| 94 300zx NA has 220K miles on it. Runs well when moving and accelerating. Does not burn oil. But will not idle well or hold a constant RPM. The Idle issues and constent RPM issues at 2000 RPM (will not hold 2000RPM or 750RPM) results in failed California smog static tests. Actually the test was aborted as they could not get it to stay in the test RPM range after trying about 20 minutes. We finally got it to complete the 30 second tests but since it took so long it wanted a 3 minute test and we could not attempt to do that. It took about 20 trys each to get it to pass the 30 second tests! Other than the idle type issues it runs quite well. Spins tires in 1st and 2nd and sometimes third unexpectedly when wet out. Accelerates smoothly in WOT position or almost WOT.
I have cleaned and changed out many things listed below but right now I am trying to figure out a simptom with the Throttle Position Sensor (TPS).
If both the TPS switch and sensor connectors are connected to harness it does not idle well or hold a high 2000 RPM consistently. It varies about 500 to 750 RPM. At idle (around 800-900 RPM it pulses down to 400 or so 3 to 4 times and back up to 800 or so, holds there 10 -15 seconds and then pulses back (drops out) down to 400 or so again. See video on youtube: "94 300zx NA unstable Idle with Throttle Position Sensor connected" [ http://www.youtube.com/watch?v=Zj80B20EAqY ] If I disconnect both connectors for TPS SWITCH and Variable Sensor it idles around 1400-1500 fairly steady. Not a 500 RPM dropoff. See youtube video: "94 300zx NA stable Idle with Throttle Position Switch and Sensor disconnected" [ http://www.youtube.com/watch?v=yfbWoCRI7f4 ] If I only disconnect the TPS variable Sensor (varies from .45v at idle to 4 volts at WOT or so) it will idle at about 750 consistently and does not drop out of have the 500 or
so RPM idle issues. See youtube video: "94 300zx NA stable Idle with Throttle Position Sensor disconnected - but unstable connected" [ http://www.youtube.com/watch?v=Pl4NNjJaj9Y ]
SO MY QUESTION.... What is making the idle drop out and be so unsteady when the TPS Sensor cable is connected?
What changes when that TPS Sensor is connected? I have checked the voltage from it at idle when it drops out and it appears to be still at .45 volts so it should not be effecting the ECU. But with it disconnected the ECU has to use the TPS "Switch" which seems to idle fine at 750. So something is different and making it do something to cause the idle drop outs/ hunting. Any Ideas? What I have done so far to try to resolve idle and flucuating RPM issues:
- Changed Spark plugs to NGK Laser Platnum PFR5B-11 but did same thing with older NGK FPR6G-11 all burning light brown.
- Change out Fuel Filter to WIX 33022
- Changed both HO2 Sensors to Bosch 13091 (one was throwing a 53 cold for passenger side and other failed 2000 RPM test, did NOT plus 5 times or more in 10 seconds.
- Cleaned Jim WOlf Cone Air Cleaner
- Cleaned MAS Mass airfolow sensor with MAS Spray NOT Carb Cleaner. FOrgot to try to ohm it but it will not run withit disconnected so it appears to be working.
- Cleaned both Throttle Bodies with Throttle body cleaner and brush.
- Removed and Cleaned Idle Air Adjustment unit IAA. Valves were dirty and connections coroded but cleaned itup and did not make a difference except that it will idle without TPS on now. [ http://home.swipnet.se/e-solutions/IdleTech.html ]
- Ohmed out all Coil Packs. Them are all at .9 OHM on my meter.
- Checked that all Injecters are clicking the same as each other. They were all normal (I guess)
- Tried to check for vacuum leaks. Not hearing anything. If I pull off a small hose like on passenge side (11 - air duct to air galley) it does not seem to make any difference. If I remove the elbow hose to IAA (larger one) it kills engine. I tried spraying some Propane around some but did not notice any spikes.
- Verified that EGR goes up with finger and it starts to lower idle some.
- checked timing at a #1 plug and at Loop on Power Transistor unit PTU both the same at a bit over 15 so I moved CAS to 15.
- Vacuum hose between IAA and Air regulator is new, os is Air regulator to Balance tube. and Fuel Damper and Fuel Pressure Regulator to balance tube are new hoses. Other:
- tried to check PCV vales but the hoses are all hard and they would break if I take them off and I do not have new hoses yet. So the PCV or hoses or both may be bad. Most hoses
are old and hard. - a 300zx Z mechanic did my CLutch, presure plate etc, Timing Belt and fuel pump and all seals and stuff about 40 thousand miles ago. He also changed out the Coolant Temp Sensor - Most electrical connections are brittle or broken but still connected and seem to function. (Coil connectors, Injector connectors, various sensor connectors. I have been
cleaning them up as I go.
Websites I have referenced so far in my search for answers:
How to solve engine hesitation and stumble problems on the 90+ By Steve Chong [ http://zhome.com/ZCMnL/PICS/hesitation/hesitation.html ]
CURING ERRATIC IDLE (Part 1) [ http://home.swipnet.se/e-solutions/IdleTech.html ]
z32 1992 area on [ http://www.twinturbo.info/ ] Mine is NOT a Twin Turbo.
Finding Vacuum leaks [ http://home.swipnet.se/e-solutions/Vacuumleak.html ] I did not try this yet. Idle Air Adjusting (IAA) Unit Replacement [ http://twinturbo.net/nissan/300zx/tech/iaa/iaa.html ]
Throttle Body Cleaning [ http://twinturbo.net/nissan/300zx/tech/tb/tb.html ]
ECU Diagnostics [ http://www.ttzd.com/tech/diagnostictech.html ]
60K Maintenance [ http://www.ttzd.com/tech/60ktech.html ] Good info
Any ideas on what causes this unstable idle when the TPS is connected?
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19:24:15 01/19/13
