Shadetree mechanic has a 1990 Pontiac motor in a 1993 Trofeo. It quit and then runs rough. Smooths out at times, roughens again. Checked hoses, MAF. There'* no EGR valve. The coils all give a good spark when taking wire off at coil. Won't pull car much of the time. Dies.

Sounds like it runs rich and misfires. The catalytic converter seems really hot. What else should be looked at.

need more input on exactly what you swapped.

The motor came out of a full size Pontiac 1990. It had been running for months--I'm not sure how long. The guy that had the swap done sort of salvages older GM vehicles and gets them running and sells them. This one died on the buyer recently, so he bought it back from him.

The motor has a sensor in the intake manifold lower part behind the throttle body that has no wire connected into it. Apparently that'* the way it'* been since the engine swap was done!!! But the car apparently ran well enough for months. The father drove it himself and then sold it to someone.

I watched while they played with it in my neighbor'* drive. The spark wires when pulled off the tower with the motor running would spark up to an inch with a hot blue spark. Each seemed to be about the same in strength. They had the MAF out and it looked clean on the wires inside. The FPR had been replaced sometime in the past. The father pushed on the shraeder valve and gas spurted out so he felt the pressure was high enough. Fuel pressure would be one of my concerns.

Motor just runs rich and misses and stumbles more at times than others. Doesn't want to idle, has to be held up in rpms.

It'* not the usual car troubleshooting I see. I thought maybe someone might know what'* up...

I would assume the trofeo had an L27 and the mechanic put a LN3 in as a replacement? Sort of an odd thing to do, but I suppose it is doable. If you just look under the hood you can tell which it is by the engine cover. If it says Fuel Injection 3800 you have a LN3. If it says 3800 Tuned Port Injection, you have an L27.

That unhooked sensor is the IAC valve if its to the lower right of the throttle body.
It needs to be hooked up to get the correct air / fuel mixture.

If the cat gets red hot, like glowing anywhere around it, then that'* clogged. When you say running rich, how do you know?

No, that can't be it if it ever ran good. I'm thinking coolant temp sensor.

What confuses me, is that this motor is supposedly a '90, but has no EGR. I don't think such an engine existed in '90..?

something sound funny.
If its the temp sensor unplugged the motor will run in open loop.