RPM stumble only at 2500 under load

Tiny
NEEDZHELP
  • MEMBER
  • 1986 NISSAN 300ZX
  • 3.0L
  • V6
  • TURBO
  • RWD
  • MANUAL
  • 215,000 MILES
Relevant upgrades:
560cc injectors, blow through z32 MAF, Isky cams, Jim Wolf springs, HX35 turbo, intercooler, Walbro 255 fuel pump, wideband O2 sensor, and Nistune.

So, what happens is that the car starts and idles fine. In neutral, there's no problem revving the engine. However, while driving, it bogs down and runs real lean when it gets to 2500 RPM. After 2500 RPM though, the AFR goes back to normal and everything runs just fine in the higher RPMs.

I've tried swapping the ECM with two or three other working ECMs. Swapped coil, distributor, MAF, spark plugs and wires, and the PRW-2 ignition module.

What else should be checked?
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Sunday, January 18th, 2015 AT 7:49 AM

10 Replies

Tiny
HMAC300
  • EXPERT
What kind of exhaust it may not be enough back pressure on system causing this.
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Sunday, January 18th, 2015 AT 10:53 AM
Tiny
NEEDZHELP
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Upgraded from stock to 3" mandrel bent from the turbo all the way back. Hi-flow catalytic converter. Very typical exhaust upgrade for my setup.
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Sunday, January 18th, 2015 AT 1:52 PM
Tiny
HMAC300
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From our description it really sounds like not enough back pressure or not running rich enough. Are those the injectors that are recommended for that type of setup as we don't run into this type of stuff very often. And is a 3" exhaust recommended for that type of setup as well. That seems like an awful large exhaust system for 3.0 L engine. I'd think 2 and half or three quarter would work out as well. You might try contacting Nistune support as I think you can download the way your car is reacting and they can tell or make adjustments for you I think. To me you have thrown a lot all at once at this car and you are either suffering from not enough back pressure or leaning out on th e injector side and support can give you an idea to richen it up enough to take that problem away
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Sunday, January 18th, 2015 AT 3:50 PM
Tiny
NEEDZHELP
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For the upgrades that have been done, a 3" exhaust is the recommended upgrade. There are a bunch of injectors from different cars that are compatible (or easily made compatible) with my setup. Nistune has been contacted. I sent it to the nearest place who handles Nistune to reflash it, but it didn't change anything. The AFR is slightly rich at around 11-12 everywhere except 2500 RPM. My O2 gauge just pegs lean.
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Sunday, January 18th, 2015 AT 4:43 PM
Tiny
NEEDZHELP
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BTW, I did say there was an HX35 turbo on this. You may have missed that. When you have a turbo, back pressure isn't something you worry about.
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Sunday, January 18th, 2015 AT 6:11 PM
Tiny
HMAC300
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The try some different injectors you really need someone who can hook this up to a computer and watch what is happening on a dyno. That will be the only way to straighten it out.
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Monday, January 19th, 2015 AT 6:25 AM
Tiny
NEEDZHELP
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I've had it on a dyno with a very reputable guy here in VA. Spent a good amount of money on that trying to figure it out!

The injectors had been serviced prior to installation, but I am going to check the wiring and see if there are any issues with that. While I'm in there, I might as well swap them again.

I thought that if there were a problem with the injectors, it'd be with the signal to them, instead of the injectors themselves. But hell, I've no idea what's wrong at this point, so if someone tells me to change the freaking valve stems, I'll do it.
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Monday, January 19th, 2015 AT 7:00 AM
Tiny
HMAC300
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I don't have a lot of experience with this that you are doing, however I do know that when you start adding boost you need to normally replace injectors with ones that are more compatible with the boost you are using as it throws more air so you need more fuel to it. ARe the injectors stock, if they are you need to add ones that have more poundage like instead of 30# go to 34# etc. If you are running a lot of boost like 8 lbs cut it down do 5 if you can. Yes it may be a tad slower but if you drive this on the street who the hell cares. You won't have the stumble. I'd also wait until nistune gets ahold of you until they may figure something out. Also is the cam what is recommended or is it the biggest one you cold stuff in it? All these factors have something to do with the way an engine runs. I kind of think you aren't really knowledgeable about this and have stuffed a bunch of money into something like a lot of people do. I don't know cause I don't know your abilities car wise. All I can tell you is good luck. One other thing if the car runs ok now then don't start screwing with the wiring cause that is where problems start wait til you hear from Nistune, you can still drive it.
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Monday, January 19th, 2015 AT 7:42 AM
Tiny
NEEDZHELP
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First of all, I had to tell you that back pressure issues don't happen in a turbo system. Second of all, the first item in my upgrade list is injectors. I don't know if you get the right to call me out on my lack of knowledge when you can't even get through the list of upgrades. Stock injectors are 260cc, btw.

The cam is moderate, I know two other cars with very similar setups running the same cam without issue. There are much more outrageous setups in the Z world that have not come across the issue of running lean at one spot but being normal everywhere else.

And c'mon man, you're not really going to advise someone to continue driving a car that runs lean, are you?
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Monday, January 19th, 2015 AT 8:59 AM
Tiny
HMAC300
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From my impression of your question and replies is that I am thinking it's running lean when you have your foot into it and not normal driving. If it does it under normal driving then no but if it only does it with your foot in it then don't drive with your foot in it. And yes I saw where you added injectors but they may not be big enough for your application. Like I said wait until Nistune gets back with you on this. I have no further comments other than good luck
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Monday, January 19th, 2015 AT 9:37 AM

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