Today: What's Causing Low-Speed Sluggishness?

Dear Car Talk

Dear Car Talk | Feb 04, 2014

Dear Tom and Ray:

I have a 1999 GMC Suburban with a 5.7 V-8 engine and 140,000 miles. Lately, the truck has had problems getting moving from 0 to 30 mph. When I press on the gas, the truck acts as if it is being suffocated. The exhaust sputters and pops. When I floor the gas, the truck takes a couple of seconds to react, usually with a jarring burst forward followed by more self-suffocation. The truck, once moving past 30 mph, works fine. I had a relative inform me that my catalytic converter is "clogged" and that I need a new one. Do you agree?

-- Al

TOM: Well, I've learned from painful experience that you should never trust a relative who gives out advice on car repair.

RAY: Yeah. Especially if he happens to be your brother.

TOM: This guy's wrong, Al. If your catalytic converter were plugged, you'd have the opposite problem: The car would have no trouble going 0-30 but would bog down at higher speeds. So that's not it.

RAY: What's more likely is that you're having fuel-delivery issues. I'd start by replacing the fuel filter, especially if yours has been in there since Bill Clinton was downing Big Macs in the Situation Room.

TOM: If that doesn't fix it, my guess is that your fuel injectors are not working well. The injectors might not be spraying fuel into the cylinders in the proper pattern, and that's causing imperfect combustion and momentary flooding, and making the car bog down at low speeds.

RAY: Once you get past 30 mph, there's a lot more fuel and more pressure going through the injectors, and that's what makes the spray pattern correct itself. Plus, the car is already moving, which reduces the demand on the engine and makes everything more forgiving.

TOM: Your injectors could just be dirty. We rarely see that anymore, because late-model cars have all been running on the newer, cleaner, Tier 3 gasolines that we have these days. But your car is older, and spent many years running on older, dirtier fuel.

RAY: So start by trying a home remedy: Get a couple of cans of BG 44K, if you can find it. Or try Chevron Techron. They're both fuel-system cleaners. Run a bottle of one of those through the system with a tank of gas and see if you get any improvement. If you do, try another bottle or two and see if you can bring it back to where you want it.

TOM: If that doesn't clear it up (it probably won't), your next step would be to find a shop with what we call a Wallet Vac.

RAY: Its real name is the Motor Vac, and it's a machine that forces a solvent through the fuel system at high pressure to clean out gums and varnishes. It works pretty well, but the service will cost you a good $100.

TOM: And if that doesn't work, then you need a mechanic to help you figure out whether it really is the injectors, or something else. He'd start that process with a complete engine scan.

RAY: You don't want to just go replacing fuel injectors without being pretty certain that they're the problem.

TOM: They're not as expensive as the catalytic converter your relative was egging you to buy, but you'd be annoyed if you spent hundreds of bucks replacing your fuel injectors based on nothing but a newspaper column, and then still had your low-speed stumble.

RAY: And we'd be annoyed, because then you'd be adding to our already-tall hate-mail pile. So get it checked out in person, Al. Good luck.

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