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Car tries to take off at idle?

Started by _tyre fryer_ , Aug 05 2020 04:24 PM

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#1 _tyre fryer_

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Posted 05 August 2020 - 04:24 PM

Hi guys, I think I know the answer to my own question here but can't find anything on google so I would like some more opinions. 

 

Car is a LC with 304 injected 5 litre and T700 box. When I had it rebuilt, the trans guy said he would 'upgrade' the converter to a higher stall (cost very little so I doubt he did). The car  has always felt like its ready to leap away at idle. Like I have to stand on the brakes at the lights (around 700RPM) otherwise the thing just takes off. If I release the brakes from idle, the car will happily pull itself up over 20Km/h. I'm presuming that the stall converter is too low for a car with approx 300kg less kerb weight than it was designed for (factory box is out of a VQ statesman) and it would be rectified with a hi stall. But I'd like some confirmation on this before spending $1000+ on a new converter. Am I missing something?

Also, the diff gears are 3.08 but I have 3.9s ready to go in. 


#2 yel327

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Posted 05 August 2020 - 04:35 PM

What is the idle speed in park? At first read it sounds like it needs adjusting down. A standard VQ 4L60 converter shouldn't do much with the engine idling at the factory set idle speed in drive even in an LC. Will it do much on a hill or just on flat ground?


#3 _tyre fryer_

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Posted 05 August 2020 - 05:13 PM

You know what? I think you may have a point there. Can't currently check the car as I'm replacing the fuel pump and waiting for some bits in the post. But I think it does rev unusually high now you mention it, it definitely drops RPM when you go into gear. 

Reading into the causes of high idles in these things, looks like there is gonna be a bit of detective work involved! But still a lot cheaper than buying a big converter. From what I can read it should have a base idle of 500-600RPM?


#4 claysummers

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Posted 05 August 2020 - 08:24 PM

Check for any vacuum leaks, then adjust idle down with the car in drive. You will need to chock the wheels by the sound.

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#5 warrenm

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Posted 06 August 2020 - 07:28 AM

The idle speed may need to be altered in the tune, if it's efi.


#6 Dr Terry

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Posted 06 August 2020 - 11:30 AM

The idle speed may need to be altered in the tune, if it's efi.

Unless some 'clever' mechanic has fiddled with the throttle stop screw.

 

Dr Terry


#7 gtrboyy

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Posted 06 August 2020 - 03:26 PM

Sounds similar to my vp ute.

 

When did 304 conversion turned out to be vacuum hose leaking.

 

Still pushes on the brakes a bit at idle but nowhere near as before....engine has small cam + other basic mods.

 

Got 3k stally from Accelerator convertors $750'ish iirc also got 3.45 or 4.11 gears for it to go in...just need big ass transcooler & motivation/time


#8 _tyre fryer_

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Posted 14 August 2020 - 07:33 PM

Okay, finally put the new fuel pump in so I can test it. In park it idles dead on 700RPM and in drive it's about 550. alleviating any pressure off the brakes makes it want to just take off.

I only had a minute to investigate so I sprayed some start ya bastard over all the vacuum hoses and nothing changed, then I wound the idle stop screw out (already was out all the way). So now I'm thinking its the IAC or maybe the ECU needs playing with?

Would appreciate anyone else's thoughts on this.


#9 rexy

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Posted 15 August 2020 - 12:36 AM

Are your brakes and booster in good order?


#10 S pack

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Posted 15 August 2020 - 09:15 AM

Okay, finally put the new fuel pump in so I can test it. In park it idles dead on 700RPM and in drive it's about 550. alleviating any pressure off the brakes makes it want to just take off.

I only had a minute to investigate so I sprayed some start ya bastard over all the vacuum hoses and nothing changed, then I wound the idle stop screw out (already was out all the way). So now I'm thinking its the IAC or maybe the ECU needs playing with?

Would appreciate anyone else's thoughts on this.

The same thing happens if I release the brakes in our VE and VZ when sitting on level ground, idling in Drive or 1st. Both are factory standard V6.
 


