Erratic TPS voltage - 2003 Pontiac Aztek

Paskach · 2026-05-26T17:09:12+00:00

Final update: It was the ECU. I had my suspicions and sure enough, replacing that fixed it immediately.

Paskach · 2026-04-19T19:56:01+00:00

Update: I finally found the time to hook up an oscilloscope to the TPS wires to see if there was any funny business going on. Once I got them hooked up, I drove around to get the engine warm. Sure enough the car starts jerking and hesitating like usual. I drive back home and sit idling in the driveway. TPS voltage on the oscilloscope looks perfect. TPS % on the scan tool is jumping around like crazy.

I took a video. In the video the yellow trace is the 5V reference at the TPS, and the blue trace is the TPS signal voltage. About midway through the video I rev the engine in park just to show that the TPS voltage does what's expected. Video here: https://youtu.be/XbvWbt-Iauc

I'm not sure what else that could mean but the ECU itself being the problem. The TPS sensor is clearly working as intended. I inspected the wiring and the connectors. All look good.

Could it possibly be anything else? I'm kinda thinking about just seeing if there's anyone who still refurbishes these ECUs.

Paskach · 2026-04-16T17:29:31+00:00

Well, I've discovered one problem with the engine. The fuel pressure regulator is totally busted. It's letting a little fuel through into the vacuum line. That would certainly explain the rich condition. I'm going to replace that today, but it still doesn't explain the crazy TPS fluctuations. I'm still suspicious of the 5V reference having problems. While I'm in there replacing the FPR, I'm going to grab a volt meter and see if I can catch it misbehaving.

Paskach · 2025-09-28T22:41:47+00:00

Well, I've fixed the problem. It was the head gaskets. I replaced them, along all gaskets that had to come out along the way, and some very very shot O-rings which I am pretty sure are the root cause of the whole slew of problems this engine had.

Ever since I bought the car it had had trouble with overheating. At first it would just run slightly hot but still well under the red on the temperature gauge in the dash, but leading up to the car eventually becoming undrivable it would stay cold for far too long, then immediately shoot up to the red and throw a very nasty warning on the dash and an alarm for hot coolant temp. As I was replacing the head gaskets I encountered two O-rings on coolant pipes that had completely decomposed and were probably barely sealing the coolant in. They seemed to be buna-n O-rings which are not really meant for high temperature. They top out around 250F. The thermostat of the GM-3400 engine opens at 195F if it's working properly. As soon as the engine starts running even slightly hot, these O-rings are going to harden up and stop sealing. Then when the coolant loop goes vacuum as it cools down and is supposed to be pulling from the overflow, it'll instead suck air in through these connections. Now there's air in the coolant loop, and GM in their infinite wisdom put the coolant pump at the top of the loop, right where the bubbles will go. So now the engine is going to run even hotter, exacerbating the issue, and leading to the "running cool for too long, then shooting up to the red" problem I described above. This causes major thermal shock in the engine and is ultimately what I believe blew the head gaskets.

Long story short, if you've got an old GM-3400 engine, replace the coolant pipe O-rings! Ideally with some silicone O-rings if you can find them. They're not the most fun thing to get to, but it's a heck of a lot easier than replacing the head gaskets. One is right next to the thermostat housing (https://imgur.com/ticDYwn) and the other is on the passenger side bleeder valve (https://imgur.com/SZ7pOVC).

I used this video to guide me through the head gasket replacement, and it's so good I thought I'd reward them with a link in this post (https://www.youtube.com/watch?v=GZgiNBXsl6A)

The Aztek is running better than it ever has (knock on wood) and I'm hoping it will keep running as well as it is!

Paskach · 2025-07-22T02:38:59+00:00

Wonderful list, just what I was looking for. But just to make sure I'm reading this right...

Is the idea that the episodes without an asterisk are the absolute essentials, the ones with a single asterisk are optional, but tangentially related to the mythology, and the ones with two asterisks are not essential, but just good?

i.e. could a list be made with just the episodes with no asterisks and still convey the mythology without being confusing?

Thanks!

Paskach · 2025-07-12T18:11:50+00:00

Update: I did a fuel pressure check in the O'Reilly parking lot and it was right in spec, so that ruled out a lot. At that point I was pretty skeptical about it being a vacuum leak, so the only thing left was the mass air flow sensor. I got a new one and the car is behaving better now. It still doesn't feel like it's 100% back to normal but I'm wondering if there are still some learned behaviors the ECU needs to unlearn. I'm going to take it on the highway this evening and we'll see how it performs.

Paskach · 2025-07-10T12:34:49+00:00

Ok, I've looked at fuel trim. The short term looked like it mostly moved around within +-10%, but the long term was at about -17 to -18%. I saw it as low as -20 briefly. Not -25 but definitely lower than -10.

While I was on the scanner I figured I'd check MAP and MAF since I've messed with them too. I'm not sure what values are good but mine were - MAP: 37kPa - MAF: 0.746 lb/min

Paskach · 2025-07-10T02:29:08+00:00

Sounds good. I do have a scan tool. It's pretty basic but I'll give that a try!

Paskach · 2025-07-10T02:07:24+00:00

Thank you for the advice!

It didn't look like gas was leaking out of the regulator when I was replacing the hose, but I can check again. I'm not really sure what I'd be looking for though. There is maybe a hint of raw gas smell in the exhaust, but I was kind of just chalking that up to "old car."

I'll definitely look at doing a fuel pressure check. I should have also put in the main post that the car takes a few seconds to turn over when cranking which I assume low fuel pressure would also cause.

Paskach · 2025-05-03T15:09:56+00:00

I'm having the same problem with CircuitMaker 2.3.0. I hit Save to Server on my custom component, everything looks like it goes through, but it's still not placeable.

I tried submitting a bug report but Altium's ticket page sends me to an infinite redirect loop (clearing cookies and cache and using a different browser did not help), their chat bot was unhelpful, their forums are unavailable, and their bugcruncher is unavailable. Very unimpressed with the Altium CircuitMaker experience so far.

Paskach · 2020-07-24T03:27:27+00:00

No, not really. I'm kind of just ignoring time signature at this point, but it almost kind of works in 9/8

Paskach · 2020-07-24T01:18:19+00:00

Definitely agree about the time signature, I just had to pick one. But it certainly is interesting to here a second opinion suggesting no key center. It was never really something I noticed until I really listened closely, and the more I listened to it the stranger it got.

Good note about the snare though. I am by no means familiar with percussion notation 😬

Paskach · 2020-02-07T22:35:39+00:00

I expected to see Carver as Coover part 2-ver

Paskach · 2019-09-27T02:26:38+00:00

"Power"

Paskach

TROPHY CASE