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.