Pathfinder Engine Stutter

[ettore]

I have a 1993 Pathfinder with ~305,000miles on it; 295,000 of them were not done by me, and I have no previous history on the truck.

When I got the truck, it ran perfectly well; pretty poor gas mileage, but on par with what I would expect from the truck. However, recently it started with a new problem, and it all started on a day it went from around -4°C to +10°C overnight, and probably rained.

If my truck is cold, it will always run fine whether I am driving for 2 minutes, or 2 hours. However, if I turn it off, go get groceries or something, then go back to the car, it will misfire. It will misfire until I can get the RPM up to about 1,800RPM, then it runs fine. However, the next stop I get to, it will misfire again. This keeps up until I have driven for about 4 minutes, then everything is happy again. Power seems about as peppy as I would expect the Pathfinder to be.

Since it runs fine on the first go, I assume it isn't the wires. I pulled all but the PITA plug #6, and they were all pretty ok (.38-.4 gap instead of .32 to .35, so I squashed em down to proper gap). The plugs all looked pretty ok condition wise, however they aren't the OEM plugs. The only reason I haven't done plug #6 is because I need a special extension since plug #5 (closest to #6) took a 1m long breaker bar to remove, #6 is a pipedream that it will come off with that little Nissan tool.

I am not rich, so throwing random money at the truck to try and fix what is VERY likely a simple problem is not in the cards. However, if I could narrow it down to one of a few problems, I'd be all ears.

I have access to a SnapOn scanner, timing light, compression tester, oscilloscope, most other general tools, and I have a pretty firm grip on mechanical stuff.

[smj999smj]

I would first do a fuel pressure leakdown test to rule out the possibility of one or more injectors leaking down during a hot soak. This could cause the plug(s) to get fuel fouled at start-up. If the fuel leakdown test checks out ok, chances are it's an ignition related problem. I don't recommend any spark plugs other than NGK in a Nissan engine and the original equipment plugs were either standard copper or V-powers, neither of which is expensive (about $2-3/plug). A lot of ignition problems tend to develop at the distributor on these engines, including bad distributor caps & rotors (stick with genuine Nissan), play or binding in the distributor shaft bearing and bad position sensors. Being that it happens after a hot soak, I would not be surprised if that's an issue. It would be hard to tell without being able to duplicate the problem and seeing the ignition patterns on an oscilloscope. I'm assuming you already tried checking for stored trouble codes and didn't find any.

[94_pathy]

Check your vacuum lines, especially around back of engine. Sounds like problem I had and was a line in the back that had come off.

[smj999smj]

Check your vacuum lines, especially around back of engine. Sounds like problem I had and was a line in the back that had come off.

Yeah, there is the vacuum line to the fuel pressure regulator back there. If it splits or falls off, it'll cause the fuel pressure to run at unregulated pump pressure, causing an overly rich mixture.