sudden problem, starts but dies, runs if I hold the accelerator but dies otherwise

[CowboyErik]

Just parked it yesterday, then started, get in, and starts but dies right away. If I hold accelerator, it runs but won't idle unless I hold accelerator in some, is that the TPS sensor?

[Mile High VX]

Could be a few things.

Any CEL or codes?

The TPS is where I would start.

[WormGod]

Yep, no CELs, could be a bit of a baby witch hunt. I would suggest FPR, TPS, fuel filter, and a good solid cleaning of the MAF (never hurts to do this one every now and then). Start by just removing each individually and inspecting, cleaning, reinstall and see if there is a difference. Best way to find the culprit. GL.

[hughesdt]

I am curious why the IAC, idle air control, wouldn't be the first thing that comes to mind. Mine gets sticky usual causes high idle. Shot of carb cleaner and I am good for a while. But I have never seen a code from it.

[PK]

I am curious why the IAC, idle air control, wouldn't be the first thing that comes to mind. Mine gets sticky usual causes high idle. Shot of carb cleaner and I am good for a while. But I have never seen a code from it.

You nailed it.

PK

[CowboyErik]

INSTALLED NEW TPS SENSOR, didn't work at first, shut down still, a few minutes later after checking fluids, it worked perfect. Very sudden failure, and a sudden fix. Very generated any codes....caustiously going to test drive tommorrow near by so that if it fails again I can limp home!

[WormGod]

In my VX's 16 years of life, I've only ever replaced the IAC valve once, and that was just cause I was doing a lot of other stuff and figured, why stop there, heh. Makes sense to check it though since it is a valve after all. Typically, my idle issues are MAF, but who knows with these clunky Iron Giants.

[DaGambit69]

I had the same idle problem... Replaced the MAF... no deal... Took out the IAC... Cleaned it multiple times, screwed it all the way down, reinstalled it... It idled high for about 5 minutes and then went back to normal idle.

[CowboyErik]

that problem was my TPS, come to find in my toolbox a previous bad TPS, so at 130K, my VX has burned up 2 TPS sensors

[ipd]

idle set screw can cause that, iirc. either a missing one, or one set improperly. that's from a different platform, but probably applies to this vehicle too.

[CowboyErik]

since my VX has burned up 3 TPS sensors in 135K, I am keeping a spare with the torx bit tool in the glove box lol

[CowboyErik]

I am at 140K, have been fine all year, no problems since last October 2016 post, now suddenly, TP sensor (TPS) again. Have swapped it out. I disconnect ground, remove the 3 wire harness's around the TP sensor, replace it, re connect ground. Car won't start on first try because I think Brain Box is learning new sensor, then on 2nd start, presto. This was 3 weeks ago. Today it did it again. I still had previous sensor. Put it back in and it's RUNS again. I have never thrown any codes like the books says it might. No codes at all. I have cleaned all the connections and checked for loose or frayed wiring. Any ideas?

Manual Says nothing relevant to me:

The throttle position (TP) sensor is a potentiometer connected to the throttle shaft on the throttle body. The PCM monitors the voltage on the signal line
and calculates throttle position. As the throttle valve angle is changed (accelerator pedal moved), the TP sensor signal also changes. At a closed throttle
position, the output of the TP sensor is low. As the throttle valve opens, the output increases so that at wide open throttle (WOT), the output voltage
should be above 4 Volts.The PCM calculates fuel delivery based on throttle valve angle (driver demand). A broken or loose TP sensor may cause intermittent bursts of fuel from
an injector and unstable idle because the PCM thinks the throttle is moving. A hard failure in the TP sensor 5-Volts reference or signal circuits will set
either a DTC P0122 or DTC P0123. A hard failure with the TP sensor ground circuit may set DTC P0123 and DTC P0112. Once a DTC is set, the PCM
will use an artificial default value based on engine RPM and mass air flow for the throttle position, and some vehicle performance will return. A high idle
may result when either DTC P0122 or DTC P0123 is set. The PCM can detect intermittent TP sensor faults. DTC P1121 or DTC P1122 will set if an
intermittent high or low circuit failure is being detected. The PCM can also detect a shifted TP sensor. The PCM monitors throttle position and compares
the actual TP sensor reading to a predicted TP value calculated from engine speed. If the PCM detects an out-of-range condition, DTC P0121 will be set.

[Thelgord]

Bad ground for the sensor? I hate chasing electrical issues, even if they are common.

[CowboyErik]

I guess the real question is, what else I am resetting when I disconnect the battery, swap out TP Sensor and restore normal operation that would cause same problem. To me it seems like the CPU forgetting what the TPS's position on the last shut down and can't re-learn it.

[89Vette]

I can't answer the question about ECM reset.

Did you get a code? It sort of surprises me that the computer would need to learn the TPS position (lowest voltage at the stop screw) but I guess that's possible if you can't adjust it to a specific value (like my older GM TPI car). I would also think the most likely cause of a TPS code would be drop in voltage (indicating a "flat spot") in the variable resistor of the lever. If the computer loses track of the throttle position (even for a millisecond), it doesn't know how much fuel to deliver.

If you can't observe intermittent value at the sensor, I'd start wondering how to find the wire(s) in the ECM end to see what value is making it to the computer. Or, more easily...take it to a shop. I'm sure an ODB2 port scanner/computer can display what value the computer is seeing....and if it's losing voltage at any opening. I can "see" the value on my 89 ODB1 car with computer software monitoring ALL sensor values fed into the computer (ECM).

On my 89, TPS diagnosis would be a simple matter of moving the lever open, close, open, close, open (slowly) and observing the voltage being sent to the ECM. If it ever hits 0 or is lower than the lowest/highest expected value, replace it. Otherwise, look to other possible causes. It could even be the ECM itself! If so, using a scanner/software to monitor what the ECM is seeing could answer your question.


(On the car I know best...my 89 GM TPI...you set the TPS to the correct low (idle) value via adjustment screw. That MIGHT be because older computers don't "learn" TPS low/high travel values? They start at .5V and go up from there....adding a correspondingly higher amount of fuel for higher voltages.)