Hello All,

I've been having a nagging issue with the car stalling out over the past few months and it seems to be getting progressively worse. The conditions under which it occurs are pretty specific, so I'd love to hear your thoughts on how I could go about diagnosing this.

First of all, the car starts perfectly every time. It runs well and spends plenty of time on the track for HPDE. There are no performance issues or OBD II (Service Engine Soon) codes.

There are a TON of speed bumps around here and the car is low, so I spend a lot of time speeding back up only to slow down to a creep to roll over the speed bumps. Occasionally, when I transition from being slightly on the gas to coming completely off of it (to press in the clutch and brake) the car cuts off. More often than not, instead of cutting completely off, the car will stumble (engine RPM down to 300-400) for a second or two....giving it some throttle at this time will help it recover and get the idle back.

At first I thought this was an issue with the oily K&N filter messing up the MAF sensor. I've cleaned it many times and there seems to be no correlation between clean MAF/dirty MAF and this problem.

My mechanic pulled out the scan tool and it seems when this issue occurs it appears almost like the TPS loses the signal and doesn't register a reading. Without knowing exactly what I should be looking for (as for scan tool readings and symptoms to help with diagnosis), does this indicate the TPS is failing?

Going back a few years, I had the same exact problem just after getting the new motor installed and it ended up being a small wire that connects to the TPS was bent badly. Once my engine builder fixed the wire it was an immediate fix to the problem. Based on where I am now, I'm thinking there's actually a problem with the TPS (sensor) or I need to go back and look at all of the wires around it carefully.

Anyone else had similar experience or care to share some advice on what I should be looking for?

 

I have some video of the issue, however the files are huge. When I get home tonight I'll try to trim them into something small enough to post.

 

Problem with the TPS is that they get a "wear" spot in the area of the sensor where your throttle spends most of its time. If that wear area is really bad you can lose signal intermittently.

Try a new TPS.

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Thanks LS6427. That's precisely the type of input I was looking for. That makes sense -- I'll go pick one up and see if that fixes the problem.

 

I picked up a new AC Delco TPS, installed it and the issue still persists intermittently. The car drove great the first day and didn't have any issues with it. When I went to start it later that afternoon it started acting up. Started fine, however the idle was choppy and bounced up and down until finally stalling out. I could push on the throttle and hold it to keep the car running.

I have a basic scan tool and recorded the TPS data to make sure the new one was operating properly -- it's functioning from 0-100% without any gaps or spikes in the range. The rub is that the problem only manifests itself when the throttle is at 0% and the car is in neutral.....that means there is no change in the TPS on the scan tool when it happens because it reads as 0% constantly but the idle RPM is fluctuating wildly between 475-950.

Any thoughts on a next step?

 

Jeez. Well, the only way your idle can increase is the IAC valve is opening and closing. The PCM is what tells it to open and close. So some sensor is giving the PCM erroneous information causing the IAC valve to wig out.

Only thing to do is clean the obvious sensors...MAF and IAT sensor in the lid with MAF cleaner.

Even a bad battery can cause issues with the idle.

Can you read the o2 sensors switching with your scan tool..?

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Still chasing this issue around and have a few more interesting things going on to add to the mix.

1. Cleaned the MAF and IAT well. Slight improvement in overall idle quality, however didn't fix the problem. I have a K&N FIPK (which I can't stand), so I clean these regularly as the oil pulls through and gets on the MAF sensor.

2. The battery seems to be just fine. When the idle drops, I get a big drop in voltage however I believe that's more "effect" of the issue, rather than the "cause".

3. I believe my scanner will provide the O2 sensor readings although I haven't logged the data yet. I'm not sure what the desired readings/output should be, so I will research that further prior to scanning it.

2. I've had an issue with my coolant temp gauge (pops up quickly and then goes back down very quickly) and the operation of my fans. If the gauge is reading higher, then the fans will run even after the car turns off. Interesting enough, if I put the key back in the ignition and turn it to accessory, the fans will turn off for good as soon as I turn the key back to the off position.

I'm not sure if the fan/temp sensor issues are related in any way to the idle issue, but there's little harm in exploring this further to see if I can get lucky with a 2-for-1 fix!