I recently bought a 1996 I 30 with a ruined engine. The previous owner had run it out of coolant. I replaced the engine and it runs fine for the most part. The problem is: when sitting at a stop light with the A/C on and the transmission engaged (Automatic) the engine misfires. It logs the code PO400 indicating an egr flow problem. Does the egr temperature sensor contribute to this type of scenario? Measuring the resistance through the sensor, it falls slightly below published specs for the temperature. It moves through the resistance ranges smoothly as the engine warms up. I have tested the egr vacuum solenoid and it operates according to the manufacturer's specs.Do these engines have a history of plugging or malfunctioning egr valves? (VQ30DE)

It sounds like you have a stuck open EGR. It might not be much (ie you can remove and clean it). The old Is tend to build up some carbon (depending on fuel quality) in the valve assembly and nearby restriction points.

If you have a scanner I would monitor EGR temp sensor voltage while revving the engine briefly upto 4k. It might also be a good idea to check the vacuum hoses near the EGR and BPT valves and just verify that there is no vacuum to the valve at idle (this would make removal counterproductive).

Steve,After working my way through the egr system, I found a faulty diaphragm in the BPT valve and a carbon plug in the intake manifold where the egr tube joins the manifold. That eliminated the egr code.

However, I still have an intermitent misfire problem. This occures while idling with the engine under load. Remove the load from the engine and the misfire doesn't seem to occur. Performance is good. Mileage is good. (the last tank of fuel I averaged 27.6MPG) Everything seems to be normal with the exception of the pesky misfire. At this point no codes are being set.

I have used carb cleaner to help identify any induction air leaks. At this time I haven't located any vacuum leaks.

Any ideas as to where I might continue looking for a cure for this problem?

Thanks Ed