1984 Suburban 6.2L diesel surging

[NZurban]

Hi, have had my Suburban two weeks now, haven't done much except tidy up a few bits in the engine bay and replace the turn signal wiring as a new one came with it when I bought it. I think I'm missing a small piece that goes under the horn button as the horn doesn't work. Anyway little side tracked there. One issue I have is the engine doesn't run smoothly at idle, kinda runs a bit like a Top Fuel dragster at idle where you can pretty much hear each cylinder firing, if you know what I mean. Also when in drive but braked waiting at the lights etc it kind of surges, rocks the vehicle forwards, won't sit and idle smoothly when in drive. The previous owner said he'd been told it was a vacuum leak in an injector line, which would be hard to diagnose as you wouldn't hear anything leaking out. Does anyone know if this is correct or should I be looking elsewhere to diagnose this???

 

[84 M1008]

You more than likely have the box style fuel filter housing. They are notorious for leaks causing air in the fuel feed line to the injection pump. 86 that box filter housing and convert it to a spin on filter and an anti backflow valve. It will start easier also.

 

[Blue Ox]

Your problem sounds like an air leak in the fuel system. Put a piece of clear tubing on the pump return and look for air. It should be clear with maybe champagne sized bubbles at the most. If you're seeing significant bubbles you're going to have to move the tubing around to try to see where it originates.

I know everyone hates the box filters, but they're kind of like Quadrajets. They're fine if you know how to work with them. The main leak point is the O-ring on the vacuum sensor. Change that and they're usually fine. Also check that the bleeders are tight. Don't overdo. They break easily.

 

[NZurban]

Thanks for the replies. As far as I can see the only fuel filter I have is mounted to the firewall, I don't see the box style filter in the engine bay. My mechanical pump has been replaced with an electrical unit mounted to the right inner guard. From the engine manual I can see where the fuel return line runs. When placing the clear tubing on the pump return so I need a fitting on one end to securely attach it to the pump return outlet or can I just stuff in the hole?

 

[Blue Ox]

The box filter is the one on the firewall. Unless your truck has been previously farked with.

The return hose goes from the overflow valve on top of the injection pump to the return crossover on the front of the engine. It's only a few inches long. Just substitute a short piece of clear tubing for test purposes.

Are you sure the electric pump is working correctly? If it's not pumping it will force the injection pump to pull fuel through it. The IP can easily pull more than 20 in of vacuum, which will find any weaknesses in the fuel system.

If the fuel pump is operating normally it should produce 4-5 psi at the injection pump inlet. Past the fuel pump leaks will show up as an external fuel leak because at that point the system is under pressure. But you can still suck air anywhere between the tank and the pump.

There are other possibilities, but air sounds the most likely and easiest to verify. Let's start with that.

 

[hatzie]

The Stanadyne 80 box filter is on the firewall.
You can use the round cartridge GM 10226035 Stanadyne Fuel Manager that sits in the rear intake valley on the 1992 & later 6.5L on the earlier 6.2L diesels. The only gotcha is finding or fabbing a mounting bracket. It's heated and has the same sensor compliment as the old leaky Stanadyne 80 Box filter.