[loched]

I have a 1991 Senta SE, manual. At idle after warmup it stumbles between about 500 and 800 rpm. It idles normally, stumbles down, jumps back up, then back to normal....repeat on a loop.
With slow normal acceleration its absolutely perfect, but with any hard acceleration it bucks until it gets up to speed or I ease up on the accelerator.

Any ideas as to what would be causing these issues would be welcome, thanks

 

[rogoman]

I have a 1991 Senta SE, manual. At idle after warmup it stumbles between about 500 and 800 rpm. It idles normally, stumbles down, jumps back up, then back to normal....repeat on a loop.
With slow normal acceleration its absolutely perfect, but with any hard acceleration it bucks until it gets up to speed or I ease up on the accelerator.

Any ideas as to what would be causing these issues would be welcome, thanks

Are you getting a "check engine" light? If yes, perform an ECU code readout to see what code is being set.

The stumbling is a misfire. Several things that could cause misfires:

• Is the misfire occurring on any particular cylinder? What brand of spark plugs are you using? You should be using OEM NGK plugs; other brands such as Champion or Bosch many times cause driveability problems in Nissan engines.
• The cam position sensor may be marginal.
• Incorrect fuel pressure. Tee-in a temporary fuel pressure gauge between the fuel feed hose and the fuel rail. The readings at idle should be as follows:

• with vacuum hose connected to the fuel pressure regulator: 36 psi
• with vacuum hose disconnected from the fuel pressure regulator: 43 psi

• There may be a major intake system vacuum leak. To check the intake system for a vacuum leak, attach a vacuum gauge to a full vacuum source. With the engine fully warmed up, the reading at idle should be 18 - 20 InHg. At 3,000 RPM, it should be 21 InHg. If readings are under 18 InHg, check the intake manifold nuts to make sure they are tight. The gasket may have failed; spray a water mist at the gasket to see if the gauge reading changes. Also check the intake plenum bellows at the throttle valve and at the MAF for cracks or loose clamps.
• Dirty fuel injector(s). Run some good injection cleaner, like BG products 44K, through the system; give the cleaner about a week or two to do it's job.

 

[loched]

A little slow on the update, but all is well now!
Turns out, the bucking was caused by spark plug gap that got way to large. Replacing the cap, rotor, plugs, and wires eliminated the misfire at idle....best guess, since the misfire was non-existent at higher rpm, the problem was probably the cap or rotor...more spins, more contact, less misfire