Help Engine Stumble

Raptor660r · July 26, 2008

I have a 99 GMC Suburban 5.7L with about 125K on the clock

Problems

During acceleration if I push the gas more than 1/4 of the way down the engine starts to stumble and jerk around. It does not clear its self by stepping on the gas, it just gets worse. Sometimes it sounds like a small backfire from under the hood. The problem has come and gone for a few months now, now it happens all the time. It almost feels like the exhaust system is shaking real bad.

Background

Yearler this year (Jan), the fuel pump went out, I replaced it, and there were no side effects at that time. I checked and replaced the exhaust hanger rubbers. I recently did a tune up with new plugs, wires rotor cap, and button. I replace the fuel filter. Changed the transmission fluid and filter.

Today I check the TPS, and MAP sensors, and they checked good.

I see other people have had similar problems, none of the posts seem to have a resolution.

Raptor660r · July 26, 2008

I also just check the routing of the plug wires, and checked the distributor cap for tightness. I moved one spark plug wire, it was touching the valve cover. I drove it down the street, and back. Its not as bad, but the stumble is still there. Also while driving I hooked my code reader up and pulled 1 code.

P0122 TPS / Pedal position sensor A circuit low input

I tested the TPS again, and all is well.

Do you think I should just replace the TPS?

L29er · July 27, 2008

does your truck have the electronic throttle plate? i have heard probs with wires in harness over engine rubbing through causing lotsa wierd things have you tried cleaning your MAF sensor?

Raptor660r · July 27, 2008

I am not sure what you mean by electronic throttle plate. I have a 99 GMC Suburban. I have not cleaned my MAF. I don't know how. I will search out how to do it.

pennzoil29 · July 27, 2008

You might want to clean the throttle body also. It will get loaded up with carbon, and the throttle plate won't function properly. As for cleaning the maf, get some maf cleaner from the parts store,"CRC" makes the best, and follow the directions on the can.

Raptor660r · July 30, 2008

Now I am getting a code of P0133 O2 sensor circuit, slow response pre-converter, left bank

Can an O2 sensor cause this problem? What side is left bank?

minibike132 · August 1, 2008

The P0133 is most likely a result of dumping too much fuel into the exhaust from the original problem. Not saying the HO2S isn't bad but it probably isn't the root issue. The P0122 indicates an electrical fault in the Throttle Position Sensor circuit. You can clean all of that stuff, it won't hurt anything it will probably help run better, once the TPS is fixed.

Raptor660r · August 4, 2008

I have replaced the TPS and the Bank 1 Sensor 1 (driver side pre-cat) O2 sensor. The problem is still there. I tested the fuel pressure at 58psi which is 2psi low for what the book calls for. I wonder if the new pump I put on is bad. My wife took it on a small road trip and had all kinds of problems with it. It drives normal for a little while, once its warmed up thats when the problems begin. You can not accelerate up a hill, you can only push the gas pedal down 1/4 of the way. Anymore that that, the engine will not rev. I am at a loss, and do not want to throw anymore parts at it.

Rightnow I am thinking one of 4 things below (in no order)

2nd cat Clogged

Pressure Reg bad

Fuel Pump Bad

Bank 2 O2 sensor bad

staged455 · August 15, 2008

Having the same issues with a 97 3500. The 02 sensors are actually going to 0 voltage and showing the engine going lean when it starts stumbling. Fuel pressure drops as low as 53 pounds or so when it's stumbling and leaning out. Seems to start around 3000 rpm if you stand on it or if you are towing a trailer. No codes are being set. Not sure if it could be a spider issue or maybe an injector issue. If you ease down on the throttle while setting still you can take it up to 4000 easy but if you hit the throttle hard it starts stumbling and going lean. Being under load with a trailer has the same affect.

minibike132 · August 16, 2008

Ok Joe and Mike, let's look at fuel for a minuite since you both have pressure gauges. Install gauge and run the trucks for a minuite or so, shut the key off and let sit for about 10 seconds. Then key on engine off and record max pressure. Then turn the key off and observe pressure drop after 1 minute and record. Then watch for another 5 minutes and record the drop. Next turn off the ignition for 20 seconds then turn it back on and note key on pressure then start and record the amount of drop between key up and started pressure. Let me know what all those are.

Please post those numbers. Joe also check your throttle position sensor wire at the connector, and you didn't say if you had new codes after replacing the things that you did.

Mike What engine do you have?

Raptor660r · October 7, 2008

Alright, I checked the fuel pressure. It only reads 58 psi after the the 2 seconds the pump runs. Then as it sits, it slowly looses pressure. The haynes says its supposed to be 60-66 psi. I took it into a repair shop. They hooked it up to a diagnositc machine, and said the fuel injectors/meter was bad. I told them I could replace it myself. I found a fuel spider on Rockauto.com for 300. Now my question is, do I have 5mm or 6.5mm injector tubes or does it even matter?

Raptor660r · October 10, 2008

anyone?

Brucer1 · October 12, 2008

anyone?

Hi Raptor660R,

I have a 2001 GMC Sierra that is doing the same thing....under light acceleration when the weather warms up especially I get a stumble. I have been trying to solve this for 2-1/2 years now. TPS, EGR, sparkplugs and wires. If you have any insight let us know...Thanks!

Raptor660r · October 18, 2008

Well

I think I have solved the problem with a little help from my local repair guy. He told me 4 out of 8 of my fuel injectors were not working to the full potential. So I ordered the complete fuel spider from Rockauto.com. I think I paid 315 with the 85 dollar core. They shipped it the same day I placed the order. I will install it tomorrow and let you know how it works. By the way, I ordered the spider with the black tubes. They are 5mm. I have a 99 GMC suburban with the 5.7.

Raptor660r · October 19, 2008

Well I think I have fixed it. It took about 4 hours to replace the spider. Not an easy job. I give it a 5 of 10 on the difficulty scale. I did it in my yard with basic tools.