L28et woes

[240zphilly]

So, long story short, the car pulls fine to 3800-4000rpm and then doesn't pull anymore. I could swear I have heard of this problem before but cannot find the correct combination of keywords to bring me the search results I need. Any suggestions?

 

Combo is:

82 280zx L28 P90a (previous owner claims less than 5k on rebuild)

stock intake/ exhaust/ turbo

aftermarket intercooler

70mm tb

factory fuel with the addition of a RRFPR and JSK rail

greddy profec electronic boost controller

7ish psi (previous owner said he used to crank it up to 14, I dont trust the fueling) 

compression test reads 90-90-90-90-90-90 +or-1

 

This issue literally happened overnight on an otherwise fun, great running 280 and I am not sure why or how to correct it. Any help would be appreciated.

Edited December 19, 2012 by 240zphilly

[BluDestiny]

Does it sputter or hit a fuel cut? Can you describe how it acts after 4k?

 

Also stock EFI right?

[240zphilly]

No sputtering or strange act really... Its like it runs out of breath, its not abrupt either, it fades from about 3800 to 4200 and really wont rev any higher. This only happens under load, also,, I can free rev it whenever.

[Brad-ManQ45]

7 psi w/intercooler is probably getting close to maxing out fuel around 4500-4800 rpm.  More like 10psi w/o intercooler.  I bet your RRFPR ain't working.

[redzedturbo]

My car done something similar and it end up being air intake temp sensor was bad over fueling it bad. Im running megasquirt though. i would check your head temp sensor if using stock ecu or as said above fuel reg. Also check for weak spark and correct plug gap. You may be blowing out spark under boost if gaped to tight  or using the wrong heat range of plug. Possibly afm? good luck in any case man.

[FricFrac]

..wouldn't that be blowing the spark out if it was gaped too big not tight?  The bigger the gap the harder it is to spark...

Edited December 22, 2012 by FricFrac

[tyler031734]

Who the hell screwed up compression? Stock comes in at 140psi ~7.3-1

 

Do a leak down test. Every time someone sold me a car with a "rebuilt" engine. It needed to be rebuilt.

 

The 81T car I hauled home from the pickNpull yard had 150PSI!! on all 6. whoo hoo! rebuilt engine! lol

 

I run mine at 10psi with inter cooler. Pulls good to 5500. stock injectors 2.5" mandrel bent exhaust. no fuel issues. 

 

Check your spark I would think. Hydro lifters get -24* btdc solid are -20* as long as your TPS is working and adjusted right.

I call the TPS Idle position checkers, they dont check throttle position, only if your pedal is at idle position or not. For spark at idle and fuel cut on decel.

 

after that id check sensors that affected spark. No funny noises or smells from rich to lean during the problem lean me twords spark issues.

CHTS, Knock Sensor, so on so forth.

[evildky]

Sounds like the typical afm issue, check the connectors forst might be that simple.

[ArchetypeDatsun]

Not to thread jack but mine does the exact same thing the only difference mine is running Z31 ECU & MAF. I don't know if you have took your spark plugs out and seen what they look like? I took mine out and they were crusty coal black.

[240zphilly]

Thanks for the input, everyone. I am still trying to narrow down the issue. Will check some of the more obvious suggestions before moving on to the more complicated. Tyler, are you sure about the compression?

[NewZed]

That's pressure that you're reading, not compression.  The gauge used can have a big affect on the number you get.  The big tubular adapters that come with some gauges can drop the pressure reading by 60 psi.  The adapter tube and gauge volume get added to the combustion chamber volume so the effective compression ratio drops.

[240zphilly]

I figured that was the case when I bought the gauge. Literally picking the cheapest one at o'reilly. My purpose for getting it was that if all of the cylinders were the same, the motor should be in good shape.

[240zphilly]

Or my logic for using it, rather. If the readings were 90-90-90-90-40-55, for example, I would be alerted to something being very wrong in there.

[Mateo]

My 82 was doing this. Disconnected battery.  Disconnected ECU connectors.  Cleaned connections with vinegar and baking soda, then rinsed with water.  Then cleaned with electric cleaner.  Let dry over night with a lamp projected on the connectors to make sure they were dry.  Applied dielectric grease to the connector.  Then she ran past 4krpms, still stuttered a little around 4500 rpms but would go through it.  Pulled spark plugs regapped from 1.0mm to  0.98mm.  Now it runs right through pulls hard at 10psi till 5500rpm 

[Tony D]

My first thought was exactly as Mateo found: generally as corrosion builds, the performance starts suffering at the top of the rpm range and eventually gets to the point where most people drive...then they notice it and start looking (usually evidenced as posts of "My Z won't rev above XXXX rpms under load, but when I check it in the driveway by hand I can redline it."

[240zphilly]

My first thought was exactly as Mateo found: generally as corrosion builds, the performance starts suffering at the top of the rpm range and eventually gets to the point where most people drive...then they notice it and start looking (usually evidenced as posts of "My Z won't rev above XXXX rpms under load, but when I check it in the driveway by hand I can redline it."

 

 

My 82 was doing this. Disconnected battery.  Disconnected ECU connectors.  Cleaned connections with vinegar and baking soda, then rinsed with water.  Then cleaned with electric cleaner.  Let dry over night with a lamp projected on the connectors to make sure they were dry.  Applied dielectric grease to the connector.  Then she ran past 4krpms, still stuttered a little around 4500 rpms but would go through it.  Pulled spark plugs regapped from 1.0mm to  0.98mm.  Now it runs right through pulls hard at 10psi till 5500rpm 

 

Thanks both, Im not sure how comfortable I am using water on the connections... I dont have a lamp or a garage to assure that it dries properly. Is there some other substance that would not potentially mess up the connector any further that I could use?

Edited January 14, 2013 by 240zphilly

[BluDestiny]

a hair dryer on low. Or just use a really fine wire brush. They are electrical connectors, just use common sense when cleaning and drying them. 

[aarang]

Use electrical contact cleaner and try some Caig DeOxit ( I think that's how you spell it). I have to use the cleaner and DeOxit about once a year on my ZXT harness.

[FricFrac]

Thanks both, Im not sure how comfortable I am using water on the connections... I dont have a lamp or a garage to assure that it dries properly. Is there some other substance that would not potentially mess up the connector any further that I could use?

... replace the main EFI connectors with new ones then you KNOW you've got a good electrical connection. Otherwise you're polishing a turd.... Replacing the connectors also fixes the issues with keeping the AFM connector attached (missing clips, broken tabs, etc).

[FricFrac]

Also it never hurts to rebuild the AFM and make sure your battery cables are in very good condition. On any new-to-me car I replace the battery cables unless they look new.