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Discussion Starter · #1 ·
My truck sprang a P0304 (#4 Cyl Miss) code about a year ago. Following the path of lowest cost/easiest fix/should do it anyway first, and with lots of help from various Nissan forums, I went through a process of elimination: Complete tune-up (plugs, wires, coil, cap), no change. Checked compression – 175/173 PSI with #4 highest, OK (spec is 149-178, not bad for a 240K engine). Changed fuel filter (nasty), checked fuel pressure (in spec) and fuel pressure regulator (OK). Pulled intake manifold inspection plug for EGR at #3 cylinder, passage was clear. Checked vacuum lines for EGR – they were somewhat rotten, and got replaced. Verified EGR vac line routing.

This left the last thing to try – change the intake manifold gasket. I had put this off as there was no indication that it was bad, a propane test revealed nothing, and there was no sound of intake leak. However, because this is a common failure on this engine, I finally tackled the task. The existing gasket was sound, but I was glad to have done the R/R. I found two coolant lines that I did not even know existed (and replaced in the process), cleaned up several nasty parts, replaced the steel heater line (about to rust through), and fixed a rusted vapor return line and broken body ground (note to Fronty owners, the cabin A/C drain drips right on the vapor return pipe, and this pipe is not PVC jacketed. Mine had rusted through). Also found the air cleaner wingnut I had lost several years ago. None of this did anything to solve the P0304 code problem. I tried Seafoam, this made the problem even worse. Now instead of a steady CE light, I had a flashing one any time I was overrunning the engine or running cruising on level ground above 55MPH.

Finally, today, I gave the EGR system a second going through, verifying the hose routing was OK, all the valves functioned properly, and the vac lines were not leaking or pinched. I then checked the EGR passage through the plugs in the top of the intake manifold. The passage running front to back on the manifold (between the throttle body and the engine block) has four plugs in the top of it and one in each end. I removed the plugs for cylinders 1, 2 and 3 – the passage below the plugs was clean (I did not worry about the #4 inaccessible plug – it had to be open if I was on the right track – see below). I rooted around a bit with a screwdriver and found that there is a drilled hole from this passage into the intake runner below it (this hole is the real reason for the plug – machining access to drill the hole). This hole was plugged on all three cylinders. I cleaned out the hole and the area around it (pretty clean otherwise) and vacuumed out what I could, verifying as I did so that the passage from cylinder #1 cleanout to #3 cleanout was open. I shot some throttle cleaner into each hole and closed them back up. I put another shot of throttle cleaner down the throat and started her up. Started a little hard and sputtered for a minute, but after she figured things out, ran perfectly smooth. I was sure enough that I had found the problem that I left the CE light on – and sure enough, on the fourth trip it (finally!) failed to come on.

What was happening was that the recirculated exhaust gas was swamping cylinder #4, as this was the only passage that was open. Opening the other passages reduced the EGR level at #4 and eliminated the miss.

Some things to note – the hose from the EGR solenoid valve to the BPT valve has an orifice in it – suggest getting an OEM part (not expensive) to replace this. Also, there is a short, hidden hose under the BPT valve that connects the diaphragm on the BPT valve with the EGR passages via a stainless steel tube. There is also a heater supply line under the manifold that had a loose clamp on it – it is only accessible from the top with the manifold out, or from the bottom with the starter out. Finally, there is also a short horseshoe bent hose from the bottom front of the manifold to the water outlet neck.
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