Truck Trend Garage: 1989 GMC Sierra Troubles At Idle

Expert Advice

Alex Steele
Dec 17, 2009
Photographers: Steve Austin
Q: My 1989 GMC Sierra 1500 Regular Cab has the original 5.7-liter 350 V-8 and has clocked almost 227,000 miles. For the past four or five months, it's developed idling issues. It will start normally and run fine, then the next time I start it, it will not start unless I press the accelerator. When I have to start it this way, it also will not idle at stops unless I continue to press the accelerator (while I have the brake depressed). This will last, on average, about a week and a half, then it will start as normal again. About three weeks later, it does it again for another week or so. I use only 87-octane gas, no ethanol. The fuel pump and fuel filter were replaced less than two years ago. I also added Heet fuel line antifreeze/water remover, gas treatments, and fuel cleaners into the gas tank, but the problem persists.
A: You're getting your money's worth at 227,000 miles, but of course problems do begin to mount up. I can't give an accurate diagnosis to the condition without being on hand with my scanner, fuel pressure gauge, spark tester, etc. But the fact that your engine occasionally won't start normally without depressing the accelerator suggests the engine is flooded with fuel. Opening the throttle supplies the engine with extra air to compensate for the overabundance of gas. A couple items you can check yourself will require only a quick peek under the air cleaner at the throttle body assembly (where the carburetor used to be with two big fuel injectors on top). Turn the ignition on to prime the system with fuel (apply pressure at the injectors) and then look to see if any fuel is dripping out of either injector. If so, replace that bad injector. Next, remove the vacuum hose leading to the fuel-pressure regulator. Check for visual signs of gas in the hose or the port at the regulator. Also give it a sniff test to find gas. Any sign of fuel at the regulator indicates the internal diaphragm is cracked and leaking and the attached vacuum hose is sucking excessive fuel into the combustion chamber. The technically correct way is to check the regulator with a vacuum pump to be sure it's holding vacuum. Again, these are just a couple of quick checks you can do yourself. But if everything looks good, have a technician evaluate the condition and test for other possible causes.
Photo 2/2   |   01z 1989 Gmc Sierra Troubles At Idle
How To Reach Alex
If you have a technical question regarding your pickup, SUV, or van, feel free to contact Alex, a master technician with the National Institute for Automotive Service Excellence. Send a letter to him in care of Truck Trend Garage, 831 S. Douglas Street, El Segundo, CA 90245, or e-mail us at Please include the VIN with your question. Due to the volume of questions received every month, we cannot guarantee that everyone's question will be personally answered or will appear in the magazine.

Can't wait for help with a problem you're having with your Truck or SUV? Ask the expert we trust here at Truck Trend Garage—visit Alex Steele at

- OF



Subscribe Today and Save up to 83%!

Subscribe Truck Trend Magazine

Subscribe to:

Truck Trend

Subscribe Diesel Power Magazine

Subscribe to:

Diesel Power

Subscribe Truckin Magazine

Subscribe to: