RPM jumping?

[db4g63]

Im pretty new to the whole car scene and wanting to get into it. Ive got alot of guidance from friends and family but still much to learn. To my problem:
Im stuck with a 420a at the moment and the rpms when idling have a slight jump ~50rpm.. Seems to be no problem with throttle and higher rpms.. When coming to a stop if stopping to fast the car would choke and attempt to die. Ive replaced the Igniton coil, wires, and plugs. I know for a fact I have 1 bad o2 sensor. Would this be the cause or maybe bad injectors?

 

[2qik4u]

the o2 sensor is the first thing you should replace. it can cause the rpm to change as the sensor try's to cycle back and forth. replace it and go from there. acctually are yo throwing any codes!!! get a scan first and if your throwing any codes deal with that before anything else.

 

[db4g63]

o2 code for #2 i believe and did have a misfire code on #4 i believe the other day but that was before i replaced all listed above

 

[Boosted_talon98]

Im pretty new to the whole car scene and wanting to get into it. Ive got alot of guidance from friends and family but still much to learn. To my problem:
Im stuck with a 420a at the moment I know for a fact I have 1 bad o2 sensor. QUOTE]

Be careful what you say..Most people that own a 420a LOVE the 420a.. I know I love the hell outta it.. SOUNDS amazing.. anyways if you know for a fact you have a bad 02 then change it

 

[LD25Delta9]

Rear o2 won't cause that. All rear o2 does is monitor emissions. It's not a metered reading. Sounds like you might have a small vacuum leak. Do a smoke test and see what you find. If you don't know how, here:

Cap off all the vacuum ports on the Intake manifold but 1. Rubber band a plastic bag over your Throttle body inlet. Light up a cigar and blow the smoke into the 1 vacuum port left on the IM. If you don't start seeing smoke, start plugging in your vacuum line one by one, and check it each time you add another line. When/If you start seeing smoke, that's your culprit.

 

[ramsack]

my 1g currently has a bad o2 sensor and it will sometimes dip down in rpms and almost die when i let off the throttle to come to a stop, and then i will get a high idle. the rear o2 like others have said is just to monitor if the cat is doing its job. you may also have a bad front o2 sensor or an exhaust leak.

 

[pgarland]

Had a similar problem with mine, changed the Idle Air Controller (IAC) and that fixed the problem. I also recommend cleaning the throttle body thoroughly (you can use carb cleaner for this).

 

[david-b]

All good ideas.

Would take off the throttle body and extensively clean it, including the IAC. Also check resistance of it. If faulty, replace. Replace throttle body gasket since it's off as well.

And then check for vacuum leaks. Never heard of anyone using a cigar... but that's a damn good reason to have one.

 

[db4g63]

Sounds good I searched for the iac and only found one on eBay for 1g can I pick this up frm my local autoparts store.. Also not sure about some things: I recently had to put in a used engine and was installed by someone else. Before this I had no troubles with anything but my o2 sensors. Maybe something wrong with this engine that's causing this too?

 

[2qik4u]

before you buy any parts I would test the parts that are there already! and change the o2 sensor before you change anything else.

 

[pgarland]

You can buy a replacement IAC at Autozone or Advanced or even Napa. Check it before you replace it though, check this thread for how to check it: http://www.dsmtuners.com/forums/420a-bolt-tech/369833-p0505-idle-air-ecu.html

 

[Boosted_talon98]

you will know if front o2 is bad it will dump the fuel in.. Clean plugs, fire it up let it idle if your plugs are black and (all your fuel is stock) then replace front o2...

 

[db4g63]

Thanks for the help guys but i think i'm going to suck it up and call it quits here. Spent about $2000 just to put a motor in to find this problem and being a full time student and working a full time job i don't have the money or time to do this kind of a thing. So i'm looking to trade or sell it within the next few weeks. I know giving up isnt the way to learn but i need something that will be a little more reliable for ~100-120 miles a day.

But for those wondering: When the engine was put in all electronics and all sensors were tested and supposedly the IAC was working along with everything else.