Poor idle, 2500 cut off, MAF is fine - Zilvia.net Forums | Nissan 240SX (Silvia) and Z (Fairlady) Car Forum

economix · 05-24-2016, 08:04 PM

The topic of the 2500 limp mode cut off has been discussed and normally it's the MAF, but my MAF is working just fine - motor stalls if removed while running.

Motor is an S14 SR (S13 chassis, 1989)
Relative mods include -
810 cc injectors (DW) on a CS fuel rail
Adjustable FPR (set to 38-40 psi)
BC 264 cams/retainers/springs
Rocker arm stoppers (bit loud if you ask me)
Z32 MAF
Enthapy tune for 18 psi
Rs3871 turbo
New IAVC
All new gaskets (head gasket and up)
Most anything on the motor besides the block is new (which also concerns me)

What else might cause this 2500 rpm limp mode? I've heard coolant temp sensor (which I replaced) as well as boost leak could do this - but there is no boost to actually leak nor do we observe a leak.

I've dialed in the CAS some to even it out but no avail. I've also dialed in the two set screws on the throttle body and messed with the IAVC just for good measure (not that those would resolve this issue).

Don't flame. Really stumped since it sounds like an issue that's easily resolved though it has been anything but.

Tyler_240 · 05-25-2016, 04:37 AM

You could check your TPS voltage. It is supposed to be .45v closed and 4v+ WOT. Also check it while opening the throttle and ensure it steadily increases and does not have any dead spots.

Just because you unplug the MAF and it changes the idle does not mean that MAF is good in my opinion. I would borrow a SOHC MAF or try a known good.

Is your 02 Sensor working?

economix · 05-25-2016, 05:05 AM

Brand new O2 sensor, so it should be good as well.

I'll give the TPS a run and mess with the MAF some. Also going to check for an discrete leak somewhere just in case.

Is frustrating to say the least when everything is new and should be working proper. The harness is a WiringSpecialties so that's in excellent shape also.

economix · 05-25-2016, 05:05 AM

Thanks for the input btw!

Tyler_240 · 05-25-2016, 06:54 AM

NP. We've all been there.

You may want to verify your grounds too. You should have battery -> tray -> Intake Manifold. Coilpack Harness to Firewall behind valve cover. There are (2) grounds on an S13 SR Injector Harness, not sure if its different for S14, that attach to Intake Manifold. And I have my alternator chassis grounded to my frame as well. I would also verify your ECU Harness is plugged in gudentite and you may even want to consider re-grounding your MAF harness. I know some guys have had issues in the past when under load it would only screw up.

Is your wastegate pre-load set correctly?

economix · 05-25-2016, 07:29 AM

Thank Tyler. I think it may be some grounding as well. Battery is in the trunk, so that's easy to eliminate. I did run another ground to the MAF from chassis last night while working on it, but I need to check the other connections. The bay was repainted Jet Black so maybe it needs roughed up in some areas for better contact. I'll give that a try tomorrow night and post here again with or without success to either carry the discussion or close it out for others.

Tyler_240 · 05-25-2016, 08:02 AM

If you rough it up, use di-electric grease to prevent corrosion and ensure you are using SS bolts. Most of the bolts in the engine bay are M6x1.00 IIRC.

Kingtal0n · 05-25-2016, 08:24 AM

If you have a fuel cut limiter at 2500 its the maf or wiring for the maf, or the ECU interpretation of the maf signal. Saying " the maf is good " doesnt automatically make the maf good. nothing else in the computer can cause a 2500 fuel cut.

KAT-PWR · 05-25-2016, 09:39 AM

Quote:

Originally Posted by Kingtal0n

If you have a fuel cut limiter at 2500 its the maf or wiring for the maf, or the ECU interpretation of the maf signal. Saying " the maf is good " doesnt automatically make the maf good. nothing else in the computer can cause a 2500 fuel cut.

This. Don't try to over complicate things. The FSM can walk you throug how to actually check the MAF

economix · 05-26-2016, 02:11 PM

Ordered a new MAF next day air for this weekends retry. Will post some sort of result with added grounding, new-new MAF, and some electrical diagnosis on the TPS just to gauge where that's at.

ultimateirving · 05-26-2016, 04:17 PM

Also just doing a visual check for vacuum leaks may not be enough.
Build yourself a boost leak tester and pressurize the system to find leaks that are impossible to see.

