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High idle and snappy throttle after full throttle acceleration 850 1994

posted by  adrian on  Mon Nov 15 12:56 CST 2010
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last visit: Sun Jul 21 12:55 CST 2013
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Hi all,

I'm kind of stumped with this one. I recently aquired a 1994 850 5 speed Volvo. When driving normally (light footed, moderate acceleration) the engine runs great. No problems at all. But when I press the accelerator to the floor to merge onto a highway, the throttle is very snappy from accel to decel and vice versa. Once off the highway and at a stop light or stop sign, the car idles high (~1500 rpm or higher) and does this until I turn the car off for more than 20 seconds or so. Once I start it up, the engine idles fine and throttle response is normal again - not snappy.
This leads me to believe there is something electronic that i'm triggering and it is sticking.

This weekend I did a lot of troubleshooting. I have done the following...

- Cleaned IAC and tested with known good one. Problem still exists.
- Tested TPS simply by unplugging it. Got the typical backfire on decel so i'm assuming its good. Problem still exists.
- Replaced oxygen sensor and fixed vacuum leaks. Problem still exists.
- Removed intake manifold to clean out and clean/test egr valve. Bought a vacuum pump and tested egr - held vacuum quite well and was fairly clean. Cleaned intake, egr, flame trap and throttle body. Problem still exists.

I'm kind of stumped now. I tested the vacuum solenoid going to the egr valve and it holds vacuum. After a significant amount of reading on turbobricks, brickboard and my chilton manual, I feel it has something to do with the egr system but I just can't pinpoint it.

Any help on this matter would be fantastic as i'm at my wits end with this thing.

Thanks a bunch,

Adrian






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