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Discussion Starter #1
I've been through the ringer with this car. Bought it from my brother 3 years ago (THANKS BRO) and now have lots of $$ in repairs. This issue has bee ongoing and never have found a solution.

VEHICLE: 2001 Sentra GXE w/Auto Transmission

PROBLEM: Occasionally misses just a bit when idling at a stop. Once up to highway speed, it will start to miss badly and shake the car, almost like the motor mounts are bad (they aren't), but only at around 45 to 50 mph, on occasion at higher speed, and not when accelerating, just when cruising at that speed. NO CODES.

WHAT HAS BEEN DONE: Cam sensors have been replaced (it had actually died on the road and this fixed that problem). Spark Plugs and Coils have been replaced. New shocks/struts. All new tires (balanced of course). MAF replaced. Reliable mechanic had for a day and a half and couldn't get a read on it - not sure if they just weren't getting a code or if they did further tests. No coolant/oil mixing.

Any ideas are appreciated!

John
 

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One of the first things to do is perform an ECU code readout with a portable scan tool to see if any fault codes are set. The tool can be purchased at most auto parts stores. Post the actual codes here on the forum so that we may be able to help you further. If there is one or more fault codes set, they can help point to the malfunction. If you have a copy of the FSM for your vehicle, the code readout procedure is described there along with a listing of codes. You can download a copy of the FSM sections from the Nico Club web site; here's the URL: http://www.********.com/FSM/ (replace the asterisks with "nico club" spelled as one word). The section EC.PDF is the one you need to read.

At this point you'll have to determine if there's an ignition or fuel delivery problem:

* Testing fuel delivery:

Tee-in a temporary fuel pressure gauge at the output side of the fuel filter. The readings at idle should be as follows:
- with vacuum hose connected to the fuel pressure regulator: 34 psi
- with vacuum hose disconnected from the fuel pressure regulator: 43 psi

One or more fuel injectors may be dirty. Run some good injection cleaner, like Techron or Redline SL-1, through the system; give the cleaner about a week or two to do it's job.

* Testing ignition:
Pull several coil packs to test; use a spare spark plug in the coil pack to test; ground the plug base with a jumper wire to the engine block; see if you're getting a spark while trying to start the engine.

* 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. A good place to connect to is the charcoal canister. There should be three hoses connecting the carbon canister. Two of the hoses go to a purge valve that's built into the canister. The second larger hose off the purge valve should be your vacuum line. 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.
 

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Discussion Starter #4
After a couple more days in the shop it has been narrowed down to a bad valve or valve seat. Not gonna happen on this old car. I'll just drive it until it dies.
 
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