Kingtal0n · 05-27-2016, 12:31 AM

dont mess with the tps. the tps wont cause a fuel cut. you can unplug the TPS on an sr20det and drive around like nothing is any different.

everybody loves to check their tps around here and hoot and holler about the tps. I've never checked my tps, and never on any other car either. I might have changed one, once, and I keep a spare. But that is not typically a part you associate with diagnostics/poor performance. Not as much as everyone would have you believe.

cotbu · 05-27-2016, 01:07 AM

Quote:

Originally Posted by Kingtal0n

you can unplug the TPS on an sr20det and drive around like nothing is any different.

Kingtal0n · 05-28-2016, 09:22 PM

Quote:

Originally Posted by cotbu

WHAT? NO

Well, what does yours do? I've unplugged mine and it makes little difference. Tried it on my redtop, my S15, my 98, all of them dont seem to care. its the maf it relies/depends on for fueling. Tps is just for a little squirt of gas, engine might stumble without it but for the most part doesnt seem to care drivability wise.

Only other thing is you lose the fuel cut for tps closed position. So if you run an atmopsheric bypass, the engine pops and backfires more when you lift because it doesnt know the tps is closed or open. Again, I am not suggesting you leave it unplugged, but strictly for diagnostic purposes the tps does very little and has very little to do with engine tuning/fueling besides that one pump shot you get when you slam the throttle open.

economix · 05-29-2016, 08:29 AM

Ok, so tried a lot of things, same 2500 rev limp.

New MAF, MAF was seeing the .5-5v needed

Checked TPS, was running at 1.8-2.5v most of the time which is good (0-4v running, or .35-.65 closed)

Checked all grounds, added 2 new

Checked all boost/VAC and IC lines

The ECU flash should be correct for the mods but I may send it in to be looked at just in case.

Think it's time to add a consult port and see other parameters. Just ordered a pigtail from wiring specialties.

Tyler_240 · 05-30-2016, 07:51 AM

Checked for codes?

You could unplug your knock sensor temporarily. You could be seeing false knock due to vibrations from more solid motor mounts which will retard your timing.

When you say you check boost/vac lines, did you plug your intake and pressurize the system? It should hold 10psi~ or so for a good bit, don't use to much air, but you should easily hear it whistle if there is a unnoticable leak. I hit an armadillo once and it jacked up my intercooler. Believe or not, the front was fine, its when it crashed into the core support that it damage the back of it and it took me awhile to track that leak down.

Kingtal0n · 05-30-2016, 07:58 AM

I dont think you can unplug the KS on an sr20det and expect the engine to run well. pretty sure that pulls all the timing. I think what you mean is to resistor/bypass it.

2500rpm limiter means the ecu does not see the maf, it isnt plugged in. wiring/ecu

Tyler_240 · 05-30-2016, 08:44 AM

I could be wrong, but I'm pretty sure I ran my stock #62 ecu w/o it plugged in. The previous owner f'd up the wiring...I think you use the resistor to clear the code if you want #34 IIRC, but a knock sensor code shouldn't perma mess up the timing, only if its working but sending false signal does it work, right?...correct me if i'm wrong.

EDIT: I agree that it really does still seem like the MAF. You should check for continuity between the ECU and the MAF signal wire.

