Codes??

[powerband]

Trying to help a 93 240 SX DOHC 5spd:

Do sensor problems ever set codes without causing check engine light to stay on during running?

Real clean car, @90K miles . This week developed problem - Starts and runs fine until warmed, then stumbles and idles very low or stalls on decel. Once warm will start fine but cuts out, or loses power then stumbles and jerks. Basically undrivable. If I let sit for a little while it will run OK for a short time until warms up then hesitates and stalls again.

Thorough inspection does not find any induction plenum leaks or hose splits. My gut feeling is MAF problem or other critical sensor but dash check engine lamp (orange dash lamp) LED lights with key on but does not stay lit once started.

Are sensor problems like MAF, TPS etc ever present without check engine lamp?. I did find a lot of info here when I searched on MAF . But I have not done check codes procedure on ECU since CHECK ENGINE lamp is not lit.Also the info in forum seems to be for OBDII which I believe is post '95 and second item to check in Chilton's and posts is the IAT but the '93 doesn't have one I can find.

Help, hints not anecdotes appreciated, THANKS - PB

[NISTECH]

Yes it is possible for a sensor to fail on OBDI cars without throwing a code. This is because on OBDI sensors operation is not really monitered only max and min output voltages are observed. Since the car runs ok cold my thinking is more O2 since that is one component not used during cold start up. once the vehicle reaches operating temp it switches over to closed loop and uses the O2 to control fuel mixture. during cold start up it uses the Air flow meter and coolant temp sensor inputs to control fuel mixture. hook a volt meter up to the O2 and see where the voltage is I suspect it is in the negative voltage range should be 0 to 1 volt.

not sure i understand your meaning at the end " Help, hints not anecdotes appreciated"

[powerband]

Thanks for the insightful reply. That makes sense as far as the O2 sensor being out of the ECM loop until warm.Could you possibly give a brief explanation of what the output of the O2 sensor is (or does), and how to test it. (Why am I looking for @1 volt)

Thanks again PB

PS The sig line on my first post was just a plea for info without opinion and conjecture.

[powerband]

I needed to verify wether the car should be running and can be cold when I test the O2 sensor for @ 1 volt.

Thanks again PB

[NISTECH]

the main reason a car stays in open loop till the engine warms up is because the O2 has to be hot to measure oxygen content in the exhaust. any readings taken off it cold would be inaccurate. After a car I experianced today I think I am going to lean toward your initial hunch of the airflow meter. try wiggling its connector and see if it effects the idle at all even cold..

the o2 sensor should sweep from 0 volts to 1 volt . low voltage would be lean raised voltage would be rich. The o2 should be able to achive at least .8volts when richened with a propane torch or carb clean. and drop below .175 volts if you create a good vaccum leak. if your o2 cant achive both those readings even if only for a second or 2 it is no good.

[powerband]

I'm back on this thread concerning the problematic 240SX with a new question or two....

As I left off last time, the car would run poorly with loss of power until warm then stumble and cut out and not idle.

No codes are being set with the ECU but the O2 sensor test doesn't seem to read correctly. - I warmed up engine put into diag mode-II, ran around 2k RPM for a few minutes and get an occasional CEL lamp increasing or decreasing engine speed but not the 5 short flashes I believe I'm looking for. I understand from the previous posts that the O2 sensor isn't input to ECU until it warms and goes into closed loop mode.

I bought a (used) MAF sensor which supposedly is from a KA24DE but it has a different connector and a slightly smaller bore. The original '93 MAF has a larger 4 pin connector (which only uses three pins) and is a JECS/Nissan. The "new"/used MAF is a Hitachi with a three pin connector but came with the plastic intake plenum which matched the '93 KA24DE's.. Ohming out the "new" MAF, it was different static values but I was able to figure out the ( + / - / Signal pinout) wiring to connect to the harness. With new(used) MAF the car now starts and runs with good power and cruise but idles poorly (sputters) when cold and once warm, hunts on idle and stalls out with no effect turning idle air screw. A few questions arise: 1> MAF's - are they calibrated to the vehicle or fairly universal?. In other words is the connector difference and bore differnce significant (or what application is the used one) ? 2> Since engine runs well accellerating and cruise but at idle sputters when cold and when warm hunts and stalls, does this sound like wrong MAF or can different MAF's be "tuned" into the setup?. Should I try to get it to idle?. 3> I keep thinking KEEP IT SIMPLE! so I'm trying not to disturb other systems until I can nail this down but any suggestion are welcome. I wanted to simply check the PCV but found it obstructed under the intake .

Thanks for listening, maybe this will clear it up in my mind

PB

[Q45tech]

Each MAF is different and calibrated so what ever the peak air flow would be at 6500 rpm equals roughly 4.3 volts.

A 4.5 liter Q45 would flow 233 grams per second and yield 4.3 volts and a 2.25/2.4 liter engine would flow/consume half the amount yet the correct MAF would still produce 4.3 volts [[ more or less]]....

THE MAF MUST MATCH THE ENGINE DISPLACEMENT.1.6, 1.8, 2.0, 2.4, 2.5, 3.0, 3.3, 3.5, 4.5.......then there are special for the turbo charged versions. Probably 20 different MAF versions.

0.2 volts difference can be significant at high flows it might change the injeector opening time 20%

The ecu is programmed with a different exponential equation to match the injector size and the engine displacement to the MAF voltage.