TPS Question, Stumble when cruising Options

[warrenoliver]

I have a 73, 2.0 with F.I. and points. I have been chasing down a stumble or hesitation that occurs anywhere between 2500 and 4000 RPM. This stumble or pause or whatever is similar to what I used to have before I replaced my TPS board with a new one. With the old board I had a stumble at 2800 or 2900 and was noticeable when I was trying to maintain a certain speed. The new issue is that I have an immediate stumble just as the car begins a slight deceleration like you would have when cruising at a constant speed and then you let up just slightly or like when you back off the accelerator just a bit when you begin down a slight incline at highway speed. This never happens when I am accelerating only just as it begins to decelerate.

When I took the TPS off the car to inspect and clean the board and the contacts I noticed the device that is connected to the copper fingers that slide along the contact board is fairly loose. This plastic rotor thingy rotates with the throttle and moves the fingers. The rotor thingy is very loose in the hole of the TPS body and I can wiggle it around a bit. Is this normal? Is it supposed to have much play? Do these things wear out?

It was acting up again tonight as I drove home on the highway so I pulled over and unplugged the TPS and it seemed to make the hesitation or stumble go away so it seems to me like it is related to the TPS.

Any ideas? I would like to keep the TPS plugged in because acceleration sucks with it disconnected.

Thanks in advance for your help

[worn]

Hi Warren,
I would want to make certain it was the tps cause I could think of other culprits. Sounds like hesitation during off-throttle response. I think this could be hard to nail down. In a related note I suspect you are now afraid to face our pile of rust at the autocross. I see the tps issue as a leveling tool. Now catching my son with my own car is another story, but backdating the exhaust seems a winner. I think I have spares in all of those parts so let me know if you need a part.
Warren

[r_towle]

Have you ever cleaned your distributor advance plates?

[PancakePorsche]

I have a 73, 2.0 with F.I. and points. I have been chasing down a stumble or hesitation that occurs anywhere between 2500 and 4000 RPM. This stumble or pause or whatever is similar to what I used to have before I replaced my TPS board with a new one. With the old board I had a stumble at 2800 or 2900 and was noticeable when I was trying to maintain a certain speed. The new issue is that I have an immediate stumble just as the car begins a slight deceleration like you would have when cruising at a constant speed and then you let up just slightly or like when you back off the accelerator just a bit when you begin down a slight incline at highway speed. This never happens when I am accelerating only just as it begins to decelerate.

When I took the TPS off the car to inspect and clean the board and the contacts I noticed the device that is connected to the copper fingers that slide along the contact board is fairly loose. This plastic rotor thingy rotates with the throttle and moves the fingers. The rotor thingy is very loose in the hole of the TPS body and I can wiggle it around a bit. Is this normal? Is it supposed to have much play? Do these things wear out?

It was acting up again tonight as I drove home on the highway so I pulled over and unplugged the TPS and it seemed to make the hesitation or stumble go away so it seems to me like it is related to the TPS.

Any ideas? I would like to keep the TPS plugged in because acceleration sucks with it disconnected.

Thanks in advance for your help

Try another used TPS to see if symptoms change or improve. There is no real good substitute for a brand new Bosch TPS. Keep your eye out for a new one either here or theSamba. The lack of the two most critical items being the TPS and the trigger points is making it hard to keep these Djet systems running properly these days.

[warrenoliver]

Have you ever cleaned your distributor advance plates?

Yes, I recently resoldered the ground wire and cleaned and relubed them then. I thoroughly disassembled and cleaned the distributor a year ago also.

[r_towle]

stumble on decel may be vacuum related.
The main system that would be affected is timing...

You may want to check the vacuum canister.
A simple test would be to unplug the vacuum lines to the distributor and cap them so they dont leak.
Drive the car and see if it happens.

A slightly more difficult test, again to test that cansiter, is to remove the canister off the distributor so the advance plates move freely.

Think about how old that canister is...rubber diaphram....oil in the line, rust etc....

If you are certain the plates are working fine and they snap back to rest upon decel, then maybe there is some friction in there just caused by the age of the canister.

I think that when you unplug the TPS as you stated in your test, you are enriching the fuel to the motor because everything defaults to rich to prevent the motor from burning up when you have a system/part failure.

Overall, it is either a vacuum leak somewhere or timing is stuck to advanced or retarded for that specific time in life.

Look for those two things..

Rich

[warrenoliver]

stumble on decel may be vacuum related.
The main system that would be affected is timing...

You may want to check the vacuum canister.
A simple test would be to unplug the vacuum lines to the distributor and cap them so they dont leak.
Drive the car and see if it happens.

A slightly more difficult test, again to test that cansiter, is to remove the canister off the distributor so the advance plates move freely.

Think about how old that canister is...rubber diaphram....oil in the line, rust etc....

