Sudden LQ4 problem??

[2014sierraAT]

Ok guys I’ve got a gen 3 lq4 out of an 05 Silverado with the 4l80 it started life with. The motor/trans is in my 70 c10. When I first bought the truck I had it serviced, and had it tuned by a reputable guy we have here locally. I put probably 3500 miles on the truck in the first 6 months I had it with ZERO issues whatsoever. Recently I put a k&n cold air intake/filter on the truck, (engine is bone stock by the way). Some time after the k&n swap I started noticing big dips in the rpm, (at idle) which was causing my oil psi to fluctuate also that worried me. Now it’s not only fluctuating 500+ rpms at idle it’s also doing it at highway speeds, and causing trans to shift really hard. I spent about 1.5 hours driving the truck yesterday with the tuning guy, he adjusted tables that were altered when I put the cold air intake in on. But the truck didn’t have one issue like I’ve mentioned while he was with me on his computer. Any ideas??

[mongocanfly]

air leak maybe? after filter install

[2014sierraAT]

I haven’t been able to find any loose vac hoses. It ran fine for a couple months after I put the k&n intake on. I’ve got the throttle body pulled gonna clean it up. Forgot to add when the guy had the hp tuner hooked up it wasn’t throwing any codes. Also noticed today with the key in the “on” position the fuel pump will cycle randomly for the 2-3 second intervals like it does when you first turn the key over. Shouldn’t it only cycle once upon initially turning the key over?

[Wgesnerjr]

Two ideas:
Loose connection on the ignition to PCM wire. Would make it think you were turning the key on and off?

Might be a bad connection from the PCM to Fuel Pump?

[dayj1]

Originally Posted by Wgesnerjr

Two ideas:
Loose connection on the ignition to PCM wire. Would make it think you were turning the key on and off?

Might be a bad connection from the PCM to Fuel Pump?

^^^^ that. Possibly a bad ignition switch, but all your symptoms sound like the ECM thinks you’re turning off the ignition switch at random times or you have an intermittent connection to the fuel pump.

[2014sierraAT]

Ok I’ll check the pcm trigger wire. Would that cause the truck to idle itself up though or is that just another issue altogether?

[beaterku]

Try cleaning your MAF. The oil from the K&N gums up the sensor. This can cause drivability issues.

[ls1nova71]

Originally Posted by dayj1

^^^^ that. Possibly a bad ignition switch, but all your symptoms sound like the ECM thinks you’re turning off the ignition switch at random times or you have an intermittent connection to the fuel pump.

I agree, it sounds like the ignition switch is on it's way out. Do you have an ignition relay powering up the LS harness or is it just run directly off the ignition switch? The original ignition switch is really isn't up to the load of a newer drivetrain so you could be overloading it without an ignition relay, but I see people do it all the time and then wonder why they have problems. GM uses them for a reason.

[2014sierraAT]

Ok so have been going through y’alls suggestions.

1st. I pulled the maf, cleaned it and reinstalled still had the self revving issue. I pulled it off again and found another (used) maf/tps off a friends truck with the same result.

2nd. Pulled, shook, moved around harness trying to replicate fuel pump cycle with no luck. I did find all 3 grounds from pcm, all appeared to be well grounded.

3rd. Cycled ignition switch several times, got under the dash and I WAS able to move a big wire on ignition switch (labeled ignition switch ACCY) enough to get the pump to cycle on demand BUT it was killing the power to everything radio, gauges, etc. I’m not losing all that when the truck revs itself or when the pump cycles.

I put a meter on my ignition trigger wire that powers my fuel pump relay and sat there and waited for it to cycle itself and the voltage went from 11.93 to 11.85 and came right back to almost 12v when the pump kicked back off..so didn’t see a significant drop when it cycled.

I did notice after the pump cycles the throttle body makes some kind of movement because I can hear it click a few times. Also with the key in the ON position the alternator makes a slight humming noise, is this normal?

Ls1nova whoever wired the truck modified a factory harness and wired in a small secondary fuse panel with 2 relays. It’s wired exactly the same as the LT1swap document. I also found 2 wires taped up and tucked down into the fender that come out of the firewall one labeled Altenator B+, and the other Altenator excitor. (It’s a painless harness). Should these be hooked up to the alt?

[homemade87]

( I did notice after the pump cycles the throttle body makes some kind of movement because I can hear it click a few times. )


I do know the tb will do a test when the key is powered on . It will move just enough to see if it is working proper . This also points to an interment power problem . Each time the power is cycled the tb will do this , also the fuel pump will cycle .

Im my opinion you have a power supply problem to the ecm or a ecm giving problems . Power supply problem would be my first guess .

I would find all connections and remove and inspect them , looks can be deceiving . Could even be a relay giving problems . I would change it just to be sure .

[BR3W CITY]

FWIW on the talk of ign. switches; I had the internal graphite posts on my ign switch wear down, to the point that they would only make partial or intermittent connections. Doesn't sound like your issues, but its worth noting that they can be disassembled internally to check the posts.

