LT1 Based Engine Tech 1993-1997 LT1/LT4 Engine Related

Too hard throttle response at low rpm

Old 04-08-2019, 01:41 PM
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Too hard throttle response at low rpm

Hi :-)

My first post here, but have been lurking around for some time now.

I just bought my first camaro.

1997 with LT1

Edelbrock Performer heads

BBK nickel plated shorty headers

MSD opti spark

MSD wires


The engine has new pistons, rods, and bearings all over about 25.000km ago. (Due to lack of engine fluid)

It runs really great and pulls strong. But I have noticed a strange problem with low rpm. People always seem to complain about lack of throttle respons. My problem is kind of reverse. It has too much throttle respons from idle / low rpm.

It idles fine and stable. A little high around 1000 rpm.

If I very slowly press the gas pedal from idle, it jumps straight up to about 1600-1700 rpm. When I release the pedal it drops down to idle speed as normal. Its impossible to make an easy throttle from idle, and hit a rpm between 1000 and 1700(ish) rpm. It jumps from idle to 1700.

Same goes when cruising at slow speed with low rpm.

An example: If I try cruising with cruisecontrol below 1700 rpm, it kind a accelerate > engine brake > accelerate > engine brake.

I meassured the voltage on TPS. Idle shows 0,55v and WOT shows 4,95v - No jumps when slowly goes WOT

I tried reset the IAC valve - no luck

Does anyone have any good ideas ?

Thanks in advance.
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Old 04-10-2019, 09:10 AM
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Re: Too hard throttle response at low rpm

Welcome to ! Not every day we add a member from Denmark.

Need a a bit more info about the car:

- automatic or manual transmission? Idle speed programming depends on transmission type.

- if automatic, does it have the stock torque converter?

- just heads, no upgraded camshaft?

- who tuned it for the performance upgrades?

Have you observed the opening of the throttle blades as the pedal is depressed? An engine cannot suddenly increase RPM under load without a sudden inrush of air.

Sorry for the delay in your post appearing in the forum. The site software put it into the “moderation” queue, requiring approval, probably due to the IP address not being from the US.
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Old 04-10-2019, 11:52 AM
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Re: Too hard throttle response at low rpm

Thanks for reply.
Yeah forgot about the info.
- 6 speed manual
- Just heads - stock cam
- All vacuum hoses / lines are brand new. No leaks.

I tried twisting the TPS a little so instead of showing 0,55v on idle, it shows 0,85v. Just to try eliminate the possible jump there might be down there. Of cause I am using the throttlebody and not the pedal, and it still jumps from idle straight up to 1700 rpm - So still no luck. Throttleblades moves smoothly.
I also went for the procedure resetting the throttleblade gap, but it was already within specs.

Lastly I just spoke to the previous owner yesterday. Some guy tuned it (remap only - no custom tune) 12 years ago with Hypertech Power Programmer III for a camaro 94 :-/
That was done while the engine was still stock. And I guess it would need a whole different tuningmodule for the 96-97 PCM (?)

So I guess its a tuning problem at low rpm, if that makes sense ?
Or am I wrong about this ?
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Old 04-10-2019, 05:42 PM
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Re: Too hard throttle response at low rpm

No, I am not saying it's a tuning problem. Just wanted to know the extent of modifications and whether any tuning had been done, which might have changed the idle speed settings.

I cannot see how anyone could tune an OBD-2 (96/97) PCM with an OBD-1 (93-95) Hypertech tuner (maybe because I never tried it). And even if they could, there is very little that the Hypertech can actually change with regard to idle, TPS, or IAC behavior. The Hypertech does include a standard "canned" power tune, but it is generally useless, and has been shown to actually reduce performance on even a stock engine. Only the 96/97 OBD-2 versions of the Hypertech could accept custom, removable tuning modules.

The stock programming for an LT1 with the T56 manual trans, coolant warmed up (above 44-degC) is 800 RPM. If your coolant temperature sensor is reading lower than actual temperature, it's possible the PCM is elevating the idle speed for cold operation. It isn't unusual for the stock tachometer to read 100-200 RPM higher than actual idle speed. So your idle speed doesn't seem to be too unreasonable.

If you have access to an OBD-2 real-time scanner, hook it up to the data link connector and read the true RPM from the PCM. Also read the actual coolant temperature. If you have a scanner, or scan software, it would be helpful to observe the idle air control IAC) "counts" (steps of opening) at idle, and how the PCM reacts to the throttle blades opening.. If the throttle is passing too much air at closed throttle, either through the blade clearance or a vacuum leak at the throttle body gasket, the IAC will typically show low (less than 20) or even 0 counts. What you read from the PCM is what the PCM is telling the IAC to do. There is no feedback to the PCM as to the actual position of the IAC.

When you were twisting the throttle blades by moving the throttle body quadrant, was the intake elbow off, allowing you to physically see the throttle blade movement? Again, the engine cannot accelerate rapidly without a sudden increase in air flow. Slowly opening the throttle blades should not allow enough air for that to happen. I'm wondering if someone incorrectly attached the accelrrator cable to the throttle body quadrant.

Does your car have traction control? If it does there will be a large black servo-motor sitting on top of the driver side valve cover. That system could be acting up in some way. Then there's the possibility of the cruise control module causing soe sort of erratic acceleration.

Last edited by Injuneer; 04-10-2019 at 05:48 PM.
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Old 05-07-2019, 01:14 PM
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Re: Too hard throttle response at low rpm

Just to get back here with an update.

- I got Jet DST and everything looked stock compared to a stock file. Except for some added spark advance in the upper RPM area. But the cable provided with the software are only able to tune. It cant do a simple datalogging or reading of sensors.. So had to get another cable for that.

- Went for datalogging and everything seemed fine at first. I noticed my tacho is waaay off, showing about 300 rpm too much. Is that normal ?
So actually the dead spot was from idle (800rpm) to about 1350rpm. All sensors working, AFR are ok - a little rich in some places. MAP sensor was ok. All was good.

- Then the engine started leaking oil from the front. Split the whole front of the engine and of course the noob who assembled the engine, destroyed the seal for the waterpump on the timing cover when he was installing it. Now it has new seals everywhere in the front of engine. No more leaking.

- Then the clutch went south. Got a new clutch installed last weekend.

- So, on to data logging again. I simply couldnt find the issue untill I looked at timing. When idle, spark advance was at 20*. As soon as TPS hit 2%, then spark advance would jump to 40*.. Some spike there!

On the table "Closed TPS Spark Advance vs. RPM
RPM 400 set to 26*
RPM 800 set to 30*
RPM 1200 set to 30*

Then went to table "Main Spark Advance vs. RPM vs. MAP"
Smoothed everything out in the rpm 600-1400 / MAP 35 - 50kpa area, so it was compared to the other table at around of 30*
Went for the "graph" and smoothed out the last small spikes..

The engine now runs great. The dead spot between idle and rpm 1350 is completly gone :-D
And it pulls great..

Will do some finetuning on the fuel tables later on...

Thank you guys, for the replys. Cheers
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