Author Topic: Where is the throttle position sensor?  (Read 8660 times)

Offline bd

  • Global Moderator
  • Senior Member
  • *****
  • Posts: 6438
Re: Where is the throttle position sensor?
« Reply #15 on: June 11, 2017, 11:18:40 AM »
None of the "sensors" on your truck affect drivability.  The "primary condenser" in HEI systems suppresses electrical interference that otherwise could propagate through the vehicle wiring.  The change in radio noise between the hood open and closed suggests that RFI in your truck is actually emanating from the ignition secondary and being received through the radio antenna.

Collectively, the symptoms you've described imply a failing ignition coil, although the ICM and pick-up coil can mimic similar effects.  Replace the complete distributor replete with cap, coil and vacuum advance - replacing individual distributor components, one-at-a-time, can cost >150% of a complete assembly.  You should be able to find a complete assembly for about $100.

Edit:
Make sure the hood is suitably grounded to the body by installing a 1/4" wide, flexible braided strap or 12-gauge wire between the hood and firewall sheetmetal.  Allow sufficient length for hood closing and opening without binding.
« Last Edit: June 11, 2017, 01:14:19 PM by bd »
Rich
It's difficult to know just how much you don't know until you know it.
In other words... if people learn by making mistakes, by now I should know just about everything!!!
87 R10 Silverado Fleetside 355 MPFI 700R4 3.42 Locker (aka Rusty, aka Mater)

Offline Irish_Alley

  • Tim
  • Global Moderator
  • Senior Member
  • *****
  • Posts: 13319
  • Family is not an important thing. It's everything.
Re: Where is the throttle position sensor?
« Reply #16 on: June 11, 2017, 04:28:16 PM »
im with bd about it being in the distributor. you can get the icm and coil tested at a parts store but theres no way to test the pick-up coil. i know you already replaced the cap and rotor. so at this point i would take the icm and coil off to get them tested. since it happens all the time when the engine is cold or hot one of them should test bad. if you do as bd suggested you could also keep the old distributor for backup
If you can’t tell yourself the truth, who can you tell it to?~Irish_Alley

When you have eliminated the impossible, whatever remains, however improbable, must be the truth ~Sherlock Holmes

Offline bnoggle

  • Newbie
  • Posts: 41
  • Newbie
Re: Where is the throttle position sensor?
« Reply #17 on: June 12, 2017, 05:54:30 PM »
Had the icm and coil both tested, both were bad. Replaced the distributor, and now I don't have any power. It runs well but does not shift or speed up properly. The radio static is gone, and the timing was reset properly. I'm not sure what's wrong with it. Also when it shuts down there is pre-ignition. I turned the idle down to 450 rpm.


Sent from my iPhone using Tapatalk

Offline bd

  • Global Moderator
  • Senior Member
  • *****
  • Posts: 6438
Re: Where is the throttle position sensor?
« Reply #18 on: June 12, 2017, 07:38:46 PM »
New or reman distributor?

By "pre-ignition," do you mean the engine knocks and keeps running when shut off (aka, dieseling, after-run)?  Is there an idle solenoid on the throttle linkage side of the carburetor?  Is it connected and adjusted to positively extend when the ignition is switched on and then fully and immediately retract when the ignition is switched off?  What are the curb and base engine idle RPM settings that you used?

With the engine running and while using a timing light, if you apply vacuum to the distributor vacuum advance does the timing advance?  How much?  Does the can hold vacuum?  Then, with the vacuum advance disconnected, does the timing advance if you rev the engine?  How much?  At what engine RPM does it stop advancing?

With the engine off and the distributor cap removed, can you rotate the rotor ~10° CW against spring tension?  Will it return to its original position when released?  Is the rotor rotation consistently repeatable by hand or does it stick/hang up?
Rich
It's difficult to know just how much you don't know until you know it.
In other words... if people learn by making mistakes, by now I should know just about everything!!!
87 R10 Silverado Fleetside 355 MPFI 700R4 3.42 Locker (aka Rusty, aka Mater)

Offline bnoggle

  • Newbie
  • Posts: 41
  • Newbie
Re: Where is the throttle position sensor?
« Reply #19 on: June 15, 2017, 03:09:25 PM »
New or reman distributor?