Edited by S pack, 15 August 2020 - 09:15 AM.

#11 rexy

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Posted 15 August 2020 - 12:13 PM

My HQ was fond of doing this until I added the vacuum pump to the brake system. Had to really press hard on the brakes to keep it in place.
Don’t know that I’ve ever had an auto that doesn’t start moving froward in drive when the brakes are released?


#12 _tyre fryer_

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Posted 15 August 2020 - 06:12 PM

My brakes and booster work well, i was testing it today and when you put it in gear the car really lurches even with your foot on the brakes. If you release the brake it doesn't crawl off, it takes off. I feel like I could pull out onto a main road in front of traffic just by releasing the brake!

 

So reading a few commodore forums, I suspected IAC or vacuum leak. Took the TB and gave it a thorough clean and played with the IAC (retracted it in). That did nothing. I took the IAC off (still wired up) and turned the ignition on but it didn't retract, which I believe it's supposed to. Then when i started the car, I noticed the the sound that I had alwas thought was the induction noise was the IAC, it's BLOODY LOUD. Like, easily noticable in the cabin with the bonnet shit. Is it supposed to be that rowdy?

 

I also ran the car with the IAC disconnected from the wiring and the base idle screw all the way out and it made no difference. From my understanding, the car should have stalled at this point. 

 

So I guess I'll just buy a new IAC and see if that fixes things.


#13 _tyre fryer_

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Posted 20 August 2020 - 04:04 PM

So after a bit of mucking around with a fuel supply issue, I have the car back to normal. New IAC ready to go in but as a quick fix I followed the 'base idle reset' procedure to reset the IAC valve.

Well, that has made a big difference, now at 500-550RPM in park and 450 in gear. The induction noise at idle has halved. Definitely way less lurching and less brake squeezing. Coming off the brakes it does still want to take off, which brings me back to my initial question, will a larger stall converter reduce this further?


#14 Dr Terry

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Posted 20 August 2020 - 06:55 PM

To me it sounds as though the idle control motor isn't working properly.

 

For example let's say the correct idle speed for the Holden 5L EFI is 550 RPM. This should be maintained under most conditions.

 

Once warmed up, the engine will idle at this speed. When drive is selected the idle motor will allow for this and it should remain at exactly 550. If the A/C is switched on, again the idle motor will adjust for this, hi-beam headlights etc. etc. 550 under all conditions. 

 

If, as you say, it's higher in park than in drive, you have problem.

 

This is all if the motor is factory spec, What I haven't seen mentioned is the type of camshaft fitted. Unless the ECU is reprogrammed to allow for a modified cam, it will be too lean & not happy at idle.

 

I would spend money on a good retune of the chip to match any larger cam & any other mods you've not mentioned.

 

Unless you've got a "big" camshaft you shouldn't need a hi-stall converter. A hi-stall converter is only generally required when the motor simply won't idle at a reasonably low speed & you it to have to set the idle at a higher speed to keep going.

 

Dr Terry


#15 Dr Terry

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Posted 20 August 2020 - 07:04 PM

Another thing that I didn't mention in my last post was the method I use to adjust the throttle stop screw.

 

Firstly, because the idle motor is of the stepper variety, so the idle speed won't alter if you just unplug it. The only way to eliminate the motor's effect is to plug the airflow thru it.

 

I run the engine until it fully warmed up (at 90 degree plus) & then remove the air cleaner tube & plug the idle air channel with my finger. This is in neutral with everything else switched off.

 

The idle speed should drop around 100 rpm. If it drops too much or just stalls the idle stop screw should be adjusted up until you get that 100 rpm drop.

 

Obviously if it doesn't drop the idle stop screw is set too high. Adjust it downwards.

 

If you have access to a scanner the idle motor "steps setting" should be around 20 to 30, Ideally 25.

 

Dr Terry


#16 gtrboyy

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Posted 21 August 2020 - 01:41 PM

Think mine is 900'ish in park then little lower in gear...does have baby cam & rev tacho increments are vague tbh...stally will help hunting on brakes but kinda of think vacuum issue for your brakes if it's not iac you're looking into.

