Poor fuel economy, runs rich, goes into safety boost at WOT

[madkiwi]

Been chasing this issue for the last year, still not resolved. 1993 300zx TT, auto transmission with JWT Popcharger, JWT ECU and 13.5 psi jets, Z1 SMICs, Stillen cat-back exhaust. Everything else is Nissan genuine with the exception of hoses (because Nissan does not make them in silicone). I will not use aftermarket replacements in a car this unique...

Poor fuel economy, I average around 16 mpg around town. I don't really hammer the throttle, and yet have black soot on the lower edges of the exhaust within a couple days of washing the car. But when I get it on out on the interstate the engine goes into safety boost 75% of the time when I push boost above 5 psi. I can actually watch my Autometer boost gauge drop from 9-11 down to 6 psi while my throttle position is unchanged. Then I have to park, shut down the engine and restart to clear that, and sometimes it will happen again, sometimes not. Happens on hot days, and on cool ones.

Here is the stuff that has been done-

In 2013-
New knock sensor in factory location (code 34) -expensive fix
New O2 sensors (codes 33 and 53)
Replaced all the vacuum hoses (including a split hose going to the LH wastegate actuator)

This year-
Installed colder OEM plugs (NGK PFR6B-11B)
Replaced cooling fan and fan clutch
Replaced fuel pressure regulator and dampener
New fuel filter
Replaced LH turbo recirc valve (leaking to atmosphere) and all the recirc hoses with Z1 silicone
Replaced OEM intercoolers with Z1 SMIC set, including new silicone intercooler hoses

Tested, checked and verified the following-
Code 55 all the time, every time since replacing the knock sensor and O2 sensors last year.
Both O2 sensors switch at least 5 times every 10 seconds
Fuel pressure is perfect (35 psi at idle, 40 psi at -5" vacuum, 45 psi at 0, 49 psi at 5 psi boost, 61-62 psi at 10 psi)
Coilpacks perfect (all either 0.7 or 0.8 ohms)
Injectors perfect (1 at 12.0, 4 at 12.1, 1 at 12.3 ohms)
Balance tested with Datascan software, all cylinders functioning normally
CAS, TPS and MAF connectors all cleaned and repacked with dielectric grease
CTS went bad (was within specs before it died), replaced and verified the new one was to spec
Had a Motorvac treatment performed
Timing checked, it was 15 degrees BTDC
Car idles perfectly
Had compression test performed, high was 168 psi, low was 153 psi
Did boost leak test, but no audible air leaks (after tightening a loose clamp on the LH intercooler)
Verified that timing mark on crank pulley at 0 when #1 is at TDC
Checked CAS cam key for wear- looks new
Live at 3,200 feet, and use 91 AKI ethanol-free fuel

Plugged in a Consult connector and drove around with it plugged into my laptop. Recorded readings all appear normal, at idle my TPS is at 0.46 V, at WOT it is 4.02 V. At idle my injectors are averaging about 2.2 ms, at WOT with 10.5psi of boost (5,500 rpm+) they are 15.5 ms (71-72% of duty cycle).

When my mechanic checked the compression he used a borescope to look inside the combustion chambers. He said there was a little carbon, but not bad for an engine with 89,000 miles. One cylinder had some scratches in the wall, but that was the cylinder with the best compression, so not an issue. But I decided to start using Redline Si-1 fuel system cleaner, put one full bottle in with my latest refill, and plan on adding 2 oz per tankful for ongoing cleaning. The gas we have here in Montana may be up to snuff for AKI, but I have been told the detergent additives are either useless or in very low dosages. I am hoping this may make some difference.

Burns no oil. Tire pressures checked monthly, last time I had to add air was after she came out of storage in the spring.

The only anomaly I have is that when I tested the MAF per the manual, my voltage with ignition ON is 0.44 volts, supposed to be 0.8 (at idle it was 1.245, manual says 0.8-1.5 V). I cleaned the MAF sensor with MAF cleaner spray, no change. But at WOT (with the boost and rpms above) the MAF is showing 4.09-4.12 Volts, I have been told this is normal.

Anyone else have any ideas? Should I replace the MAF based on it being out of spec on one of the 2 tests (50% fail is still a fail, right?)

Mark
(cross-posted everywhere)

[Nicko500]

Did you tried to bypass Knock sensor with resistor ?

I had to do that to fix my safety boost issue, while car was running fine.

[Ace2cool]

MAF is nominal reading. .4V with the engine not running will make about .8V at idle.

I'm wondering if the knock sensor isn't picking something up and clearing before you can check it. Because a sudden loss of boost and not clearing until you stop the car and restart it definitely sounds like a KS issue to me.

[DCaff300ZX]

Quick question for a thought that just hit me reading this- have you done the switched voltage mod?
Reason I ask is the mod also has the side effect that it makes the ECU not save fault codes past shutoff- so if you have done it, it may partially explain why a shutoff "resets" things for no apparent reason.