By "pre-ignition," do you mean the engine knocks and keeps running when shut off (aka, dieseling, after-run)?  Is there an idle solenoid on the throttle linkage side of the carburetor?  Is it connected and adjusted to positively extend when the ignition is switched on and then fully and immediately retract when the ignition is switched off?  What are the curb and base engine idle RPM settings that you used?

With the engine running and while using a timing light, if you apply vacuum to the distributor vacuum advance does the timing advance?  How much?  Does the can hold vacuum?  Then, with the vacuum advance disconnected, does the timing advance if you rev the engine?  How much?  At what engine RPM does it stop advancing?

With the engine off and the distributor cap removed, can you rotate the rotor ~10° CW against spring tension?  Will it return to its original position when released?  Is the rotor rotation consistently repeatable by hand or does it stick/hang up?
By pre ignition I do mean diesel ink. I managed to fix that by turning idle way down. Apparently when I replaced the distributor it somehow changed my idle speed. There is no idle solenoid on the carburetor. Curb is ~450, base is set from factory (aka I forgot to check). The timing does not advance much, maybe 2 degrees when vacuum is applied. The can holds vacuum, and was new with the distributor. Timing does advance about 2 degrees when vacuum is disconnected and engine is revved. I cannot tell what rpm it stops advancing as the interference in my engine bay is too much for my tachometer to give a good reading with engine revved, but I would guess it is about 4,000 rpm based on sound alone. The spring tension in mechanical advance seems good. It is the same as in the old distributor. Rotor seems to advance and return freely.


Sent from my iPhone using Tapatalk

Offline bd

  • Global Moderator
  • Senior Member
  • *****
  • Posts: 6438
Re: Where is the throttle position sensor?
« Reply #20 on: June 15, 2017, 09:45:06 PM »
By preignition I do mean dieseling. I managed to fix that by turning idle way down. Apparently when I replaced the distributor it somehow changed my idle speed. There is no idle solenoid on the carburetor. Curb is ~450, base is set from factory (aka I forgot to check). The timing does not advance much, maybe 2 degrees when vacuum is applied. The can holds vacuum, and was new with the distributor. Timing does advance about 2 degrees when vacuum is disconnected and engine is revved. I cannot tell what rpm it stops advancing as the interference in my engine bay is too much for my tachometer to give a good reading with engine revved, but I would guess it is about 4,000 rpm based on sound alone. The spring tension in mechanical advance seems good. It is the same as in the old distributor. Rotor seems to advance and return freely.

If there is no idle solenoid, then curb idle and base idle are the same.  Dieseling results from too much air entering the engine and/or improper ignition timing.  Poor engine power can result from insufficient or poorly implemented ignition timing.  W/o an accurate tachometer or timing advance light, diagnosis of the distributor timing curve can be difficult.  However, two degrees vacuum advance is insufficient.  Manufacturer references are available that accurately describe timing curves by distributor number and application.  Unfortunately, I don't have them at my disposal.  Strictly as a guess without any reference materials, I expect vacuum advance should be in the range of 8° - 16°.  Mechanical advance probably should be in the range of 20° - 30°.  Where is the base timing set?

Remove the #1 spark plug and bring the piston to TDCC.  Peer through the spark plug hole with a bright penlight or substitute a long straw touching the top of the piston to verify that the piston is at the very top of its stroke.  Once you confirm that the piston is all the way up, look at the timing marks on the balancer and see whether they align to zero.  This will determine whether the outer ring of the balancer has slipped, throwing the timing mark out of compliance.

