Nissan V6 engine runs fine until 2800 RPMs

[scott14450]

For all you motorheads out there, here is a question that has left me and many mechanics absolutely baffled...

I own an '88 Pathfinder, V6 (throttle body injection), with 110K miles. I drive it daily, and it runs fine. Overnight, the following problem surfaced - the engine sputters at 2800 RPMs and wont go past that mark! It idles real nice runs absolutely fine before it reaches that mark. It doesnt matter what gear I'm in, what kind of load its under, the position of the throttle (which rules out the throttle position sensor, right?), the engine runs fine exactly until it reaches 2800 RPMs, and then begins to sputter and jerk until I lay off the throttle. (Although it seems to have a little less power and might miss while going up steep hills)

There are no vacuum leaks I've found, no obstructions in the exhaust (I temporarily disconnected the cat, and the problem doesnt go away) From a visual inspection, the injectors are getting plently of fuel at the point of sputtering. I changed plugs, wires, and cap, without solving the problem. This model, to my knowlege, does not have any type of rev limiter. What could the problem be?

Although, by use of the throttle, the engine wont go past 2800 RPMs, I was able to get it past that mark by downshifting while going down a steep hill. Here's what happens: the engine still runs above 2800 PRMs, though pressing on the gas pedal has no effect, save a slightly different sound from the engine. As the engine slows to 2800 RPMs, pressing on the gas will cause the violent, on-again off-again jerking, until it goes below the mark and runs fine.

I am at my wits end! If anyone knows a quick fix, or if this is a common problem with the old Nissan V6s, please let me know. Thanks!

Scott Roney

[Kurtdg19]

Try doing a compression check on the cylinders and see if their where they are suppost to be at or around. If thats all right, I would pull out the timming gun and make sure the timming advances over the 2800rpm mark.

[RandomTask]

Even though the injectors are getting fuel, they may not be getting enough to pressure for the higher RPM's. Try switching the fuel filter, it may be clogged. If this doesn't do it, you can either install a pressure gauge to see what type of pressure you're running, or you could just switch out the fuel pump. Good Luck, let us know how goes.

[curtis73]

This sounds fun. I would say its a fuel problem, except that you say it occurs at any load. I would say that its a compression issue, but it would change a little with load. Typically with a compression issue, the pressure differences relate to throttle position vs. VE, so the fact that its absolute at 2800 makes me think its not a compression issue.

I think its something more simple. Its gotta be something with the engine management. Either the ECM is dying (a generic answer) or its one of the peripheral sensors. Crank position sensor, MAP sensor, or something else that might do it.

What it boils down to for me is (since its stuck at 2800) it sounds like the timing is only advancing far enough to support that much piston speed. That's the only thing I can think of that would make such an absolute cutoff point.

Look to the ignition components, sensors, and ECM. That's my guess.

[scott14450]

So I put a timing light to the '88 Pathfinder. And here is what is happening: at low RPMs, the timing is fine. As the RPMs increase, the timing naturally advances, but it keeps on advancing until it is way out of range (of the tickmarks) and the engine sputters at 2800 RPMs. So, it seems I have a timing advance problem, and I would assume the problem is either with the mechanical advance in the distributor or the ECU. How do I find what exactly is the cause? And if this is the case, what is the best (and cheapest) way to fix? Can I just replace the whole ECU with another Ive found in a junkyard?