Howdy all,

I've been working on my truck for the last few months (and made several other posts on these forums to troubleshoot, thanks to those who helped with the other issues) after installing a new 5.9l magnum, front end, and virtually everything else in the engine compartment besides the brake booster and master cylinder. I'm so close to the finish line now that I can see it but I'm having some really weird issues that I can't seem to diagnose or find answers to elsewhere.

For the last few weeks I've been able to get the truck to turn over and start rather easily and idle relatively well (550-650 RPM) and get to temperature (around 210 or so both on the gauge and my cheap scan tool) but the second I apply any additional throttle, whether it be in park or in gear, it sputters and then suddenly revs to 1.5k-2k RPM before slowly dying whilst backfiring out of the exhaust. I originally thought that this may be a vacuum issue; I got a test kit and measured the PSI at the throttle body to be only 5, and the MAP sensor was reading 1.5-2 at idle. So great, I have a vacuum leak and maybe a bad sensor as the one that is on there is 22 years old and the only sensor I didn't replace on the whole truck. My first thought was "huh, that's odd that the computer didn't throw any type of code seeing as both of those values are astonishingly low" and proceeded to test with starter fluid and cap off all vacuum ports to check if the leak was in the manifold or not. After testing with the fluid I determined there wasn't a leak in the manifold gasket, but still only got 11-12 PSI at the throttle body.

Now this is where it gets even more confusing; I had my mechanically inclined cousin over yesterday to help and diagnose my problem, and whilst testing with the ports capped, the coolant temperature quickly shot up from ~180 to ~240 in a matter of 30 seconds or less. We shut the truck off and checked to see if there was coolant in the top return hose from the thermostat and it appeared that the hose was empty, which means that the thermostat didn't open. However, when we opened the radiator cap a ton of pressure and room temperature coolant rocketed out (which from my knowledge can mean that there is a leak in the head gasket and combustion gasses are entering the coolant system). We decided to try again but open up the radiator cap to see if there was any flow which would indicate the water pump is actually moving fluid around the system, but saw no movement in the coolant and no bubbles rising to the surface, nothing. Yet, even after running for a few minutes, the odd coolant temperature gauge issue didn't return.

Throughout all these tests, the cherry on top is that the voltage readings for the O2 sensors never budged above or below 0.000 volts.

The truck was purchased from a lien sale and was a complete lemon I decided to make my project truck; the cats were cored out and a bunch of crackhead mods were on it. With all that being said, I think the real issue may actually be with the PCM/ECM. There's too many oddities considering everything is bone stock and brand new. In the event that it isn't, and somehow my MAP sensor, O2 sensors, coolant temp sensor, are all bad and I have a combustion leak into my coolant I just purchased all the necessary parts to replace them, but I wanted to see if anyone on here had an inkling as to what was happening before I spent more time and money going down this rabbit hole.

Thanks!

 

Unplug your upstream O2 sensor, run 'er n see what happens. Next, run the engine to temp WITHOUT the radiator cap on. You might just need to bleed the coolant system.
Then report back.

 

Thanks, will do. To note, when I started the engine for the first time with the cats and o2 sensors hooked up, both the upstream sensors produced a significant amount of smoke. At the time I figured it was from the thread lock that came with the sensors but I'm thinking they might've gotten cooked.

 

The O2 sensor have heaters in 'em, that warm them to like 900 degrees..... so, cooking stuff off isn't a surprise. If your hands were even slightly dirty when you installed them, then so are the sensors, and that will burn off almost right away.

You should see 18-20 inches of vacuum at idle. Vacuum will decrease with throttle opening. If it dies as soon as you give it ANY gas, I would see if your scan tool will read live data, and see what the TPS is doing..... also verify the map sensor agrees with a vacuum gauge. (keep in mind, MAP sensor is Manifold Absolute Pressure... so, have to do some math to convert inches of HG, to an actual pressure.

 

Did you set the fuel sync? Did you check fuel pressure? You are in CA so do you have 2 or 4 02's?