OK, your dad primes his fuel injected engine with gas and you fixed it with a TTL logic probe...what kind of family is this anyway?

I learned from my dad to never be afraid of taking things apart to see how they work and he taught me just about everything I know about mechanical things. I have spent countless hours working with him on all kinds of vehicles over the decades. We make a good team. He is the expert on the mechanical side and he relies on me to be the expert on the electrical side.

One evening, my dad asked me to come over and help him with his truck. He said he had to prime it to get it started. But wait a minute, I thought. Isn't it fuel injected? I had never heard of anyone priming a fuel injected engine. My dad's reply: "Yeah it's got fuel injection, but I'm telling you, sometimes it only starts if I prime it and this is one of those times". Well, I had to see this in person, because, as my grandfather used to say, I don't believe anything I hear and only half of what I see.

I took my trusty toolbox and headed over to my dad's place. Turns out his truck has throttle body injection so it looks like a carburetor with two injectors mounted on top. Sure enough, he wasn't kidding. It was cranking and there was plenty of spark but it would not fire. As soon as he poured a little fuel in the throttle body, the engine fired up just fine and ran completely normal. The bad thing was, the problem was intermittent and there were no trouble codes from the Engine Control Module (ECM). As soon as the engine warmed up a little, it would start on it's own and could be OK for several days or weeks after that.

He told me he replaced the fuel pump and that didn't help. Then he replaced the throttle position sensor and that didn't help either. Now he was thinking about replacing the fuel pressure regulator. I don't like shotgun troubleshooting so I said we should check a few things first and unplugged the injector connectors.

Both injector coils measured fine with an ohm meter and they were getting battery voltage on one terminal of the connector. To test them, I connected power to one side of the coil with a jumper to battery voltage. Then we cranked the engine for a few seconds and momentarily grounded the other side of the coil. A few drops of gas came out of the injector. So I knew it was not plugged and the fuel pump was working. Same results with the other injector. In fact, the few drops of fuel were enough to get the engine to start.

We didn't know if we should expect a blast of fuel or just a few drops under the conditions we were testing and there was no port for testing the fuel pressure. So we decided that the fuel pressure regulator could be responsible, especially if it was intermittent. We bought one and installed it but it did not solve the problem.

OK, now I was determined. We were not replacing another part unless we knew for sure it was bad. I had never owned a vehicle with fuel injection, so now it was time to scour the net and learn about all the sensors and what they do.

The next time it wouldn't start, I took my B&K 15-MHz scope with me. I could see that the injectors did not fire at all until we primed the engine. When the engine did start on it's own, the injectors would fire slowly when cranking and then they rapidly fired. But what was keeping them from firing sometimes?

Over the course of a few more evenings, we were able to rule out quite a few things. For instance, the temperature sensor would not prevent the engine from starting. Neither would the oxygen sensor since it's output is ignored until a certain temperature anyway. In fact, most sensors could be disconnected and the engine would still start. The neutral safety switch would prevent the engine from cranking if it was not working. We verified all connections at the ECM connector. In fact, I even took the ECM apart to examine the solder connections. I was not impressed with the quality of the soldering and touched up a few joints, but I knew that the problem was not due to this.

I had eliminated all possibilities except one. There was a signal labeled "ref" that ran from the ignition module in the distributor to the ECM. The ECM could advance the ignition timing instead of the old vacuum control and centrifugal weights used in distributors back in the day. I didn't know if the "ref" signal was an output from the ECM or an output from the ignition module and I couldn't find any information on the internet about this. However, I could see it pulsed slowly when cranking and then really sped up when the engine started.

Well, since the truck was starting again on it's own, there was nothing more I could do but wait until the next time the truck would not start. By now we had been working on this for several weeks and I knew what all of the sensor outputs should look like and was convinced something had to be different when it wouldn't start. My dad was ready to take it to the dealer, but I convinced him to give me one more shot. Besides, he could always prime it, right?

My opportunity came about a week later when the engine would not start. Finally, I was able to see something that made sense, I hoped. There was no signal on the "ref" wire at all during crank but there was spark from the ignition coil. What is going on here? I had one idea left. I knew that when the truck would start on it's own there was pulsing on this wire, so I went home and picked up a 5-volt power supply and an old logic probe kit I had for testing TTL logic.

I know my dad thought I was crazy when he saw me show up with the power supply and logic probe but he agreed to crank the engine as I lay under the dash fiddling with the logic probe, scope, power supply and wiring harness. As soon as I injected a few pulses onto the "ref" wire, the engine started!

I concluded that during engine crank, the ECM fires the injectors when it sees the "ref" signal from the ignition module in the distributor. This is probably about once per revolution. When the engine fires and rpm increases, the ignition module outputs a "ref" signal that is more like once per 120 degrees of crankshaft rotation since this is a V6. I suspected there are two sources for the "ref" signal and the once per rev source was intermittently defective, but the other one was still working fine since the engine had spark and could run if the rpm got high enough.

We replaced the ignition module and my dad never had to prime it again!

David Kanceruk is a senior software developer at Parker Hannifin in Winnipeg, MB, Canada. His first job, at age 16, was at TV repair shop. Afterwards he worked in test engineering at a disk drive plant then moved into product engineering to do hardware design and firmware development. His house has a high population of X-10 control modules and an assortment of embedded systems he designed including a control system for bathroom heaters, fans and heat recovery ventilator plus an award-winning wireless sump-pump monitoring system. He enjoys working on cars and listening to music.