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Discussion Starter #1 (Edited)
I have a 1999 Chevy Tahoe that hesitates bad sometimes when you accelerate hard as soon as it gets warm. When it’s cold it runs great. I have changed plugs, wires, distributor cap and rotor button about 6 months ago as well as the fuel filter this week. But it is now getting terrible gas mileage. Maybe 200 miles to a tank which is a 21 gallon tank. The truck is regularly maintenance and i run 93 non ethanol gas. Fuel pump was change about year and a half ago that was my next step. The truck is straight piped and tuned with the cats tuned out. The only engine codes it gave me were all 3 oxygen sensors and eventually it went away and engine light hasn’t came back on. Any input would be great. I was leaning towards ordering an msd distributor Kit or another fuel pump.
 

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‘01 Silverado 2500HD 8.1/Allison 5sp xcab long bed
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Before spending any money, check your fuel pressure
 

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A cold engine requires more fuel in the air fuel ratio to run smoothly. As the engine warms, AFR should lean out because a warm/hot engine requires less fuel to run smoothly.

Your engine computer is not leaning the mixture and you are running rich.

Now, before you jump to the expensive decision to replace the engine computer; don't, it's probably fine, just doing what the inputs tell it to do.

One of the inputs is the temp sensor that the computer uses, test this sensor.
 

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Discussion Starter #4
I went ahead and changed that coolant temp sensor. Didn’t help it at all. Ran great while it was cold and as soon as it warms up back to the same thing. Will be testing fuel pressure soon.
 

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I believe that engine has 2 temp sensors, did you change the right one? And sorry, I don't know which is which.
 

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T, I don't know if that's the one the computer uses, if you don't mind, I'll ask Greg, @Cowboytrukr
 

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‘01 Silverado 2500HD 8.1/Allison 5sp xcab long bed
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The T’stat housing one is the one computer uses. The one in the head is for the gauge
 

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Discussion Starter #9
Yep that’s the one I changed. I will be testing the fuel pressure when I get back in town from work. What psi should I be looking for. And should I test it when the motor has warmed up that’s the only time it has the problem. Thanks
 

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kennythewelder has posted the fuel pressure numerous times but I've never saved any of the threads. I know he says the Vortec 5.7 needs 60psi min to start reliably but will run OK in the upper 50's. So look for 60 to 65 psi. The engine temp should not affect the fuel pump, but of course the amount of time the pump has been running could. One of the common ways the pump fails is getting hot and sticking. Hitting the bottom of the tank with a 2x4 or something has freed up many of them and gotten people home.

On the '98 I had years ago, my pump stopped while the fuel level was real low. After I had it towed to a shop, it fired right up as soon as the tow truck let it down. I drove it home 20 something miles and drove around a little, then changed the pump. I kind of wondered how much more I could have gotten out of it as long as I kept the fuel level up. In fact, I thought I had ran out because I knew it was low and forgot to exit at a station and was trying to make it to the next one. We put gas in it and it still didn't start, then I listened under the tank and didn't hear the pump run. It started on starting fluid. It might have lasted a lot longer as long as I would have kept the fuel level up to keep the pump cool.
 

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Oh yeah, go around to the back and see if it smells real rich. If it does, your fuel pressure regulator could be leaking. That will cause the mileage to be real low and cause various bad running issues. The regulator is inside the intake manifold, so it leaks into the engine. When the regulator was leaking on the '98, I had hesitation, stumbling, coughing on acceleration and under load like going up a big hill. At that point it had so many miles I was afraid to do any hard accelerating.
 

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The 2x4 trick in the form of a 12" rubber mallet saved my bacon once when I was 10 miles from home with a failing pump.
 

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Fuel pressure is definitely the next thing to check, the cheapest too. If that checks ok, I would move to ignition module next. It's easy for a tired module to get hot and start to act up. It is located inside the distributor. I bought a cheap complete distributor to use as a test unit since the module and cam sensor are inside it. It only takes about 15 minutes to drop it in and test the vehicle. If it works, I leave the cheap unit in it until a nice AC Delco arrives in the mail. If your problem is in anything electronic, don't go cheap! Delco or Delphi products only, you'll thank yourself when you aren't changing the same part every year.
 
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Discussion Starter #14
Will be checking fuel pressure Friday morning. Out of town working. Jack if I’m not mistaken I believe the ignition control module is on the passenger side of the motor on above the valve cover in that area. I know the cam shaft sensor is under dis cap and rotor button. I changed that not to long ago. And if fuel pressure checks out okay. My next step was to order the whole distributor kit. Only thing I know to do on top of everything ive already changed. Gas mileage sucks but shop prices are outrageous just to look at it. Thanks for the help much appreciated
 

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I have a 1999 Chevy Tahoe that hesitates bad sometimes when you accelerate hard as soon as it gets warm. When it’s cold it runs great. I have changed plugs, wires, distributor cap and rotor button about 6 months ago as well as the fuel filter this week. But it is now getting terrible gas mileage. Maybe 200 miles to a tank which is a 21 gallon tank. The truck is regularly maintenance and i run 93 non ethanol gas. Fuel pump was change about year and a half ago that was my next step. The truck is straight piped and tuned with the cats tuned out. The only engine codes it gave me were all 3 oxygen sensors and eventually it went away and engine light hasn’t came back on. Any input would be great. I was leaning towards ordering an msd distributor Kit or another fuel pump.
Replace the 2 upstream oxygen sensors. They determine the fuel mixture. The downstream sensors don’t need to be replaced unless you’re getting a catalytic converter code. Even then it would probably be the cat not the sensor. Buy a $20 OBD2 reader that connects to your smart or tablet via WiFi or Bluetooth. I prefer the Wifi OBD2 readers. I like this one for the money and functionality. KOBRA OBD2 Scanner & WiFi Car Code Reader – Clears Check Engine Lights Instantly – Diagnose 3000 Car Codes - Wireless Car Diagnostic Scanner – Auto Scanner for 1996+ Vehicles (iOS & Android Devices Only) https://www.amazon.com/dp/B01C3HAHCS/ref=cm_sw_r_cp_tai_--KgFbM20EACG
 

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I have a 1999 Chevy Tahoe that hesitates bad sometimes when you accelerate hard as soon as it gets warm. When it’s cold it runs great. I have changed plugs, wires, distributor cap and rotor button about 6 months ago as well as the fuel filter this week. But it is now getting terrible gas mileage. Maybe 200 miles to a tank which is a 21 gallon tank. The truck is regularly maintenance and i run 93 non ethanol gas. Fuel pump was change about year and a half ago that was my next step. The truck is straight piped and tuned with the cats tuned out. The only engine codes it gave me were all 3 oxygen sensors and eventually it went away and engine light hasn’t came back on. Any input would be great. I was leaning towards ordering an msd distributor Kit or another fuel pump.
Just out of curiosity, what type of plugs did you use?
 
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