Questions and Answers

GMC Vandura Hesitates, Stalls Q. Vince, Great site. I've been to a bunch of weekend mechanic Q&A sites today and yours is the best. Here's the stats: 1993 GMC Vandura    Tiara Conversion 262 V-6, Automatic transmission 108,000 miles P/S, A/C, A/C blower in rear, Cruise

Problem:
After the engine warms up for about 10 -15 minutes of driving, there is a terrible hesitation when I press moderately to heavily on the throttle. I have stalled it a couple of times recently and the van flat refused to go above 35 for a while this morning. If I apply gentle pressure to the throttle, I get smooth, even acceleration. But once I crack it open too quick, the engine jumps coughs and knocks!

What I've done:
I have had the coolant flushed, the transmission fluid change (with that new drain the torque converter system), the oil changed, the fuel filter changed, the fuel injectors serviced and cleaned, and the EGR valve changed. I personally have changed the spark plugs, cap, rotor, wires, air filter and the PCV valve.

I queried the ECM and there were no diagnostic codes given other than the standard "12". I listened to all parts of the running engine with a short PVC pipe trying to find a vacuum leak and heard nothing. I run with 3 parts 87 octane and 1 part 92 octane in an attempt to compensate for the age of the engine.

I've bought gas at about 12 different stations since it started. It doesn't matter if I'm cruising at 55 trying to go to 65, if I'm stopped at a light and trying to pull away, or I'm sitting in the driveway in park - when I stomp on the throttle I get the same response.

Some other background info:
I do not get the problem in the first 10 - 15 minutes of driving. I live just north of Tampa, FL. It has been in the high 90's pretty consistently since this problem began in June - and it's not a dry heat here, it's a wet stanky heat. I first bought the van in early February with 98,000 miles.

My first thought was vapor lock but even after the engine is warm and the problem has begun I can hold the fuel lines all the way to the connection at the intake manifold with bare hands. I also read that vapor lock should be impossible in newer pressured fuel system vehicles.

HELP!! I'm stumped!

Thanks!
Frank

A. It could be a lot of things but I would start by looking at the fuel pressure. The pump may be on it's way out. If you have TBI, pressure should be 13 psi, give or take a pound or two. When it falls on it's face, the pressure should remain constant. You could try driving it with the air cleaner off and when it acts up spray some carburetor cleaner into the TBI and see if it picks up. Make sure the timing is correct also.