I'm thinking of running my '93 Jimmy (W) down to Oregon next week for the Mount Angel Oktoberfest. Since the beginning of July it hasn't been getting the mileage it should. In combined driving I was getting 17-18mpg. The last couple months it has been down in the 14mpg range. The beginning of August I replaced the water pump, fan clutch, thermostat, fuel pump (Delphi, natch), fuel sender, fuel filter, cap, rotor, and plugs. The mileage remained low.

The truck also seemed to be a little low in power in the higher RPM range. I checked the ignition timing with the timing wire disconnected and it was 0° BTDC. I checked the timing again with the wire connected and it was around 16° BTDC. The timing advanced with engine RPMs. At around 1500 RPM the timing was about 30° BTDC. That all seems normal to me.

I moved on to fuel and that's where things got weird. I've replaced injectors and nut kits on four Vortecs so I'm used to testing fuel pressure and looking for the signs of leakage. I put the fuel pressure gauge on the Jimmy and turned on the key. It pumped up to 55psi and bled down to about 52psi after ten minutes on the first test. That's with a brand-new Delphi pump in the tank. I cleared the pressure in the gauge and ran another test. On test #2 it pumped up to 60psi and only dropped to 58psi. I did a third test and it pumped up to 58psi and then the pressure went *UP* to 60psi and held for ten minutes.

Given that unusual data I pulled the manifold tuning valve and looked inside the plenum. The passenger side of the plenum looked clean and golden which makes me think the fuel pressure regulator is leaking. I took the top of the plenum off and did a few more fuel pressure checks to see if something was visibly leaking. I didn't see any leaks. I put a little compressed air on the diaphragm of the fuel pressure regulator and it didn't leak. I ran three more pressure tests and each time it would pump up to 60psi and hold for ten minutes.

The spider and nut kit were new in June 2010. They were both leaking badly when I bought the truck at that time. It was the first thing I did when I bought it. The spider is a Sorenson brand from Autozone. I'm going to take it out of the truck and have it tested at Dr. Injector on Monday. I can't seem to figure out if it's really leaking or not. A couple tests seemed to indicate a leak but after the first couple tests it held pressure OK. The pattern of washdown in the manifold really looks like the pressure regulator is leaking.

Another variable: my friend Alex was watching the scanner in the truck while I was under the hood. While it was running during the timing tests he noticed the long-term fuel trim was at 160. That was with the truck warmed up and in closed loop mode. Last year when I got the tabs for it the truck wouldn't pass emissions. It was high on hydrocarbons and carbon monoxide both at cruise and idle. I paid for the waiver and didn't do any more diagnostic work on it at the time.

A leaking fuel pressure regulator would certainly explain the high emissions. Would it also explain the lack of top-end power? What about the high fuel trim number? I'll know more about the injectors after I get them tested. Does anyone have any more ideas in the meantime?

 

I had the injectors tested this morning and they passed so I took it home and put everything back together. I did a couple more leakdown tests and the first two didn't hold pressure for even a few minutes. The next two were perfect and I think I may have figured out why: for the first two tests I just turned the key on without having run the engine. There was no manifold vacuum to affect the fuel pressure regulator. After the first two tests I started the engine and let it run for about two minutes before shutting it off and measuring pressure. Once there was vacuum it held just fine.

Now I'm going to drive it around some and see how it does. In the meantime, if I can replace the TCC valve and install a shift kit in my '98 I may drive that down to Mount Angel. We'll see just how much work I can get done in the next two days.

 

I could be wrong here, but wouldn't a high fuel trim number mean NOT ENOUGH fuel?

 

I always get confused by fuel trim numbers. I never know whether the higher numbers mean it's getting more fuel or less fuel. I'm still somewhat confused by the specifics of how the computer determines how much fuel to spray into the intake manifold.