Motor Age Garage: The Not So Nifty F-150

Jan. 1, 2020
An ?85 Ford F-150 pickup that doesn?t want to start puts Contributing Editor Curt Marsh through the diagnostic paces.
The Not So Nifty F-150Faulty F-150VEHICLE: 1985 Ford F-150
DRIVETRAIN: 5.0 EFI, automatic transmission
MILEAGE: 133,927
COMPLAINT: Hard start after heat soak.

This truck proved that fuel systems can and do fail in a variety of ways.

“Hi Curt. When ya wanna look at my old truck?”

He didn’t have to tell me who he was. I recognized the voice. That’s one of the nice things about being in a small independent shop. You get to know your customers. That old ’85 F-150 has seen better days, especially with more than 130K on the odometer, but the average miles per day is down to just over two-and-a-half. He just keeps it around for when he needs a “truck.” The call came last summer, and he was using the truck to haul stuff for the yard. 

It seemed that the truck was hard to start after a heat soak. When he brought it in, he said that now it was hard to start most all of the time, but it was worse after a heat soak. He also said that it acted like it wasn’t getting any fuel. 

Above: As you can see, the deadhead pressure measured after the frame-mounted pressure pump is fine. There was no leak down. Below: The KOER pressure was within spec, but just barely.(Photos: Curt Marsh)
What we can find out The next morning, we drove it around to get it heated up. It started a little hard, but it wasn’t too bad. In my part of the country, traffic is pretty heavy the first thing in the morning, so we didn’t have to get it up to any speed to heat it up.  Sure enough, after a heat soak, it didn’t want to start. We found a technical service bulletin (TSB) for a leaking regulator, but there was no fuel in the vacuum hose connected to the regulator. Besides, that wouldn’t be consistent with it acting like it wasn’t getting any fuel. We also found a TSB about a bad fuel pump electrical connector at the frame-rail-mounted pressure pump, so we checked the connector. It was good. 

Because the design engineers, in their infinite wisdom and knowledge, didn’t think it necessary to provide a fuel pressure tap on this engine, we opted to amp probe the fuel pumps before we opened a fuel line connection. All three pumps looked good. Oh, by the way, did I mention that it had dual tanks? However, the switching valve wasn’t the problem. It checked out fine. 

We teed the gauge into the line coming out of the pressure pump on the frame rail. We clamped off the hose in the tee to check deadhead pressure and to check for leak down from the pump to the tanks. Everything looked normal there. 

As luck would have it, when we released the clamp and hit the key, the engine started right away. The key-on/engine-running (KOER) pressure was 28 psi, which was within the manufacturer’s specification of 28 to 38 psi. Granted, it is at the low end of the spec, but remember that the engine started fine at this point. While the engine was in the mood to start and run, we figured that we ought to check the amperage waveforms of the injectors. Because the intake plenum covers the right bank injectors and connectors – and getting to the left bank connectors isn’t exactly a walk in the park – we decided to pull the connector cover off of the Electronic Control Assembly (ECA) and amp probe the injectors there. They all looked fine. Not only did the amperage draw and initial ramp look good on all of them, they also looked clean. If anything, the pintle bumps occurred a little early, indicating that they were coming fully open pretty quickly. 

We shut off the engine to let it heat soak again. When we tried it after the heat soak, the truck didn’t want to start. However, this time I tried going to clear flood mode. That made it start. Hmm, that’s interesting.  

All eight injectors looked about the same. The amp draw and ramps looked fine and according to the pintle bumps they were all clean.

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