Sudden power surge

[jacob360]

I have a 91 240sx with a KA24DE. It has 150,000 miles. It runs well, and at WOT pulls ok, but at 5500 at WOT it gets a very sudden burst of power and pulls hard all the way to redline. It isn't a gradual increase in power, it's very abrupt. Is my engine suposed to pull this strong all the time? I wouldn't mind it

Thanks for any insight.

I originally posted this on the Online Mechanic forum, but it got no response. Hopefully someone here can help out.

[NISTECH]

I seen it but had no idea what changes at that specific rpm. does the car actually burst with acceleration or does the engine rpm just burst with little change in forward momentum?

[jacob360]

Thanks for the reply, and sorry for not being clear.

5500 RPM at WOT, the car bursts with acceleration.

[NISTECH]

sounds like it is loading up then when the air fuel ratio leans a bit BANG it accelerates like it should from about 2500 rpm. when your wot switch closes the engine goes into full enrichment and the mixture control system goes into open loop. but regardless of where the rpms are anytime that switch closes it will go into this state. So it should not just be limited to 5500 rpms. your buddy had metioned his car didnt do what yours did when he drove it. but compared to his car does yours lag at lower rpms and his seems stronger. this would somewhat back up the loading up theory.

[jacob360]

Yeah, you are definitely on the right track.

My car does lag more at lower rpms. Also, when I go to WOT, there is a pause before the engne reacts.

So are you thinking it's in the TPS? I had considered that. All my tools are locked up at work, except for my digital graphing multimeter, so I can check it out (I'm an apprentice/entry-level technician).

[NISTECH]

actually i dont think its the tps but that component does alter the computers control over the system at and above 75% throttle opening. when it goes full open fuel control is no longer controled by o2 inputs and more on airflow meter and tps readings. take a look at your coolant temp sensor connection and check its ohms at different temps to see if its within spec. if it is reading colder then it should it could be running the car richer then it should. in most cases though there is a cold start problem associated with it. flooding usually. go ahead and check your tps sweep if you have a graphing meter set it in graph mode and do a sweep slowly it should be at around .45 to .50 volt at idle (car at operating temp). and should sweep up to above 4 volts. I dont know the sample rate of your graph meter but if posible graph your o2 sensor as well .45 volts should be median and it should sweep between .2 volts to .8 volts about 5 times in 10 sec's . also if you have carb clean or propane would be better while watching your graph give it a quick shot of either. you should see it shoot up over .8v then give it a slight vaccum leak it should drop below .175 volts. do not pull your f/p reg vac line to create the leak cause that will defeat what you are trying to do. try using the brake booster hose just lean it a bit to allow it to suck air. if those come out ok there is one more test on it and that is reaction time from full lean to full rich. if your graph has a freeze button you can do this test. drop it full lean then snap throttle and freeze. you gotta be quick doing this test or you will miss it. the rise time from below .175 to above .8v should be no more then 100ms if all these tests are ok your o2 is fine. let me know if these tests reveal anything.

[jacob360]

Thanks! I'll do it later this afternoon, I have to go to a BBQ for lunch.

I really need to get a FSM.

[deftdrummer]

just leave it and get a "Vtec" sticker!

[MikeMurphy]

Actually mine is exactly the same! 5500rpm I get that really cool inner-ear feeling of acceleration. I find that mine isnt very abrupt though, so I dont worry about it. Overall its not very noticable. If you see any stock dynometer graphs you will find that 5500rpm is usually the peak anyhow. I would be interested in what you find this to be though.

[Nismo_Freak]

So are you thinking it's in the TPS? I had considered that. All my tools are locked up at work, except for my digital graphing multimeter, so I can check it out (I'm an apprentice/entry-level technician).

Tip-In Retard... the ECU retards timing for a second or so when the throttle goes WOT, it also enrichens the mixture. It's to prevent detontion.