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The hesitation or stumble problem I've dealt with on my '93 NA can best be described as the feeling you get when the A/C kicks on even though you know the A/C isn't running. If you know your engine well you can feel it's being robbed of power. It's a flat on the torque curve. This issue was most apparent at lower rpms (1,000-3,000) and would usually wash out above 3,000 rpm (or at least I couldn't perceive it above that speed). Idles smooth. It wasn't effected by hot or cold engine temperatures. When going from a decel back to accel you were never sure if you'd return to 90% power, 95% power and fluctuating, or 100% power but it didn't happen only during turn-arounds, it happened from a stop light too. It happened at full throttle and part throttle but not always, just sometimes (when it felt like really pissing me off). This was unsettling if you're a person who's concerned about the driveability of the car especially in hard corners. What was frustrating was that almost no one could perceive the issue except me. People were thinking I was chasing a ghost saying things like, "Taylor, I think you should learn to live with it" and "Ummm, I - don't - feel - anything!" People thought I was just loosing it. Well now the problem's fixed and I can't be more proud of my decision to never let it go. Here's a synopsis of what I dealt with over the past two years: 1. I replaced the passenger side O2 sensor based on bad results of the crude test outlined in the FSB -no benefit whatsoever.
2. Bought ConZult in hopes of better diagnosing the stumble hesitation problem -BIG MISTAKE! CONZULT SUCKS!!!
3. Bought upgraded ECU with specially designed consult interface in an attempt to work around the ConZult NA connection issue -that didn't work either. Bottom line with ConZult is the software is not robust. It's very nice software when it works on a TT (sometimes intermittently depending on your vehicle), but CONZULT ABSOLUTELY DOESN'T WORK ON NAs... period! 4. Built intake plug and pressure tested the plenum for vacuum leaks. Recognized the EGR valve stem leak is expected, otherwise no intake leaks.
5. Vacuum test indicates a very slightly swinging needle on an old gauge. Buy a new gauge which indicates vacuum is A-OK. I believe the test was inconclusive.
6. Diagnosed the #2 new style pintleless injector was defective through FSB ohm test. And you thought the new style injectors were immune to the problem?!
7. Pulled and dremeled the plenum and replaced all six injectors with new OEMs, new PCVs, and did TB bypass -engine runs noticeably smoother at idle, power is about the same, hesitation still exists.
8. Hesitation issue persisted after VTC solenoids and EGR were turned off.
9. Desperate, I questioned the timing belt setting by the previous owner. Do timing belt job. Timing belt was A-OK so I rounded out the Timing Belt job with the full 120K maintenance.
10. Compression test: 176 / 179 /182 / 179 / 180 / 174 (video link of an intermediate result). Not the greatest but they seem reasonable. #6 being only a slight aberation.
11. Rich at RDZ diagnosed and replaced the driver side O2 sensor -major increase in performance! I have no explanation for this conspicuous power benefit, but the hesitation issue persists.
12. Rich also swapped out the ECU, ECU program chip, and CAS with no impact on hesitation.
13. I continue looking over all the sensors in the engine now using ECUtalk(link) + Blazt cable and friends' borrowed laptop. The only potential issue is a lot of timing scatter that Greg (SpecialtyZ) helps me diagnose as the CAS misalignment issue recently published by JWT.
14. Middletown Nissan's owner, Dave, graciously offers his top mechanic, Scott, who connects the factory Consult to my car with ease! Scott recognizes my timing concerns but can't come up with an explanation.
15. I borrow Agner's oscilloscope, become intimately familiar with every detail of its operation and the NA's ignition system (video link), culminating with me driving around and trying to diagnose the problem real time with an oscilloscope.
Yes, that's right, an O-scope and, yes, recording real-time data (video link).
16. In the end the oscilloscope is inconclusive, but if I had to hedge my bets I concluded there was nothing wrong with the apparent excessive timing scatter at the lower RPMs.
17. Not finding anything wrong with any engine sensor data I decide the problem must be mechanical and start pulling apart the engine in January with the intent of completing a valve job. 19. Take the heads to Precision Automotive machine shop in Simsbury, CT who does absolutely nothing but sits on them for a week. Frustrated, I take back the heads and based on BiggTDog's strong recommendation take them down to Mike Roger's Machine Shop in Branford, CT.
20. Mike recognizes that a #6 intake valve is bent explaining the slightly lower compression number on that cylinder. Mike also recognized the strong misalignment of the CAS relative to the exhaust cam, installs the JWT CAS pin, and performs the necessary realignment procedure. We have no idea how only #6 intake could have gotten slightly bent. You have any ideas??
21. After weeks and weeks investing every spare minute to put the engine back together I finally fire it up for the first time on April 1. No leaks, no fires, no bad noises, and engine idles smoothly. I've driven a car for 3 days total and conspicuously the hesitation issue is gone. It's smooth power from 1000 rpm up to 6000 rpm! Finally! And what a relief! This saga is OVER! Lessons learned? Based on my belief that the problem was the #6 intake valve the ONLY way to recognize this was the compression test: #6 was slightly lower but well within the FSB tolerance. Therefore I believe the FSB differencial tolerance band, 14 psi difference between the highest and lowest compression number, is too large. If you do a compression test and your engine is on the ragged edge of 14 psi even if the vacuum test is ok, it's a sign! Take it seriously! It could be the reason your car isn't running smoothly, and more importantly it could be your ONLY indication if something mechanical is wrong! A big thanks goes to all those folks who've helped me contend with this and other Z issues over the last two years including Agner.S(CT), BiggTDogg, DVDBURN, BPC23, Greg(SpecialtyZ), Ash, Mike Rogers, flash-to-pass, BDE, Keith, Art S., Peter, Gordon of Witchhunter.com, Middletown Nissan (Dave and Scott), and others of CTZCC, you guys are great!
Zarcasm(Hfd, CT)
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14:14:10 04/17/11
