of the other Bosch component, but today I swapped the ignition amp for the second hand one, and still exactly the same problem, so very unlikely that two different units would produce the same fault. I also replaced the other 'mystery' component, and that also had no effect on the problem. The misfire is always at the same 3,100rpm, in third, fourth or fifth - first and second it doesn't do it, but I think that's simply because their lower gearing is not putting so much pressure on the engine. It never does it at any other rpm! So tomorrow Volvo are having a further look, and I have saved them some time, and me some money, changing the first two components.
The problem is linked with asking for maximum power, and whatever is doing it is not flagging up any codes, so rather a mystery. It's not really a question of driving style, as I am not a foot flat on the floor driver, but there are times when that acceleration is needed for safety reasons - I live in Lincolnshire, and going south join the A1 at Colsterworth, where the newly redesigned junction has a very short slip road with a right angle bend at the start, which when the road is busy (often!) means you have no choice but to floor the accelerator to have any chance of safely merging with the traffic. It was this sort of occasional use of the throttle that drew my attention to it about six weeks ago - this engine has never done anything like this before, and my driving style is unchanged, and generally gentle on the car!
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Old Feb 19th, 2018, 21:13   #7
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I wonder if the problem has been there since before you had the car? If yo're normally a gentle-ish driver, you probably wouldn't have noticed it until having to give it the beans.

It sounds like the same idiotic road designer/architect had a grimy paw in your dual carriageway as designed the Elveden junction from the A11 round my neck of the woods!
On slip going towards Norwich from Elveden - 90 degree turn onto the on-slip from the approach then a very short on-slip. On the off-slip coming from Norwich, very short braking area in the slip, then another 90 degree turn!

Obviously designed by a non-driver!



Rant over!

It's odd how it happens at exactly 3100rpm though. I'll give it more thought.
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Old Feb 20th, 2018, 00:46   #8
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Thanks again Dave - it is very odd about the exact same rpm, and this one of many things causing head scratching!! - no chance I would not have noticed it before, as I have been driving the same routes regularly for years - I teach in two Cambridge colleges, and play concerts all over the UK sometimes carrying early keyboard instruments that I play - which is why I love this car, as the modern ones aren't big enough! So for all the 12 years I have owned this car it has had to do lots of the same sort of driving, with the occasional hard acceleration needed. This fault only manifested about six weeks ago, so quite sudden onset with no lead in.
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Old Feb 26th, 2018, 18:35   #9
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Hi Dave - problem now solved, and not what we really expected with such a precise rpm every time - it was a fault MAF sensor. I had kept the original which had caused one problem a couple of years and more ago, most likely at that time just the connecting block not quite right. The replacement back then was a non- genuine equivalent by Vemo which was obviously not very good quality. It was also a quite different pattern of sensor wiring in the air flow from the Bosch original. Changing the sensor back to the old Bosch cured the problem instantly, and Volvo can get an original one still - heritage parts from Sweden - so will once again replace it whilst this is available, as hopefully then it might last the car out!
The other work to replace ignition parts not wasted, as the engine is now running better, smoother and cold starting better than for some time. Thank goodness!!!
I wanted to come back and give the end result, as it may be of interest to others in this matter.
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Old Feb 26th, 2018, 21:09   #10
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The only conclusion i can suggest is that at 3100rpm, regardless of load, acceleration etc, the engine is consuming a specific amount of air. The MAF measures how quickly that air is rushing in, the ECU already knows the speed of the engine from the coil signal, it's expecting to see a certain input from the MAF, depending on load, acceleration etc and if the MAF is faulty, the signal will be outside of these parameters.

Because of this basic premise, my idea is that certain points are measured across the rev range and the 3100rpm point has gone faulty.

Just a wild guess, maybe right, maybe wrong but holds water in certain respects.

Either way i'm glad you got it sorted!
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