 

vp ute will hunt but just need to add tad more pressure on pedal...still on stock stall but think 3k stally will fix that especially when f280 355 with 4.5k stally in vh sle could do it easier.


#17 _tyre fryer_

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Posted 07 September 2020 - 04:25 PM

Another thing that I didn't mention in my last post was the method I use to adjust the throttle stop screw.

 

Firstly, because the idle motor is of the stepper variety, so the idle speed won't alter if you just unplug it. The only way to eliminate the motor's effect is to plug the airflow thru it.

 

I run the engine until it fully warmed up (at 90 degree plus) & then remove the air cleaner tube & plug the idle air channel with my finger. This is in neutral with everything else switched off.

 

The idle speed should drop around 100 rpm. If it drops too much or just stalls the idle stop screw should be adjusted up until you get that 100 rpm drop.

 

Obviously if it doesn't drop the idle stop screw is set too high. Adjust it downwards.

 

If you have access to a scanner the idle motor "steps setting" should be around 20 to 30, Ideally 25.

 

Dr Terry


Thanks guys,

 

played around with a lot of this over the last couple weeks. Terry, first things first, I replaced the IAC. Then did the idle reset (disconnect battery for a min, reconnect then warm up and hold revs at >2500RPM). Worked like a dream! Idle sat dead on 500-450RPM in whatever gear. Still a little bit of lurching but no biggie, can barely hear the IAC.

 

Went inside smiling. Came out and started it the next day, back to square one! (even after warming up). So I did the IAC block with my finger and set the idle screw, sounds just perfect with no IAC. Then in this process, I had my thumb on it for a good minute, the car dropped the idle down to the previously described RPM and all was well. Turn off, start again, back to bloody 900+RPM! the IAC is deafening and with my foot on the brakes, it literally does a little wheelspin as it drops into gear.

 

To answer your question, the engine does have a 'baby' camshaft (can't currently find specs) and supposedly a chip to suit (was told it was an 'SV500' chip), I have had all the best intentions on getting it tuned but was hoping to get it nice and driveable, club regoed and some headers before I dropped money on that.

 

I don't know a lot about htese AC Delco units but I wonder how hard it would be to reduce this idle stepper setting as it seems to be purely a computer problem, atm it's tempting just to plug the IAC port, seeing as I have literally no accessories.

 

Thanks again!


#18 Oversteer

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Posted 08 September 2020 - 03:55 AM

Sounds like ecu is unhappy, could be a fault with another sensor etc causing issue, you need a scan tool to check faults, is CEL wired in ?

 

Those early Delco units were well ahead of their time, but a bit ugly now with batch fire and chip rewrite to program...you need special equipment.

 

Maybe block off iac and see if its ok to drive, get your other parts etc then go for tune/diagnosis.


#19 _tyre fryer_

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Posted 10 September 2020 - 11:14 PM

So, I simply taped over the IAC port and set the idle stopper screw so it ran at 450RPM.
So that obviously wasn’t an adequate permanent solution. I pulled out the IAC, pulled the pintle all the and popped it back in without connecting it.

It’s like driving a WHOLE NEW CAR! I’m not constantly fighting it, I can just rest on the brakes, doesn’t lurch into gears, there’s no deafening IAC intake sound. I’ve gone from pondering selling it a week ago to being super excited about working on and driving it.

Thanks for all your advice, saved $1000 on a stall converter. I guess when it’s all up and running I’ll take it to someone for a tune and get them to set the idle right. For now it’s no issue as I run no accessories and I rarely drive it in the cold.

#20 yel327

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Posted 12 September 2020 - 10:30 AM

Sounds good. Out of interest, do you have the speed sensor connected? I remember my mate’s WB with a VP 5.0L setup having IAC noise and idle issues. It was due to no speed sensor. It was a manual though.
We had to get an in-line 2 pulse per rev one for it, just screwed into the Muncie and then the cable screwed into it.

Edited by yel327, 12 September 2020 - 10:31 AM.

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