Did you ever check for side-play in the throttle shaft of the carburetor (see prior post)?  Do you still have the original distributor?
Rich
It's difficult to know just how much you don't know until you know it.
In other words... if people learn by making mistakes, by now I should know just about everything!!!
87 R10 Silverado Fleetside 355 MPFI 700R4 3.42 Locker (aka Rusty, aka Mater)

Offline bnoggle

  • Newbie
  • Posts: 41
  • Newbie
Re: Where is the throttle position sensor?
« Reply #21 on: June 18, 2017, 09:13:31 AM »
By preignition I do mean dieseling. I managed to fix that by turning idle way down. Apparently when I replaced the distributor it somehow changed my idle speed. There is no idle solenoid on the carburetor. Curb is ~450, base is set from factory (aka I forgot to check). The timing does not advance much, maybe 2 degrees when vacuum is applied. The can holds vacuum, and was new with the distributor. Timing does advance about 2 degrees when vacuum is disconnected and engine is revved. I cannot tell what rpm it stops advancing as the interference in my engine bay is too much for my tachometer to give a good reading with engine revved, but I would guess it is about 4,000 rpm based on sound alone. The spring tension in mechanical advance seems good. It is the same as in the old distributor. Rotor seems to advance and return freely.

If there is no idle solenoid, then curb idle and base idle are the same.  Dieseling results from too much air entering the engine and/or improper ignition timing.  Poor engine power can result from insufficient or poorly implemented ignition timing.  W/o an accurate tachometer or timing advance light, diagnosis of the distributor timing curve can be difficult.  However, two degrees vacuum advance is insufficient.  Manufacturer references are available that accurately describe timing curves by distributor number and application.  Unfortunately, I don't have them at my disposal.  Strictly as a guess without any reference materials, I expect vacuum advance should be in the range of 8° - 16°.  Mechanical advance probably should be in the range of 20° - 30°.  Where is the base timing set?

Remove the #1 spark plug and bring the piston to TDCC.  Peer through the spark plug hole with a bright penlight or substitute a long straw touching the top of the piston to verify that the piston is at the very top of its stroke.  Once you confirm that the piston is all the way up, look at the timing marks on the balancer and see whether they align to zero.  This will determine whether the outer ring of the balancer has slipped, throwing the timing mark out of compliance.

Did you ever check for side-play in the throttle shaft of the carburetor (see prior post)?  Do you still have the original distributor?
No play in the throttle shaft. I checked the marks yesterday when I re lashed the valves, they appear to be between 2 and 3 degrees off. I timed it that way and it runs better, but there is still a significant engine miss when the engine gets warm. At cold idle it is smooth. I do still have the original distributor.


Sent from my iPhone using Tapatalk

Offline bnoggle

  • Newbie
  • Posts: 41
  • Newbie
Re: Where is the throttle position sensor?
« Reply #22 on: June 20, 2017, 10:34:08 AM »
I have also noticed in the past few days that I have an unburned gas smell coming out of the tailpipe when cold, but once it warms up and starts to shake the gas smell goes away. Jos could the two be connected?


Sent from my iPhone using Tapatalk

Offline bd

  • Global Moderator
  • Senior Member
  • *****
  • Posts: 6438
Re: Where is the throttle position sensor?
« Reply #23 on: June 20, 2017, 12:29:50 PM »
So, when the engine runs rich the symptoms disappear.  Try adjusting the base timing to 12° to see what happens.
Rich
It's difficult to know just how much you don't know until you know it.
In other words... if people learn by making mistakes, by now I should know just about everything!!!
87 R10 Silverado Fleetside 355 MPFI 700R4 3.42 Locker (aka Rusty, aka Mater)

Offline bnoggle

  • Newbie
  • Posts: 41
  • Newbie
Re: Where is the throttle position sensor?
« Reply #24 on: June 25, 2017, 09:17:58 PM »
So, when the engine runs rich the symptoms disappear.  Try adjusting the base timing to 12° to see what happens.

Engine has been adjusted to 12 degrees before TDC. It runs better, but there is still a significant gas smell coming from the tailpipe when after the engine warms up. It also begins to shake a bit still when this happens. I tried timing to several different points and 12 degrees is definitely the best. Could it need to be an mixture screw adjustment?

