Engine deceleration issues

[69gmcc10]

I am currently working on a stock (to the best of my knowledge) 00 5.3l 2wd Silverado that I purchased to eventually transplant the drive-train of into my 69 GMC. Until then, I am trying to work out what little quirks the drive-train has. I have resolved most of the little issues with a through cycle of sea-foam and new MAF, but now I am working on the idle and don't know where to go from here.

The issue I am trying to correct now is that the engine does not decelerate very fast and when the trans is in park the rpms drop to 1000 mark and then they hesitates and stays there for a second or three before settling at 500. does anyone have any suggestions on what to look at?

I thought it was a leak in the intake gasket, but after allot of WD40 sprayed on the gaskets I came up with nothing. Also, the only codes the ecm has are intermittent rear O2 sensor failures on each bank. where do I go from here?

[clinebarger]

1. Verify the Coolant Temperature Sensor is accurate.

2. If the Idle Air Control counts read 0 at a fast idle, troubleshoot the IAC and IAC circuits. (Tap on the IAC, If idle returns to normal, Replace IAC)

3. Monitor the throttle angle (or percent) on a scanner. It should always be 0% at idle.

4. If higher, sweep the Throttle Position Sensor signal voltage output. Verify the voltage always returns to the same value at idle. (Can be done with a volt meter, Dark Blue wire at TPS)

5. Check all other sensors using the 5 volt reference from the Powertrain Control Module (PCM) for pulling the reference voltage low. (Can be done with a volt meter, Grey wire at the TPS) If you find an issue here, Let me know so I can tell you what sensors use the same 5-volt reference, Could be something you don't need in a swap....Like a fuel tank pressure sensor.

Tech Tips: The PCM will idle the engine at a normal speed when the TPS voltage returns to the learned base idle value. This is also seen as 0% throttle angle. If this throttle angle is more than 1% at idle, the PCM will assume the engine is off idle and raise the engine speed to prevent a decel stall. Note that another sensor using the same 5 volt reference as the TPS can momentarily glitch the reference voltage low. This will cause the TPS to send a lower base idle voltage to the PCM, which the PCM quickly records as the new learned base idle voltage. When the reference voltage returns to 5 volts, the base TPS signal voltage increases to it's previous value. The PCM now interprets this as being off idle. This may cause the throttle angle to increase to more than 1% and idle the engine at a fast speed. This "glitch" can occur very quickly and may not be visible on a scan tool.

[69gmcc10]

Originally Posted by clinebarger

5. Check all other sensors using the 5 volt reference from the Powertrain Control Module (PCM) for pulling the reference voltage low. (Can be done with a volt meter, Grey wire at the TPS) If you find an issue here, Let me know so I can tell you what sensors use the same 5-volt reference, Could be something you don't need in a swap....Like a fuel tank pressure sensor.

Thanks Cline, you always jump in with awesome help. Much appreciated! I will go down the list of suggestions this week.

Quick Question, if a fuel tank pressure sensor can cause this issue, could it also be caused from the "P0463: Fuel Level Sensor Circuit Signal?" My fuel gauge works intermittently and from time to time this code shows up. I didn't think it had anything to do with engine idle so I didn't mention it and I wont replace it because like you said it will go away when I get the trucks computer gets flashed for the transplant.

thanks again!

[69gmcc10]

1. temp gauge in dash reads 195ish and IR temp gun reads 210ish on the head next to the sensor. is this enough of a variation for me to be concerned?

[clinebarger]

Originally Posted by 69gmcc10

1. temp gauge in dash reads 195ish and IR temp gun reads 210ish on the head next to the sensor. is this enough of a variation for me to be concerned?

Not enough, Temp sensor sounds fine.

Did you tap on the IAC motor?

The fuel level sensor is not on a 5-volt ref. Not the issue.

[69gmcc10]

Originally Posted by clinebarger

Not enough, Temp sensor sounds fine.

Did you tap on the IAC motor?

The fuel level sensor is not on a 5-volt ref. Not the issue.

I took the motor of, cleaned the plunger tip (filthy!), re installed and tapped it. no change. I did notice that when I first start it up, hot or cold, it sweeps just fine for about the first 20-30 seconds.

