TROUBLESHOOTING: No-Start Suburban

Dear Roy, The vehicle is a 1990 Chevy Suburban 1500 with a VIN K engine and a 700R4 automatic transmission.

We installed a brand new General Motors (GM) crate motor (the one specified for this vehicle), and now the engine will not start unless primed.

The Throttle Position Sensor (TPS) reads 5 volts at engine off, key on. The scanner does not indicate clear-flood mode, although that is what appears to be happening. No fuel can be seen coming from the injectors.

If you spray a little carb cleaner in the throttle body, the engine will start and the TPS reads normally. Engine idles is OK, but it does have a surge every few minutes.

Idle Air Control (IAC) counts are around 145, and the vehicle drives fine. All sensor values appear normal and no codes are present.

I'm not sure what I'm missing here, but this one has me stumped. Thanks for any insight you can provide. Mike McArthur, Master Technician Merchants Tire and Auto, Newport News,VA

Dear Mike, If you are reading 5 volts on the TPS signal line with a closed throttle, you can bet that the system is in clear flood mode when cranking. Whenever the TPS is above a certain voltage during a certain rpm range, the system will shut down the injectors. These limits can vary among manufacturers but are typically anything over 3.5 volts (TPS) and engine speed under 300 rpm. What concerns me is that the TPS voltage returns to normal after starting.

I would check the following:

• Is there a bad ground to the TPS?
• Is the TPS sticking?
• Is the TPS defective?
• Is the 5-volt reference shorting to the signal wire?

Use your multimeter direct at the TPS to perform these tests. My guess is that you got some cleaning fluid in the TPS during the engine installation.