Truck Repair: Sluggish performance with 98 GMC Sonoma, gmc sonoma, vacuum leak


Question
Hi Van,

I have a 98 Sonoma with a vortec V6. Lately the motor has been making a very fast "ticking" sound when ever I excelerate. I have noticed that my gas mileage is getting worse and I am starting to loose horse power. Occasionaly after I finish accelerating, it will have a slight hesitation (or slip) while it slips out of "passing gear" (sorry for lack of technical terms). Other than that it starts and idles fine. I maintain it very well, but since it has now come off warranty, I would like some info before a mechanic puts it on the bench to begin troubleshooting the problem.

Thanks in advance!

Answer
Hi Shane,
If that ticking could actually be referred to as clattering, there are several causes for it, and the knock sensor looks for that noise, and retards the timing to compensate for it.
It used to be called low octane knock, which could be one cause. But I would expect you to find a vacuum leak, which could be a hose or connection, the egr valve not seating completely, the brake booster leaking, the pcv valve dirty, etc.
A dirty fuel filter could even cause it, since too much air for the delivered fuel can cause it.
But that is assuming the noise is what I said.
It is also possible that a push rod has some slack, due to a rocker arm retainer backing off, a lifter not working, a bent push rod, or something along that line.
But that should be there weather you accelerate or not, where the earlier suggestion would only happen during acceleration, or under load, like going up a hill.
Good luck,
Van