Hi, i have a 94 940 with the 2.4L Bosch fuel injection system. I replaced my oil separator and flame trap, coolant temp sensor, intake gasket, fuel injectors, fuel pressure regulator and spark plugs and now the car has a weird idle. It surges up and down from 1000 to 2000 rpms. I have checked for vacuum leaks 50 times. Check my tps and iac and both are good. I also ohmed and checked voltage on my coolant temp sensor on pins 13 and 5 on the ecu and that was good. The car does run 10x better when the AMA is unplugged. Is there anyway to know for sure thats what it is before i drop that much $$$? Thanks

 

I seem to remember some of those old cars - If the throttle position switch is telling the ECU the engine is supposed to be idling - and the rpms go above about 1800 rpms, the injectors are turned off until the idle speed gets back to near normal. That means if there is a large air leak, the idle can bounce around from 1-2000 rpms. The air causing the high idle can come from a un-metered leak (vacuum line/manifold gasket/injector seal/broken brake booster check valve/etc) - or past a misadjusted (open too far) throttle plate or through the idle motor/IAC/aux air valve (whatever you want to call it)

How much air is passing through the iac? How much air is going past the throttle plate when "closed"? You should be able to block it off and adjust the throttle plate to let enough air in for the engine to idle at the normal speed (750?) (I used to just use a clamp to clamp of the the idle motor hoses down preventing air flow - but that would probably break a 35 year old hardened hose today!) Then reconnect the iac air path and the iac will compensate for extra load on the engine, cold startup needing more air - you know - just do it's job.

Are there any codes stored in #2 or #6 position?

 

I think you'd want to clear codes, then turn the car on and try to keep it running for a bit so it collects latest codes, turn it off, then check for those most recently stored codes. Unplugging the AMA previously may have left a bad AMA input fault in the system memory--not entirely sure, though, fact check me before erasing potentially useful diagnostic info!

 

Thank you all so much for the info and tips! I tried clamping the iac hose off and the car would idle down perfect so i replaced the iac this evening with a new one and the car runs like brand new! I guess the old one wasn't working like it should even though it would open and close with 12v applied. On the bright side i have learned a ton about this car and how it works in the process of diagnosing everything lol thanks again!