This is a 96 940 2.3 Turbo Estate, owned by a family member and has just had the Turbo replaced by a somewhat poor mechanic (there were a few bolts not done up, gaskets missing etc). During the work something seems to have been broken/disconnected/disturbed.
If you rev the engine then let it go back to idle it stalls, this happen alot when slowing down quickly at lights, just as you change down from
3rd to 2nd and you have to bump start it in second (not good for the cat I know). It seems slow to react to the throttle change, if you just rev it a little it drops but then recovers.
It ticks over fine and colds starts ok.
I have just overhauled the head and cooling system (for overheating due to corrossion/no anitfreeze etc), and its still the same. I can't see any obvious missing plugs.
I've had another go at it today, cleaned up the thottle body but didn't have time to completely clean it, also cleaned the idle air valve but it works fine and wasn't very dirty. Checked all the hoses as best I could. After cleaning and new gasket the tickover was high, caused by a slightly incorrect of accelerator linkage bar, popping it off restored the tickover (maybe it was just the position I put it on in, only needed one turn or so to correct). Next I'll try some carb cleaner and/or a swop with known good unit.
There is no wooshing, all sounds very normal. Tickover is correct speed and tickover mixture gives the same reading on my gastester as the good car. Power and driveability are otherwise normal, if I am correct in assuming a B230FK is a bit slower that the B230FT.
The engine actually revs up ok and doesn't conk out or particularily dip (any more than a good car) on the de-rev when still. It seems to be more when power is applied under acceleration something gets 'set' and doesn't get unset or takes too long to reset. It just goes past the tickover point and quickly conks out. However if you slow down in a high gear so that it reaches tickover slowly it doesn't conk out.
When my good car (95 B230FT) de-revs it slows down as it gets closer to the tickover rev as if it is noticing. Also if the car is still moving it keeps the revs at 1200 or so until you stop, the faulty one doesn't do that.
It feels like there is a minor sensor signal missing, how does the ECU know the car is moving? How does it control the revs? what does the Throttle closed switch do? Maybe I should swop out the ECU aswell.
The owner is quite sure it happened during the turbo replacement, previously the dump valve was stuck open (gunked up engine due to no oil changes). What relevance might that have?
I've been reading up and the manual (surprisingly) has some useful info. Firstly while trying to follow your advice I found that the idle air valve has to be open (its variable I assume, rather than open or closed as the manual states) to idle properly. The basic tick over was correct (it conks out).
Secondly it says the idle valve will only open when the throttle closed switch is closed. I get the feeling its something to do with that, either the throttle closed switch is not working or the info is not getting to the ECU. Should be easily tested.
But why then does it idle ok all other times?
Either its not opening the idle air valve or its not detecting the throttle closed switch.
The diagnostics may say something about that, but am I correct in assuming if there are any codes stored the engine check light should come on? Its not on, so I assume there is nothing stored and no point in me checking.
I don't know how much difference there is in the control system between your
940 and my 760T, but I've found the idle passage on the bottom of the throttle body is a very troublesome area. If your TB is like mine, there is a knurled adjustment at the bottom. That knob should set the physical bypass amount. Deposits love to build up in the passage and cause the sort of symptoms you are having. When that happens I count the number of turns to bottom out the adjustment, then unscrew the thumbscrew. I spray carb cleaner from the intake through the adjuster hole and spray it into the adjuster hole, then put the adjuster back in and screw it out the number of turns it was before.
Hmm, I don't know, the 940 idle bypass comes into the manifold from above and from the inlet pipe nearer the radiator, but the pipe loops round 180 and goes down to the bypass valve. The pipes and engine side of the throttle body where were pretty clean (depite the rest of the engine being an absolute pigsty).
I am still thinking the throttle closed switch is not closing maybe due to carbon on the TB barrel (but it does click), and the engine then assumes idle as a get-around-after-a-delay from the throttle position output, but really it should report a fault then (if the programmer had any sense).
I need to get at the car again, but the owner has it at the moment.
Ok throttle body is now spotless thanks to some intake cleaner, but still the fault remains.
Another theory is the Turbo dump valve is sticking closed, and the pressure is keeping the throttle open until it dissapates, or buggering up the mixture or something. The owner got the turbo reconditioned for a stuck open dump valve and general age, but the reconditioner might have not renewed it.
It matches up with the time the fault was introduced, and the power requirement (Rather than reving up at standstill) to induce the fault.
Aux boost gauge will be applied tomorrow (why didn't they put one on this car?), should prove or dissprove.
Boost gauge shows normal pressure, but on removing the dump valve there is a hole in the diaphram and I didn't have anything I wanted to risk trying a temporary repair. Blocking the dumpvalve pressure feed didn't help.
Will see if that sorts it, makes some sense under circstances but I don't really understand why.
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