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Truck not running well after 30 minutes

Greetings everyone,

I have been restoring this 1996 Chevy K1500 V6 Vortec engine, 5 manual speed. Its shell has 237,000 with a replacement engine that has 64,000 miles. Almost everything has been replaced, from gas lines, brake lines, cat, clutch, etc. I was having many error codes regarding sensors, and It turned out that the computer was bad and was sending too much voltage to sensors, and I guess they were getting damaged. I replaced the computer and the dealer programmed it. Replaced basically every sensor, the spark plugs, wires, rotor/ distributor, coil and replaced the fuel regulator. I finished about three weeks ago and I thought everything was good, perhaps because I was only driving 10 minutes to work. Today I took her for a long drive on the highway and after 30 minutes, the engine started to hesitate, like there is no power (like it is running out of gas -but the tank was full). So I will park, turn the engine off, restart, and the truck is fine again for 10 minutes or so, then the hesitation would come back, turn truck off, turning right back (so it is not like I am letting it cool off, then the truck drives fine again! I never heard of a problem like this… and there are no engine error codes (yet)… One last thing, on the highway, the truck will no pass 55 MPH!

Your suggestions are greatly appreciated,


Fuel Pump? You need to conduct a fuel volume test.

Could also be some electronic component is getting hot after 30 minutes.

Sorry, I forgot to mention that fuel pump and fuel filter were also replaced! Even though i would think that a bad fuel pump would fault all of the time, not after 30 minutes

Possibly your gas tank isn’t venting properly. Plugged evap canister or vent valve?


Agree, try with the gas cap loose or removed.

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Thank you guys, I know where the evaporator canister is on this truck, any ideas where the vent valve is? Is the evaporator canister just filled with coal that could be simply replaced? Should I try driving without the gas cap to see if that would make any difference?

Lastly… is your thought that the truck works fine the first 30 minutes because after that pressure should be released from the gas tank? Just wondering

Again, thank you for your thoughts!


If you mean the EVAP purge solenoid, it’s on the intake manifold (#14 in pic below). There’s also a related vacuum switch (#16).

I’d say yes, try driving with the fuel cap loose. Cycling the key when it acts up may be triggering something in the EVAP system that relieves any vacuum in the tank, at least temporarily.

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A vacuum can be created in the tank if the EVAP system is not working. Taking off the gas cap will relieve that vacuum. BITD gas caps had a little hole or valve in them for this purpose.

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by vent valve do you guys mean the EVAP purge solenoid as shown by bugmenot? Also, I just found out the store sells carbon replacement for the canister evaporator… so for 5 dollars I am willing to try that right away!

There may also be a fuel tank vent valve in the top of the tank (see pic), though I don’t see one listed for the pickups. The vent may be part of the sending unit. This is what the tube to the canister would attach to.

Fuel Tank Vent Valve

There’s three tests I’d focus on for this situation. The first is to verify the battery voltage is correct. Ask your shop to connect a volt meter in the passenger compartment. You can monitor it as you drive. It should always read in the 13.5-15 volt range when the engine is running. Second, a test to verify the crank position and cam position sensors are working correctly. Third, ask you shop to monitor the fuel rail pressure during a test drive to see if the pressure is going too high or too low coincident with the problems occurring. The observation that it won’t go over 55 mph is actually a good thing, b/c obviously it should, and so you have a repeatable bench-mark to test against.

Modern engine computers usually include a precision voltage source circuit that powers all the sensors. Just using battery voltage for the sensors isn’t precise enough as the battery voltage varies with temperature, engine rpm, and whatever else in the electrical system is being used, like the headlights, rear window defroster, engine compartment fans. Sensors don’t usually need much power, so if that voltage regulator circuit was damaged I expect the cause was one of the wires going to a sensor was shorted out, or shorted to battery voltage. Either could damage all the sensors. So it is critical that your shop verifies the voltage at each sensor is correct, usually it is a 5 volt precision reference. Good idea to monitor that along with the battery voltage in the test above.

This may seem sort of overwhelming, but I expect whatever the problem is, while finding it may be a challenge, it’ll be pretty simple to fix.

Hello Keith and It_s_Me!

So after doing my homework, I found that the store sells the carbon filters to replace the old ones from the evap canister. As I was removing the evap canister I discovered that one of the lines was damaged. The line got too close to the manifold and melted closed. So I was able to cut the hose, found the other half, repair it in a way so now is far away from the manifold. That line ended on a sensor that was replaced but cannot remember the name of it.

So I take my hat off because you guys know that the problem had something to do with the evap canister! Since this problems only happens after 30 plus minutes of driving, I will have to wait for the weekend to go on a long drive, in the meanwhile, do you mind explaining to me the purpose of the evap canister and what is the point to send a line to that sensor? Is that gas vapor that is going through that line?

Thank you so very much for your help!

The charcoal canister absorbs fuel vapors from the gas tank. The valves and hoses get that vapor introduced into the engine when it’s running, so it is burned. The result is a lot less unburned hydrocarbons in the air, so less smog and fewer health problems. I don’t know just when and why valves open and close.

Good sleuthing job! You can see a lot just by observing.