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Hello,
I have been battling a poor idle/hesitation problem with my '89 780T for two weeks now. I'd appreciate any suggestions.
The car: '89 780 Turbo with B230FT and LH2.2, 230k miles, no major mods.
The symptoms: rough, surging idle, with it occasionally dying. Hesitation at cruise when not under boost. Smooth under boost but has less power than normally. The hesitation really feels like missing: every second or two the engine bucks a bit.
What I have checked:
A/F meter indicates that mixture goes to lean when idle drops, but I don't know if this is cause or effect. Mixture does not go to full rich under full boost as I'd expect it, but I don't know if it did before, as I did not have the A/F meter in there before.
Fuel pressure is around 40psi at idle, as it should be. However, it rises to about 65 psi under boost. I am not sure if this is normal.
My first hunch was vacuum leak, so investigated under the hood with a flammable spray, to no avail. Finally I blocked the whole intact tract off from AMM to intake manifold and pressurized it from an air tank. Found two small leaks, fixed them, did not make any difference. I guess the intake manifold gasket could be leaking, but it did not react to my flammable spray, and none of the nuts are loose. If there is a better way to check, I'm all ears.
Here's a list of items I swapped with a used one after a new fuel filter had no effect:
AMM, fuel pressure regulator, fuel rail with injectors, idle air motor, spark plug wires.
I measured the resistance of the engine temp sensor, it's within spec.
Pulled all the plugs to see if any are different, but they all look the same. Slight powdery deposits, dark on the ring, grey on the portruding part.
Throttle position switch clicks when it's supposed to. Did not check with voltmeter, but I doubt it's the problem since it's not just idle that's affected. Am I wrong?
Car does not drive noticably better with a disconnected oxy sensor or with the vac line to the fuel pressure regultor plugged.
As you can see, I have already spent considerable time diagnosing this problem. If you have any suggestion, please let me know!
Thanks,
Balu
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'89 780, 208k mi; '80 245, 360k mi
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