idles fine then throws code P1258
 

Just finishing my swap for 2004 LS 5.3 hooked up to a TH350 in my 72 blazer It'll start fine and idle for about five -ten minutes then starts missing and it sounds like it's running on four cylinders. No code until it starts missing. I looked up the code and sounds like an overheat that makes it go into a safe mode. Sprayed carb cleaner around intake and no rpm change so I don't think it's a vacuum leak. Currently my temp gauge is not hooked as it is mechanical and I don't have electrical gauges. The electric fans do come on after a while and I do have heat through the heater. Is this caused by no temp gauge hooked up?

Re: idles fine then throws code P1258
 

If its showing overheat,then it may be..you really need a gauge on it..
But, did you fill the engine with coolant thru the top hose and let the air work it's way out?..
Sounds like you may have the engine airlocked..
Take the top hose of the radiator and fill the engine til you quit getting air bubbles..
Also do your heater hoses have a block valve or bypass valve? ..not all the time , but I've seen it happen, if the heater hoses are blocked off ,it wont circulate the water to open the thermostat
Your wiring harness/ecm should be reading temps from the drivers side head..that turns on the fans
You can use the pass side head for your in dash gauge..I drilled and tapped my pass head for the mech gauge sender for my dash gauge

Re: idles fine then throws code P1258
 

This ^^ or the temp sensor for the computer is not getting a reading via not plugged in, bad sensor or wires not crimped correctly

Re: idles fine then throws code P1258
 

Burped quite air out of the block and I'll give that a test run this afternoon. Is there any way of installing a mechanical temp gauge with the 72 sending unit without tapping the water pump or rear head?. I saw a t connection to splice into the radiator hose but it was a 3/8fip connection instead of the stock 1/2 fip size.

Re: idles fine then throws code P1258
 

Not really going to be an 'easy' way to hook up your original mechanical temp gauge. But if you have a way to read codes, you should be able to see what the PCM is seeing for temperature. I would go by that.

Re: idles fine then throws code P1258
 

Ran for twenty minutes with no code or limp mode. Must of been an air lock in the engine block. Now to figure out how to get a real mechanical temperature gauge. Thanks for all the help

Re: idles fine then throws code P1258
 

glad you got it..!!
just drill and tap the pass side head...took me maybe 5 min to do mine with mech gauge

Re: idles fine then throws code P1258
 

What location did you use for the drilling and tapping, passenger side at the back of the head?

Re: idles fine then throws code P1258
 

look at your drivers side head..see where your temp sender for the ecm is...in the same location on the pass side head (but in the back) there is a plug..i removed that plug..then its as simple as drilling the hole out and tapping it to npt...put some grease on your drill and tap and itll help catch most of the shavings ...then i stuck a shop vac over the hole to pull out any straggler shavings

Re: idles fine then throws code P1258
 

someone also makes a turned down stock sending unit for LS heads. Or this https://www.amazon.com/Engine-Adapte.../dp/B015NG418C