New PI Swap, good cold idle, bad hot idle

Busted Knuckles:

Welcome to StangNet! And, congrats on tackling a decent under-the-hood project -- it will make a big difference to the power and driving enjoyment.

There are 2 coolant sensors on the '98. One is just the indicator "gauge" in the instrument panel and the other one is the ECT (engine coolant temperature) sensor. Many, many things depend on the ECT so it has to be hooked up correctly after the PI swap. If you dropped it then it should be replaced. If you cut the wires to move the ECT to a different location then, for sure, check the joints. If you drilled/tapped the t-stat housing to install the ECT there then there might be a bubble of air in there insulating the ECT from the true engine coolant temperature.

Do you have access to a datalogger (XCal2 or Predator or other)? Data is gold, imo, and it is tough to diagnose what is going on without data. The Haynes manual has a procedure to test the ECT sensor (checking resistance when cold and when hot then comparing to the allowable values).

If coolant got to the O2 sensors somehow then the sensors may be shot. (Datalogging can tell you if the O2 sensors are working. There is a method to backprobe the O2 sensors to check their health but I do not have details, just a fuzzy recollection that a method exists).

Last thing is to go over all electrical and vacuum hose connections. Unplug and then re-plug them together to confirm that every one is correctly seated.

The DPFE (flow sensor for the EGR gas) is active at low- to mid-cruising conditions. EGR is not used at idle or WOT.

HTH,

Chris

Busted Knuckles:

You know, if the ECT sensor is good then I would not replace it. If it ain't broke...

As long as the cap is good on the reservoir tank the system will be pressurized and it doesn't matter if it is water or the usual 50/50 water/antifreeze solution. You have to have the reservoir filled to the MIN level. If it is lower, air will be trapped in the system.

I don't know if this is correct but I warm up the engine and, with the engine running, start to squeeze the upper rad hose starting from the t-stat housing and working towards the reservoir. The idea is to work the air towards the reservoir so it can burp out. Then I turn the engine off and sqeeze the lower rad hose for a minute or two to force any air into the reservoir.

I have carefully opened the reservoir cap while the engine is hot but I would NOT recommend it because your coolant is mainly water and it will flash when the cap is loosened (and you'll get scalded).

I hope this works for you. Please post back with an update.

Chris

Check the o2's. Mine were shot and the only way i found out was because when it would got into closed loop(warm up) it did exactly what yours does and my mechanic told me thats what it was, so i replaced them and viola problem solved