I hope someone can help me with a problem I'm having with the '92 240 LE coupe that I recently purchased (my first 240). While cruising along at a fairly constant 40 to 45 mph (70-75 kmh), in 4th gear at around 2500 rpm, the car surges as if I was pumping the accelerator a bit. It is not to the point of being uncomfortable, merely unpleasant and annoying, though I have (a couple times) experienced a jerk as opposed to the more common 'surge'. The ambient temp is about 30 to 25 deg. fahrenheight (0 to -5 deg C), dry weather.

I recently adjusted the idle, which was consistently at around 1500 rpm while in neutral at a stoplight, via the screw at the end of the throttle linkage near the air intake butterfly. Air filter is clean (new) and a recent oil change. Using 91 to 92 Octane (winter) gas from Petrocanada (usually).

Is it possible that this engine likes to rev higher while cruising (ie. in third at the above speed) and plugs are becoming fouled (yeah, I guess I'll check them if this persists but it's COLD)? Could it be that there is more to adjusting the idle than just screwing in the screw a bit (mixture also needs adjusting or?) ?

The Fix

I took the car to the local Nissan dealer. They hooked it up to the computer and couldn't find anything wrong with it. I decided to pay for an hour worth of diagnostics/time and they eventually did find that the 02 sensor was not working quite as well as it should - apparently the signal was somewhat delayted causing the surging. Anyway with the new sensor the car is running as it should. Cost Sensor $140, labour $75>> (1.5 hrs) Cdn $.


check procedure

To check your O2 sensor. Here's the procedure for all 1991's and up:
1 - Turn on ignition switch (do not start engine).
2 - Turn diagnostic mode selector on ECU fully clockwise.
3 - Wait a few seconds.
4 - Turn diagnostic mode selector fully counterclockwise. You are now in MODE-II.
5 - Start engine and let fully warm up (temperature up to normal).
6 - Run engine at 2000 RPM or higher for about two minutes under no load.
7 - Check that the inspection lamp (red LED) on the ECU goes on and off more than 5 times during 10 seconds at 2000 RPM.
If the O2 sensor checks out OK then check the EGR valve.

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Re. 2

Re your surge problem....same problem has existed with my 89. Check for vacuum leaks throughout the system. This would include ALL vacuum lines AS WELL AS valves- EGR valve, BPT valve etc. This is basic engine diagnostic stuff a good tune up facility should check on the first pass. You could check for vacuum leak between the EGR and BPT valves...this is where mine was leaking. I suspect this, since your idle speed is very high- and it's very unlikely that mechanically it moved out of adjustment...I think you have a hole in a line. If this is the case, you'll have to go set everything back to where it was...Don't be surprised if it won't want to idle well once you plug a hole!

On my car, I think the diaphram in one of the valves is bad, but I haven't had time to check further. Still has a slight stumble between 2500 to 3000 RPM, but it runs much better, especially when cold. Hope this helps!

Re. 3

Check the connection to yr MAF (it's right downstream of your air filter box if your intake is stock. You'll see an aluminum "throat" between the  filter box and the plastic tube running past the radiator to the throttle  body -- you should see a snap in wiring connector running to this. On  mine (and apparently on others' 240's too) this has come loose and caused  caused mine to idle wildly, up and down from zero to 2000 rpm  w/occasional stalls at stoplights... Fiddled with the connector and  instantly cleared things up...