Off boost misfire/hesitation/sputtering when hot at cruising speeds/partial throttle

[dylanc]

Hey ya'll,

Been trying to diagnose a problem. I'm getting a hesitation/slight bucking with partial throttle applied at cruising speeds. It only hesitates when it is hot (85 degrees + oil temps). AFR gauge will show an erratic reading spiking to full lean, back to normal, then repeats.

WOT - everything is perfect, spools fine, no hiccups or misses

Idle - Doesn't miss or bounce up and down, however it is leaner than usual [before was between 14.5-15.5 AFR at idle, now it is between 15.5-16.5 (sometimes will go higher to 17+ and feels like it wants to stall but only happens once in a awhile)]. Possible vacuum leak?

Thought it may have been a dead spot in the TPS, however, tried disconnecting it and still had the same problem when it got hot, ran great and smooth at 70 degrees oil temps.

The problem has gotten progressively worse, it first started with a slight miss/hiccup at 0 psi going up hills, but now it is doing it much more frequently on flat stretches as well.

Car has new plugs (1000km) and coilpacks (5000km Splitfires), new injectors and fuel pump (3000km on both).

Watchya guys think? Help is much appreciated, thanks.

[Bee-RSkyline]

I had similar problems, turned out to be a combination of vacuum leak and incorrectly adjusted tps closed settings in nistune

[garetth]

What spark plugs and did you gap them? Try cleaning maf also

[dylanc]

Bkr7e's, and yea they're gapped. Plugs ain't the issue here, will be testing for vacuum leaks tomorrow, tested TPS tonight with a multimeter and TPS is in spec.

[DarkCaporaL]

I recently had what felt like intermittent misfires at cruising speed. It was nothing major, just the odd hiccup here and there, I first noticed it on the drive from Ottawa to Toronto for Skymeet.

I wasn't able to pinpoint the problem and it progressively got worst. That is until one day I was on the highway only to notice my revs were limited to 2-3000 rpm. Obvious MAF failure.

Car also stalled a few times at red lights if the hiccup happened to occur when coming to a stop. It also stalled once a minute after I started it (idle bounced and then stalled). Every time it restarted normally as if nothing happened.

All this to say, don't rule out the MAFs. What may seem like a misfire may be the MAF connections (classic solder issue inside the MAF) interrupting for a second which drops the car into limp mode for only a fraction of a second but makes it look like a misfire.

[Skym]

Also intake airleak behind MAF's can cause stuttering. If a intake airleak before MAF's, can make ECU hit boost cut.

[dylanc]

Interesting, yea I initially thought it was MAFS but I have a spare set of refurbished ones at home which I tested and problem still exists with the MAFs that are known to work. Thank you for the replies.
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[judasentinel]

TPS voltage issue usually causes idle hunting. If you dont have that, it's unlikely it would be the TPS.

Air leak is a possibility, but is could also be the following:

1. Fuel pressure. I see you have a new fuel pump installed, but I had this issue years ago and turned out to be due to reduced fuel pressure caused by a bad walbro 255.

2. Ignition timing. Ive seen similar stuff also happen due to the timing belt skipping a tooth, thereby causing a retardation and loss of power, which seemed to go away under full load.

3. Another thing to check is the spark plug gap. When I reduced the gap on my plugs to 0.8mm, it got worse. 1.1mm worked better.

[Skym]

17's A/F ratio can mean intake or exhaust manifold leak or fuel pump as mentioned above. The fact it happens when on boost or when engine is warm points towards those area's (check fuel pressure, check for intake airleaks between intake, exhaust manifold, head). RB26 engines are known to blow intake gasket's (Tomei metal gasket set solves it and should have metal intake, exhaust, headgasket).

[SkylineUSA]

What will lean you out? Injectors, fuel related. I would think under pressure, a vac leak would likely wreck havok on the A/F, which you are not getting. Could it be ECU, fuel curve screwing up? More than likely, no. I have seen MAFs screw up, but they scew up under boost as well. Fuel pressure reg, not likely, since once again under boost everything is smooth. Hope you solve it.

[dylanc]

Skym, my on boost performance is perfect, the problem only exists off boost. On boost AFR is perfect as well, 11.5. Only lean on partial throttle (off boost) and idle.

Intake and exhaust mani gaskets have around 1500kms on them.

Checked fuel pressure today, its dead on and tested a new FPR as well, problem still exists.

Going to smoke test the system tomorrow and see if anything pops up. Thanks guys.
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[dylanc]

SkylineUSA, thanks for the response I'll have my injectors flow tested if I don't have any vacuum leaks.
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[Skym]

If off boost only, stock narrowband o2 sensors affect A/F ratio. Sputtering can be o2 sensor being lazy (running rich longer and creates a popping sound from exhaust) to switch from lean to rich and rich to lean.

[dylanc]

Hmm, wish that was the case however I have deleted the narrowband O2 sensors and adjusted my power fc to not reference them (car is tuned for no narrowband O2's).
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[judasentinel]

Injectors.......??