nissan 570sx

I'll try to keep this as short as possible.


I got my LS1 swap done last spring and it has been running excellent up until a couple weeks ago. I'm now getting an intermittent hard start / rough idle (slight misfiring). It does always start, but every so often it will take some extra cranking to fire and then it idles really low and wants to die. If I rev it up and let off the RPMs drop down to <400, a couple of times it has died. It isn't temperature related as sometimes it will do it when the engine is cold and sometimes when it's hot. The ECT is reading correctly according to the scan tool, and I can unplug it/jump it and see the reading change.

The weirdest part is that it can be running great, I'll shut it off, restart, and it will be idling like crap again. Shut it off, restart, and it can be running great. Or sometimes it will idle like crap start after start, or sometimes go days without a problem.

Things I've checked:
-fuel pressure is a steady 58psi at all times (Vette FPR)

-MAF signal is strong and changes as it should when depressing the throttle, no hiccups or bad spots (I have access to a Genesis scan tool)

-TPS signal is also correct

-no vacuum leaks

-replaced the PCV

-cleaned the air filter (I even ran it a bit in the garage with it off to make sure it wasn't restricted)

-checked plugs

-rechecked all the wiring (I modified the harness)

-cleaned the MAF and throttle body

-it does not throw any codes

Now on to what I believe is the problem and I'm hoping for an agreement or disagreement here.

On the scan tool with it idling correctly the MAF frequency is around 6-6.5. The IAC count is about 55 and the IAC airflow request matches the MAF frequence of 6-6.5

When it's idling incorrectly/misfiring the MAF frequency is still around 6-6.5 at idle. However, the IAC count is up at 150+ (I'm guessing that's maxed out?) and the IAC airflow request is 11-14. When this is happening it definitely seems as though it's running incredibly rich (although the O2s are still cycling), the short term fuel trims are about 15-20% lower than normal. I can definitely smell unburnt/partially burnt gas.

I believe what is happening is the IAC is sticking closed, the PCM is trying to open it (hence the 150+ IAC count and the much higher IAC airflow request). What does anyone think of that? Is there something I'm overlooking that I should check? I've probably checked more than I listed and just not remembering it. I'll edit this if I do think of anything.

Help greatly appreciated.

itsslow98

iTrader: (18)

When it is idling bad i would unplug the IAC and see how it changes things. You can remove the IAC and clean it if i recall correctly. Plus you could always find a buddy with a known working one and swap it out to confirm thats the problem.

I was gonna say IAC before i even saw what you checked.

nissan 570sx

I forgot to mention I did try unplugging the IAC and it didn't make a difference when it was idling bad. I don't remember if I tried with it idling correctly.

I did take the throttle body off and clean all the carbon/oil/black **** around the IAC with carb cleaner and Q-tips.

If I knew anyone with an LS motor around here I would just try swapping the IAC but I don't.


Also, I forgot to mention it has not thrown any codes at all.

nissan 570sx

Swapped the IAC today with a new one. The first couple starts it idled fine. Just ran to the store, when I went to leave it was doing the same thing - idling low/almost dieing.

thunderstruck507

iTrader: (18)

Strange, I had the exact same problem and a IAC fixed it for me.

eurautodave

iTrader: (1)

my volvo had this problem before i plugged in the cam sensor. i would start there.

nissan 570sx

Ok, I have today off so I looked at it some more.

One thing I'm not sure if I made clear in the first post is this: I can start it up and it will be idling like ****, shut it off, immediately restart and it will be running perfectly. Or just the opposite. Or it will run like shirt back to back, or run perfectly back to back. I don't need to let it run or stay shut off for more than a second for it to change. However, if it's idling like **** it will be won't change at all until I shut it off, same goes if it's running right. It will not change at all until it's shut off and restarted.

That leads me to believe something is or isn't happening at startup. I'm trying to think if there's some sensor that it only reads at startup?

Also, I don't have access to a scan tool today so I'm kind of diagnosing blind. I've basically tried unplugging things while it's running to see if they have any effect, so here's a list:

idle changes when unplugged
MAF
MAP
O2 sensors
TPS

idle doesn't change when unplugged
ECT (but I have verified it is working properly
IAC (brand new)
Cam sensor
IAT


One thing that I found weird is that I can disconnect the PCM ground and it will still start and run, it doesn't seem to have any effect on anything. Why is that? Shouldn't it need the ground?


I also checked for exhaust leaks but there aren't any and the header bolts are all nice and tight.

nissan 570sx

So I went to take the 240 to work this morning (I've been on vacation the last week). When I turned the key to run the fuel pump didn't turn on. I cycled it a couple times and it still wouldn't turn on. Decided to try cranking it and it took a few seconds to fire, but it did eventually and of course was idling like ****. It actually died a couple times. The fuel pump is run off one of the relays I put in originally (with the signal coming from the PCM obviously). So I guess I'm going to check those relays again, maybe it's as simple as one them going bad.

nissan 570sx

Well if anyone cares I figured out what it was finally. After completely redoing almost all the wiring I did when I first did the swap (mostly getting rid of the five aftermarket relays I put in and reusing unused relays in the original Nissan harness) I found the B+ constant power wire to the LS1 PCM had a small cut on the heat shrink on a solder joint and was starting to corrode. I fixed that last Saturday and it's been fine since then.

Camarrow SS

Did you ever figure out what was going on with it, I'm very curious cause it sounds like I'm having the same issue. I recently rebuilt my ls1 with forged parts and lowered the compression to 8:5 but otherwise stock Ls1 with STS turbo. It starts like crap and hangs out around 400-500rpm for about ten seconds and then idles up but in a few it will stumble and then come out of it. Also its leaning out on the higher rpm's even when we tell it to be 11.5 it won't hold it, it ends up in the high 13's before I let off. Fuel press is good when driving, replaced tps and Map sensors no fix. Seems that if I unplug the tps while running and then plug it back in, it starts way better but idles high. Any suggestions. PLEEEEASE HELP !!!!!!!!!!!!!

nissan 570sx

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