Misfiring under partial throttle

[Nurokil]

Ok so I hope I explain this well enough. Just got the project up and running, 91 Fastback Sr swapped ect..... Got it titled and on the road yesterday. Driving to work today because you know i couldn't wait any longer to drive it, and on the interstate and realize when i am just barely touching the pedal to maintain speed it kinda cuts out. There is no drop in RPM's just kinda for a lack of better works.... bummm.... bummm. bum..... Almost like it is not getting enough fuel. If i push a little more on the gas it's fine. Only when i am maintaining speed does it do it. No idel problem no normal acceleration problem and even when i get on it nothing, only when i am trying to maintain speed. Any help would be much appreciated thanks all!

[sticky_steve]

That all depends on the last time you changed out your fuel filter. If its been a while, then you might as well change it out for a z32 filter ( it flows more fuel anyway and something that should have been done when the swap was done). And if it still does it, you might need to upgrade the fuel pump.

[Nurokil]

Without going through everything gonna just try and give the meat and potatos. Red top SR, No idle issue, no acceleration issue. When just maintaining speed motor acts rough. By rough I mean kinda cuts in and out Mostly between 2800 and 4000k rpm's. Thought was a fuel issue and talk to a guy from my wifes work. (ASE Cert. dealership mechanic) He says sounds like a ignition problem but doesn't have the right machine to diagnose it. If i accelerate it does just that without a skip of a beat. If anyone as dealt with this and found a cure or just thinks they might know what it is PLEASE feel free I am open to anything. On a side note anyone think it might be the coil packs?Thanks in advance all!

[Nurokil]

It was upgraded with the swap, changed out for a new z32 and I have a walbro 255. Nothing has changed. I started a different thread with a better explanation. think it maybe something with the ignition.I really appreciate you even making a comment tho. I gave up on this thread, started a new and what i think maybe better and then seen you had left a comment.

[the converted]

I merged your threads because there is no need to have two threads on the same problem.

If it was a fuel issue it would be doing it at full throttle, not cruising. I would start with double checking your timing, and if there is no improvement there, I would check your MAF.

[treymeiste]

i had the same problem for a while... i messed with my timing, didnt fix it, i was also running no o2 sensor, bought a 300zxtt o2 sensor hooked it up and didnt do it till i sold the car. easy way 2 check get a dvom {digital volt ohm meter} and check your signal wire on the sensor should read betwean .1 and 1.1 at all times, if out of the range replace the sensor

[theother1wstolen]

i had a similar problem on a car once turned out to be the brand new plugs, try getting a set of cheap autolite coppers and see if that helps.

[wheelman]

Forgive me if my idea isn't relevent to an SR engine, I'm new to Nissans but not engine problems.

The problem the OP describes sounds a lot like a deceleration fuel cut off (DFCO). I don't know if the SR ECU has that feature but I'd be very surpised if it didn't.Anyway I swapped an LT1 into a 240Z and put a custom tune into it's ECU. When I'm cruising at very lite throttle and let off just a little to maintain speed it sometimes crosses the DFCO threshold setting for the TPS. Everytime this happens the engine cuts out because the ECU thinks I'm decelerating and cuts the fuel to improve mileage.

Maybe the OP is encountering the same issue with his car.

[justin240]

if you drive your car hard at all on any kind of a constant basis plug wires wear out, when they do they cause backfiring, not all the time, just when shifting or maintaining speed, sometimes when in gear and running lower than 2k rpms and coasting, but my point is i have to change mine almost every month or 2 or i get the backfiring problem that you are describing, change your wires and go for a drive

[jetroy240]

Sounds like u have a spark plug misfiring under load. Any codes? If spark plugs did not fix it, try to swap known good ignition coils. Inspect the coils first, u might have oil there, probably valve cover gasket leaking. If no oil leak, I would inspect the tip of coils for any burnt marks or wear.

[tercel drifter]

one million dollars bet its a boost leak i had the same thing when i swapped my second sr everything was runinning good but when i was cruising regular speed and holding it it would sorta bog. tightened all my couplings and solved the problem

[jetroy240]

yep, I agree, a boost leak will do it.

[rogoman]

If there's no boost leak, check out the TPS with an ohm meter.

[Nurokil]

What exactly do I do to check/fix a boost leak. What all do I check and hod do I go about that?

[nuts510]

I tend to agree I have ran into several cars with a low-speed light throttle Miss and it didn't turn out to be corrected when a installed am o2 sensor. I have also seen when the car sits a while the injectors get about half clogged up in that light throttle the problem shows up. But I would start with the 02