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It’s become a relatively common sight that my 1991 Volvo 940 GLE ends up on jack stands in my garage. I think age is starting to catch up to the old guy, but believe it or not, it has never left me stranded in a parking lot or on the highway. If you’ve been keeping up with my posts, it’s not that my car dies on me, but I’m trying to stay ahead of the failing curve of 25 year old parts.

This brings me conveniently to my topic today, what happens when fuel pumps fail, and how you can pinpoint your problem.

For a few weeks, I had free use of both of my cars. Figuring that the our 2005 Volvo S80 would be the reliable steed, I took the time to replace the timing belt on my 940. As luck would have it, the post office misplaced one of my pulleys, and being reluctant to put my car back together knowing I needed to install the new pulley later on, decided to drive the S80 for the next couple weeks.

You can probably see what’s coming. The following are all indicators that your fuel pump might be on its way out. However, as is standard for most car diagnoses, these indicators are not exclusively fuel pump problems, but put together, you can make a pretty good guess as to what your problem is.

 

Jack_940 The 940 in the garage in preparation for a timing belt change.

 

Indicator 1: Your car dies immediately after starting.

One morning, I started up the car, only for it to promptly die immediately. It was a cold morning, so I didn’t think too much about it. I started up the car again, and worked fine for the next few days. As they say, out of sight, out of mind.

Indicator 2: Your car hesitates or is rough on acceleration.

While accelerating from a stop, the car hesitated abnormally. Now, I know what turbo lag is, and this wasn’t it. As I stepped on the gas, the car would respond in surges. Sure, this could have been down to a number of reasons, and that’s why I didn’t think too much about the problem. I figured that if it started becoming consistently bad, then I would start to be worried.

Indicator 3: Your car dies for no reason while cruising on the road.

This is when I started to get worried. I was cruising on the highway out of the blue, the engine died at the worst time, as I was going uphill, towards the exit, on a busy highway. Using my previously documented procedure, I started the car up while I was still moving and continued uneventfully on my way home. Now the diagnoses started.

Indicator 4: You can hear your fuel pump making funny noises.

In short, the problem got to a point where the car wouldn’t idle properly and would randomly die. This was a problem because the S80 had become my daily driver while the 940 sat peacefully in the garage awaiting its new timing belt pulley. Having little time to figure out the issue, I brought it to my trusted mechanic who quickly diagnosed the fuel pump and/or fuel pressure sensor. I had a low voltage signal from the fuel pump, and a low pressure signal from the fuel pressure sensor. Either the fuel pressure sensor was bad, or the pump was bad. Putting my ear near the rear driver’s side wheel well, I could hear the erratic whining of the fuel pump, not a good sign.

Simple diagnoses: disconnect the fuel pressure sensor.

The easiest diagnoses in this case was to disconnect the fuel pressure sensor and see how the car performed. This will most likely cause a check engine light in the car, but the ECU will assume a worst case scenario and order the fuel pump to run at full pressure. If your car runs better without the sensor, then you’ve just found your problem. In my case, the car ran better, but only slightly.

Note: Most older cars will have your fuel pump running at full pressure all the time, with the pressure being controlled with a fuel pressure regulator. On modern cars, the fuel pump is regulated, with the fuel pressure sensor and ECU regulating the voltage to the fuel pump. If you want to know more about how fuel pumps fail, you can check out this post by Anthony: What causes a fuel pump to fail.

Conclusion: It was the fuel pump.

Going through the list of problems, and having my car die on me once again, even with the fuel pressure sensor disconnected led me to replace the fuel pump. To be safe, I also replaced the fuel pressure sensor. Hopefully you’ll be able to diagnose a bad fuel pump before it strands you on the road, but if you start experiencing symptoms similar to mine, then at least you’ll know where to start looking. Conveniently, FCP Euro sells everything you need to replace your fuel pump and fuel pressure sensor.

 

Fuel Pump sending unit Fuel Pump sending unit
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Written by :
Tim Wong

Tim is a Canadian from Windsor, with a knack for taking things apart, and sometimes putting them back together. He is a mechanical engineer by day and backyard mechanic by night. His mantra in life is to never break another bolt.


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