Problems under load- missing then dies

[nopower65]

I give up. I have replaced the FSD with a Heath remote mounted unit, put new filters in, and replaced the lift pump-- and none of these fix my problem.

Symptoms: Runs fine at idle, and light load driving (flat roads, less than 1/4 throttle).
If you try and go up a hill (Pittsburgh, PA is FULL of big hills) or give it 1/2 throttle accelleration or mre, it starts missing badly, and sometimes dies. If it dies, it is very hard to restart. To even keep the motor running you have to floor it.

I took it to the Chevy dealer and they won't even try to diagnose it -- becasue it has been "modified" (turbomaster, remote FSD, and a Heath chip).

I am frustrated beyond words. The truck does have 235,000 miles, last year we put new heads on it (and was thrilled to find virtually ZERO wear in the cylinders!).

Any help greatly appreciated.

They did check the timing, and said it was fine. Has not impacted the problem.

thanks

:confused: :confused:

[rjschoolcraft]

It sure sounds like a fuel delivery problem. Make sure there are no loose connections on the low pressure fuel lines that will allow the injection pump to draw air. And, although you've changed it, verify that the lift pump is working and that the filter is clean.

[john8662]

Also sounds like you're not getting power to the lift pump, that would lead to thinking that the lift pump had failed. Check the Oil Pressure Switch. This is a combination oil pressure sending unit mixed with a safety switch to operate the lift pump. With the truck running, unplug the connector at the lift pump, take a volt meter and test for voltage. you should see anything above 11 volts at least. If not, you know where else to look, the switch that controls that circuit, the oil pressure switch.

Another trick to verify lift pump operation is to get a container and find the drain hose off the water drain spiggot located on the thermostat crossover. With the truck running open the valve and you should get fuel coming out of the outlet hose and into your bottle (or onto the ground). If the truck starts to die, you have another indication. Otherwise you'll see a steady stream of diesel fuel coming out of the hose.

I'd also look and make sure the fuel filter is seated properly and not sucking in air.

Good luck!

[nopower65]

thanks for the help. Here's what we've found thus far: have new lift pump, it is working and delivering good pressure. Tried another PMD, same problem. Tried disconnecting the electronic filter harness on the back of the IP, same problem.

Starting to fear the worst: failed IP?

[twaddle]

Hi, Does it make any difference if it has a full or empty fuel tank?
Could be a split in a suction pipe in the tank.

Also try and check along the suction hoses and pipes for cracking, corrosion or damage just in case it is not a connection that's the problem. I had my 85 M1008 pick up give the same kind of trouble, it was a hose fretting against the chassis, wore through and started drawing air intermittantly.

Is there any way that you could rig up a jerry can with fuel and a hose directly onto the fuel lift pump? This is easy if it's a pick up that you have, a little bit trickier if a suburban.
This would eliminate or locate any fault from the tank to the lift pump.

If you can install test gauges on the fuel pressure side of the pump and a vacuum gauge on the suction side as well you can watch these to see if the pressure/vacuum changes while moving or flexing the hoses.

Good luck

Regards

Jim Twaddle
Biggar, Scotland

[twaddle]

Hello again,
Forgot to add, re. the suggestion about the filter seating/drawing air.
If the lift pump is making good constant pressure any problem with filter sealing should show up as a fuel leak as the filter is on the pressure side of the lift pump.

Jim Twaddle
Biggar, Scotland

[ucdavis]

You can find whether you have an air leak by replacing the black fuel line heading off the front of the IP to metal tubing, with clear tubing. Replace the black fuel tubing later, since the clear is not sufficiently heat resistant. Air will show in the clear tubing if present.

If its not the lift pump relay (new LP means LP is OK, but the stuff that powers the LP could be on the fritz) and LP is delivering plenty of fuel, given the high mileage you probably have a weak Injection Pump. You will likely get computer codes from a progressive IP failure: 35/1216, 36/1217 and the like. Pre-96, you can read the codes w/out a scantool:
http://www.thedieselpage.com/members/trblcode.htm
Post 95, a scantool is required; you can sometimes get Autozone or Kragens to do a scan w/their gizmo & get any code numbers that are showing.
Scan & post codes for more feedback.