05-24-2016, 08:04 PM	#1
economix Zilvia Member Join Date: Feb 2011 Location: Erie, PA Posts: 185 Trader Rating: (5) Feedback Score: 5 reviews	Poor idle, 2500 cut off, MAF is fine The topic of the 2500 limp mode cut off has been discussed and normally it's the MAF, but my MAF is working just fine - motor stalls if removed while running. Motor is an S14 SR (S13 chassis, 1989) Relative mods include - 810 cc injectors (DW) on a CS fuel rail Adjustable FPR (set to 38-40 psi) BC 264 cams/retainers/springs Rocker arm stoppers (bit loud if you ask me) Z32 MAF Enthapy tune for 18 psi Rs3871 turbo New IAVC All new gaskets (head gasket and up) Most anything on the motor besides the block is new (which also concerns me) What else might cause this 2500 rpm limp mode? I've heard coolant temp sensor (which I replaced) as well as boost leak could do this - but there is no boost to actually leak nor do we observe a leak. I've dialed in the CAS some to even it out but no avail. I've also dialed in the two set screws on the throttle body and messed with the IAVC just for good measure (not that those would resolve this issue). Don't flame. Really stumped since it sounds like an issue that's easily resolved though it has been anything but. __________________ 89 240SX w S14 SR20; 95 JDM 180SX S13 SR20; 09 Acura TL SH; 18 Jaguar XE S

05-25-2016, 05:05 AM	#4
economix Zilvia Member Join Date: Feb 2011 Location: Erie, PA Posts: 185 Trader Rating: (5) Feedback Score: 5 reviews	Thanks for the input btw! __________________ 89 240SX w S14 SR20; 95 JDM 180SX S13 SR20; 09 Acura TL SH; 18 Jaguar XE S

05-25-2016, 08:24 AM	#8
Kingtal0n Post Whore! Join Date: Jul 2005 Location: South Florida Age: 41 Posts: 4,828 Trader Rating: (17) Feedback Score: 17 reviews	If you have a fuel cut limiter at 2500 its the maf or wiring for the maf, or the ECU interpretation of the maf signal. Saying " the maf is good " doesnt automatically make the maf good. nothing else in the computer can cause a 2500 fuel cut. __________________ 5.3/4l80e/turbo 240sx playthrough: tinyurl.com/4l80er

05-26-2016, 02:11 PM	#10
economix Zilvia Member Join Date: Feb 2011 Location: Erie, PA Posts: 185 Trader Rating: (5) Feedback Score: 5 reviews	Ordered a new MAF next day air for this weekends retry. Will post some sort of result with added grounding, new-new MAF, and some electrical diagnosis on the TPS just to gauge where that's at. __________________ 89 240SX w S14 SR20; 95 JDM 180SX S13 SR20; 09 Acura TL SH; 18 Jaguar XE S

05-26-2016, 04:17 PM	#11
ultimateirving Zilvia FREAK! Join Date: Nov 2010 Location: arizona Age: 36 Posts: 1,521 Trader Rating: (1) Feedback Score: 1 reviews	Also just doing a visual check for vacuum leaks may not be enough. Build yourself a boost leak tester and pressurize the system to find leaks that are impossible to see. __________________ Z32 with lq4 swap Desert Street Scene \|\| Dssevents.com

05-25-2016, 04:37 AM	#2
Tyler_240 Zilvia Member Join Date: Feb 2005 Location: Gainesville, FL Posts: 239 Trader Rating: (9) Feedback Score: 9 reviews	You could check your TPS voltage. It is supposed to be .45v closed and 4v+ WOT. Also check it while opening the throttle and ensure it steadily increases and does not have any dead spots. Just because you unplug the MAF and it changes the idle does not mean that MAF is good in my opinion. I would borrow a SOHC MAF or try a known good. Is your 02 Sensor working?

05-25-2016, 06:54 AM	#5
Tyler_240 Zilvia Member Join Date: Feb 2005 Location: Gainesville, FL Posts: 239 Trader Rating: (9) Feedback Score: 9 reviews	NP. We've all been there. You may want to verify your grounds too. You should have battery -> tray -> Intake Manifold. Coilpack Harness to Firewall behind valve cover. There are (2) grounds on an S13 SR Injector Harness, not sure if its different for S14, that attach to Intake Manifold. And I have my alternator chassis grounded to my frame as well. I would also verify your ECU Harness is plugged in gudentite and you may even want to consider re-grounding your MAF harness. I know some guys have had issues in the past when under load it would only screw up. Is your wastegate pre-load set correctly?

05-25-2016, 07:29 AM	#6
economix Zilvia Member Join Date: Feb 2011 Location: Erie, PA Posts: 185 Trader Rating: (5) Feedback Score: 5 reviews	Thank Tyler. I think it may be some grounding as well. Battery is in the trunk, so that's easy to eliminate. I did run another ground to the MAF from chassis last night while working on it, but I need to check the other connections. The bay was repainted Jet Black so maybe it needs roughed up in some areas for better contact. I'll give that a try tomorrow night and post here again with or without success to either carry the discussion or close it out for others. __________________ 89 240SX w S14 SR20; 95 JDM 180SX S13 SR20; 09 Acura TL SH; 18 Jaguar XE S

05-25-2016, 08:02 AM	#7
Tyler_240 Zilvia Member Join Date: Feb 2005 Location: Gainesville, FL Posts: 239 Trader Rating: (9) Feedback Score: 9 reviews	If you rough it up, use di-electric grease to prevent corrosion and ensure you are using SS bolts. Most of the bolts in the engine bay are M6x1.00 IIRC.

05-27-2016, 12:31 AM	#12
Kingtal0n Post Whore! Join Date: Jul 2005 Location: South Florida Age: 41 Posts: 4,828 Trader Rating: (17) Feedback Score: 17 reviews	dont mess with the tps. the tps wont cause a fuel cut. you can unplug the TPS on an sr20det and drive around like nothing is any different. everybody loves to check their tps around here and hoot and holler about the tps. I've never checked my tps, and never on any other car either. I might have changed one, once, and I keep a spare. But that is not typically a part you associate with diagnostics/poor performance. Not as much as everyone would have you believe. __________________ 5.3/4l80e/turbo 240sx playthrough: tinyurl.com/4l80er

05-29-2016, 08:29 AM	#15
economix Zilvia Member Join Date: Feb 2011 Location: Erie, PA Posts: 185 Trader Rating: (5) Feedback Score: 5 reviews	Ok, so tried a lot of things, same 2500 rev limp. New MAF, MAF was seeing the .5-5v needed Checked TPS, was running at 1.8-2.5v most of the time which is good (0-4v running, or .35-.65 closed) Checked all grounds, added 2 new Checked all boost/VAC and IC lines The ECU flash should be correct for the mods but I may send it in to be looked at just in case. Think it's time to add a consult port and see other parameters. Just ordered a pigtail from wiring specialties. __________________ 89 240SX w S14 SR20; 95 JDM 180SX S13 SR20; 09 Acura TL SH; 18 Jaguar XE S

05-30-2016, 07:51 AM	#16
Tyler_240 Zilvia Member Join Date: Feb 2005 Location: Gainesville, FL Posts: 239 Trader Rating: (9) Feedback Score: 9 reviews	Checked for codes? You could unplug your knock sensor temporarily. You could be seeing false knock due to vibrations from more solid motor mounts which will retard your timing. When you say you check boost/vac lines, did you plug your intake and pressurize the system? It should hold 10psi~ or so for a good bit, don't use to much air, but you should easily hear it whistle if there is a unnoticable leak. I hit an armadillo once and it jacked up my intercooler. Believe or not, the front was fine, its when it crashed into the core support that it damage the back of it and it took me awhile to track that leak down.

05-30-2016, 07:58 AM	#17
Kingtal0n Post Whore! Join Date: Jul 2005 Location: South Florida Age: 41 Posts: 4,828 Trader Rating: (17) Feedback Score: 17 reviews	I dont think you can unplug the KS on an sr20det and expect the engine to run well. pretty sure that pulls all the timing. I think what you mean is to resistor/bypass it. 2500rpm limiter means the ecu does not see the maf, it isnt plugged in. wiring/ecu

05-30-2016, 08:44 AM	#18
Tyler_240 Zilvia Member Join Date: Feb 2005 Location: Gainesville, FL Posts: 239 Trader Rating: (9) Feedback Score: 9 reviews	I could be wrong, but I'm pretty sure I ran my stock #62 ecu w/o it plugged in. The previous owner f'd up the wiring...I think you use the resistor to clear the code if you want #34 IIRC, but a knock sensor code shouldn't perma mess up the timing, only if its working but sending false signal does it work, right?...correct me if i'm wrong. EDIT: I agree that it really does still seem like the MAF. You should check for continuity between the ECU and the MAF signal wire.