Advice: Tuning Up 1993 GMC C1500 4.3L V6

mark3069 · January 9, 2019

My old 1993 GMC Truck has been hard to start lately when we have prolonged damp, rainy, weather, and also my fuel mileage has went from about 16 mpg city down to about 9 mpg city. I called a couple shops in town, and they said it sounds like the truck needs a tune-up, with new spark plugs, spark plug wires, distributor cap, and rotor. I was shocked when they told me what they charge to do that work. One shop was $275 including parts, and the other shop was $150-$170 for labor only, with me supplying the parts from NAPA myself (because I get a 10% AAA Discount at NAPA). The reason I was shocked is because I haven't had a shop do a tune-up on any vehicle I have owned since about 1975, and it was A LOT less at that time. Also, to be honest, I have never had a tune-up done on this 1993 GMC since I bought it new in 1993, and so it probably is time, or past time, to get this done, and it probably will solve my hard starting and fuel mileage problems. I am thinking about doing the job myself, in order to save the shop's labor fees, but am concerned whether it is something that is better off being done by a shop, and if I was to do it myself are there any tips, tricks, or general advice that you all can give me to make the job go smoothly & easily without any problems. Since those plugs have never been out for almost 26 years, I worry about them breaking off, and I wonder if I should spray some penetrating fluid around the plug base and let it soak before trying to unscrew the plugs. Maybe there are other things I should be concerned about too, and I would like to hear your thoughts about anything that you know of regarding this job. I watched a YouTube video about replacing spark plugs, and read some comments that said it's easier to go in through the wheel well, but that involves removing the front wheels, and I would rather avoid that if possible. I used to work on my vehicles all the time back in the late 70's and early 80's, including doing tune-ups such as the one in this post, and I have replaced a timing chain, done drum brake jobs, and a whole lot of mechanical work on my vehicles. So I already have all the tools I need to do this job. But I am a lot older now (60 years) and not a limber, strong, healthy, young man anymore, and so things like this tune-up are a major undertaking for me anymore. But I am also retired on disability, and money is tight, and so I have to try to do as much of my own work as I can due to finances, and only hire out the stuff that is too big or complex for me to handle. Oh, and while we are talking about this, what do you guys think of NAPA Belden Edge spark plug wires, NAPA Echlin distributor caps & rotors, and OEM AC Delco standard spark plugs? Anyway, I would appreciate any advice on how to do this job that you all can give me, that will make the job easier for me to do myself. Thank you.

thx1138v2 · January 13, 2019

I bought my '94 Sierra C1500 new and have done all of the work on it except a transmission rebuild. I'm in pretty much the same circumstances as you although I'm 68 years old so this truck has more or less become my "hobby truck."

There are a couple of things you did not mention.

Air filter, but you probably know to do that.

Replace the fuel filter also. It's on the frame under the driver's side door. You'll need either open ended wrenches or the box ends with the fuel line cut outs. I've just used open ended wrenches but you may need the fuel line wrenches if the fittings are rusted. Definitely use some PB Blaster penetrating fluid if they are rusted.

There is a sensor called a coolant temperature sensor. There are two sensors on this truck. One is on the driver's side cylinder head and it feeds the gauge in the instrument cluster. The other is on the top of the manifold just to the left of the radiator hose and is made of brass. This is the one that feeds the ECM (computer) and is used to determine the richness/ leanness of the fuel mixture. Replace this one. You'll need to drain down some of your coolant so it doesn't make a mess when you pull the sensor out. This is probably what is causing your poor fuel mileage. I'm getting about the same mileage now as when it was new.

You might also think about flushing your brake fluid. That's not something everybody thinks of but if it's never been done, you should do it now. If your hydraulics are good you just go wheel to wheel, starting at the passenger side rear, and progressing to the driver's side rear, passenger side front, and then driver's side front, and bleed the brakes, refilling the reservoir as needed. It's pretty easy to do with a self bleeding brake setup.

I've never used penetrating fluid on spark plugs but then I've been changing mine every 70k miles or so and I'm up to 226,000 miles now. I wouldn't worry about breaking them off coming out. But definitely use some carbon powder on the threads going back in.

I don't have any opinion on the NAPA parts because the NAPA parts house nearest me is about 40 miles away and I have several others less than 10 miles away so I never use NAPA parts. One word on the "OEM" parts: they usually aren't. AC Delco and some others are made over seas and you don't know what you'll get until you pull the part out of the box and look at it. They may be to the same specs but that can also be misleading. I've never found a instrument gauge temperature sensor that reads correctly and I've been through 8 or 9. It may have the proper threads and the right size tooling so the wrenches fit but the electrical specs may not be the same and there's no way to know without installing it.

If I remember any more tips, I'll post again.

Good luck!

PS: Don't forget to reset the ECM (computer) after changing or modifying things that might affect your fuel mileage. To do so, disconnect the battery ground for about 15 minutes and then reconnect it. That does the reset. Then drive the truck around for 15 or 20 minutes both in town and at highway speeds to let the ECM relearn the variable parameters.

thx1138v2 · January 14, 2019

While you have the air filter out, remove the air filter housing and spray down the outside of the TBI with TBI cleaner. Then start the engine and spray some into the TBI, holding the throttle open sufficiently so the engine doesn't die. Also check all of your vacuum hoses while the housing is off.

While you have the distributor cap and rotor off check that the distributor is not loose. Grab the distributor body and try to turn it. If it turns by hand you'll need to set the timing and then tighten it down again. There is a single wire under the dash just behind the glove compartment that needs to be disconnected while you set the timing and reconnected afterwards. If I remember correctly the wire is brown and it may have a white trace on it. I'm not certain about the white trace. This, of course, is going to require a timing light. I usually use a white piece of chalk to mark the pointer attached to the block and the mark on the flywheel pulley to make them show up clearly under the timing light. Since this is done very infrequently, I usually borrow a timing light if possible. You might have one from your earlier tune ups.

Although I've never used penetrating fluid on my spark plugs, if you are more comfortable doing that, then by all means do so. I highly recommend PB Blaster penetrating fluid. If you use it you should let it sit overnight before pulling the plugs.