'83 ZXT Mysterious (Fuel?) Issue

[Connor280ZX]

First off, its a bone stock L28ET paired to a T5. I got this car as is because it's going to donate its engine to my N/A ZX. But, before I do the swap I obviously need to make sure the engine is running in top shape... which it isn't.

 

When i first got it (9 days ago), the engine idled fine, this the exception of it randomly dropping a cylinder if iIlet it idle for a long time, and it sounded like fuel starvation. Taking it on the road, with more than 15-20% throttle applied it would stumble, sneeze, and front fire very harshly. If i was able to get it into boost, it would smooth out and pull better. Same thing when i'd try to free rev it, Popping and stumbling and the engine would rev up very slowly. It would sometimes start right up, and other times would need to crank for an extra 2-3 seconds before it stumbled up to idle when cold. Warm starts are better.

 

First thing I checked was the ignition system. Cap and rotor are new, as well as plugs and wires. Plugs looked clean. Then I started testing sensors. (6 days ago) First being the CHTS. I could unplug it and the car would run better, but brutally rich (Black smoke, even at idle). It would rev up decently (Still a bit of a stumble) and pull a bit harder, then the engine would fall on its face at 4500 RPM, no backfiring, just wouldn't accelerate further, so I plugged it back in and it went running the same way it intially was of course.

 

So at this point I was thinking a fuel issue and started testing other components. Fuel pump seems to be quieter than my N/A's, but seems to work fine. I tried replacing the fuel filter, and no change. (3 days ago)Then, i unplugged the vac line to the FPR and the car seemed to run way smoother. When cold, the car fires up right away. Under throttle, acceleration is smooth, but the turbo lags, and it just seems to be really low on power. Doesn't backfire, or stumble, but still hesitates and revs up rather slowly in neutral.

 

I know some of the veterans on here will probably slam me for not performing resistance and fuel pressure tests (YET) but It's a holiday weekend and my local auto parts store has been closed, therefore i haven't been able to purchase any testing equipment for the electronics or fuel system. So i'd like to gather some ideas here in the meantime since I can't do anything about the car but gather information. Google hasn't done me much justice so far...

[NewZed]

Extend the fuel rail return line in to the cabin and run it under your foot so that you can press it closed as you drive.  More foot pressure = more fuel pressure.

 

You could also put a potentiometer on the CHTS circuit.  Turn up the resistance to get more fuel.  The AFM's tend to get lean as they get old for some reason.

[Connor280ZX]

Extend the fuel rail return line in to the cabin and run it under your foot so that you can press it closed as you drive.  More foot pressure = more fuel pressure.

 

You could also put a potentiometer on the CHTS circuit.  Turn up the resistance to get more fuel.  The AFM's tend to get lean as they get old for some reason.

Yeah, haha might have to come to that. However today i swapped the good CHTS from my good ZX into the ZXT and no change, so at least i've ruled that out.

[NewZed]

However today i swapped the good CHTS from my good ZX into the ZXT and no change, so at least i've ruled that out.

That wasn't the point.  Many of the older stock systems, for whatever reason, run lean with stock parts in good working order.  The potentiometer lets you add a little bit of fuel over the full fuel curve.  It's a simple tuning aid, not really a fix for a bad part.

 

But, of course, measuring fuel pressure so you know what you're actually working with is the best idea.  If fuel pressure is fine, do the potentiometer.

 

This is the common lean AFM symptom - "Taking it on the road, with more than 15-20% throttle applied it would stumble, sneeze, and front fire very harshly."

[Connor280ZX]

The potentiometer lets you add a little bit of fuel over the full fuel curve.  It's a simple tuning aid, not really a fix for a bad part.

 

But, of course, measuring fuel pressure so you know what you're actually working with is the best idea.  If fuel pressure is fine, do the potentiometer.

 

Ok, I understand that, but what i'm trying to do here is find out exactly what my issue is here. It may be old tired electronics such as the AFM like you said, or maybe it's the fuel pump. I'd rather not hack up my wiring harness until I know for sure what the issue is, but if it comes down to it, i might have to do that. I bought a fuel pressure tester today, only come to find that our rails do not have test fittings. The shop doesnt sell "T"s to put in the fuel hose either... So I guess i have to make some sort of hack job tester with a gauge, hose, and barbs or something.

[NewZed]

Mid to late 90's Honda and Mitsubishi SUV's have a metal inline Schrader valve that you can cut out and T in to a rubber fuel hose.  If you're by a wrecking yard.  Just leave it in.

 

You said you wanted ideas to try while you waited to get a gauge.  The potentiometer can be slipped in between the two bullet connectors on the CHTS circuit.  Zero hacking required, completely reversible.

[Connor280ZX]

You said you wanted ideas to try while you waited to get a gauge.  The potentiometer can be slipped in between the two bullet connectors on the CHTS circuit.  Zero hacking required, completely reversible.

Well that sounds much more suitable. Is there a specific potentiometer I should be looking for? Since i don't need to cut anything, I'd like to try this out.

[NewZed]

Here's a write-up for the 280Z thermostat temperature sensor.  The CHTS does the same, from a different location.

 

http://atlanticz.ca/zclub/techtips/tempsensorpot/index.html

[Connor280ZX]

Alright, so i finally found a pressure gauge that works with this car and here are my results:

 

Idle: 29-30 PSI with FPR plugged in, 40 PSI with FPR unplugged.

 

WOT: 32-33 PST with FPR plugged in, 40 PSI with FPR unplugged.

 

So I'm pretty sure that at this point its the fuel pump unless you think otherwise.

