Stumped!!
Stumped!! Ok Ive got a dbc harness I built for a guy an he installed it started up idles fine but at 1400 rpms it surges up an down.
Checked the pcm on a buddys truck an it ran fine.
He replaced the maf/tps/injectors/map/cam/crank sensor same problem.
BUT it will take throttle with the tps unplugged!!!!!
I gave up built him a new harness an pcm from another truck with nothing but vats delete, egr deleted.
Same issue
But yet if he unplugs the tps it takes throttle fine.
I'm no rookie to building these harnesses I build them all the time but I'm at a loss anyone have any ideals?
Checked the pcm on a buddys truck an it ran fine.
He replaced the maf/tps/injectors/map/cam/crank sensor same problem.
BUT it will take throttle with the tps unplugged!!!!!
I gave up built him a new harness an pcm from another truck with nothing but vats delete, egr deleted.
Same issue
But yet if he unplugs the tps it takes throttle fine.
I'm no rookie to building these harnesses I build them all the time but I'm at a loss anyone have any ideals?
Staging Lane
Joined: Dec 2006
Posts: 70
Likes: 1
From: Minneapolis
I'm just throwing my 2 cents in, it sounds like the harness/PCM isn't the problem, especially if you have the same exact symptoms with almost a completely different setup. The TPS isn't exactly life or death for the LS, the PCM can detect that the engine is changing speeds based off of other sensors, cam, crank and MAF, I presume. It's possible there is a vacuum leak causing everything to try to compensate for vacuum loss, try spraying some brake clean around the intake to see if you have any obvious leaks, make sure the intake o-rings are making a good seal.
I would think you would want to hook up a OBD2 reader and look at live data, it's possible that the TPS is set incorrectly causing the idle to hunt and the computer is trying to play catch up.
Good luck, hope that helps some.
I would think you would want to hook up a OBD2 reader and look at live data, it's possible that the TPS is set incorrectly causing the idle to hunt and the computer is trying to play catch up.
Good luck, hope that helps some.
Teching In
Joined: Nov 2011
Posts: 30
Likes: 1
From: Utah
I was having a bad surging problem with my swap and replaced everything trying to figure out what it was. I ended up not having enough power going to the fuses and relays i added. I had used an existing constant 12V power wire under my dash from my old computer and I guess it wasn't supplying enough amps. Ran a wire straight from my battery and it fixed the problem. Worth checking your power connections, or running a temporary wire from the battery to check.
Engine is on a stand it's not been put into a car yet. The wires are going directly to a battery but also found out his exhaust only goes 6" past the O2s "possible issue yes" but not sure it would cause this wouldn't really be much different than open headers.
Staging Lane
Joined: Dec 2006
Posts: 70
Likes: 1
From: Minneapolis
I would think its also running open loop during initial startup, it shouldn't really be trying to correct from O2s until it gets warmed up, but I never really rule anything out unless it's fact. Are you watching live data? Usually you get a high idle right away, so the 1400 rpm sounds about right for a cold start, but what one not sure of is if it's dying or not. Does it accelerate if you crack the throttle and hold it at lets say 2000 rpm? If you warm it up for 5 mins or so, does it change in any way? What are you using for a fuel setup on your engine stand?
It's not idling at 1400 it's that it won't throttle past 1400 rpms once it gets to 1400 which is right at 8% tps it starts surging up an down. But if you unplug the tps before starting it will accelerate fine
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If it's just on the stand, is the intake tract complete enough that the MAF gets a good signal. I was having some wierd stuff going on once when I just ran the maf right on the TB without any tubing. I failed the MAF in the tune and it ran fine.
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