My Truck Sneezes

Six months ago we purchased a '91 Chevy Pickup and 3 months ago the truck started to “sneeze”. Sometimes while accelerating the truck makes a “tooft” noise from in front of the driver under the hood, and with the noise I have a momentary loss of RPMs. This can happen a number of times in a row, making pulling out on to busy streets difficult and dangerous. Most of the time, I take my foot completely off the gas and then re-apply it, and the issue goes away. It happens both with a cold engine and a warm/hot engine. This problem started when it was cold out (around 10 degrees Fahrenheit) and the mechanic I talked to about it thought it was the E10 87 gas I was putting in. I switched to a 97 octane/no-ethenol fuel and the problem went away, until about a week ago when our temperatures finally broke 50 consistently. The problem is back and with it is a minor fluctuation of RPM when idling in neutral. The truck also created a new symptom, I was driving at about 50mph and the truck acted like it slipped on ice… I had evened out my speed and the truck seemed to start coasting then got back to driving at speed, it jerked the whole truck, like driving over a patch of ice. I don’t know if it has anything to do with the original problem but all symptoms may be the same thing…

The truck is a 91 Chevy 1500K, 5.7L V8 and has 74,000 miles (mostly highway towing). It had been sitting 2 years, not really being used, before we bought it. We had the fuel injection system cleaned when they did the oil change, right after we purchased the vehicle. Any help is appreciated.

It sounds like your truck is running borderline lean. I did a job at a gasohol plant where they gave me a pickup truck to drive. It did the same “tooft” and stumble at irregular intervals. The guys there told me, “Oh yeah, we add extra alcohol to that truck’s tank.” Dunno, you might try another fuel system cleaning. Or it might be an O2 sensor issue.

On the “ice” episode, are you saying the engine revved up and the it felt like the rear wheels were spinning and sliding sideways?

Sounds to me like you need to reset the Distributor Timing. Over time as the timing chain stretches the timing of the engine changes…the distributor needs to be adjusted for this as it only has a limited amount of adjustment inside of it. Its quick and rather easy to do…give it a shot. Its something I would quickly try.


insightful: For the “ice” issue… the truck didn’t change RPMs, and the rear wheels continued to track correctly… But the truck lurched forward like the tires were spinning out (but were on dry pavement) and the lurch is felt throughout the truck and always freaks me out… I don’t really know how else to explain it…

Just a guess, but whooshing sounds similar to what you describe can come from a defective brake booster, which sits on the firewall immediately in front of the driver. That’s something fairly easy for a shop to check. If it is that, the other symptom could be associated with the brakes locking up, or that brake fluid is getting aspirated into the intake manifold. Take a look, is your brake fluid low?