If you are certain the plates are working fine and they snap back to rest upon decel, then maybe there is some friction in there just caused by the age of the canister.

I think that when you unplug the TPS as you stated in your test, you are enriching the fuel to the motor because everything defaults to rich to prevent the motor from burning up when you have a system/part failure.

Rich,

Thanks for the ideas. I will try disconnecting the vacuum cannister and see how it goes. As far as removing the cannister, so the plates can move freely - why would the advance plates move without the vacuum advance moving them? Wouldn't they be stationary?

I didn't know that removing the TPS causes it to default rich, thanks for that tip.




Overall, it is either a vacuum leak somewhere or timing is stuck to advanced or retarded for that specific time in life.

Look for those two things..

Rich

[r_towle]

Advance plates move to full advance due to centrifugal force...the spinning.

The vacuum advance and retard, if equipped, moves them slightly advanced a bit sooner then the rotation does to give you smoother acceleration..

You can run without the vacuum advance and you will still get full advance, it will happen a bit later...so long smooth accel will suffer, but fast accel works fine.

[warrenoliver]

Advance plates move to full advance due to centrifugal force...the spinning.

The vacuum advance and retard, if equipped, moves them slightly advanced a bit sooner then the rotation does to give you smoother acceleration..

You can run without the vacuum advance and you will still get full advance, it will happen a bit later...so long smooth accel will suffer, but fast accel works fine.

Aha! Thank you for the explanation. Fast is good (IMG:style_emoticons/default/piratenanner.gif)

[warrenoliver]

Hi Warren,
I would want to make certain it was the tps cause I could think of other culprits. Sounds like hesitation during off-throttle response. I think this could be hard to nail down. In a related note I suspect you are now afraid to face our pile of rust at the autocross. I see the tps issue as a leveling tool. Now catching my son with my own car is another story, but backdating the exhaust seems a winner. I think I have spares in all of those parts so let me know if you need a part.
Warren

Aaah little grasshopper. You are still in trouble because I don't think I do much coasting at the AX. Remember in my original post I said acceleration is good. When is the next AX? Is it the Friday night one at the Go Kart track?

I may take you up on your offer to try some spare parts. I want to try Rich's suggestion first and if that doesn't work I may want to try swapping out the TPS if you have one for the 2.0

[warrenoliver]

stumble on decel may be vacuum related.
The main system that would be affected is timing...

You may want to check the vacuum canister.
A simple test would be to unplug the vacuum lines to the distributor and cap them so they dont leak.
Drive the car and see if it happens.

A slightly more difficult test, again to test that cansiter, is to remove the canister off the distributor so the advance plates move freely.

Think about how old that canister is...rubber diaphram....oil in the line, rust etc....

If you are certain the plates are working fine and they snap back to rest upon decel, then maybe there is some friction in there just caused by the age of the canister.

I think that when you unplug the TPS as you stated in your test, you are enriching the fuel to the motor because everything defaults to rich to prevent the motor from burning up when you have a system/part failure.

Overall, it is either a vacuum leak somewhere or timing is stuck to advanced or retarded for that specific time in life.

Look for those two things..

Rich

Finally, an update on this frustrating problem:

I took the distributor apart and found the distributor plate ground wire loose again so I repaired that. I also checked the rest of the distributor and it seemed to be in good working order.
I keep coming back to the TPS because it runs better when it is unplugged so I bought a good used one from BDStone -Thanks for the part! (IMG:style_emoticons/default/beerchug.gif) The TPS came too late for me to put on before my trip.
I took the car to a PCA gathering in Door County this weekend and had problems the whole weekend with it "stuttering" or hesitating just as I ease up on the throttle. It made it so it was not as much fun as it should be.
Monday evening I replaced the whole TPS unit with the good used one. I retimed the engine and drove it a short time - worked great.
Next day I drove to work and not a single episode of hesitation so it looks like it is fixed. It is really fun to drive now that it is running well. (IMG:style_emoticons/default/piratenanner.gif)
I am not sure what it was but I suspect it is the rotor device that drags the contacts over the traces on the board. At this point I don't really care since it is now running well.

[bdstone914]

Good to hear it fixed the problem.

[Ian Stott]

What do think causes the RPM to be slow in coming down? I also have the stumble when I let up slightly when cruising on the highway at a constant speed, when in traffic for example. When I accelerate the rpm stays up at the shift point for quite a few seconds.

Ian Stott
Moncton
Canada

[DRPHIL914]

What do think causes the RPM to be slow in coming down? I also have the stumble when I let up slightly when cruising on the highway at a constant speed, when in traffic for example. When I accelerate the rpm stays up at the shift point for quite a few seconds.

Ian Stott
Moncton
Canada

First thing that comes to mind here would be a weak throttle return spring. Second most likely is your accelerator cable is worn, sticking in the slave . What condition is your pedal cluster in?
Check those items and I think you will find your problem.

Good luck