This one has me scratching my head..lol..when you first start this car up it has a very high idle then after about 1 minute the idle drops down to about half of what it was but is still high..at this point it will start surging..you can actually see the engine move as it surges..as the engine heats up the rpm will drop down lower but it never drops to the proper rpm for driving or totally smooths out..if you try and drive it before it has warmed up for a while you can go 50 klms an hour without touching the gas peddle..it had codes..P1684,P0123,P0108,P0171,P1294,P0508..I've change the IAC motor, map sensor, TPS, thermostat, and PCV valve..I took the throttle body off and used carb cleaner to clean it up like new..I followed all the vacuum lines and visually inspected them and listened for a vacuum leak..after I changed the MAP sensor I cleared the codes by disconnecting the battery for about half an hour.. since then I have drove it for a few hours and no new codes have appeared.. using the key method it goes from P1684 to ----- done.... using a code reader it comes up with 0 codes..any help in solving this puzzle would be greatly appreciated..

 

How about a faulty ECT not indicating it heating up properly focing too much fuel. But then you had code for lean. Would still look at a basic tune of plugs and wires, then good fuel injector cleaner such as Techron concentrate or Redlines SL-1. Would look at that Engine Coolant Temp sensor as well. Se what you have then.

 

Today I had a code come back it is P0171...the surging engine at idle has turned into bucking in drive with an occasional small backfire..without touching the gas pedal you can sometimes reach speeds of 70 klms an hour ....when you come to a place that requires you to stop if you shift the auto trans into neutral it will go to a smooth very high idle as you are slowing down.. then when you stop the car the idle drops down and begins to surge...if you stand at the back of the car and listen to the exhaust you can hear a constant miss and if you put your hand in front of the tailpipe you can feel different pressure coming out as it surges...

 

Sounds as if you have an air leak such as a manifold gasket. The code indicates your fuel trim has gone way to rich (adding more and more fuel to reach Stoichometric or ~14.7:1 air fuel ratio) so you have false or unmetered air entering you combustion chambers or your primary (upstream nearest the engine in your exhaust) O2 sensor is reading faulty. Your symptoms indicate false air that your PCM is trying to adjust for but is outside its capability.

Check from your throttle body back to see if you have a leak. A bottle of propane or carburator cleaner would be helpful as this getting sucked in will raise your RPM when it hits the leak. CYou can search in this manner. Careful with propane and combustables around the engine though. Do not forget about the brake booster as a possible leake source and check you check valve. Happy hunting.

 

Thanks for the reply ewetho..took some pliers and squeezed the hose that runs to the brake booster car still ran the same..from what your reply says it's possible for the "(upstream nearest the engine in your exhaust) O2 sensor is reading faulty" and therefore may be faulty..so I think I'll change that tomorrow and if that don't cure it I guess it's time to take off the intake manifold..another thing that occurred to me as a possible cause was a stuck EGR valve but I couldn't find one..lol..does this year neon have an EGR and if so where the heck did they hide it..hehe

 

Not sure right off but it has one on there somewhere. But EGR makes it run rough as the Ehaust gases don't burn for crap and in fact if open a lot will cause it die out. Stuck open EVAP maybe but not likely.

 

What I was thinking with the EGR was..if it was stuck open a bit it would send exhaust gasses back into the intake which in turn would cause the cylinders to have a lean A/F mixture which the upstream nearest the engine in your exhaust) O2 sensor would read and the computer would try and correct by telling the injectors to feed the engine more fuel..which could cause RPM surges..then because of these surges the MAP sensor would be sending wrong data to the computer further confusing it..does this sound logical at all?????

 

I found the problem this morning..I disconnected the O2 sensor and it still ran the same..so I figured it can't be that..I still couldn't find an EGR..lol..so I took off the intake manifold and right along the seam on the bottom it has a long crack..so tomorrow I get my parts and I can put it back together and see how it runs..thanks for all your help ewetho...

 

Fixed runs great...

 

Way cool, have a great holiday weekend.