[stupid_fast]

This sounds like what my ZX did when I first turbo swapped it.   Mine would build boost, then pop out the intake / pov loudly and lose power... ran ok at low throttle. 

Ended up being bad connections on the harness & ECU connectors, as well as loose terminals.   But in the process of troubleshooting I pretty much replaced and went over everything ... I can't remember what exactly fixed the issue. 

 

 

 

Download the FSM from xenons130 and go through all the ef/ec tests.  Ignore the AFM one however, every one will give you a different resistance number. 
Inspect your EFI connectors while you do this, make sure there is no corrosion.  

Check the ECU connectors and maybe retention the pins.  Check your battery connections make sure everything is tight. 
Go through standard tests, check timing, compression, vacuum leaks. 
make sure your injectors are firing well, it may be worth it to soak them in techron and test them externally.

If you think its your fuel pump- is the fuel filter new?  Replace it! 


After that your issues should be minor and the car should drive well. 

 

Every S130 turbo ECCS I've seen runs RICH 10:1 at WOT unless the AFM is corrected. These cars run very rich from the factory.  A wide band or dyno is needed to set up the AFM properly. 
 

 

Where are you located in the bay area?? I'm in Antioch/brentwood.   There's a couple other hybridz members in the area as well who are knowledgeable with the nissan ECCS systems. 
 

Edited September 13, 2015 by stupid_fast

[Connor280ZX]

Stupid_fast, you think it's an electrical issue? I've gone through the FSM procedures last week, reseated the ECU connections, checked all of the sensors, compression check etc.

 

Even with such low fuel pressure, you think it's EMS related? I'm getting 39-40 psi MAX even with the regulator unplugged. FSM calls for 44PSI or something at WOT. With the regulator in I'm getting 32 MAX. Fuel filter and hoses are brand new. It pops and chugs throughout the rev and the throttle range, not nearly as much without the regulator. I'm in El Granada btw.

Edited September 13, 2015 by Connor280ZX

[NewZed]

Alright, so i finally found a pressure gauge that works with this car and here are my results

 

WOT: 32-33 PST with FPR plugged in

 

FSM calls for 44PSI or something at WOT. /

 

With boost, pressure should increase. Go beyond "or something". It's very important.

[Connor280ZX]

With boost, pressure should increase. Go beyond "or something". It's very important.

 

Though unfortunately it doesn't at all. I can't get it past 39-40 under any circumstances. I'm gonna try pinching off the return line.

[Connor280ZX]

When I pinch off the return line (FPR plugged in) The pressure rises slowly. Watched it come up to 55, then I removed the pinch fearing I would blow a hose, then it quickly dropped back down to 29. However, with the pressure up the engine would rev so much quicker and smoother than i've ever seen it do... I've also noticed that injector #1 loses connection (Stops firing) occasinally. Once i wiggle the connector a bit, or restart the engine it will come back on though. Problably need to clean the contacts, there was white paste all over it.

[NewZed]

With boost, pressure should increase. Go beyond "or something". It's very important.

 

Though unfortunately it doesn't at all. I can't get it past 39-40 under any circumstances. I'm gonna try pinching off the return line.

I meant learn more stuff.  "Or something" means that you "don't know".  7.4 psi (15 inches Hg) of boost plus 36.3 psi ambient = 42.7 psi.  That's maximum fuel pressure at maximum boost.  Not WOT with no load.  No boost, max. pressure = 36.3 psi . Or ~38 depending on which book you're looking at.  Some FPR's run a pound or two high, so 38 psi is common.

 

The 1983 FSM is pretty bad, so good luck.  There's nothing in there about 44 psi though.

 

stupid_fast was right, give the engine a better tune than new cap and rotor. You're kind of spinning your wheels, and about to replace good parts, without a general systems check.

[Connor280ZX]

Ok, I understand but I thought I already mentioned that I had already checked all of the major electronic components (ECU, AFM, VCM, CHTS) and everything checked out fine. I'm going off the 1982 FSM, when I said "or something" I meant that I hadn't remembered 100% exactly what it was. Once I get home, I'll find the page and take a picture of it if you'd like.

 

I have no way of checking the fuel pressure when I'm actually driving the car. I'd need $200 more in equipment to route a gauge to the dash.

 

The pressure could infact be dropping when I'm driving under load since it drops under no load.

 

But, replacing good parts?? Come on, it seems pretty obvious at this point, so I just wanted to get final clarification here.

[NewZed]

The numbers don't support the need for a new pump.  They are are correct for no boost.  Not clear.  That's all I'm saying.

[Connor280ZX]

The numbers don't support the need for a new pump. They are are correct for no boost. Not clear. That's all I'm saying.

Well, let me clarify further. Max pressure when free revving: 32PSI, Max pressure when idling 29PSI, max pressure under load: less than 32PSI since it won't accelerate unless I unplug the FPR, which I shouldn't have. If I pinch off the return line so I get 50+ PSI the engine runs like a raped ape unless I lay on the throttle too long which then the pressure decreases too much and I have to let it build up again.

[Connor280ZX]

Took the good off so I could see the fuel pressure gauge, and once I floor it under load it drops about 10-15 psi.

[NewZed]

 If I pinch off the return line so I get 50+ PSI the engine runs like a raped ape unless I lay on the throttle too long which then the pressure decreases too much and I have to let it build up again.

This plus the latest supports the fuel pump proposition.  I was just looking for a solid case.  But.  It could also be a clogged filter, hindering flow from the pump.  That would be cheaper.  Anyway, it seems like a supply problem.  At least you're down to two possible items to check.  Assuming it's not something in the tank.