TBI high idle on '91 motorhome p-30 454

[adamstarr]

Hi there,

I have a 91 motorhome with a TBI 454 with 80K on it. Starts great and idles and runs perfectly for 5 minutes, then the injectors start dumping fuel into the engine and it starts running rich - real rich, and pretty poorly. Have replaced the fuel pressure regulator diaphram, cleaned injectors, replaced ECM, spark plugs, and checked fuel lines / sending unit for irregularity.

I thought that it may be a thermostat problem that is failing when it comes to temperature? Perhaps an oxygen sensor?

Why is it starting fine and idling fine for several minutes - then racing like a bat outta hell?

Thanks,

Adam.

[maxwedge]

Welcome to AF. I'd suspect the coolant temp sensor, try and scan it and see what the readings are or just replace it.

[Blue Bowtie]

Scanning for data should reveal some useful data. Check both the CTS and the data on the MAP and TPS.

Instead of scanning, you can also test the CTS with an ohmmeter and compare the resistance to the following table:




You can also get data on the MAP with a voltmeter:




I suggested checking the MAP to see if the engine is producing the correct vacuum. Vacuum leaks can cause a higher than programmed idle RPM. So can a TPS which is poorly adjusted.