Hi All,

I'm working on a 1995 Blazer with a 4.3L V6. It was running rich when it came in. We took off the air tuner and found a leaking fuel pressure regulator, so replaced that. We also cleaned the badly plugged up EGR port and replaced the upper intake gasket.

With those done, it still had a bad miss, especially with power braking. We found that the plugs were well worn, two wires were worn through to the center conductors, and the cap and rotor were shot. All were replaced.

It still has a miss! It's worst during power braking, but noticeable even at an idle. The Modis shows no codes, and, sadly, mode 6 doesn't have misfire counts. (We suspect that the misfires are randomly distributed over all the cylinders.)

We think it's running lean, as a shot of carb cleaner down its throat clears the miss right up. We noted that the fuel pressure is about right, at 55 psi, but it does bleed down within 20 seconds after the engine is shut off. The O2 sensor is switching between rich and lean as desired, and the temp sensor seems to be reading the right temperature.

From what I've read around the web, the "spider" is most suspect, but I don't want to just throw parts at it. What should I be checking next?

Thanks for the advice,
Jeff

 

Key ON, engine OFF, fuel pump running, pressure must be 55psi to 61psi and it must remain above 50psi for at least 10 minutes afer the pump shuts off. With a misfire and excessive leakdown, could be a poppet valve stuck open.

95 was a hodge podge year for the 4.3L: It could have any one of: 3 different computer control systems, 2 different ignition systems, 2 different fuel delivery systems. Do the plug wires attach to the cap horizontal or vertical? If horizontal, (OBD1.5 with EDI) don't waste time with aftermarket caps, use AC Delco only, yes, it does make a difference. Put a dab of silicone dielectric grease on each terminal of the cap, inside and out. With your Modis, check camshaft retard, preferred spec is zero degrees +or- 2 degrees. Are there any DTC's pending or in memory?

 

Hi Captain,
Thanks for the response. KOEO, we get just above 55 psi fuel pressure. This is the value when driving, too, for what it's worth. It only takes a minute or so for the pressure to bleed off to 0, though.

This 4.3 has a tower (vertical) cap. The cap, rotor, wires, and plugs were all shot, and were replaced with OE parts, except for Belden wires.
There are no DTCs. Sorry, but I don't recall the camshaft retard. I can't look until Monday morning, but will do so then.

Just to finish the story to this point, I didn't mention before that when driving, you can hit the gas, and it struggles to take off. After 10 seconds or so, it gets past a certain load/acceleration point, and takes off as though nothing was wrong. I liken it to the miss being worst during power braking. It's smoother without as much load.

I can tell you that the fuel trim is right at 128, and that the O2 sensor is switching between rich and lean pretty regularly. The O2 sensor, at least, seems to think things are fine.

When we had the upper intake off, replacing the leaking fuel pressure regulator, we ran the fuel pump and checked that the injector "spider" wasn't leaking and all 6 ports squirted. That doesn't mean it's perfectly functioning, but it's something.

More later!
- Jeff

 

This vehicle has OBDI with HEI ignition, (no camshaft retard adjustment). Your KOEO fuel pump running pressure at 55psi is right at the bottom of the allowable range. The leakdown is excessive and may be contributing to the low fuel pressure.

Next step is to check fuel pump maximun output pressure and leakdown, at the fuel filter. All fuel pressure and flow must end at the tester, (commonly called "deadhead"). When the pump is running, pressure should be 73psi to 108psi and must remain above 50psi for at least 10 minutes after the pump shuts off. If it fails one or both parts of this test, there's a problem in the tank. If it passes both tests, there is a leak in the plenum. Post your results.

 

We did the deadhead test, and the fuel pressure was over 80 psi. From the way it was behaving, it looked like a bad fuel pressure regulator, even though we had just replaced the old one that was leaking. We got a second one, replaced it, and found the problem fixed. You gotta hate that occasional bad, new part!

Thanks for the help, Captain. Knowing what the deadhead pressure should be helped. I still want to believe it's in Alldata someplace, but information gets pretty shaky in there for some of the older cars.

- Jeff

 

Glad you found the problem, and thanks for posting back with the fix.