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LS1 Revs high and runaway when moving

s_ikari2015

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There are no leaks. Sprayed every possible part of the intake, injectors, IAC valve, spark plugs... not a single bit of difference in idle. It idles fine. It's not the cable.

It's not IDLE... it's REVVING when MOVING.

Sitting still it idles as it should no problem whatsoever. As soon as the car starts rolling it rockets up to 1500rpm without ANY throttle input or anything. Just rolling down the driveway.

Then driving, it tries to go to 1500rpm if it's under, and occasionally it will run away to 3000rpm. It's not "kind of around" 1500 and 3000.. it's EXACTLY 1500 and 3000.


WHAT THE ACTUAL **** IS THIS THING DOING!

It's not the Tune because my old ECU does the same thing... so what has the engine builder fucked up when they took the engine out, tore it to pieces, put it back together then put back in the car?
 

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Do you have a basic OBD2 scanner - even a cheapy $20 bluetooth one? Would start looking at some of the live data, looking at sensors that are doing things they're not supposed to.

Inclined to not think it's an engine build issue just because you'd think it would do it all the time then, regardless of if it was moving. This sounds super funky, like it's some kind of weird ground or electrical issue. AFAIK, all sensors are supposed to get their grounding from the ECU so there's not a massive voltage drop on the sensor negative between the sensor and ECU (block/head grounding is more for spark ignition). Maybe next step is to look at if anything is undergrounded or "over"grounded.

Would also turn attention to the speed sensor as this seems like the only thing that would do anything in that situation - got any speedo issues?
 
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VS 5.0

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Would ABS sensor be involved given it only happens when moving ?
 

s_ikari2015

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Speedo works perfect. Had a mate with a tech 2 set the right pulses per second when I changed the diff gears years ago, so the speed sensor is still sending the right signal because the speedo is still GPS accurate.

ABS seems so far out there... but it makes me think I could at least eliminate it by removing the ABS fuses, and also the cruise control fuses. Just in case.

I have a decent OBD2 diagnostics tool, with SRS included. There are zero errors. I've even run it while driving to show live data. Nothing out of the ordinary. Throttle position even reported as closed (the usual low percentage) when it was revving to 3000rpm.

So, the injector and spark timing is being adjusted by the map tables to make it rev its nuts off without throttle input or idle air control valve adjustment. So what sensors do timing map tables rely on? It isn't the ECU because it does it across two ECUs, so it MUST be reading input data, or failing to receive input data.

MAP Sensor
MAF has been removed and tuned out. OBD2 reader shows permanent constant value for MAF so I know that's working.
Cam angle sensor
Crank angle sensor
Knock sensor

Is there any other sensor that would be used to adjust timing tables?

If the cam is clocked wrong could it contribute to this?
 

s_ikari2015

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I spent several hours reading all sorts of threads allover the place and found someone experiencing almost identical problems.

It's a symptom of the Throttle Cracker and Throttle Follower tables in the ECU. They're factory so thus why it happened with both my ECUs. They "crack' the idle air control valve when it detects speed (car moving) to prevent stalling and anticipating gear changes. It's literally a controlled vacuum leak.

So, after fitting a bigger cam and tuning appropriately, if the Throttle Cracker table isn't adjusted, and the there's too much air leak around the throttle body butterfly valve, then it can result in excessive revs when the "cracker" table activates.

In my case I can feel the air being pulled around the throttle body flap (ever so slightly) but add that and the 2mm factory hole that's drilled into it and it may have resulted in too much leak. I've covered the hole with aluminium tape and adjusted the throttle stop screw a little to hopefully get a better seal.

I then test drove and the cracker revs sat at 1100rpm, and dropped to normal idle much quicker. So, warm engine maybe working normally, or I was just lucky.
Tomorrow in the cold morning I will try again, as it has never failed to be problematic on those cold mornings.
Fingers crossed.
 

s_ikari2015

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So it didn't do it this morning until I put the AC on do defrost the windscreen. Even then the AC only made it rev to 1500rpm. Once the engine was warm after 5 or so minutes driving the AC didn't do it.

However, on the drive home it DID do it again, but only wanting to rev to 2000rpm. So, I think I've identified the cause (Throttle Cracker), but there may be additional vacuum leaks or problems exacerbating the symptom.

I've partially taped over the IAC valve opening in the throttle body to restrict the airflow and I'll see how that affects it over the coming days.
 
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s_ikari2015

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Yeah, still trying to go to 2000rpm. Also, it's misfiring and rattling once the oil is warm. Booked it in with the engine builder because something is fundamentally wrong.
 

s_ikari2015

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I suspect the cam was degreed wrong and the valve timing isn't right. Misfiring, occasionally backfiring, excess of vacuum and a bad metallic rattling sound? Yeah, valve timing being out ticks all those boxes. Still gotta wait until the 24th when the engine builder can take the car, but before then if I can find my boroscope I'll have a look in through the spark plug hole to see if there's any evidence of the valve contacting the piston.

The other possibility, it could be a failed lifter. They're brand new and came with the cam kit, so that would be very unfortunate. They are hydraulic so they may not have been prepped right. Also, it may have failed due to oil starvation because the engine light comes on under heavy acceleration. Again, the oil pump was replaced during the rebuild and the oil pickup was "adjusted". So something was done wrong in the build.

I have no choice but to drive the car to get to work right now, my other car has a busted wheel bearing and I don't have the equipment or time to fix it, and have to wait for my other mechanic to be available. I've not had much luck with my cars in the last 6 months, in spite of the nearly $15k I've spent on the two of them.
 
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