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96 K1500 350 Vortec Struggles to Crank/Turn Over When Warm/Hot

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Okay first I want to say that this topic may seem like one that has been beat to death. The only problem is that my truck has no problem starting when warm/hot, it just struggles to turn over when warm/hot. Cranks just fine when it is cold or been sitting for a while just like it used to (one quick turn of the key).


A little background on whats been done recently. I have been chasing a coolant leak that started with the radiator hoses. They have all been replaced. The radiator has been replaced. Then the heater core started leaking so I replaced it. Water leak still present so I decided to replace the lower intake manifold gaskets. While doing that job I also replaced all the exhaust manifold bolts and gaskets and all four O2 sensors. Put everything back together and that's when the problems started.


Before I removed the Distributor I marked where the housing was in relation to the engine and then marks where the distributor was in relation to the housing. I put the distributor back in according to my marks. Truck started/cranked like normal when I first started it up. Noticed a check engine light but wasn't too concerned at first. I initially had a fuel leak on top of the upper intake where the fuel line come in. Apparently there was an O-ring that I didn't see fall off the ends of the fuel lines when I removed them to get the intake out. Got that sorted out and took it for a test drive. Didn't get a mile down the road and it started bucking about every 20-30 seconds while going down the road. Went to Autozone and had them read the CEL code and it gave me a P0340 (I think that's the number, lost the piece of paper) which was the cam position sensor. I was pretty sure the truck was just out of timing. I don't have a scan tool to adjust the timing so I carried it to a local mechanic. Before I carried the truck I did notice that it was harder to start then normal when it had warmed up. When I say hard to start what I mean is that it starts and fires just fine, it struggles to turn over. Its almost like it has a weak battery but I know it's not the battery because I have had it tested and put it in another vehicle and it starts and turns the other vehicle over just fine. So I was kind of figuring that the timing had something to do with the starting issue when the truck was hot.


Got it back yesterday and it bucked shortly after I drove it away from the shop and hasn't bucked since, so I am pleased with that. The only issue is I still have the hard to start/crank issue when it warms up. So my question is where should I start looking? And I do know that with these trucks best practice for adjusting the timing is doing so with an engine speed greater than 1000 RPM's, so with that said could I still have a small issue with the timing causing the hard to crank issue if the shop didn't adjust the timing at the correct engine speed?


Thanks for the help guys, I'm stumped.

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grab a fuel pressure gauge. shut it off hot check fuel pressure any more than a 10 lb drop and its dropping raw fuel to the engine causing the hard to crank when warm hot basically a hydrolock situation now the distributer in these truck are non adjustable the distributer has the cam shaft position sensor in it it has to be set +1 -1 dead center for the check engine light to be off. also what i have ran into with mine is the oil sending unit went bad it would buck and down shift no tq converter lock issue easy way to find out after its running pull the relay for the fuel pump if it shuts off oil sender is bad this issue is for 95-2000

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I have often seen the bucking you're talking about with a clogged fuel filter.


Otherwise, this problem can often be attributed to the cam retard not being properly set. The cam retard (also known as CMP offset) is supposed to be 0, but +/- 2 is within spec. The further out it goes, the crappier it runs. And, it doesn't turn the CEL on until it gets somewhere around +/-14. And, yes, it must be read with the engine above 1000 rpm.


Professional scan tools can read the CMP offset. A friend of mine has an obsolete Snap-On MT2500 that works fine for this. Start the engine and run it slightly over 1000 rpm. If the CMP offset is out of spec, turn the distributor slightly and check again. Keep doing it until it is within spec. I have seen people turn the distributor until the light goes out, then spend a ton of money trying to figure out why it still runs crappy. You must use the proper scan tool and make sure it is within spec.


I know it doesn't solve the strange slow crank problem, but this is what I can offer.

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At this point go.back to where u started. How is your water pump? If you need to retime your enginenow, might as well replace that beforehand. Assuming your fuel pressure is fine.

With an improper water pump, i would think the the engine temperature regulation is compromised.


Are there engine temp sensors on the 5.7?

Edited by Jprun25
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Update: Timing is within spec, Fuel system checks out, Water temperature is being regulated as it should, and water pump was replaced not too long ago (within the last two years). I am staring to think it is a heat issue with the starter (original) so I am going to replace that this weekend and see what happens. I will update once installation is complete.


Thanks for the feedback.

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remember, a coolant leak has minerals..you mentioned intake had one.

you may be scrubbin' some walls in the ring lands...expanded to heat.


that is terrible news, I would hope it isn't the problem.


a scanner that can monitor, and even show a right bank lean is autel ms609.

I bought a new one needing a firmware update cheaper for the chore needed..

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