Did we ever figure out what that hesitation was?

[Mr Hardware]

I've now owned 4 I6 AU's, and every single one seems to be afflicted by a slight hesitation on decent accelleration from standstill. I'm 99% sure it's a petrol delivery issue, possibly not being able to get enough (too much?) fuel into the chambers in time.

Symptom: Sitting at intersection, want to use anything above 50% throttle as soon as clear to do so, car hesitates slightly before pouring on the power. Doesn't hesitate or stutter from then on, until takeoff from standstill happens again. Also doesn't happen if you only use say 20% throttle.

My Gas AU does it when running on petrol, but not on Gas. On gas the throttle response is exacting and without any hesitation whatsoever. Flick it back to petrol, and she'll hesitate.

What's the story? Injectors? Fuel pump? something in between?

[GT 276]

I have same problem with an au xr8 ute!

Recently fitted and had supercharged tuned can drive under 50% throttle fine as soon as I hogh it any more I get a delay then kicks in!

I spoke to someone who said it could be TPS throttle position sensor

[PridenJoy]

My XR8 ute does this exact same thing on LPG (SVI) and to a lesser degree on petrol also, initially i thought it was ignition related so i changed coils and leads but still there, keen to know a definitive cause of this.

[Hitman]

Could it be the ecu limiting the torque to save the transmission?

[Mr Hardware]

Well my coilpack is only 40k old and it is doing it just the same as it's ever done.

I don't think that the AU has any sort of torque limiting from the ECU. Certainly after the initial hesitation it storms away at full noise from 0kmh.

The only thing that leads me to think it's a petrol delivery issue is that it does it on petrol, but not on gas. But I suppose there are other factors, my gas doesn't use the o2 sensor for example. I read some old threads and I couldn't find one that definitively displays a resolution to the issue; most people seem to try a bunch of things all at once (replace coilpack, leads, plugs, fuel filter, fuel pump, reset ecu, tighten intake manifold, clean tps) and then it goes away, sometimes.

I've had this present on 3 different petrol only AUs as well, one was VCT and it was most pronounced on that example.

[madas]

I had the same problem on my AU111 and found the intake manifold gasket at 6 broken. Used some carby cleaner and sprayed it around the gasket which identified the problem straight away.
Replaced the gasket and also the cam position sensor and now goes like a power of ****.
no hesitation what so ever all for under $100 and a couple hours and scratched up hands

[mik]

mines good ,no hesitation although it does have the trademark falcon btr tall 1st gear lethargy, not that its slow, but even with the 3.45 xr6 diff, for the first 20 feet off the mark its not exactly a hell cat ,
i look forward to day when it comes to engine freshen up ...... time .... we tweak grandpa`s axe a bit a bit.

i always run good plugs in mine and use 95 or better, currently got like the old racing style plugs with the 4 prongs on it, hasn't made any noticeable speed difference (i dont think), but to be honest these days don't really give it the hiding i once did to find out.

i just went and checked fuel pressure 2.5 hours after i moved it this morning, it is still showing 23 psi on the gauge i have on the fuel rail, that is still within leak down rate specs ( i think )
i didnt check the running pressure but its usually just over 40 psi, i remember a bloke on the forum say he had trouble with his and 40ish is a bit low, but touch wood mine still seems ok at this stage , and mine is all original fuel system at about 230 klicks..

[Dauphin]

I haven't noticed this before. Are you able to make a video of it? I'd be interested to see/hear it.

[Jarry]

I've had this off-idle hesitation at WOT issue too.

Been chipping away at it for years now. Have changed coil pack, leads, plugs, cleaned TB, checked TPS and T-MAP, changed intake manifold gasket, changed intake tract to BA...but it is still ever so slightly there.

When I switched from 5w-30 to 10W-40 engine oil, there was a big improvement.

I notice it is worse in summer.

I agree that it could be a fuelling issue. I recently replaced my fuel pump. This might help if the problem is due to the fuel line and injector rail getting a bit of heat soak when sitting at traffic lights, and a better flow rate from a fresh pump might help keep the fuel cooler... but it is too soon for me to tell.

It could also be a rich bog, hence it being worse in the heat of summer.

[martos]

Originally Posted by Mr Hardware

I've now owned 4 I6 AU's, and every single one seems to be afflicted by a slight hesitation on decent accelleration from standstill. I'm 99% sure it's a petrol delivery issue, possibly not being able to get enough (too much?) fuel into the chambers in time.

