Surge and dies

[Fauz]

My 6.2 n/a has recently developed a surge at idle 200+rpm when I let off the throttle to shift the engine dies. I replaced the fliter and checked the lines at the IP.

[DmaxMaverick]

Sounds like you are getting air into the fuel line upstream of the fuel lift pump.

Could be a loose connection, cracked/abraded line between the tank and pump, or failed tank sender/pick up.

The lift pump is probably fine. If you can get the engine started, they will normally idle fine with a bad lift pump.

Best way to check is to pressurize the fuel tank and watch/listen for a leak. Use a gas cap, tap a tire vavle stem into it and give it about 5 psi. You can go as high as 10 psi if necessary, but risk tank rupture above that.

The only other thing I can think of that would cause it is a blocked or partially blocked fuel return line, but it's rare.

Good luck

[dieselbegreat]

Is there any way a malfunctioning vacuum pump could cause surging?

[Fauz]

I pressurized the tanks and found no noticable leaks. I did notice a lot of pressure venting out of the caps when I removed them, after the engine has been running.

[britannic]

How many miles on the inj. pump, injectors and engine?

[N Wilkens]

I had similar problems (with air leaks) when the motor mounts became so wore out that the motor would occasional compress the fuel hose against the forward cross member. The intake hose that connects the mechanical fuel pump to the metal fuel line on the frame eventually failed, causing the engine to stall out. However, in the regular course of driving, it kept bumping the hose (before it completely failed) causing small surges of air into the system. This led to some very strange idle surges, at least until the hose was completely cut through.
Incidentally, replacing the formed rubber fuel hose has been a HUGE pain. My local GM dealer says the part is unavailable, and a regular un-formed fuel hose pinches closed whenever it gets warm. Currently I am using a steel braided fuel hose, but it still doesn't seal correctly, causing the fuel to drain back into the tank if the truck is parked facing uphill... Does anyone know if the formed fuel line is still available somewhere? (I have a 1985 CUCV Blazer, J engine).

[Fauz]

Approx. 89,000 miles. I'll will check the lines around the crossmember, should I be concerned about the tank pressurization when the engine is running? I opened up the injector pump to check the flyweight and it looked okay, should I have looked at more when I was in there? Thanks