Thread: Misfiring below 3,000 RPM

First off, it's a B13 SE-R

I just got done rebuilding her, and now I've got a misfire for anything below 3,000 RPM. If it's above 3,000 RPM, it runs *great*. Also, it doesn't idle very well, either. Tends to stumble a lot and die sometimes. Lastly, if I am in gear and let off the gas, I hear a LOT of exhaust backfiring coming down below 3,000 RPM, making me think that there is some unspent fuel making it into my exhaust. Also, I seem to be getting pretty crappy gas mileage.

List of what I've already done:
Checked MAF, tested good
Checked injectors, resistance was 10.5 ohms across all 4 of them
Replaced TPS, set it to .49 volts closed and 5.something @ WOT
Replaced spark plugs with NGK's

I have tried to set the timing (going into timing mode and what not) but it will misfire and die if I disconnect the TPS sensor. This makes me think that it's the AAC or IAC valve.

EGR valve has been cleaned recently, along with all the plumbing.

Check my signature for mods if you need to know.
Thanks for any input!

Also forgot to mention that it surges while idling - it'll go from about 750 RPM to 1,500, and back down again, especially when it's cold. Then it will start misfiring, and struggle to stay at about 400 RPM or so. Once it warms up, it evens out a little, but still has the same problems, to a point.

you have a vacuum leak somewhere it sounds like, double check all vacuum lines

well how is your coolant temp? normally a bad coolant temp senor will cause the car to have bad idle, and start and die right away.

try setting the tps to .44v closed

did you regap the spark plugs? if so what are they gapped to?

is your pcv valve any good? this can cause idle issues also.

getting that timing set is #1 along with the idle.

did you get the camshaft timing set right?

iacv valve can effect the idle and make it idle high if the valve is stuck open.

sorry for all the questions just trying square it away

Thanks for the help guys. I'll double-check the vacuum hoses and get a gauge if I need to.
Spark plugs came pre-gapped from NGK at .044"
Camshaft timing is right, now. Thank God.

I've discovered something today - the IAC valve is the name they give the AAC/FICD as a whole.
AAC tested good
FICD tested good
(thus IACV is good)

The Air Regulator (that thing that hangs out under the intake manifold) might be bad - I am in the process of swapping it out with the other one again, since it seemed to be more closed than the one in there right now. See pictures here:
http://www.sr20-forum.com/showpost.php?p=46699&postcount=10
I'm going to install and test it as soon as the liquid gasket is dry.

Also, the plugs look a little funky. I'll post pictures soon.
I'll check the PCV valve momentarily and post results as well.
If that checks out, I'll test the coolant temp sensor and see what I find there as well.

I always re-check and/or re-gap spark plugs, no matter what they say they're "pre-gapped" at. I've had NGK's come "pre-gapped" at .044 but actually were at .032.

Wow that's way off. Well I did check one of them, and it checked good, but I'll check the others. Here are some pics of one of the spark plugs. This plug was brand spanking new 3 days ago, now it has a black ring around the base, and yellow deposits on the little arm. What does that imply?










The other 3 are the same way as well.

Well I think I figured it out - I pulled off the electrical connector for the coolant temp sensor, and looked inside and it was all green and corroded. Scraped it off, pinched the connector down a little tighter, and now it's working great. I'm probably going to just order a new one from the stealership because I couldn't get all the corrosion out, but we'll see how long it stays the way it is.

Thank you all for your input on this one!
Rick

yahs a bad coolant temp sensor can cause some nasty problems, however i do recomend you try to adjust the base idle and adjust the timing correctly too. do you have it overly retarded or overly advanced in any way or kinda in the middle?? adjusting that will definitly sort alot of problems out

Actually now that it's idling correctly, I'm going to go through timing mode and set it the right way. Last I checked it was at about 17* BTDC