Symptom: Sitting at intersection, want to use anything above 50% throttle as soon as clear to do so, car hesitates slightly before pouring on the power. Doesn't hesitate or stutter from then on, until takeoff from standstill happens again. Also doesn't happen if you only use say 20% throttle.

My Gas AU does it when running on petrol, but not on Gas. On gas the throttle response is exacting and without any hesitation whatsoever. Flick it back to petrol, and she'll hesitate.

What's the story? Injectors? Fuel pump? something in between?

You say it stutters on take off on petrol?
Tried using plugs with smaller gap than the recommended?
As an example,if it's recommended 1.4 gap,try 1.2.
Just a suggestion that worked for me on a 5.6 stroker.I had 1.3 but went too 1.0.
Mine wasn't too bad on take off but 3rd 4th & fifth gears it would hesitate until revs came up.Found spark was arcing to side of plug until higher revs.
Try "NGK normals" as a tester.They're cheap.

[Jarry]

Top suggestion on the plugs gaps.

But it doesn't explain why Mr Hardware's car is all good on gas but funky on petrol (when you're supposed to run a smaller gap on gas right?). I still reckon that the OP is on the money and it is a fuelling issue.

FWIW, I run a 1.1mm gap on the factory iridium plugs, where I think the spec is 1.3mm.

I did this to try and help preserve the life of the coil-pack after a Fuel Miser one died on me after 4,000kms.

[Mr Hardware]

Yeah I just run ordinary $4 NGK plugs, but the .8 gapped ones for Gas.
I will take a look at the intake manifold gasket & related bolts. If I chose to swap the gasket, it'd be a simple case of unbolt intake manifold, scrape off old gasket, put on a new one, bolt it back together? I wouldn't need to muck around with anything else, would I?
I'll attempt to make a video, however it'll be hard to show the delay. I'll wait until brisbane isn't a giant swimming pool as if i attempt WOT from idle today, i'll end up on a current affair.

[Jarry]

Originally Posted by Mr Hardware

I'll wait until brisbane isn't a giant swimming pool as if i attempt WOT from idle today, i'll end up on a current affair.

Classic!!!!!

Originally Posted by Mr Hardware

it'd be a simple case of unbolt intake manifold, scrape off old gasket, put on a new one, bolt it back together?

I think the general consensus is:

• remove TB,
• remove manifold bolts,
• loosen manifold supporting rods front and rear,
• pry the manifold away from the head,
• then slip out the old gasket, then in with the new one....

After pulling out the old gasket, I just sprayed some aerosol brake cleaner down there in a lame attempt to "clean" things up.

Having these items on hand made life a lot easier:

• 10mm swivel head ratchet spanner
• Loctite 243 Blue
• Silicone to seal the metal heater hose o-ring - the one that seats in the thermostat housing ('cos no doubt the o-ring is stuffed, if not already weeping)

On the BF you would need to get to the bottom bolt of cyl #6 from underneath the car (due to the "KTM" external trans cooler... KTM... BRaaaap!!!!), but on the AU you can do it all from above. I waited til the engine was cold so I didn't scald my arm on the metal heater hose.


My mechanic reckons you can do the manifold bolts with an impact driver and flexible extension, but I couldn't decipher if he was taking the p!ss or not...




Good Luck!!!

[Maka]

Try changing your fuel reg perhaps, anyone tried cleaning theirs at all & put it back together without replacing the o ring's? I did & then it had a hesitation off idle, replaced it with another unopened one, hesitation gone. Mine was a hesitation too not a misfire just above idle. I replaced the manifold gasket, coilpack previous too plus running a BA inlet pipe into opened to 3inch stock airbox. Goes fine now!

cheer's, Maka

[Mr Hardware]

Okay, that's a thought too. What's a fuel reg? Sorry, I've been 99% LPG for so long i don't know where to find it.

[Maka]

Fuel regulator is on the end of your fuel rail near the firewall, round thingymabob with a black tube running out of it. I would do the other stuff including fuel filter first before looking at it though, my original one had over 300kay on it & it wasn't that dirty, probably shouldn't have touched it. Good luck Mr Hardware!

cheer's, Maka

[Maka]

I should add the fuel reg is a tight fit too, so be careful not to damage it either when removing it. Iirc i may have damaged it when removing mine, slight dent on the body of it.

cheer's, Maka

[Jarry]

Hey Maka,

I had the issue before pulling out the fuel regulator to check the gauze filter....

It was clean as a whistle (at about 120,000). I regret pulling it out too, as I warped the casing

Was lucky enough to still get it to seal though... Phew!