cold start issue with GMC truck

rfuerst911sc · 10-04-2010, 05:50 PM

I have a 1997 GMC pickup with a 5.7L throttle body injection. Just this past week it has developed a hard cold start condition. Lots of cranking and then she'll start. Runs/starts fine once it's been started. I have no check engine light. I've done oil/air filter maint. but have not done plugs/wires/fuel filter. The truck has 107,000 miles. It seems to me that she's not holding fuel pressure but that's a guess. Any advice on what to check ?

Brian 162 · 10-04-2010, 06:13 PM

I would check the fuel pump pressure and replace the filter. When the fuel pump went on my 97 Tahoe it was around 180,000 km's.

Scott R · Scott R's Garage

That could be a sign that the MAP may be failing on a TBI motor. Any other times that it starts running rough or smelling rich at idle?

The tbi system is pretty simple. It gets a signal when cranking from the ignition switch to fire the injectors. It's critical that it gets a good 12V from the ignition to send that initial injector fire signal. If the battery is too low it doesn't send the signal and the car can be very hard to crank over, but once it catches and the ECU is powering the injectors it will run fine. Also bad grounds will cause this process to get messed up as well.

That signal goes from the ignition switch to the ECU (via the pink wire in the main loom) under the hood. Check for chafing on the wire bundle as it crosses the manifold to the firewall. Sometimes the shielding gets old and the wiring wears through, but that's rare.

The last items is the control pressure regulator can leak. It's in the TBI unit and after time it can leak and cause excessive, or no fuel pressure conditions. However it's normally a rich condition when they fail. The circuit is a simple supply and return to the tank. The fuel pumps do go bad at times, but normally that's a no start condition at all times.

SCcaretaker · 10-04-2010, 06:52 PM

My 02 5.7 had this issue, changed fuel filter and cleaned the fuel delivery system, and the start issue went away. As per Scott R, the fuel pressure should be checked, and make sure it does not fall off while not running. Pump replacement is pricey, not something to guess at! The owners manual addresses some of this issue in the starting the engine if flooded section.

BGCarrera32 · 10-04-2010, 07:10 PM

If that's cold as in cold after sitting overnight, then the first thing I would do is get a fuel pressure gauge on it and find out what the pressure leaks down to after several hours. There is likely a spec printed in a book somewhere. If you have no fuel pressure in the morning it will likely start as you describe. Other thing to try is consciously turn the key to the "on" position at first start in the morning to prime the fuel system for 5-10 seconds, then crank it over. If that helps then you probably have a bad check valve at the fuel pump.

juanbenae · juanbenae's Garage

GM truck fuel pumps are notoriously bad. its in the tank too boot and a pain in the ace or expensive to have done. an old mechanic suggested not to run the below 1/4 tank. the low fuel causes the pump to run hot and go out early. above a 1/4 the fuel surrounding the pump aids in cooling.

Scott R · Scott R's Garage

Quote:

Originally Posted by car 311

GM truck fuel pumps are notoriously bad. its in the tank too boot and a pain in the ace or expensive to have done. an old mechanic suggested not to run the below 1/4 tank. the low fuel causes the pump to run hot and go out early. above a 1/4 the fuel surrounding the pump aids in cooling.

You can actually leave the in-tank pump in and install an external on the frame rail. A pump from a 1988 Ford F250 is the exact 14psi pump for the TBI system. It's about $60 and really never dies, it's also nice and quiet and comes with a bracket.

yetibone · 10-05-2010, 04:28 AM

If it really is a TBI engine, check the grounds at the thermostat housing. One is ECM ground, and the other is sensor ground. They were notorious for corrosion, thus causing some driveability issues.

Check, also the oil pressure switch because after the first KOEO fuel pump prime, the oil pressure switch is responsible for powering the control side of the fuel pump relay. If there's no closing of that circuit in the switch, the fuel pump relay won't close and power the pump.

If it's a Vortec engine (big drum air filter on the passenger's side, most likely what you have in '97) check the fuel pressure at the fuel rail. That induction system needs AT LEAST 55-65 PSI to start the engine, and 45-55 to run the engine.

rfuerst911sc · 10-06-2010, 02:18 AM

Thanks for the tips guys I now have some things to check. Yesterday I cleaned the throttle body and it seemed to start much easier but that may have been just from the flammable cleaner itself.

rfuerst911sc · 10-06-2010, 02:20 AM

Quote:

Originally Posted by Scott R

You can actually leave the in-tank pump in and install an external on the frame rail. A pump from a 1988 Ford F250 is the exact 14psi pump for the TBI system. It's about $60 and really never dies, it's also nice and quiet and comes with a bracket.

I like the idea of an external fuel pump but 14 psi doesn't sound high enough to me ? Do you have experience running this pump ? Do you have a part number ?