98 B4000 4.0L - P0171 and P0174 lean codes

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OK. It's been a few months since my post.

A recap. I was having cooling system problems which was a result of a bad headgasket. Eventually, I toasted the engine when the gasket let go and all the coolant came out.

Fast forward to this month, I finally got a replacement engine installed. It took a while but I did it.

With everything back together and the truck runniing good I am getting the P0171 and P0174 codes intermittently and alternately. They appear 95% of the time after the engine comes up to full temp and I make a stop where the engine is off from anywhere to 5 minutes or 20 minutes like if one goes to the grocery store.

I can clear the codes and drive from cold start until the cows come home and it will not store either of these codes unless I turn off the engine wait a few minutes and restart then drive after a full warm up.

I replaced the MAF and oil fill cap which reduced the CEL occurance. Now it will store a code but not light the CEL and sometimes the code self clears. The MAF is aftermarket since it 's less expensive than OEM and I did have trouble occasionally with the old oil cap. So I don't know which one made the slight improvement. Would an OEM MAF be worth the try?

I'm also thinking that I may have a very small vacuum leak in maybe one of the plastic vacuum lines from moving stuff around when removing and installing the engine but I can't find it.

Truck idles and runs fine otherwise.

Any ideas?

Thanks,
Doc

 

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update:
Found what I think is a vacuum leak where the intake plenum and EGR return pipe connect. A heavy squirt of carb cleaner caused the engine to change RPM. Put some sealant around the connection as a quick fix and to verify if I need to pull the intake plenum.