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Unstable Idle at Start Up 700 1989

posted by  Bean on  Sat Nov 13 15:57 CST 2004
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last visit: Mon Aug 30 03:53 CST 2010
[RELATED]

After sitting for 8+ hours (overnight or while at work) my '89 765T, that recently had a motor transplant, has a very unstable idle. The idle will get low enough (~500rpm) that the car wants to stall but most often it catches itself, engine sounds almost as though it's missing on one cylinder. When it happens I just rev the engine up to 1500 - 2000 rpm for about 30 seconds and it settles down to a normal, stable 750 rpm idle. The car runs like a champ after these episodes.

History:
The '89 runs on LH 2.2, the new motor from a '94 940T was running on LH 2.4. When I swapped the engine, I kept the LH2.4 injectors, ECT, and knock sensor which, according to a BB guru, would work just fine with the LH2.2 system. They appear to work fine aside from these starting idle issues.

It seems to me that something, when warmed up, settles down (expands?) to allow the car to run normally. What that something is I have little idea... I plan on checking the whole ignition system including the coil/wires/plugs/ cap/rotor and hall sensor. Afterwards will check the ECT with the thought that its resistance reading at "cold" may be out of spec. Another thought that crossed my mind was fuel pressure, I'm thinking fuel check valve but from my research it's symptoms show during hot starts. Fuel pumps are both running, main pump does make some noise but I consider it to be normal.

Any thoughts as to where I should start checking first?

EDIT: The Idle Control Valve is clean and seems to work fine, another thought is the Power Stage...

Thanks,
Bean

--
'80 242GT 93k, '94 945T 139k, '89 765T 68k (new '94 B230FT)






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