LT1 Low RPM popping under load. - Chevy Impala SS Forum
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post #1 of 11 (permalink) Old 08-16-2010, 07:30 PM Thread Starter
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LT1 Low RPM popping under load.

Hey everyone, new to the forum.

I have a 94 LT1 5.7 bolted to a TH350.

idling in park and neutral the engine revs fine.

But when driving, if you step on it from a stand still it will start popping and bogging out and you let off and its fine. If you ease into it it will start doing it around 1500 rpm. Sometimes you can power through it but you can feel the hesitation until you let off the gas and just maintain the speed.

new coil, wires, opti, plugs, icm.

also the fans run constantly when the engine is running.

Any thoughts or ideas would be great, thanks in advance.
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post #2 of 11 (permalink) Old 08-16-2010, 07:45 PM
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Welcome!

Because you say the fans are always running I'd suspect the coolant temp sensor on the water pump.

Also check fuel pressure ~ should be 41+ w/ key on/engine off.
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post #3 of 11 (permalink) Old 08-16-2010, 07:56 PM
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I agree. It is also running in low octane mode and pulling timing fully. have you ever seen the burst knock tables on these things? They pull almost 12 degrees on a rapid throttle opening when in low octane mode.

Chris

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post #4 of 11 (permalink) Old 08-17-2010, 01:58 PM
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How do you get out of low octane mode?
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post #5 of 11 (permalink) Old 08-17-2010, 02:22 PM Thread Starter
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My fuel pressure is good, about 43 then bumps up when the engine is running. Also holds pressure for a good while, I don't seem to have any leaky injectors, not to say any are not clogged.

I'll try and check the CTS here in the next day or so, I did a bench test with an ohm meter the other day and it seemed ok, im betting it spikes radically when it warms up... but it seems to act up like this cold and warm.

thanks!

I'm a little unfamiliar with the the octane settings on the LT1, thats something thats programmed and can't be adjusted manually, right? and I'm not sure how octane levels affect the driving characteristics. I know that running low octane in a high comp engine will cause pinging, but this is MUCH worse than a ping. It almost feels like there is either way too much fuel getting dumped in or not enough. when I try and power through it the popping the stop, but as the car pulls you can feel the hesitation, like 2 or 3 cylinders are not hitting. This is as best as i can describe it so far, i got a video of driving the car but you cant really hear the popping too much. I'll post it if you guys think it will help, it was mainly just to get an idea of what RPM it starts acting up.

Last edited by wrightlimousine; 08-17-2010 at 04:12 PM.
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post #6 of 11 (permalink) Old 08-17-2010, 06:22 PM
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Take the time to fill out your user CP so the rest of us know where your from. Maybe a local can come by and check it out in person... And no the low octane mode needs to be programmed out. I also seem to remember the engine needs to have a VSS signal to properly deliver fuel and to determine fuel trim cells. It has to do with calculated load and some other things that are not commented out in the ecm for tunercat and lt1edit. Let me look into that.

Chris

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http://www.mediafire.com/?0elo4c935uc5s
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post #7 of 11 (permalink) Old 08-17-2010, 09:33 PM
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Question #1: Is your CEL on?

Question #2: Who tuned your PCM so that you can run a TH350?

And, a statement. GM calls what your car is in "limp mode". It ignores most sensor inputs and runs a conservative tune. This is for the safety of the engine, as it only does it when something screws up. It also turns the fans on full time.

2005 Pontiac GTO
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post #8 of 11 (permalink) Old 08-18-2010, 08:45 PM Thread Starter
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Ok, so I left out that its a 94 bolted to a TH350 bolted in a 79' Cutlass.

rocko, I do not have a VSS hooked up at this point. I have it its just not plugged in and i need to get the toothed ring out of a 4l60. I don't know why low octane mode would be programmed in, it should be a 100% stock PCM.

cletus, the check engine light is NOT on. However, i noticed that it does not come on at any point in time. It may be a bad bulb but i just put in a new one so it should be ok. The PCM is out of a 94 Z28 with a 6 speed so there should be no reason a non computer controlled TH350 wouldn't work with a non computer controlled 6 speed... aside from the VSS which is unplugged. Oh my other thought was maybe the TH350's vacuum modulator causes a strain on the LT1s vacuum system?

I'll try and get on my user cp in a few and get you updated. you can also check my car domain at cardomain.com/ride3855732

Last edited by wrightlimousine; 08-18-2010 at 08:47 PM.
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post #9 of 11 (permalink) Old 08-18-2010, 09:14 PM
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OK, there's some more details. How's the wiring go from '90's B/F-body to '80's G-body? I'd say that's most likely where you CEL is getting lost. Also, its not so much 'low octane mode' as it is open loop 'limp mode'. All PCM's for our cars have this. Like I said before, its purpose is to protect the drivetrain from damage when the PCM knows an important sensor isn't working. Get your VSS and CEL issues fixed.

2005 Pontiac GTO
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post #10 of 11 (permalink) Old 08-19-2010, 06:23 PM Thread Starter
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the harness is from a 94 camaro and is the one that came with the PCM. There is not any real splicing of the two harnesses. My 79 had no existing computer so all the wiring that was there was for the dash sensors and the HVAC system. All I did was used a fused wire and wired up the constant power for the PCM, then used the keyed power as a signal to turn on a few relays to run power from the battery to a separate fuse/relay block that gives power to all of the sensors the PCM controls.

in the camaro harness there are bulk head connectors that run between the PCM and fuses and relays. I noticed that the impalas seem to have the harness as one whole thing with relays and fuses already connected.

still need to get out and do some checking on the car but i wanted to keep you guys updated and cued into what im working with.

again HUGELY appreciate the help! if there is any other info you guys might need i missed out on let me know. I'm keeping my fingers crossed that its the temp sensor and not anything else.

Oh, about open loop and closed loop, just around town less than 45 mph it will not go into closed loop but after i get on the freeway i notice once the RPMs are more around 700 when stopped than 1000 suggesting it gets into closed loop on the freeway. however even after a drive on the freeway it will still give me the same drivability problem.
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