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LS1 Revs high and runaway when moving

s_ikari2015

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Hi I need serious help. Just spent nearly 10 grand on my V2 Series 1 Monaro LS1 (cable throttle) with a full engine reseal, head work, cam and tune and ever since it has a problem that happens more often than it doesn't.

As soon as the car starts moving, even in neutral (6 spd manual) the revs jump to around 1500rpm. Same if I put the clutch in and coast, it stays where the revs were or climbs to 1500rpm if it was lower.

Even worse, sometimes the bloody thing runs away to 3000rpm! It just goes nuts! Spot on 3000rpm.

About a second after coming to a complete stop it drops back down to the standard idle, around 750-800rpm.

Most of the time it just "floats" when changing gears, no matter the engine temp.

I've replaced the IAC valve, and has had no change. I've removed the intake pipe to visually see the throttle body when it's idling high and it's not opening (I thought maybe the tune had someone messed with the cruise control module, yeah, left field I know). I've unplugged the IAC when it was idling fine, and the car just sat there idling fine. I turned it off and started it again, it went to around 1000rpm for a bit, then up to 1500rpm until I plugged the IAC back in, then a second later it was back down to 750/800rpm normal idle.

I've taken it to the tuner, and no freaking surprise the damn car drove perfectly for them. They suggested it might be a bad engine earth, so I fitted a new earth straight from the drivers side head to the negative terminal. First drive after, everything was fine. Second drive, back to being mental.

So, how does the ECU actually control the idle? What sensors does it use to determine that?

Is it just me, or is the tune cactus? They had to replace the ECU because my eBay special MAFless mailorder was "locked" and they couldn't use it. Have they supplied a dodgy ECU, or have they screwed the pooch on the tune?


Please help, I've lost the love for my Monaro, and that has made me deeply depressed.
 

_R_J_K_

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So, how does the ECU actually control the idle? What sensors does it use to determine that?

Is it just me, or is the tune cactus? They had to replace the ECU because my eBay special MAFless mailorder was "locked" and they couldn't use it. Have they supplied a dodgy ECU, or have they screwed the pooch on the tune?
The tune doesn't change, it can't go bad, and you don't tune to fix problems (unless that was the problem in the first place). Who replaced the ECU? Who did you take it to for tuning? Does it happen all the time or only when it's hot or cold? Issues with acceleration? When did it start happening? Does it do it at a normal idle? Does the idle jump around or does it just hang high constantly? Does a scanner bring up any codes? May also help if you mentioned where you are. TBH it doesn't sound tune related to me, classic symptoms of a vacuum leak, but it could be something to do with the throttle follower/cracker in the tune or the idle table has not been changed to compensate for the cam.

As for idle control, the IAC obviously, and the ECU will get RPM readings from either the CAS or on some cars the coil (I'm not super familiar with LS1s so IDK) to get to its target idle RPM by adjusting the IAC, and also takes coolant temp readings to idle higher or lower depending on the engine's temperature to get it up to temp if it needs to (a lot of people mention strange idle issues in cooler weather).

Have you tried replacing the MAP sensor? And are you just using ebay stuff or something decent? If you still have the MAF have you also tried plugging that in?

First thing you should do though is look for vacuum leaks or splits in any of the piping and lines (any line attached to the manifold), or the possibility that one of the intake seals is under-torqued or slightly out of alignment and is letting air in. The very first thing I'd do is start looking for vacuum leaks with a can of Start Ya' Bastard or something similar, basically you spray it around the engine until you notice a change in idle - heaps of videos on YouTube about it.
 
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crew chief

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Cam specs? There are quite a few tables that affect revs as _R_J_K_ stated.
Another tuners opinion might be in order.
 

s_ikari2015

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I'm South of Perth, Rockingham region. I'll give the Start-Ya-Bastard a go just to eliminate that, but I very much doubt it.

It has always done it since the cam and tune, and it's a constant rev, not jumping. When it runs away it will sit at exactly 3000rpm, when it just "floats" it will sit at 1500rpm.

IAC valve isn't the culprit, how can it be when it's unplugged? I let it sit at 800rpm, then unplug the IAC valve and a few seconds later because the ECU can't see the IAC anymore it will increase in revs to around 1000rpm, as I'd assume a "safemode". So, how else does the ECU adjust revs?

It's also cable throttle and visually it isn't moving when the idle goes high, so it's not the cruise control module either.

So far 8 times out of 10 it does it, revs up to 1500rpm when moving, even in neutral, then slows back down to normal 800rpm idle a second after the car stops moving. Start the car, it does the NORMAL high rev thing until the sensors warm up then settles to 800rpm. Leave it in neutral, take the handbrake off and start rolling backwards out the driveway and it spikes up to 1500rpm and stays there until a second after completely stopping.

Is it just the one speed sensor, or are there two, one for the ECU and one for the dash? Because the dash speedo works fine all the time.

There's no change in throttle input, no other conditions are changing other than the car sensing movement. It ONLY happens when the car is moving.

So it HAS to be the tune causing the high revs, but is it reading the speed sensor incorrectly, or is the Powertrain Interface Module causing problems because it hasn't been linked correctly with the new ECU?

Could it be compatibility issues between the ECU the tuner installed and the PIM? I hear what sounds like relay clicking when the car is stationary and idling coming from the passenger footwell, but when I stick my head down there it seems to be coming from the center, not the sidewall where the PIM is located. The clicking stops when the car is moving and doing the 1500rpm thing. Should I try to get someone with a Tech 2 to re-link the BCM and PIM?

I have my previous ECU, could I use this to help diagnose? I know the spark and injector timing won't be great, but it should still run and if it's a bung sensor then it should still do the same thing, right?

Oh, and no codes (I have an OBDII reader that also does SRS) and no engine lights.
 

s_ikari2015

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Today I put the old ECU in and it started right up and straight to 750rpm idle. Of course it was a little rougher than the tuned ECU. It sat there idling happy, throttle blips didn't float, it didn't rev when rolling.

Put the tuned ECU back in and it started up and immediately went to 1500rpm for about 3 seconds before settling to 800rpm. Smoother idle, throttle blips didn't float, it didn't rev when rolling. Hmm okay. So I've left the tuned ECU in there and went for a drive. It was perfectly normal, no rev float, no runaway.

I will drive it again later tonight, then again a couple of drives tomorrow just to see if unplugging and plugging it back in has fixed whatever weird was going on.

If it doesn't the old ECU is going back in and then test driving the same. If it's the PIM instead (possible cause to the clicking in the passenger footwell) it should rev float and runaway with either ECU.
 
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s_ikari2015

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So, tuned ECU back in and first drive was perfect. Second drive was perfect. Third drive, first thing cold morning and it started, revved to 1500rpm for a while before settling, then didn't rev up when rolling out the driveway... however as soon as I started driving away it revved itself up to 2000rpm and kept itself there until two seconds after coming to a complete stop.
Fourth drive, exactly the same thing, revving to 2000rpm all by itself until two seconds after complete stop. Oh, and throttle blips when stopped and the revs float at around 2000rpm for a second.

Don't have time to piss fart around swapping the old ECU back in tonight, will try tomorrow night instead.
 
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