[solidaxel]

Originally Posted by BR3W CITY

FWIW on the talk of ign. switches; I had the internal graphite posts on my ign switch wear down, to the point that they would only make partial or intermittent connections. Doesn't sound like your issues, but its worth noting that they can be disassembled internally to check the posts.

Just reading this may have solved my studder/fludder under WOT
,,,,,,,,,,,, hard wire a new 12vdc on/off switch/fused or a relay

Thanks all for the info

[2014sierraAT]

Hopefully you can get some results solid, always a wealth of information and experience around here. I’m gonna buy a new standalone harness and go from there with my fiasco!

[BR3W CITY]

Originally Posted by solidaxel

Just reading this may have solved my studder/fludder under WOT
,,,,,,,,,,,, hard wire a new 12vdc on/off switch/fused or a relay

Thanks all for the info

[2014sierraAT]

Ok fellas, still banging my head against the wall. In the past few days I have installed a new standalone engine harness, new ignition switch, new CPS. It’s still surging like crazy at idle. The new BP Automotive harness I installed was routed inside the cab so the PCM/fuel pump relays etc are just sitting in the floor right now. Was just out in the truck while it’s at idle and I noticed the ignition relay closes when it idle’s itself up, you can hear it click. Then when it idle back down it clicks once again..opens I guess?? Any suggestions?

[homemade87]

Originally Posted by 2014sierraAT

Ok fellas, still banging my head against the wall. In the past few days I have installed a new standalone engine harness, new ignition switch, new CPS. It’s still surging like crazy at idle. The new BP Automotive harness I installed was routed inside the cab so the PCM/fuel pump relays etc are just sitting in the floor right now. Was just out in the truck while it’s at idle and I noticed the ignition relay closes when it idle’s itself up, you can hear it click. Then when it idle back down it clicks once again..opens I guess?? Any suggestions?

I assume the relays are new as well . If it is the ignition relay , it is only controlled by the switch ( power on ) . If it is cycling ,you have a bad power source threw the switch or a bad ground source . I would pull another 12v from some where and see if it goes away . If not I would look into the ground source .

[2014sierraAT]

Yes all relays are brand new. Same thing it was doing with the relays and harness I just pulled also..so what’s an easy way to temporarily wire what you’re describing to help me narrow it down?

[homemade87]

Originally Posted by 2014sierraAT

Yes all relays are brand new. Same thing it was doing with the relays and harness I just pulled also..so what’s an easy way to temporarily wire what you’re describing to help me narrow it down?

Just fine another 12v from any good source . Could pull it directly off the battery if you had a wire long enough and attach it to the 12v side for the trigger of the relay . This will eliminate the switch circuit . If it still does it get another wire for the ground and directly ground that to the pcm triggers on the relay . This will isolate both sides of the relay . If the ground eliminates the problem it is in the pcm causing the relay to cycle .

[2014sierraAT]

Ok thank you I’ll check into wiring that up. The new harness I have came all pre wired and wrapped nicely so I’m trying to avoid cutting into that if I don’t absolutely have to. I may be able to un pin the relay leads and try it that way.

[2014sierraAT]

I bought a pcm this morning also. I haven’t tried switching it out yet because it will
Need some things deleted. Don’t know it if will run right, or at all without having the front 02’s and vats deleted?

[Goats86]

definitely will not start with the vats still there

[2014sierraAT]

Originally Posted by Goats86

definitely will not start with the vats still there

Alright thank you

[ls1nova71]

Originally Posted by 2014sierraAT

Was just out in the truck while it’s at idle and I noticed the ignition relay closes when it idle’s itself up, you can hear it click. Then when it idle back down it clicks once again..opens I guess?? Any suggestions?

So your truck ran fine for 6 months, then you changed the air cleaner and it started running bad? Then you replaced the whole harness?? You may end up fixing your problem throwing parts at it, but that is an expensive way to go about it. You say the ignition relay clicked and it idles up then clicks and idles back down? Are you sure that is the ignition relay? Because if that relay turns off, so will the engine immediately. It should only be controlled by the ignition switch, not the PCM. So as the other guy suggested, unhook it from the ignition switch and run it directly to the battery. That is a wire you have to connect on an aftermarket harness so no need to cut the loom off any at all.

[2014sierraAT]

The truck ran fine even after changing the air intake. I had been wanting to change the harness anyway just to clean up the engine bay. In the fuse box that’s built into the harness there are 2 relays one for ignition and the other for fuel pump. You can hold your finger on the relay and feel it cycle when the truck idles up.

[Big Moose]

We had a truck that had the same symptoms, it would idle up to 1100 rpm +/- as soon as you'd put it in reverse or drive it would idle down, we tried all kinds of things with it, in the end we took it to the dealer and they had to reflash/relearn the ECM, apparently it had lost it's calibration