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Discussion Starter · #1 · (Edited)
I have a single turbo with vpc and fuel controller running 850 cc injectors. The car has run flawlessly for over two years. Last month, the car started to idle rough, with the rpm needle bouncing between 500 and 800 rpm. During the "bouncing", the AF on my wideband was going lean (like 16:1) and then going back to stoich. It was a fairly consistent pendulum between these two AF points in a steady pulsating fashion.

I had been running leaded race gas, so I figured my O2 sensor was out. I changed it out and nothing happened; same thing.

I then figured it was one of my fuel filters (I have the stock one and an aftermarket one). I put a brand new stock filter on and it made zero difference. I did not clean out the aftermarket one though.

Next, I richened-up the VPC idle nob. This did make a difference and now, when the car is warm, it will idle reasonably well at around stoich. However, if I try to slowly cruise through a parking garage with my foot barely on the accelerator, the lean condition comes back. However, if I accelerate normally from a stoplight, it bypasses this temporary lean condition and the car drives normally.

At high boost, I have plenty of fuel, so I doubt it can be one of my dual Walbro pumps.

At cold start, the car idles way too rich because of the richened VPC idle knob, but as it warms up it gets better. Before I changed the VPC knob, at cold start the car would idle perfect and then once warm would begin the rough idle.

So, it seems that whatever is causing the lean condition and associated stumbling is only affecting things after the car warms up and only at idle and very light throttle pressure.

I have a stock intake manifold and a breather on the exhaust-side PCV.

Anybody have any ideas?
 

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King Ding-a-ling!
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If this condition just occurred overnight, it would lead me to look towards the idle air control valve or a small air leak after the throttle butterfly. I would not adjust anything.

Brandon
 

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Discussion Starter · #3 ·
It did happen overnight and I had done no recent mods.
 

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King Ding-a-ling!
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One thing that I would check that I have seen on my wifes old 99 Lexus RX300 and my honda accord... is the IAC motor will seize or can't move to its full potential. This is usually caused either by grit in the intake system or suit from the egr system. In both cases I just removed the IAC valve and cleaned it out with brake parts cleaner. FWIW... I have never had a problem on my 13 year old high mileage MKIV. This could also be a symptom of the TPS being off cal.

Brandon
 

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Alpine Hardtopper
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You're probably correct that your fuel system isn't the likely cause. If you have enough fuel for driving around, you should have plenty to idle with. If it was running rich you could look at leaky injectors or something, but that's not the issue here.

Is this on an OBD I or OBD II car? Did you replace both O2 sensors, or just the front one? If you're OBD II you may consider getting a laptop interface for your scan port. You can read several of the ECU parameters, including the short term and long term fuel trim.

The other thing to check out is that any misfire will show up as a false lean condition. Since the O2 sensors are just sensing oxygen in the exhaust....when one cylinder doesn't fire, or fires poorly, there is enough oxygen coming out the exhaust to convince the O2 sensor that it's way too rich. But it does seem odd to have a misfire problem at idle and *not* anywhere else.
 

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Boost Junkie
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I would definately clean the IAC. These cars at high mileage or age start to get deposits on the IAC itself, which can cause this problem. Do you have cams or cam gears? If you have cam gears make sure they haven't slipped at all. If you have cams I would lean even more toward the IAC.


Steve K.
 

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Evill Supra said:
I would definately clean the IAC. These cars at high mileage or age start to get deposits on the IAC itself, which can cause this problem. Do you have cams or cam gears? If you have cam gears make sure they haven't slipped at all. If you have cams I would lean even more toward the IAC.


Steve K.

Good Advice. Let us know what happens.

-Corey
 
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Have you found a solution to correct the idle surging? I am having pretty much the same issue with my 1jz swap in a mkiv. Current diagnosis is IAC, but I am leaning more towards vac line routing or haltech tuning.

//Dylan
 

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(overspooled)
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may want to check electrical connections for vpc too...a loose power or ground could yeild similar results. also check your battery terminals. check to see if the problem changes for example...when your lights are on...
 

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Pull spark plug cover off and check the timing. Make sure the crank is at TDC and see if the cam gears line up to the indicating marks.
 

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Discussion Starter · #11 ·
I removed my fuel controller and hand checked all wiring connections. The problem is now gone. I feel there may have been a loose connection with the fuel controller that was corrupting one of the signals. Thanks for all the comments and help guys.
 
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