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Low boost???

12K views 26 replies 11 participants last post by  smason54321 
#1 ·
Hey guys. I'm not on here often, mainly because, well the Xterra is the wife's car and I'm gone alot. Anyways, I figured here is the place to ask.

So I put a gauge cluster in the Xterra with a boost and volt gauge almost a year ago. The Xterra is an 02 SC SE with 78,XXX miles. Recently I noticed that the X feels even more sluggish than normal and then I looked at the boost gauge and noticed at full acceleration it only reads between 2 and 3 pounds. Everything runs great, it just feels like the SC isn't even there...if that makes any sense.

The gas mileage has been crap ever since I bought the thing, but I'm not one to complain about mileage. Then I realized that my Frontier with all of it's lift and armor, gets better mileage than my wife's stock Xterra. I also noticed that my wife can only get 200-215 miles per tank and trust me, she drives like an old lady.

I don't have any codes when I plug in my OBDII, so any ideas from you all who may have already solved a similar issue?
 
#4 ·
Since you don't have a knock sensor code showing, check the bypass valve on the back of the SC, its a got a rod that runs to a flapper inside the SC, it could be sticking open (letting all your boost by), or the bypass valve could be going bad itself. Also check all your vac lines, make sure none are cracked or popped off, especially the ones running between the bypass valve, solenoid and map sensors.
 
#7 ·
Okay, so after checking out my vacuum lines I found a corroded line that came from the passenger rear corner of the SC and led to a small box with a gate on the back side of it. I replaced that line and immediately had 5-6lbs of boost, then a CEL came on and knocked the boost back down to 2.

My OBDII displayed P0328 Knock Sensor 1 Circuit High Input (Bank 1).

Is there a how to on replacing the knock sensor?
 
#8 · (Edited)
reset and try again...
disconnect negative bat terminal for a min or so and then try again if same code comes up there is your issue. ive had issues with other cars when i fixed a boost leak it would throw a code. but there was no issue just the car was learning off the boost leak and when fixed it was a big change and said oww c$%^. idiot light come on limp home.
 
#11 ·
Well, I disconnected the negative terminal for 15 minutes or so. When I hooked it back up and took the X for a quick spin, my qauge when up to 5lbs of boost on one hard acceleration, then dropped back down to 2. Did the battery thing again, and still can't get above two pounds.

So, I happened to find a knock sensor in the garage tonight and came across a knock sensor relocation thread and I'll see if I can get the time to attempt it. We'll see, but at least I fixed the bad line near the bypass acuator.
 
#12 · (Edited)
Today I did the KS relocation. I can turn the truck on and make one good run with 5 pounds of boost, then it goes back to fail safe mode. No codes come up with my OBDII either. I swapped the wires around and couldn't even get three pounds of boost that way and it through a P0327 low volt code. So I swapped the wires back to how I had it originally.

As soon as turn the truck off and restart it, I get 5 pounds of boost for one more hard acceleration.

Now, the KS is connected to the passenger side of the supercharger. I know that the KS is most likely picking up the S/C noises and tripping the fail safe mode. Also, my wife thinks the last time she filled up she put 89 octane in.

Has anyone relocated the KS on a supercharged engine (not counting those of you that just put it on the firewall)? If so, where did you put the sensor?
 
#13 ·
Bringing this thread back....

Update, I returned from deployment....again, and decided to tackle this knock sensor issue once more. I moved the KS to the firewall and I no longer have a retarded Xterra! The supercharger is running at full boost, and I can't believe how alive it feels. I use to think my truck with all of its aftermarket glory could smoke the Xterra, but now with the supercharger working properly, it would be pretty damn close. Too bad my wife is the one that gets to play with it. Maybe I should get my truck a Stillen :p
 
#19 ·
Find the knock sensor relocation thread. It's the "how to" that has all the pics and step by step directions to tap into the wiring harness. It's not specific to a supercharged model because it's the same between the SC and the NA motors. I'm leaving town today and will be back in a few days. If you haven't found the thread I can take pictures of the harness for you then.
 
#21 ·
Thanks for posting this info as I am currently researching the knock sensor code and an 02 sensor code. Grrrrrrrrrr!
 
#25 ·
If he relocated it to the firewall it's not doing what it was designed to do -protect the motor by retarding the timing if it senses knock (predetonation). You could mount it almost anywhere in the engine bay if your goal is merely to trick the ECU. Or better yet, buy a 560k resistor for under $2 and do the resistor mod instead. It's been argued that if you mount it somewhere "rigidly attached" to the engine block, like the intake, it will still function as intended. The firewall doesn't even meet that questionably lowered standard of placement.

Did you try the bolt on the intake that some use and still get the code?
 
#26 ·
He doesn't get the code but the truck still runs like shit after it warms up. Mine does the same thing. I believe it is another sensor (possibly o2s) causing it to freak out. Uses a shit ton of gas and has no power. I had a new engine installed in mine due to cat failure. The muffler was new (just happened to need one), and the tailpipe is already black with soot from running rich. Going to swap out o2 sensors this weekend, and clean the MAF to see if that helps it out.

He bolted his to the side of the supercharger, mine is replaced in the oem spot. Both trucks do the same thing.
 
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