Need Explanation of TPS Malfunction

[seventythree]

Can anyone explain why and how a TPS malfunction would result in engine bucking at cruising speed? Thanks

[3D914]

seventythree,

The TPS has two basic functions 1) tell the ECU when you're at idle, and 2) tell the ECU when you're accelerating.

The idle portion can be a problem if the ECU thinks you're off-idle when you're on, or on-idle when you're not, due to a bad TPS. This can cause bucking at cruising speeds if the ECU thinks you're on-idle (or doesn't detect the acceleration), as the air volume increases with the opening throttle, but insufficient fuel is being added.

The acceleration is broken into 20 incremental steps, from off-idle to full open throttle. If the TPS only processes say half of the 20 (I had one that did just that), you'll get some surging, and the power doesn't come on as smooth.

It can be helpful to pop the cover off the TPS when doing some testing, as you can visibly see whats happening, as well as use an inductance meter to check that everything is connecting properly. Sometimes its a simple matter of cleaning the very tiny hemispherical contacts to restore proper operation. I had a new TPS that had a smal burr on one of these contacts, and it actually gouged the circuit. Cleaning the burr, and bending the arm ever so slightly, allowed it to track in a clean area of the circuit.

Good luck,

[Bleyseng]

There are three tracks or traces on the TPS. One is for idle one is for part load one is for WOT. Mostly you drive with the TPS sliding back and forth between idle and partload so that is where the wear is. The traces drag metal as they wear and then the line between idle and partload gets gunked up. The ECU is responding to a bad signal as the TPS is not clear on is it idle or partload. Bucking occurs as you get gas then no gas.
It is right when you transision between idle (off gas) to accelerate (on gas).
Thats my understanding.
Geoff

[seventythree]

I believe Geoff's response is what I was looking for. Worn out traces can result in loss of acceleration clicks, which would manifest itself in form of hesitation or roughness while trying to accelerate. What I could not visualize was the nature of the idle switch closure, which would result in bucking during an off-idle condition.
Geoff, correct me if I am wrong, but are you saying that metal shavings can accumulate at the idle switch portion of the circuit board and cause intermittent shorting?

Regards

[pbanders]

Look at my parts webpage in the "Troubleshooting" section for the TPS. I have photomicrographs of worn contact traces where you can see the metal transport.

The big problem with the TPS design is that there is a LOT of wear at low throttle angles, due to the fact that you spend a lot of time cruising. This means that this area wears out first, and you get bucking when you get the additional 1.0 ms injection pulses from the unwanted contacts.

You can try bending the arms, cleaning the traces, etc., but IMO, once they start bucking, you're better off just buying a new switch. Take a look at products from CAIG, such as DeOxit, they can be applied to the trace area of a new TPS and will likely extend the life greatly by providing lubrication and oxidation prevention.

[Dave at Pelican Parts]

Not so much "shavings" as "smearing". The repeated friction of the wiper arm across the traces can eventually push the metal around. It can result in short circuits to one or more of the functions in the TPS (idle, acceleration), or an open circuit. The damage is the worst in the area where the throttle lives most of the time--light "cruise" throttle settings.

--DD

[BenI]

Didn't you get the memo on the TPS reports?




(Office Space movie reference...)

[seventythree]

Brad:

What happens if the throttle is in part load position and the idle switch is closed? Would the ECU switch to the idle circuit and reduce injector pulse duration substantially?

Bottom line is, I am loosing power for a few seconds at a time, at random, while cruising at 2000~4000 RPM. The tach stays alive, so I suspect the problem is related to fuel delivery. Any ideas?

Regards,

Hamid

[M. Hendrix]

I have a place that I notice around 2700-2900, easiest (and more likely noticed in 5th, while it's hanging there)..

It's like there is no fuel for exactly one complete firing of each cyl., too quick to call it a stumble.....

Woooooooo(bup)oooooooooooooooooooooo.. and so on.



M

[pbanders]

Hendrix and Hamid...

If your switch is adjusted correctly, there's little chance of the idle switch being closed while under open throttle. Just takes 2 degrees of opening to shut it off.

If you're losing power for "a few seconds", I doubt that it's the TPS to blame. You need to ID if the problem is due to the fuel or ignition systems. I'd be betting on an ignition problem.

Same thing for M.Hendrix - could be the TPS, but I'd look at ignition, too.

Generally, when you get bucking from the TPS, while it's annoying, it usually does not feel like an ignition misfire, which has a distinct power loss feeling. I'd suggest to both of you to get a second opinion from either an expierenced friend or mechanic.

[seventythree]

Hi Brad:

I was hoping to qualify as the experienced friend, but I guess not.

Three questions:

1) The tach stays alive during the power loss incidents, which is why I was suspecting a fuel problem. After all, our 914s feature pretty primitive ignition systems; no spark, no tach. What do you think?

2) If the fuel pump goes out intermittently, would that not result in the same intermittent power loss while the tach stays alive?

3) However unlikely under real life conditions, what would happen if one intentionally shorts pins 12 & 17 (idle switch contacts) on the TPS together, while cruising along with the throttle plate in the part load position?

Please bear with me just a little longer, for grins sake:
The problem is intermittent, so it’s a pain to diagnose. I don't suspect an ignition problem because, as stated earlier, the tach remains operational at all times; however, I will investigate all common ignition problems.
Otherwise, either I am dealing with a catchy-goee fuel pump, or there is an intermittent problem in the fuel pump circuit, e.g., the relay, the relay board, or the wiring.
Finally, what about the ECU or the MPS?

All input will be greatly appreciated.

Hamid

[pbanders]

Hamid,

I wouldn't rely on the tach as an indicator of ignition quality, as you're only seeing what's happening at the coil. What about the rest of the system? There's a reason that shops have those big Sun ignition analyzers!

If your fuel pump is going out intermittantly, you should be able to monitor either by watching the fuel pressure or by monitoring the current draw on the fuel pump. Somehow, I doubt that's the problem, as it would likely happen under all running conditions, and usually if the connection's bad, it will eventually cut out altogether.

Shorting the idle switch "on" while in part load would have various effects, depending on the setting of the idle mixture control and the type of ECU you have. On the later ECU's (75-76's), you might cut out the injection completely until engine speed dropped below the setpoint. If your knob is set to the "notch" position (where the factory marked the plastic surround, sometimes not present on some ECU's), no effect - that's what the mark means, it's the position where the idle mixture is the same as the speed control circuit would have set it (that's why early ECU's didn't have the knob, they depended on the speed control circuit to set idle mixture, too). Above or below the mark it would either richen or lean out the mixture to some degree.

Personally, I'd have someone else look at the car to see if there's anything obvious. I don't want to tell you of the boneheaded things I've learned I was doing wrong by simply having another set of eyes check it.

[seventythree]

Makes sense. I will start over and report back.

[Dave at Pelican Parts]

Hamid, your analysis on the ignition only holds for the "primary" part of the circuit, from the points to the coil. The "secondary" side, the coil to the distributor rotor and cap and plugs and such, generally won't affect the tach.

The fuel pump going out intermittently will cause a loss of power for a second or two or more. I had a bad wire in my wiring harness, and it caused a total loss of all power, but the tach was still on and reading correctly. Not fun! Usually popping the clutch would give the car its power back again. It was fun troubleshooting that intermittent problem!!! A fuel pressure gauge in the cockpit probably would have helped a whole lot at that point.

--DD