I also took off the throttle body and cleaned it real well to make sure there was absolutely nothing clogged or restricted, no change.

I am going to try and get my hands on my friends diagnostic tool by monday to check the throttle position and voltage.

thanks

[69gmcc10]

Throttle position is at .4% at idle! voltage ranged from .54 to .56 but 8 out of 10 times was .55.

I am going to check again for anything obstructing the blade, but if that doesn't work what is the next step?

[69gmcc10]

cleaned it again, no difference.

[clinebarger]

Originally Posted by 69gmcc10

Throttle position is at .4% at idle! voltage ranged from .54 to .56 but 8 out of 10 times was .55.

I am going to check again for anything obstructing the blade, but if that doesn't work what is the next step?

For some reason I didn't get a notification on your post.

So throttle percent is 4% not .4% correct?

Monitor the 5-volt ref at the TPS...If it stays constant & the blade or cable isn't binding.....Replace the TPS sensor.

[69gmcc10]

Originally Posted by clinebarger

For some reason I didn't get a notification on your post.

So throttle percent is 4% not .4% correct?

Monitor the 5-volt ref at the TPS...If it stays constant & the blade or cable isn't binding.....Replace the TPS sensor.

Not a problem Cline, I can only move so fast myself and I was not worried about you not getting right back to me.

back on point...

The bosh scan tool I used that day said the throttle blade was open "0.4" degrees, I thought it interesting it was measured down to fractional percentage. The Snap on scanner that I plugged into today said initially the blade was at 0% and after a few rpm sweeps stated that it was at 1% and it never returned to zero the rest of the time the scanner was plugged in. other numbers that I monitored were the VPS (v) was constantly .57 when returned to idle, the IAC position was 28 at idle and got numerically large with rpm increase, and DES IAC was 4.79 g/sec and returned there after rpm sweep.

[69gmcc10]

I am going to bump this back to the top just in case Cline missed it, sorry.

[clinebarger]

Do you have a TPS to throw on it, It should return to 0% every time.

[69gmcc10]

Sorry this took sooooooooooooo long follow up on!

So, while I was checking sensors and stuff to get the hesitation problem to go way the dirt caked in the engine bay of this truck from the previous oner finally go to me, it was so dirty under there the ocd in me took over and I was forced to clean the engine which i had planned to do after pulled it, but oh well. I this produced 2 results, first that engine was way cleaner, not perfect but better and second that knock sensor failed form flooding. I nearly filled the cavity with water, which was impressive noting how little water I tried to use.

Now, the reason it took so long to get back to this point is I had to save my nickles to get a new TPS, a coil (while cleaning I found a chip missing in one of the coils under the boot), 2 knock Sensors, the harness (just in case) and intake gaskets (fel-pro problem solvers). Good news, one of the intake bolts was loose when I removed it and I think there was a leak that I fixed inadvertently.

Problem, with all of that stuff done, most importantly the TPS (always returns to 0% now) the problem is still there! After the truck is warmed up and idling and you rev the engine quickly and watch the rpms fall, they will settle at about 1000 rpms for 2-3 seconds before they fall again to 600 rpms.

what gives? can this be a computer tune problem? I do plan on sending my computer to black bear for the tune before I swap the engine and trans to my 69, should I do it now and see if the tune fixes this?

pics of the knock sensor replacement. cleaned the intake for about 2 hours with simple green and some brushes, then cleaned and smoothed up all the mounting surfaces. I put rtv sealant around 90% of the rubber sensor plugs to keep the moisture out! the old plugs were so abused that they were rattling around and fell out with ease. felpro gaskets are metal, hang on the head bolts in the valley and look to be the real deal!!

[68C15]

Go after the codes for sensors 1-2 and 2-2.

[69gmcc10]

Originally Posted by 68C15

Go after the codes for sensors 1-2 and 2-2.

I don't quite follow you?

Are you asking what the actual code is on the rear O2 sensors? The code is a p0161 heat circuit

Are you telling me to have the rear O2 sensors turned off (plan to eventually) and do the tune now to see if it goes away? I was under the impression that the rear O2 sensors would not have an effect on anything related to the performance of the engine.