I'd like to point out the following test as a quick fuel delivery check that requires minimal tools/supplies. In about 15 minutes it proves whether or not basic flow components are functioning: tank pre-pump and hose (really only tested if tank is nearing empty), main pump, filter, fuel pressure regulator. One can certainly argue that a fuel pressure test is more meaningful, however many DIYers may not have the fittings and gauge to do this.

This test is based mostly on a fuel return line flow test in the FAQ - Effects of FPR Failure - but instead of starting the car to gauge flow, I simply forced the fuel pumps on. Here are the steps:

1) Disconnect the fuel return line after the fuel pressure regulator. On my '86 it's held by a screw clamp right on the regulator; on my '90, its a spring clamp at the back of the fuel rail. You can avoid spilling much fuel by ensuring the hose is angled continuously "downhill" before disconnecting it.

2) Buy about 10 feet of 3/8" ID clear vinyl tubing. Fit the tubing over the exposed nipple. It doesn't fit very tight, but if you ensure that the tubing runs continuously downward from this point, with the end draining into a small (1 gallon) gas container, you don't need to worry about it being tight.

3) Now force both fuel pumps to run for about a minute. You can do this by jumpering +12V to the left side of the tank pre-pump fuse at the fuse panel. This powers the main pump directly (it's the point in the circuit that the fuel pump relay usually drives), and it powers the tank pre-pump through the fuse. I believe for most later year 240's (both my '86 and '90), the tank pre-pump is on fuse #4, and 12V can be found on the left side of fuse #6. On some earlier models, I believe the tank pre-pump fues may be fuse #5, with 12V on fuse #7.

4) You should observe gas flowing through the tube immediately. According to the FAQ, you should see about 1/2 to 1 gallon in a minute.


The test is probably most useful in assessing no start or hard starting situations. A car that passes this test can likely deliver appropriate gas pressure to run the engine. Pump or filter problems would likely cause too low a flow; fuel pressure regulator problems could cause too low or too high a flow. This test however doesn't verify that the fuel pump relay is engaging properly. This would have to be verified independently, however if the engine starts and runs, you know the relay is at least engaging.


I had a consistent hard starting problem, yet after it started it ran well. I had just acquired the car, and realized that since the tank was almost empty, it might be the tank pre-pump or hose. I really didn't want to fill up the tank on a car I was planning on parking for a while. When I ran the test I got nothing out. This gave me some solid evidence that it might be the pre-pump, so I went ahead with the "fuller tank" test. Adding 1/4 tank made the starting a bit better; bringing it up to 1/2 tank made it quite acceptible. Running the test again, now with a half tank, gave an acceptable result. Had I tried the FAQ test with the engine running, I would have seen a decent flow with an empty tank. For some reason, even with an empty tank and a pre-pump problem, fuel seemed to flow fine once the engine had started. The "jumper" test however gave evidence as to where the problem might be, since filling the tank gave a different result.