Hey guys new to the forum but I’ve been searching here thoroughly to try and get my new to me Z running right, I’m gonna share my long winded story in hopes that someone here can give me some things to check out
First off the situation, I bought a 1990 300zx tt for a great deal. However the engine has something up with it. The gauge shows I’m only getting to about 6-7 psi boost at full throttle (previous owner claims it has boost jets to bump it to 12 psi over the stock 9), at ~3000rpm you can feel the power fall off and will not go over 4000rpm and starts jolting and cutting out. I’m fairly certain is running very rich as it starts blowing a lot of black smoke once the power falls off
Sitting at a stop if I give it very little slow throttle I can get it to rev out but whacking it open gives me the same 4000rpm limit
The last few days the idle has seemed a bit rough and steady cruising you can here the engine missing or hiccuping
Now what I’ve tried so far:
New fuel, oils and spark plugs as soon as I got it
Cleaned the maf
Boost leak test showed me a small leak on the intake pipe at the throttle body, don’t have new pipes yet but some duct tape around the cracked edges have stopped the leak temporarily and there was no change at all in performance, might be another small leak on the back of the engine but I can’t pinpoint it
I have the Nissan Datascan software, no faults on the ecu, temp sensor works, tps is working and set correctly, maf sensor appears to be working fine as well as everything else the Datascan is reading
I can post a picture of a datalog if there’s anything you guys think would be helpful

Now I’m stumped
Initially seems like a large boost leak but the system does hold the pressure for a while
6-7 psi is the wastegate pressure right? Maybe there’s something up with the wastegate plumbing? Bypassing the solenoids makes no difference and wouldn’t it still rev out if only boosting to that?
Are the turbos hurt?
Anything else that I can check?

Any insight you guys can give would be greatly appreciated, no matter what I change it’s always the exact same stumble in the exact same rpm

How many miles ? And what do you know about the timing belt / tensioner condition , any work rcts.? Can you look into the oil Cap fill opening and actually see , cyl 1 's cam lobe ? Is it nice ,shiny , or WORN ? and is tons of sludge in there ? varnish ? One of the FIRST things I do when inspecting a car is to look into the valvetrain , under oil cap .Also listen for NOISY valve-train , may need mechanic 2nd opinion on this . Now , to be sure of timing componants will necessitate taking one of the plastic "covers" off , and you know ONE side is "on top" and thus the first to be removed Some thoughts here.

About 110k miles on the dash, the timing belt is in pretty good condition and felt tight
there is an occasional lifter tick that goes away as the engine warms up, I figured I'd address that once its running correctly though
And the cam lobe looks perfect through the filler cap

Your thinking this is a valvetrain issue? I'm nearly certain its running rich causing it to bog out from my testing

Thanks for the reply

Now probly not from this new info , other thought - ignition timing OK , it's not "fluttering" around the mark at idle or especially when held super steady 2000 rpm's ?? Instability here could point a certain direction. Otherwise this all I got for now.

That could definitely be another thing to check!
When the idle is rough it will fluctuate maybe 100rpm
When engine is warm the Nissan Datascan is showing timing at 15 degrees, however I will need to get a timing light on that to verify, I’ll also do try holding it at 2000 tonight and post what the results are

Another thing to add, using the Datascan we were able to limit the fuel to 80% of what the ecu wanted to give the engine and then I was able to rev up to 5500 where I lifted in case we were too lean... this however kills the engine at idle and the engine definitely didn’t feel at full power under acceleration

One other thing since you say you put in new plugs is to pull them ALL out and compare after a "fail incident",indicating possible injector cloggs , massive injecter leakage - in ANY one or more of them , and general AF ratio ; notation and even photos here a must. Those "new" plugs form a decent "baseline" you know.
A theory - previous owner had some super bad bad gas / additives/ whatever wreaking the CAT's , O2's,injectors , PLUGS ! whatever you know , hate to suggest.

Can you drive the car normally with out boost? I would look harder for a boost leak, especially the couplers near the turbo outlets.

Thanks for the replies guys, I really appreciate it
I’ll try to check out the things recommended tonight as I leave for a vacation tomorrow
I’m also leaning on the side of either a boost leak or a wastegate/turbo problem, I can drive it around normally but the motor is coughing/ missing pretty constantly, there’s a slight bog when I first tip into the throttle and it overall feels lazy. When I hit boost there’s a lot of black smoke out of the tailpipes
Disabling one cylinder at a time gives me a heavy misfire for each one so for now I’m considering the internals as healthy

Previous owner deleted the aiv system and read did all the vacuum lines so my next course of action I think is going to be get new boost pipes, delete the prvr system and the egr system (no longer need it in Canada hehe) as well as remove and double check the routing of all the vacuum/ boost lines
Anyone know how I can check the functionality of the aac system? Or possibly give some insight to when it should be open or closed? The Datascan shows it’s activating it while I’m driving even though I assumed it would be closed unless idling
Also turning it off using the Datascan doesn’t make a difference

Update for you guys
While I was on vacation I ordered in new boost pipes, a manual boost controller, an egr delete kit and a AAC hose kit
I already have the plenum pulled (what an absolute pain ) so I’m going to do the egr delete, aiv was already deleted, the coolant bypass and the prvr delete
Couple things I noticed, plenum gasket looked awful and the balance tube was sealed with liquid gasket maker
The driver side throttle body also seams to be sticking a bit so I’ll clean that up and see if it makes a difference

Now I called jim wolf because he did the chip that’s in the car, he suggested checking the fuel pressure too, anyone ever have a fuel pressure Regulator go bad and what symptoms that would give? My new plugs were black as can be

Anything else I can do while the plenum is off? Is it worth it to upgrade injectors before I do any other power mods?

Christofehrzy wrote: ↑

Tue Aug 06, 2019 8:39 am

Anything else I can do while the plenum is off? Is it worth it to upgrade injectors before I do any other power mods?

I would at least do later style injectors with an adapter kit. What kind of power are you shooting for?

If you can afford go with bigger now, OEM sized you will be limited to 370 ish WHP with good tune.

My goal is to be around 500 but that’s coming later.. I want to first just be able to drive it
Since I don’t have any reason to believe any injectors have failed I may just leave it for now and go to a bigger size later... I don’t believe stock turbos are capable of 500hp correct?

On pump fuel, No. E85 with supporting mods you'll peak out at 500.