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Discussion Starter #1
so I've searched for hrs and read many posts about cars going into limp mode.. long story short car is 2002 sentra ser spec v. Already replaced full tb and alternator. My research shows me its probably wiring to ecu. Have not tried accel pedal replace. Or maf. Many people take it to shops and replace those three minus maf.... and ecu but still have problems.... my question is where is a link to service manuals and what wires going into ecu are for throttle body. My ecu looks very clean and I don't see any corrosion on pins or white connector? Is the problem usually at the ecu? Also have hear of ecu relay but usually people report noises and clicking problems with that not in my case... any info on troubleshooting wiring would help. Started random one day... car just looses power and won't rev up fast. Intermittent once it started... replace the TB and everything seemed fine. For a few days then came back more frequent than last time. Replaced alt. Still happened intermittently. Messed with plug to tb and tried checking voltage with key on there.. red wire shows 6.4v!? No others show more than.5 v. Shouldn't be more than 5 at full pedal correct? It now happens every time at start up which leads me to think it is wiring at plug. Any further help would be appreciated cannot ever see. To find what people did specifically to solve the problem with wiring. Thanks guys ive had the car 4 months and loved it, put a bit into it to improve performance.but this is pissing me off lol. Please help me get this baby back on the streets.
 

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I think you need to back up a couple of steps. Are the parts you put in the car factory or aftermarket b.s.? Do you have codes? Has any modifications been done to the motor, intake, exhaust? Stop just throwing parts at it or guessing what is wrong because that is not how you figure out a problem.
 

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Discussion Starter #3
Aftermarket TB and alternator. Custom homemade cai intake, Lightweight crank pulley, 2.5 cat back. As a note the problem first occurred after installing 2j medussa headder. Was fine with other mods. I do not belive that caused it..? .. the codes indicated throttle body that is why I replaced that first. My brother is a mechanic and he scanned it and said to start there after cleaning it and it still had issues. I will have home come over this week and re scan to try and help diagnose.. but any insights from. People whove solved the issue is what I'm looking for to help. Thanks
 

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If you can get the specific code that will help us help you. Also include a pic of your engine bay so I can see exactly what you did to the intake and pcv system. Note that our cars do not like aftermarket electronics. Things like alternators and starters are fine. Things like the TB, O2, MAF, ect.. should be factory replacements. I'll wait until you give the code and a pic of bay so we can actually start helping.
 

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Discussion Starter #5
Thanks, will do asap. I will look into the aftermarket TB issue, good tip didn't hear that before.
 

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Discussion Starter #6
Ok heres a picture of the intake.. the pvc vents to air. it was running for a while fine for over a month until the limp mode issue started. Only had the car about 4 months. Was totally stock when I started. My main question I have is which wires in white the ecu plug go to the THROTTLE BODY? I can start checking those as i suspect a bad wire is my culprit. But need to know which ones to check for continuity. I will get codes when my brother is available to come back.over and try to help me.. later this week hopefully.
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The plug for ecu oriented as it sits normally which wires for TB?
172269
 

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That hose needs to connected to the intake for starters, between the maf and the throttle body.
 

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Discussion Starter #8
Why? I've ran pcv to air on many cars to avoid getting my intakes gummed up with oil over time. Like I said it was running fine before and after that mod.
 

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Discussion Starter #9
Ok so we got codes. After reset it gets this code p2138. The accelerator petal is tracking voltage as it moves both 1 and 2 sensors on accel pedal are reading the same approx as it moves up and at full throttle so he doesn't think its the pedal. He recommends that the brand new TB i got might have gone to crap right away. He has had that happen at the shop before. So my next thought is maybe I need an actual nissan part if it doesn't like aftermarket. What is the evidence for this is it a common problem for aftermarket TB to not work.... if that is the case then it means my factory one went bad. Replaced with aftermarket and it went bad too... it is random with the old tb and new weather it starts in limp mode or not. We got it to start up and rev normal with both TBs and also go into limp mode with both hooked up.. hopefully that all makes sense.
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Discussion Starter #10
FYI. He did say on most cars the accel pedal voltage read outs run opposite each other. One starts high and moves low and vice versa. In this case they moved together but he said maybe thats normal on this car. Based on it tracking well with pedal preassure and being consistent at full throttle he didn't think it was the pedal.
 

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Yeah I thought the 2 app signals should mirror each other. Sometimes there are 3 reference signals. Not sure on the specv. I would assume 2 that opposite each other.

I can get wiring diagrams and info on the function of the gas pedal on this car at work. It could still be the pedal. There has to be a certain voltage window between idle and floored. Even tho it sweeps ok, it can still be low over all voltage. Also have to know what pins direct what at the tb. Voltages can range all over the place depending on what being controlled. The tb also controls tps and intake temp (?) Aftermarket tb's on this car can be sketchy. Hitatchi is oem. Like $115. And make sure you are performing both idle relearn and air volume relearn or whatever they are.

Connect the pcv vent to the intake. You can use a cheap catch can if you want. It rules out any lean possibilities and doesnt spray the engine bay with oil. Dont use a vent filter.
 

