Where is it backfiring?
Up through the Throttle body it is a lean fuel mix.
But if it is out the exhaust then it is the headers causing it.
A lot of poster have reported this after installing headers.

 

It is backfiring in the collectors of the shorty headers.

What do you mean the headers could be causing it? How does that work?

 

It is backfiring under throttle...not during deceleration.

 

Have you checked resistance between Pin 47 on the EEC and the green TPS wire yet, or the other test I recommended?

 

Yes, .3 ohms....no problem to the ecm and I have previously back probed the TPS sig return all the way to the 60 pin connector. And no falling on its face either.

 

After some family time over the weekend, I have new developments with the junker. The truck developed a continuous miss anywhere above 1500 RPMS after piddling with the wiring harness that bridges the gap between the inner finder where the 4 bulkhead connectors are and the engine.

So I took the loom off the harness and found one end of the IDM resistor had a flat spot in it and melted rosin looking junk running out of the coil side of the resistor, where it had melted to the case grounding (tin foil looking wrapping) that covers the ignition wires running to the ICM. What would cause damage to the resistor like that? Resistance across the circuit still checks good.

 

The IDM resistor is 22K ohms (22,000).

 

rla2005, I read in a previous thread (as posted by Subford) that 20-24K is within spec, and he certianly knows his stuff. Also, on a trip to the junkyard I found a few in junk trucks that showed far more evidence of damage (IE melting) and tested their resistance and they were also good, so I guess a little head damage doesn't hurt the resistor.

I'm sure that everyone is tired of me posting on this thread and whining about a busted truck, but I'm not going to leave this alone until I figure it out and maybe someone somewhere else will benefit from this. I have scrounged and found several threads here and on the FSB forum that deal with similar symptoms but have no conclusion, and I'll see this thread through and there will either be a conclusion or a carburetor!

Anyway, after fussing and cussing and reinstalling the loom on the harness from the inner fender over to the engine, I put the fuel pressure gague back on the truck just to see what it was reading now that it had developed this lovely misfire/stutter (not the original lean-ish backfire under a load). Now at idle fuel pressure is at 30-32ish lbs and when you goose it pressure jumps up momentarily but immediately falls back down to 30-32ish even when holding the throttle.

Shouldn't you have constant pressure with less vacuum? Previously (Friday when I last checked fuel pressure while driving) at part throttle I had constant fuel pressures above 30-32 lbs.

I know Subford has been telling me to check fuel pressures, but until now I have gotten results that did not suggest a bad pump.

The high pressure fuel pump is new and when I jump the terminals on the diagnostic port KOEO I get 40lbs of pressure, but Im not sure if jumping that diagnostic port kicks on the low pressure pumps. I would think it would, but does anyone know for sure? Is there a better way to test the low pressure in-tank fuel pumps that opening a line and checking how quickly they fill a container with a known volume to figrue out their lbs/hr output? The sharp drop off in fuel pressure seems like a supply or perhaps a volume problem...but I get the same problem on both tanks and I wouldn't think both in-tank pumps would go bad all at once.

 

From what you said above you have a fuel volume problem. I do not think it is not an electrical problem.
You may have a fuel filter problem as you may have two of them, one on the frame by the HP pump and one inside the selector valve.
You may also have junk inside the selector valve or in the tank plugging the sock at the bottom of the LP fuel pump.
You also may have a pinched fuel line.

 

Should the fuel pressure regulator hold pressure after it primes when you first turn the switch on?

Fuel pressure falls to nothing immediately.

 

I never questioned Bill's reponse with the IDM circuit resistance. I was questioning your previous reply of "22.4 ohms". You are a couple orders of magnitude off there....

 

The fuel pressure regulator will (should) close the port going back to the fuel tanks whenever the fuel pressure dose not overcome the spring in the fuel pressure regulator with the engine not running.
With the engine running the vacuum from the engine will pull against the spring also.

Other items in the fuel system also must close their ports going back to the fuel tank like the check valve in the HP pump.

An open injector will also cause the fuel pressure to drop but it would put out a lot of black smoke when you start it if that were the case.



/

 

Yes rla2005, I was way off. My apologies. I responded at about 1 a.m. between service calls last night and didn't really read your or my own post. Sorry.

There is no black smoke, and the check valve in the HP fuel pump is definitely no good. The truck is getting harder and harder to start.

 

I am hesitant to replace any part of the fuel system, though, because when the truck is cold you can drive it up the road with no problem until it reaches opperating temp, and then the missing and backfiring starts. Though I am somewhat guilty of throwing parts at it when it first started to act up thinking I had a bad vacuum leak or valve train problems, I don't want to continue just throwing parts at it.

I replaced the multipurpose reservoir with a known-to-be-good junkyard unit I had on the shelf just to see if the filter in it was clogged and maybe causing the loss of fuel pressure. It didn't help.

Were it just doing it on one tank I could see it being one of the LP pumps that had gone bad and as the pump ran it would "get hot" and not work as well.

As the truck acts up on both tanks and I wouldn't think both LP pumps would go bad at once, and the HP pump provides good pressure when you cross the diagnostic ports; I would question the fuel pressure regulator. But I don't think the fuel pressure regulator would over-regulate, and I don't get any tell-tale smell of fuel in the vacuum hose that comes from a broken vacuum diaphragm in a fuel pressure regulator.

 

Fixed it! Had two injectors, #7 and #2 that were working at idle but as fuel demand increased they were unable to keep up. After beating the truck around, sparingly so not to hurt the shortblock or burn a valve, I pulled the plugs and found those two very ashy and light gray.

Having two injectors acting up is what was the most problematic to solving the issue because I would get the lean "pop" on both sides of the engine. Just an FYI for anyone that has a similar problem.