Offline bd

  • Global Moderator
  • Senior Member
  • *****
  • Posts: 6438
Re: Where is the throttle position sensor?
« Reply #25 on: June 25, 2017, 11:08:36 PM »
I have also noticed in the past few days that I have an unburned gas smell coming out of the tailpipe when cold, but once it warms up and starts to shake the gas smell goes away.

Engine has been adjusted to 12 degrees before TDC. It runs better, but there is still a significant gas smell coming from the tailpipe when after the engine warms up. It also begins to shake a bit still when this happens. I tried timing to several different points and 12 degrees is definitely the best. Could it need to be an mixture screw adjustment?

Your statements are contradictory.  Why?  The residual symptoms sound like an engine misfire caused by a carburetor issue (wrong or improperly installed gasket, plugged passage, incorrect adjustment, etc) or bad fuel.  What brand & number spark plugs did you install?
Rich
It's difficult to know just how much you don't know until you know it.
In other words... if people learn by making mistakes, by now I should know just about everything!!!
87 R10 Silverado Fleetside 355 MPFI 700R4 3.42 Locker (aka Rusty, aka Mater)

Offline bnoggle

  • Newbie
  • Posts: 41
  • Newbie
Re: Where is the throttle position sensor?
« Reply #26 on: June 25, 2017, 11:28:32 PM »
I have also noticed in the past few days that I have an unburned gas smell coming out of the tailpipe when cold, but once it warms up and starts to shake the gas smell goes away.

Engine has been adjusted to 12 degrees before TDC. It runs better, but there is still a significant gas smell coming from the tailpipe when after the engine warms up. It also begins to shake a bit still when this happens. I tried timing to several different points and 12 degrees is definitely the best. Could it need to be an mixture screw adjustment?

Your statements are contradictory.  Why?  The residual symptoms sound like an engine misfire caused by a carburetor issue (wrong or improperly installed gasket, plugged passage, incorrect adjustment, etc) or bad fuel.  What brand & number spark plugs did you install?
Autozone brand spark plugs are what I installed. Sorry for the contradictory statements, to clarify at 12 degrees before TDC is where the engine runs best. By best I mean least rough. Though it runs best there it still has a significant gas smell when the engine is warm as well as a noticeable roughness when you stop at a stop sign. Otherwise the roughness is not noticeable. I played with the idle speed a bit and it didn't seem to make any difference.


Sent from my iPhone using Tapatalk

Offline bd

  • Global Moderator
  • Senior Member
  • *****
  • Posts: 6438
Re: Where is the throttle position sensor?
« Reply #27 on: June 26, 2017, 09:29:44 AM »
This is a long shot, but replace the spark plugs using ACDelco R45TS gapped at 0.035".  Be sure to check the spark plug gaps with a gauge before you install them - never assume gaps are correct out of the box.  And, don't substitute with another brand or part number.
Rich
It's difficult to know just how much you don't know until you know it.
In other words... if people learn by making mistakes, by now I should know just about everything!!!
87 R10 Silverado Fleetside 355 MPFI 700R4 3.42 Locker (aka Rusty, aka Mater)

Offline Irish_Alley

  • Tim
  • Global Moderator
  • Senior Member
  • *****
  • Posts: 13319
  • Family is not an important thing. It's everything.
Re: Where is the throttle position sensor?
« Reply #28 on: June 26, 2017, 01:43:40 PM »
and triple check the firing order
If you can’t tell yourself the truth, who can you tell it to?~Irish_Alley

When you have eliminated the impossible, whatever remains, however improbable, must be the truth ~Sherlock Holmes

Offline bnoggle

  • Newbie
  • Posts: 41
  • Newbie
Re: Where is the throttle position sensor?
« Reply #29 on: June 30, 2017, 07:51:25 PM »
New spark plugs gapped at .035 did not make a difference. Also checked firing order again and it is still 1, 5, 3, 6, 2, 4. I have noticed over the past few days a ticking sound coming from the engine when it gets warm. The ticking sound starts around the same time as the rough idle, so I'm fairly sure the two are connected. Around the same time the ticking sound started I also started having trouble getting the engine to start when it is hot. It cranks for longer than it used to before firing up. Any ideas?