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Constant enging hesitation / stumbling

(1990 Nissan 300ZX)
in Huntington, WV on March 21, 2011
I've had this problem for several weeks now. Car feels like it has a constant miss throughout the rev range. I have just done a plenum pull, installed new injectors, replaced injector and coil connectors, new temp sensor and cleaned the throttle bodies as I had planned to do this - no difference. I ran ECU diagnostics with code 55 recorded with engine off - everything OK. With engine on recorded left side O2 sensor is faulty.
I also checked the voltage on the MAF and got a reading of 0.48 volts with key on (spec 0.8V). With the engine running the reading was within spec at 1.29 volts. I cleaned the MAF with MAF Cleaner and also cleaned connectors and terminals. Re-tested - no difference in values.
Voltage on Throttle Position Senser OK at both connectors - spec in the manual states approximate values for a couple of wires - only discrepancy I got was 0.86V wher it states approx 1V.
Also timing belt etc was replaced and has done approx 50,000Km (half way throuigh its life cycle).
I am planning to change the O2 sensor this weekend, easier said than done.
Any help with this issue would be appreciated as I am becoming frustrated - do the figures recorded mean I need to replace anything.
Displaying 2 answers & 2 comments
Popular Answer
That engine has coil over plug ignition system. I have seen bad air flow meter need to check it on an oscilloscope but electrical connectors at the injectors can be a problem especially if they were changed. Have replaced lots of injectors on those. Make sure you have no vacuum leaks the hoses are hard to get to at the rear and sides of the intake manifold. Unlikely to be O2 related but watching the voltage may indicate lean or rich conditions. Have you good diagnostic procedure low charts? Wiggle the wiring harnesses in the engine compartment with the engine running if the engine changes speed or runs rough you have bad connections.
on March 22, 2011
The problem existed before any of the above work was done. The replacement has not improved or made it any worse - stayed the same.
All vacuum hoses were replaced less then 6 months ago with silicone type, PCV valves about 12 months ago - have not detected any leaks, but will look again just in case.
For diagnostics I followed the Nissan manual. Will get the MAF checked.
on March 21, 2011
first, did you do a compression check?? how about basic stuff like plugs, wires, cap and rotor? maf really cannot be cleaned. the carbon is burned on to the wire and no cleaner can remove it, the only answer is to replace it if it is out of spec.
i would hold off on the o2. just because it has a code does not mean it is bad. it just cannot control the fuel mixture correctly because of the missfire. if you fix the missfire, the code may dissapear and the o2 can do its job. replacing it will definitely not fix the miss.
once again, start with basics before getting any more involved.

on March 22, 2011
Had a mechanic look at it initially who checked the basics. He didn't have the program to run a diagnostic. Has new plugs, has spark, coil packs are fine, compression was good.
Will go through methodically over the weekend.
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