1999 Chevy Suburban Drop in RPMs while driving

1999 CHEVROLET SUBURBAN
11,000 MILES • V8 • 2WD • AUTOMATIC
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M1ARON
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I changed the fuel pump about 2K miles ago. Was driving fine but then then when going about 40-45 MPH the RPM's would drop to 1500. It will climb back up again after I press the gas pedal. This was not a problem until the car stopped after sitting at a busy red light. Would not restart and finally drained the battery from trying to hard. After finally getting someone to help me started right up with a jump. Took battery to auto parts store, battery was fine. The car keeps on starting and going with no problem except for the drop in RPMs. Thought that the Fuel Pump Relay might be bad so I changed that. But the RPMs keep on dropping at that same 40-45 speed.

Do not want to change fuel pump again, any other ideas?
Jul 8, 2009 at 9:47 PM
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RACEFAN966
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Ok so does the rpm drop when you are at a constant speed? Has is died anymore? You said you replaced the fuel pump but how old is the fuel filter? What side of the tank do you normaly run on, do you run on the lower end or upper end of the tank? See if you like to run empty it is hard on the fuel pump as the liquid in the tank 1/4 tank and better keeps the pump in the fuel which helps it stay cooler and helps it last longer. Also with gm the tight media of there fuel filters and with the ethenol in today's fuel (which absorbs moisture from the air) they should be changed once a year. Is the engine light on? Do me one test ok, drive at a steady 45 let the rpm drop then hold it steady and light step on the brake with the other foot just enough to get the brake lights to come on but not enough to cause any brake drag did the rpm come back up? Get back to me with what you can and we will take it from there.
Jul 9, 2009 at 9:42 AM
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M1ARON
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Thanks for your tips!! I had a quarter tank when the car stalled, but since has not stalled. I try to refill at a quarter tank. Its starting really good since then. The check engine light was on before stalling, but since I jumped started the light went away.

Yeah the RPMS stay at 1500 when going between 45-50..and of course the more I press the pedal it starts going back to 2000 rpm. Never changed the fuel filter...forgot all about that!! But I did the test you asked me to do. Going 45 mph i applied a light brake while stilling pressing the gas and it went right back up to 2000 RPMS!! Blew my mind....but as soon as i let go of the brake pedal, while still pressing the gas, it went back down to 1500 RPMs....

Thank you so much for your help!!!
Jul 9, 2009 at 10:16 PM
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RACEFAN966
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OK good the rpm change in that manor tells us the computer see's the brake lights and thus locks the convertor and to give the engine more power. This means the computer is working and the transmission is seeing the signal as it should. What you did by pressing the brake pedal was make the truck think you had a load thus taking it out of overdrive. Now I would change that fuel filter they arn't very expensive and it is more then needed and then track how it runs. I would also like you to check you local parts store to see if they do a free scan if so then scan you truck and write down what codes are there and then have them clear them. Get back to me with what codes are there, if there are any and I will look them up and see if any of the codes will tell us what happened. Thanks.
Jul 10, 2009 at 9:12 AM
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M1ARON
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Will change the fuel filter this weekend....is it something a novice mechanic be able to do??

And right now no check engine light is on, will they still be able to scan?

Thanks...!
Jul 10, 2009 at 10:33 AM
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RACEFAN966
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Yes you can change your fuel filter it isn't that hard to do. Once you have it in hand just look on the inside of the frame under truck about there the drivers seat is. As for the codes if the truck picked up any codes from the engine light being on before and it has never been cleared then they should still be there. Let me know how it goes. Here is a pic of the fuel filter location.


https://www.2carpros.com/forum/automotive_pictures/249564_Graphic_102.jpg

Jul 11, 2009 at 8:55 AM
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M1ARON
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Hello changed the fuel filter...it was really dirty, with no problem. Took it to get scanned but nothing came up.....the truck is acting the same... :-(

any other suggestions would be great....
Jul 13, 2009 at 8:54 AM
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RACEFAN966
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OK let go back a step and clean the Mass Air Flow Meter ok. I will need you to get some mass air flow cleaner (nothing else) and then remove the mass air flow sensor and spray the inside of it and then put it back in and see what it does. If it still acts up then we will check the TPS and the MAP sensors ok. let me know how it goes.
Jul 14, 2009 at 9:43 AM
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FLETCH900
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This is an old post, but just in case anyone else sees this while trying to correct this problem...
If the truck serges while driving a steady speed, it may be transmission related. I have a 99 suburban. Had transmission rebuilt, now at 43 - 48mph the rpms serge about 200rpms every second or so. I'm thinking it could be the torque converter or a vehicle speed sensor.
I have a brand new motor and the truck ran 100%. Transmission came out and went right back in. Nothing under the hood was touched, and now the serging problem.
It's definitely not fuel related. It runs like a scalded dog everywhere else, except in overdrive around 45mph.
If anyone knows anything, I'd love feedback.
Dec 18, 2015 at 2:58 PM