Engine Hesitation at 40 MPH
 

Hello! I have a question regarding my 1988 S-10 Blazer 4x4 with 4.3L engine. When I get to 40 MPH the engine hesitates, when I hit 43-45 MPH it will run smooth again. Any ideas on what could cause this? Thanks in advance!

JAS69

since its an 88 i'm assuming ur still running a carb. yes?

this is a fuel related problem most likely and can easily be adjusted on the carb. (possibly running too lean)

if ur not running a carb then it could be other fuel related parts such as the Fuel Pressure Regulator (FPR), Nut Kit (plastic fuel lines inside plenum leak), or Spider Injector Assembly.

88 would be TBI if still stock.

It only does this at that particular speed? Is there anything going on then that is repeatable (throttle position, engine load, etc)?

Yes, it has TBI. It will hesitate between 40-43 mph whether I'm on level ground or going up a hill. It also starts hard when cold, I'm not sure if the two problems are related.

Check your TPS reading by back-probing the wires. You should read ~0.5V at closed throttle and ~4.5V at wide open throttle. The transition should be smooth between these two values with no drop outs. I suspect the TPS sensor has gotten a worn spot from frequent driving at that set throttle position

You may want to try running some fuel injector cleaner through the truck. I prefer Chevron Techron Fuel System Treatment Plus. Get two or three large bottles and run it in the next 2-3 tanks of fuel (one bottle dumped into the tank before you fill up each time). See if that helps. You may also want to consider replacing the fuel filter at the same time.