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Intermittent Fault - THROTTLE LOPING when Driving. Revs up to about 1900rpm.

(1996 Volvo 850)
in San Jose, CA on October 14, 2011
Sometimes Throttle Loping ceases when Auto shifted into Neutral (while driving along); sometimes requires engine to be Shut Down. One one [the worst] occasion, Engine wouldn't re-start when Drive engaged, unless throttle was revving up before pulling into Drive. Dangerous when Volvo_850 accelerating/Loping windy-downhill on brakes, and needing to turn off to kill Loping !!
Displaying 2 answers & 2 comments
Popular Answer
of Contra Costa Smog and Repair on October 16, 2011
Sounds like a vacuum leak to me, check the cold air hose from mass air meter to throttle body for cracks. Cracks may not be visible unless you remove the hose.
on October 24, 2011
Well, thanks for those suggestions, but it turned out that the IDLE AIR CONTROL VALVE was the intermittently faulty link in the [broken] chain. Took a couple of returns to our Mechanics to pinpoint the source of this Loping While Driving problem, but fortunately they are very persistent and thorough.
Hi, There are many possible explanations for this. My first choice would be the air mass meter is beginning to fail. You might try cleaning it, and cleaning the throttle body at the same time. Any DTC's? If so, which ones. 2nd guess would be weak oxygen sensor. Good luck with this, Andrew
on October 16, 2011
Thank you Andrew. There were NO fault codes evident from the On-Board Diagnostic (OBD-II) system. (Assuming that's what "DTCs" means ?) The 'flame trap' has been cleaned, and any vacuum leaks searched-for, and the Coolant Temperature Sensor has been replaced. There must be a faulty electronic feed-back loop when the Throttle starts Loping up and down while we are cruising along in Drive. Weather mild, and even in rain about 14 degrees C. Initially we thought it was related to engine getting hot climbing long steep grades, but since then the fault's happened at modest workloads/speeds intermittently. We apparently have a Mechanic's nightmare ?!? as "good luck" has meant running on a Dynamometer to get the Loping in a workshop. Any other suggestions very welcome - we'll get them to look at the Air Mass Meter, etc.
Specifically, does an ECU need to 'follow' its OME-engine - could there possibly be an incompatibility between ECU and Engine ?? The vehicle may have been re-engined (still Non-Turbo before and after). Thanks again.
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