98 Nissan frontier trouble

I have a 98 frontier 4 cil. 4x4 manual transmission. The CE light is on telling me that the evap system needs work. I replaced the purge canister valve and now the light wont go off at all. You can clear it all you want, as soon as the truck starts the light come back on.
The truck also runs rough, bucks at certain RPMS has no power and bad gas mileage. Within the last two years I have replaced; catalytic converter, fuel filter, plugs, wires, cap & rotter, transmission and clutch, air filter, cleaned the MAF sensor. If you have any ideas on where to go from here I would love to hear them. Thanks.

Sounds as though there is trouble on the Frontier. It sounds as though there is a vacuum leak somewhere. Since it seems to have started when you replaced the purge canister valve, I wonder if you have a vacuum hose off somewhere.

Agree with Triedaq, but the simple fix would have been to stop overfilling (topping off) the gas tank, it would have fixed itself.

What code are you getting now?

Seems to me it may be time to send the car to a shop for professional diagnosis and repair.

The problem now may have nothing to do w/the evap, maybe something else is the cause. You’d have to retrieve the code and post it, the experts who frequent here know how to interpret the codes and would give you some ideas I think. Or you could still have a problem w/the evap. On some evap systems there are both purge and vent gadgets. Since you replaced the purge one, maybe the vent one has now stopped working.

fyi, here’s a short tutorial on evap systems and their testing.


The two codes that it is showing are, P0120 - TPS switch A and P0446 evap control circuit. Any thoughts on either of these? I have experience working on cars so maybe this is something I can tackle, but ware to start? Thanks.

Have you checked the output on the TPS? A repair manual should describe the procedure, and it’s not difficult.

The EVAP control circuit might take some work. The purge valve and solenoid are one, so the first step should be to verify that you have the correct part number and to recheck your work. You’ll want to verify that the solenoid is operating. Basically, it comes down to checking everything connected to he vapor canister.

I read in a repair manual and checked the TPS. From what I can tell it should be replaced. I did not check the PCM because I was afraid of damaging it via static electricity. Any thoughts on this system? Thanks

As long as you follow the procedures as directed, you’ll be fine. The critical paths through which electrostatic discharge could be damaging are all protected via a ground plane. To really feel safe, you could always earth-ground a wrist with multistranded wire. That’d dissipate any static you might accumulate.

This is an easy fix, the P0120 that is. It is the TPS, but you might not have to replace it. P0120 is a one trip DTC and as long as it is detected, the engine will remain in limp mode. Limp mode limits the engine to about 2500 rpm so if you can drive it normally, then the issue is at the idle position of the TPS and not in either the resistor or the wiper. These do develop a dead spot at the idle position over time.

You have a two part switch. One is the rheostat (variable resistor), the other is an actual switch that detects foot off throttle aka idle position switch or IPS. This one is probably working, this switch tells the computer that your foot is off the gas pedal and it needs to control the idle speed via the IAC (idle air control valve).

If you work the linkage, you will see a screw that screw that acts as a stop for the linkage. sometime people confuse this with an idle adjustment screw, it is not, it is there to protect the throttle body from the throttle plate digging into the bore.

Remove the air cleaner assembly. At the front of the throttle body you will see a U shaped component with two three prong plugs. The plug on the right as you face it is the TPS, the one on the left is the IPS. If you have a multimeter, measure the resistance between the center pin and either of the other pins while moving (or having someone move) the throttle from closed to wide open and back. If you are measuring between the center pin (2) to the outer pin (3), at idle you should see about 500Ω. As you sweep to WOT, it should rise to 4kΩ or more.

If you see infinite resistance at idle position, then as the throttle moves slightly off idle it jumps down to 500 or so then rises up to 4k as you go to WOT, then it is a dead spot. Return to idle and adjust the idle stop screw until you get a reading. The remove the other connector and check for continuity between center pin and the inboard pin. If you have both the continuity here and a resistance of about 500Ω on the TPS, you can erase the code and it will stay off. I did this about a year ago on my 97 Nissan truck and it has worked ever since.

If you don’t have a multimeter, then just turn the throttle stop screw in about a half turn, erase the code and see if it stays off. If not, try again but with a quarter turn instead and repeat with quarter turns until successful. If the code wont stay off after a couple of quarter turns, you will need a new TPS.

Stop filling you gas than for a few months, only put in about a half a tank so that the canister has a chance to dry out. The P0446 should go away on its own. After that, never add gas after the handle first clicks off. I have been through that with my truck as well, now I do not have that code and it didn’t cost me anything.

One last thing, the 97 might have been the last year that Nissan used the two part switch for the IPS. Everyone else had already stopped using them and relied solely on the TPS for idle and WOT position information. Nissan did abandon the IPS but I’m not sure what year. But you can still follow the procedure I outlined above, more or less.

Wow, thanks for the clear instructions that is great! I believe mine does have the IPS. About the evap, my tuck has had an evap system problem ever sense I bought it a few years ago. I could always get the code cleared and drive it to get inspected, and the light would come back on as I was on my why home from the inspection station.

This year I replaced the purge valve on the canister and now the light wont go away! I checked the wiring. I had my mechanic check it, he did,'t know. I am not one for over filling the tank, at least not intentionally, so I’m not sure. I will try what you suggested. Thanks.

check your fuse panel for a fuse labeled “engine control”, but I think if that fuse were blown, you have a lot more problems, but worth a check anyway.

Disconnect the plug to the vent control valve, turn the key to on and check for 12V or so on pin 2 of the wiring harness connector (plug lock up, pin 2 on right).

Check for continuity between pins 1 and 2 of the vent valve, you might have gotten a bad component.

Check the rubber hose to the vent control valve for leaks or blockage.

This could also be the result of the canister being saturated for too long by the previous owner. Keeping the fuel level from being to high in the tank might help it to eventually dry out and then maybe it will work again.

There is a chance that it is the pressure sensor, but that should give you a P0450 instead of a P0446.