96 5.7l truck stalling and skipping at random

[newguyinnc]

hey all. Have a 1996 k1500. I bought the truck used a couple months ago , got it cheap knowing I was gonna put some money into it. First thing I noticed was the truck skipping when I put it on the road , purely at random. About 2 weeks later it started stalling at random. I can fire it up in the morning and sometimes it'll run , sometimes it'll stall after 10-15 seconds. It'll stall after being warmed up going down the road too. When it stalls , if I put it in neutral , sometimes it fires back on its own before I can turn the key , sometimes I have to refire it with the key. It may do this 3 or 4 times in a mile , it may not happen for 2 or 3 days. So far , I've replaced the distributor , msd coil , msd 8.5mm wires , delco plugs , cam sensor , crank sensor , map sensor , egr valve , and the fuel filter. I'm getting 60psi fuel pressure key on engine off , 52psi running , 60psi revved up. If I let the truck sit 15 minutes it'll bleed down to 42psi. I'm pulling 20" of vacuum at idle. The truck idles smoothly (when not stalling), gets 18mpg , and has plenty of power. I've checked the grounds and when I had the truck running today , I was up under the gas tank when it stalled and I could hear the fuel pump run for 4 or 5 seconds after it stalled. Yesterday afternoon when I had it running it would skip like it was gonna stall several times in a row , then suddenly rpm jumped and it stalled after idle leveled off. I'm at a loss , any ideas??

 

[SAATR]

Likely a fuel issue, even given the results of your pressure test. May try swapping the fuel pump relay for a known good relay and see if that doesn't make it act right. Intermittent power failure to the pump could certainly cause what you describe. It would be good to connect your fuel pressure test gauge to the port and tape it to the windshield so you can see if you are losing pressure when it stalls. That would confirm or rule out fuel problems.

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[newguyinnc]

I swapped out the fuel pump relay for the a/c relay a few days ago , no difference , but for $10 I'll grab a new one on my way home tomorrow and try it. As far as the fuel pressure test , I had the truck running with the gauge on the windshield facing me waiting for the truck to stall. When it stalled , there was no pressure drop , in fact it went up to 56 or 58psi when it stalled. Only other thing I've tried was a suggestion from another forum to unplug the MAF sensor to revert it to speed density mode and it ran worse , so the MAF should be good. Any idea what to check next??

 

[woody31]

try having the ignition module tested,,have them test it repeatedly ,,as with heat it breaks down,,

 

[newguyinnc]

I'll def try that , it stalls after running for 15 seconds first thing in the morning when its only 55-60 degrees out tho. That's what has me baffled , theres nothing consistent about what its doing. Every time I've had it in the shop , they change something out and it runs great for 2 or 3 days , then it'll start skipping and stalling again. It started with a code for cam sensor , so we changed it. My mechanic put it on his scanner cause it wouldn't time right. So we changed the distributor. In the process , the new distributor wouldn't drop in , so we had to use the gear off the old distributor. Then it threw a code for crank sensor. We changed that. Then it threw a code for MAP sensor , we changed that. Hasn't thrown another code since , but have changed the coil , wires , plugs and egr anyway trying process of elimination. This is getting expensive in a hurry though.

 

[newguyinnc]

One thing I completely forgot about and its only done it once. I drove the truck to breakfast a couple weeks ago and it ran fine there. On my way back to work , it stalled out doing 45mph several times in about a mile and a half and had one big backfire during that round of stalling out. Only time its ever backfired like that. Should I have a mechanic double check the timing on this again?

 

[SAATR]

The MAF wouldn't be the source because it's stalling while still in open loop, which doesn't use the MAF. This is one of those situations where a datalog of sensor outputs is crucial to fixing the problem without having to use the shotgun approach. What were the specific codes that you were getting from the crank sensor, MAP sensor, cam sensor, etc?

 

[Keepinitoldskool]

Sounds like a intermittent ecm issue. Leave a scanner hooked up and view the live data. When it stalls do you loose your data feed?

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[acmeflyer]

Check positive battery cable stack. Mine was corroded and caused simular problems. ECM's don't like voltage spikes.