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First let's start with your "intake". It's a big no no to have a filter mounted directly to the MAF tube as this could cause turbulent air across the MAF throwing fuel trims off and can also cause the car to go into limp mode. You need at least 6-8" of pipe between the air filter and the MAF. Second, by disconnecting the valve vent tube and leaving it vent to atmosphere without doing anything to the PCV valve side that is connected to the intake manifold you are self-creating one hell of a vacuum leak and allowing unmetered air into the intake once again throwing fuel trims way off. My recommendation for that is to buy an actual intake or put it back to stock. Either way will yield more power and a happier computer.

On to the code. P2138 has to do with the accelerator pedal voltages which would definitely put the car in limp mode. Even though the scanner says that the voltage sweep is good it could still be bad. An intermittent issue would cause the car to go into limp mode at any random time. Knowing that the accelerator pedal is the only moving part in the circuit I would say you most likely need an accelerator pedal.

BTW you NEVER check for power and ground or continuity from the pin side of a connector. You ALWAYS backprobe the wire in between the insulated sleeve and the wire itself. Checking from the pin side will expand the pin causing no connection between male and female end.

Put it back to stock, change your pedal with factory parts and maybe your TB, and see where that gets you.
 

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Discussion Starter #13
Awesome info. Sus pec if you can get the wiring diagrams I would be interested in having those... beohsix. i will look into modifying the intake ill put stock on to eliminate possible issues while diagnosing everything.. I am getting a pedal to try that.. I have heard of a certain space needed either before or after maf. You cleared that up. I'm not sure exactly what you mean with the PVC and vacuumed leak. The intake tube has bung for pvs plugged so its sealed. Theres no unmetered air entering intake after maf. .... does it matter where maf is.. if I put it closer to TB and have the 6-8 inches after the filter? Keeping filter where it is? So again to be simple and clear are you guys suggesting it is far better to just hook the PVC to intake to prevent "lean" running.. i am obviously looking to make the most power so I appreciate the intake info. And I am doing the relearn aswell. After I disconnect the TB just out of curiosity how does that work... how does it know its been disconected... loss of power and thus memory to the electronics inside when plug is removed?
 

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Your PCV valve (Positive Crankcase Ventilation) is a check valve that is on the firewall side of the valve cover. There is a tube that runs from the PCV valve to the intake manifold on the front of it. This valve opens up when the intake manifold has vacuum (idle/part throttle) drawing air from the valve vent tube on the valve cover, through the crankcase, and into the intake manifold. By disconnecting the valve vent tube, as you have, and "venting to atmosphere" you are actually not venting anything unless you are WOT (wide open throttle). Instead the intake manifold is pulling vacuum on the PCV valve, through the engine, and through that tube you have electrical taped to the valve cover. All the air that enters the engine through the PCV system needs to be monitored via MAF if its intended route is to ultimately be routed into the intake manifold to be "burned up". Like i said you have self-created a vacuum leak. Just keep it stock until you have the money to spend on a known name cold air intake. I recommend 2J Racing for the intake and keep that hose hooked up. If your concerned about oil then get a catch can.

You don't need the wiring diagrams and honestly, seeing what i have seen so far, I don't know if you should be attempting any sort of electrical diag on harnesses. You need to trust the codes as it is very rare that a computer lies. If there is a code for an APP sensor then it typically needs an APP sensor.
 

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I agree on all that. Its a direct code refering to the app anyways, not the tb. I will do some research when I can for u later on today
 

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Discussion Starter #16
Thanks guys.. had a code for the tb and app previously thats why we changed it first.. thanks for the detailed description I appreciate the info on pvc you have made me smarter... now let's say someone wanted to be creative and eliminate the check valve... i belive someone mentioned not to vent it... im assuming this is all old school thinking and the computer wants / needs that info and would not be beneficial if it simply was vented correct!... thanks for not being assholes too thats one of my biggest pet peves is people who don't help but criticize.
 

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Discussion Starter #17
Also since you seem to know what you're talking about. Can it work to move maf to close to tb as i mentioned... its more about wanting to be different and build it myself for me.. i take pride in something I make that works lol.. this doesn't so far
 

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Sorry for the late reply

172272


172273


Hope the text is legible. You need an accurate multimeter if u dont already have one. If the new app doesnt work, there a few hits for ecu's..
 

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Discussion Starter #19 (Edited)
Thank ou. I'll show my brother that. Got new pedal installed. With aftermarket tb.. went right into limp mode.. let it idle to warm up for relearn.. did 3 part tb relearn and started and reved finr 3 times in a row.. went to drive and went back into limp mode at end of driveway... put factory tb in which sounds broken and springy when tapping on it and I couldn't get the last part of relearn to take the last part with 5 pedal pumps. It never caused cel light to blink so I think tb is indeed bad. Only thing I can think to try is get Hitachi stock tb to try next. Don't have access to codes at this time.
 

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You need to see if the same code has returned. You can prove an ecu problem with voltage monitoring. This might be an in depth problem that takes some detailed diag time unfortunately.
 
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