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Az:
I would second steve's hypothesis and recommend you replace the pressure regulator. I was having all kinds of issues with my 1985 745t's fuel system, and am still operating with a slightly lean mixture at times. While i think my mixture is still a little lean, I can say with certainty that replacing the FPR worked wonders for the stability of the tach, idle, and fuel flow.
If the FPR is shot it will deliver inaccurate prssure to the injectors. Since they can only handle (or are "supposed" to handle) about 44psi in the turbo model, pressures as high as 100psi (which it CAN run up to) can work havoc on the injectors. You're basically trying to push a liquid at 100 psi through a hole the size of a pencil head. At a certain point the pressure will start to exceed the rate at which the fuel can exit the injector, and it will back up or retard the spray. Add to all this that you can get a new FPR online for about $35 and its a worthwhile upgrade (judging from the wrecks I see in my pulling yard, I don't think most owners EVER replace the FPR).
On a side note, I have now replaced my sender assembly, pre-pump, and FPR. I've run a few cycles of cleaner through, and the thing seems to be working beautifully. I'm "trying" to replace my fuel filter, but the damned thing is corroded onto itself and won't come off. I'd really like to know what genius at Volvo thought "hey, I know, let's put the main fuel elements UNDER the car! We'll leave them in an open container so they gets corroded and beaten by road conditions... that way the part will fail TWICE as quickly AND the owners won't be abl to fix it themselves without a headache!" Along with the blower positioning/shrouding/exposure, the fuel pump/filter placement/design was a definite design flaw.
Good luck,
Rick
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