hesitation under load - 1000-2000 RPM

[cbcurtis]

My name is Chris - newbie.

I have a 91 300zx. The first issue was what I think a stuck valve. Had throttle body service done and it was fixed. I think I changed the plugs for this without success.
A year later I started getting hesitation and runs rough at medium throttle and low rpm . If you are in a high gear and heavy on the throttle it will pop. This problem has been going on for a couple of years without change. I tried the canister purge test, intake leak and O2 sensor tests per the service manual without success.
A few friends suggested it may be the EGR. I wanted to test the injectors as well so I decided to remove the plenum, delete the EGR (want to get rid of things it doesn’t need anyway) and check resistance of the injectors. Found one bad injector (>50 ohm) and two going bad (~16 ohm). EGR looked good. So at this point I thought it was the injector – I was wrong. In fact I thought it was worse when I got it back together with new injectors.
PCV valves have suction at idle – pretty much the same on both sides.
Squeezed the air regulator hose – no effect.
Checked fuel pressure: it was a few psi high both at idle and when regulator under vacuum - cheap gauge.
Removed the timing sensor and rotated clockwise: all injectors fired and sounded the same.
To check for intake leaks I tried both putting the intake under low pressure and used brake cleaner at idle– none that I could find.
I decided to buy the data scan software. At first I only had the left O2 sensors on the display and it was going rich to lean to rich in what I thought was the standard. Did a power balance test: OK.
Put both O2 sensors on Data Scan display: let side switched between lean and rich faster than left.
Replaced left side O2: right slower than left – in fact, it didn’t switch much at all staying on lean with occasional quick burst to rich.
Replaced right side O2 sensor. I thought: No way this won’t fix it – Wrong Again!
Now the engine seems to run slightly better but still has the hesitation. At low RMP and heavy throttle the O2 sensors are reading max lean and there is hesitation. As the RPM comes up and cruising they do the fast switching. At higher RPM they read rich at heavy throttle. At least they both read the same!
What is new since troubles started:
plugs (Bosch, 2 years but low miles), don’t remember if I changed these prior to after this problem started.
injectors
EGR removed
O2 sensors
I intend to rig up a fuel pressure gauge to read while driving – maybe the fuel pressure drops for some reason but unlikely – as the rpm comes up and the duty cycle increases there is no detectable problem.

I have a compression tester but you can only tighten it in the plug hole by twisting the hose. How tight does it have to be and how do you tighten it. To me, tightening by twisting the hose doesn’t seem to be enough. On the one cylinder I have tried I got 96 psi so of course I suspect the gauge is not tight enough.

So there you have it. A confused Z32 owner. Help! Is there something else I can do? Thanks!

[t.mcginley.jr]

plugs (Bosch, 2 years but low miles)

That's one of your problems right there, you need to use NGK PFR6B-11 or the NGK PFR6G-11 for NA cars and NGK PFR6B-11B for TT cars. They are pretty specific plugs, but running anything else tends to cause problems.

Also, what brand of O2 sensors did you use?

And I'm assuming you're still running old style square fuel injectors, which tend to fail pretty quickly with todays gas. The ethanol just eats them up in a hurry.

Some other things to check/consider:

Clean the MAF with MAF cleaner or electrical contact cleaner
Change your PCV valves (get OEM) if over 60k miles old
Adjust your TPS if its not already (TPS adjustment)
Change out your coolant temp sensor (http://www.z1motorsports.com/product_in ... ts_id=1723)
Ohm test your injectors again

[cbcurtis]

O2 sensors are bosch.
Yes, new injectors are standard - Nissan said they were updated to accommodate today's ethanol blended fuels.
I did clean the MAF - two different occasions.
I will change the PCV valves.
Adjusted TPS (though not the hard idle switch) and looks to be idling nicely - will check again, both this time.
Coolant temp looks good - is 170 fully warmed correct? At any rate - intend to check the resistance per the service manual.
Can I check the injector resistance at the ECU? - no way I'm pulling the plenum unless necessary! I checked them before installation.

Mechanic said check compression. I did a power ballance check so I don't suspect problem there. He just doesn't want me to throw good money on bad.

I'll let you know how it all flushes out this weekend!
Thanks,

[nissanfreak12]

All this work done, and not one word about a timing belt. With a pop in the higher RPM's, that makes me think of either skipped tooth on one of the cam gears or a vacuum leak.

When you say idling nicely, whats the idle at?

Another issue that plagues z's are corrosion at the connectors, it will have green looking crud. How do the connectors look? Which then asks another question, how does the harness look? 22 yrs old, can cause some issues. That is something to think about.

[cbcurtis]

I didn't get very far this weekend. Tried checking compression. Was depressed when I got 70/60/90/10/60/20. Put oil in cylinders got 160/180/210/90/190/200/185. I was pretty bummed at first. However, thinking back, oil was getting past the O rings so I don't believe those are accurate. Any recommendations on a pressure gauge?
You are correct - I have not checked timing. Had a new belt put in 30k ago by previous owner. No problems until 10k ago. However, it does not pop at high RPM - only low RPM at high throttle position. Engine seems strong otherwise. Took it to 100 on highway and had a lot left.
No corrosion on injector connectors. Previous owner put in a new injector harness. Pretty sure he wasn't lying. The harness has the EGR temp connector but its not CA model and it looks new.

Looking at it on Datascan, the time where it is demonstrating a problem the O2 sensors are staying on max lean during the high throttle position so I think the mixture is somehow getting fouled. Unless lean is normal at low RPM and heavy throttle I have to find out why and fix. It doesn't seem logical that the plugs would cause this. PCV sound more reasonable. Temp sensor on Datascan and the instrument gauge look in sync - 187 F during highway cruising with AC on.

What would you do?

[cbcurtis]

I should clarify - Intend to change the plugs. The more I look around the more I see agreement on running only the NGK.

[cbcurtis]

Turns out it was the spark plugs. Car is running nice now! What a relief!

[t.mcginley.jr]

Good to hear! Like I said, these engines are pretty picky when it comes to spark plugs lol