Interesting Engine Problems

Since I’d gotten her registered I’ve been driving my car around fairly regularly, or at least as regularly as the current pandemic lifestyle both requires and allows. Happily there’s been no real issues. The engine starts when asked, the car rolls, and it stops when I push the brake pedals. What more could one ask for? Well, consistency would be nice. That was called into question one morning when I climbed in, turned the key, and almost immediately stalled out. Rats.

This was a bit out of left field given how well the car had been running. I decided to see if I could “Italian tune up” the problem away and proceeded to drive to the grocery store. Sure enough when the engine got to operating temperature things smoothed out a bit. Hoping that the problems were just a fluke, I parked at the store and went in to do my shopping. When I came out about a half an hour later my issues had returned. I drove home and parked in the driveway. I was about to leave for a week’s vacation and investigation would have to wait until I came home.

In addition to being relaxing, my holiday also gave me some time to think about the problem. I’m a software engineer by profession, and when I encounter a bug that seems to pop up out of nowhere the first question I ask is “what changed?” If a system, software, mechanical, or otherwise, works fine and then suddenly doesn’t some aspect of it must have changed to produce the error state. With my car there were two things that I could imagine being the variable in the equation. Either a part had decided to give up the ghost or one of the consumable components such as gas or oil had changed in some way.

In terms of parts three came to mind immediately. First was the MAF sensor. If it was suddenly not metering air intake correctly the mixture calculations would be wrong and the engine would not work properly. I got my manual and multimeter out and did basic tests. They were:

  • Ignition OFF
    • Test resistance between PIN 1 of the MAF connector and ground. The reading should be negligible.
  • Ignition ON Engine OFF
    • Test voltage between PIN 5 of the MAF connector and ground. There should be ~12 VDC.
    • Test voltage between PIN 5 and PIN 1 of the MAF connector. There should be ~12 VDC.
  • Ignition ON Engine ON
    • Test voltage between PIN 3 of the MAF connector and ground. There should be ~2.3 VDC.
  • Ignition OFF, Unplug the MAF
    • Test resistance between PIN 2 and PIN 3. The reading should be between 2.5 and 4 Ohms.

And in case you’re a visual learning type, I made a video demonstrating the process.

The end result was that my MAF sensor was fine which is good because new ones are sort of expensive.

My next thought, parts wise, was the coolant temperature sensor. If the computer wasn’t getting accurate engine temperature data it could be messing up the mixture ratios. However I replaced that sensor not too long ago with a quality part. I find it hard to believe it went bad already.

My final theory was related to the fuel pumps. I had already replaced the in tank pre-pump, but it’s pretty loud. You hear it pretty clearly whining away while the car is running. This might be, I reasoned, because the main pump, being old, is starting to fail. The fuel pressure is regulated at a constant pressure. If the main pump was weak the pre-pump would be forced to work harder to pick up the slack. Hence the sounds. Except that this problem cropped up suddenly. Unless the pump had failed completely, which it hadn’t, it would have to be something else. 

That’s when I got the idea that perhaps the now nearly empty gas tank might not be providing the same level of gravity assisted pressure that a full tank would. I could imagine the extra weight of the gas in the tank giving a little extra help to the pumps. Was the gas level the change to the system state I was looking for? To test this out I limped the car to the gas station and filled it up. Unfortunately this resulted in no change in performance. Defeated, I parked the car and hung up the keys for the night.

The next morning, just for curiosity’s sake, I decided to start the car and see what happened. It started up a little roughly. That was expected. Then the motor smoothed out and stayed running. I put it in gear and cruised around the neighborhood a little with no problems. The issue appeared to be gone! Self fixing problems are the best kind, but I’m not going to be lulled into a false sense of security. There is still a problem lurking in there somewhere. I’m still suspicious of the main fuel pump, but I’ll wait for another day before I pull the trigger on replacing that.