1994 Nissan PU XE, KA24E, rough idle/cuts out after long warm-up

[psilogen]

I spent way too much money on this pickup, not realizing what kind of shape it was in. First car of my own, etc. Anyways, when I first bought it it would go about a mile and a half before the engine would start cutting out. The problem would show up gradually, dropping by about 1k rpm then going back to power, but the drops would get progressively more frequent until the point where the engine would just die. I brought it into the shop, where they cleaned the throttle body and replaced the air filter. Now it will drive pretty well just long enough for me to get to work, but if I drive 12 miles or so to run an errand and then try to start it, it will start running and then immediately die on the first attempt or two, then it will run alright. about 8 miles after that, it starts cutting out again, with a rough idle, and has a tendency to die at stop lights. A mile or two after that, it will die when driving.

I've looked around, primarily on these forums, and it sounds like a common culprit is the TPS or the MAF sensor. This makes some sense to my untrained mechanical brain, but the problem is that I only have about a month before I need to either drive this sucker 1000 miles across country (moving), or get rid of it. I know that a multimeter is something I should already own, and I plan on rectifying that tomorrow.

Where's the throttle body located? I suspect it's right under the air filter, but I'd like confirmation before I go to the hassle of d/c-ing a bunch of vacuum tubes, etc. Once I find the throttle body, is there a way to narrow down which element is malfunctioning, if indeed the issue is with the throttle? This truck is also a straight-up rustbucket, so it would be nice to know if there's any wires the could conceivably be causing this problem due to corrosion.

If I haven't clarified something that I should have, let me know. I can also take a pic of the engine if someone's willing to throw a big old MSPaint arrow on it to show my car-dumb *** where to find the knowledge I seek.

[-RJ-]

i would download the fsm of the car, or even the 89-90 years of a 240sx

Have you done the basic tune ups? Spark plug and wires, cap and rotor and 02 sensor?

[Rev_D21]

The TB is located right under the air filter. It could be the TPS, I have seen the symptoms you are describing be related to a failing TPS. Since the truck starts to act up when it warm I would look at things that function differently once the truck is warm, systems that either wake up or go to sleep once you have reached operating temps. Timing, Fuel Delivery, Wiring. You could have a really simple worn through wire somewhere.

http://www.nicoclub.com/FSM/hardbodyThat's the complete FSM collection and it should help. I have a 95 with a KA so if there is anything you need me to compare on my truck with what you are seeing on yours let me know.

[psilogen]

Thanks, both of you. I've started grabbing the FSMs, and I'll give it the basic look over tomorrow after work. The fact that it runs great until a pretty serious warm-up has me kind of at a loss, but I'll make sure it's not something simple before I start swapping out hundred-dollar parts.

[Rev_D21]

Keep us posted.

[psilogen]

I popped the air filter assembly off, and can now see into the throttle body. While doing this, I noticed that the MAF plug did not appear to be properly connected. Obviously that is not an ideal configuration. I've yet to check the ECU (still poking through the FSMs), but do you think the unplugged MAF would explain the problems, or should I poke around a bit more before putting it all back together and driving across town?

[Rev_D21]

It could have a bad effect on performance for sure. I do believe that Nissan programmed a default value for the computer to use when any sensor fails. The MAF being on of major importance I would imagine the computer didn't know exactly what to do.

I would drive it to see if the symptoms went away.

[psilogen]

Woah, okay. The truck got mad as hell when I plugged the MAF sensor back in and started it up. Ugly sound, engine rocking back and forth, some lingering smoke from the exhaust. It started up fine when I unplugged it again. Either the MAF is shot, or I have to reset the computer when I plug it back in (do I have to reset the computer when I plug it back in?). I'm still looking through the FSMs on the subject, but they are scanned images so it's pretty hard to search for text.

edit: found the diagnostic chapter in the Engine Control fsm. I plan on picking up a Fluke DMM asap, and testing the circuits, if they're fine then the MAF definitely has some problems.
Modified by psilogen at 8:27 PM 9/24/2008

[Rev_D21]

Sounds like you are looking in the right area.

[psilogen]

I'm having a hell of a time finding a MAFS for under $300 (that is about a quarter of what I paid for the whole truck). Are the DE MAFSes backwards compatible or is that too much to ask of engine engineers?

[Rev_D21]

I wouldn't count on it. There are some KA power Hardbodies in my area junkyards with the MAFs still in place. Have you tried any yards in your area?

[psilogen]

I've tried one yard so far, no luck, but I have some free time coming up.