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what causes engine rpms to lope and engine to die

Customer Question
1990 chevy lumina euro 3.1ltr...
1990 chevy lumina euro 3.1ltr tbi auto trans
engine dies when approaching stop at stop signs and lights at both intown and highway speeds. went to local gm dealer for computer diagagnostic only thing found was low fuelpump pressure replaced fuelpump still same problem please list all possible causes and fixes thanks
Submitted: 10 years ago.Category: Chevy
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Answered in 16 hours by:
11/25/2006
Chevy Mechanic: RIP, Lead Technician replied 10 years ago
RIP
RIP, Lead Technician
Category: Chevy
Satisfied Customers: 5,591
Experience: A.S.E Certified Master Technician, AC Delco service training, Ca State Smog License, L1 level
Verified

Possible causes of erratic idle and stalling when comming to a stop include:\

  • Engine vacuum leak, causing excessive air to enter the engine's intake. Check for any obvious whistling sounds, and using propane to issolate any leaks around the TBI and intake manifold.
  • Possible failing engine sensor such as MAP (manifold absolute pressure), ECT (engine coolant tempresture). These sensors should be tested via a scan tool, and lab scope. More commonly they will cause the computer to overrichen the fuel mixture.
  • Possible excessive fuel entering the TBI, this can be the result of a failing fuel injector, or fuel pressure regulator which are installed inside the TBI unit. Commonly you can check for this condition by looking down the TBI, and visually checking for liquid fuel around the interior, or for fuel 'dripping' from the injector(s). Whenever there is excessive fuel entering the engine, it will throw off the computers mixture calculations and during the compensation it will cause a surging idle and stall.
  • The idle air control valve (if equipped on your model) may be faulty.
  • The engine computer (ECU) may be faulty, and not controling the mixture or idle correctly, this needs to be verified by first testing and elliminating all sensors and engine performance. Then the power and ground supply of the ECU needs to be tested, before it can be finnally condemned.

-Rip

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Customer reply replied 10 years ago
Relist: I still need help.
all answers given were checked by my local dealer.
was told that all sensers were working properly.
also i mistakenly id'd tbi its mpfi with 194000mi
i dont know if this makes any differance or not
would air in the fuel system cause this to happenbecause iv'e changed the fuelpump,filter,and the o ring around where the distributer should be having to remove the tb and fuellines
Chevy Mechanic: RIP, Lead Technician replied 10 years ago
Customer
  • Did the idling concern begin after the o-ring was replaced? Or after the fuel filter?
  • What was the fuel pressure the shop tested, which prompted the replacement of the pump?

-Rip

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Customer reply replied 10 years ago
#1 no it has ben a problem since i purchased the car in june
#2 yes local dealer tested fuel pump and all other sensors you mentioned
engine is mpfi not tbi with 194000 mi on it does that make a differance
Chevy Mechanic: RIP, Lead Technician replied 10 years ago

Generally, the solutions are the same wether it's TBI or MPFI. The only difference is that you have 6 injectors verses the two, and you cannot visually tell if they are leaking fuel. I'm not to fond of the dealer diagnosis or any other repair facility, unless you received written technical data following the diagnosis. To have all the above sensors correctly tested, view scan data, fuel injection testing, and computer control is anywhere from 5-7 hours of diagnostics and if done at the dealer, it can be roughly $450-650 - just for the diagnosis. So if you had the vehicle in there and paid for a diagnosis, they most likely came to an assumption the fuel pump was faulty and stopped there, telling you everything else checked out fine. Of course, they'll continue to advise you of more failures for the additional time. Therefore, we can go around in circles trying to find the cause of the surge in the idle, if your told one component tests good, when it may be faulty. The reason the engine is surging and stalling, is that the computer cannot maintain the idle per specification, most likely due to a fuel mixture imbalance. First the fuel mixture imbalance can easily be identified by the Fuel Intergrator or Fuel Trim values in the computer. Does your repair order from the dealer state what the Fuel Trim values during the diagnosis? They entail what the ECU is doing to compensate for a mixture concern. If the engine has a vacuum leak, is not getting enough fuel, getting too much fuel, etc ... the fuel trim will change to correct the problem, than thru a dedicated scan tool we can view this data. If all the sensors do infact show up good on an oscilliscope and multimeter tests, than the ECU can be deemed faulty. Problems like this are what keep good vehicle's off the road, most any good shop with quality mechanics will want a good hourly rate at an unbearable amount of time to diagnose the concern, with the $$$ of expensive equipment and techs they have. If you don't agree to the time, well they guess on the cause, and move on. Most lower quality shops, will simply guess at the cause from the begining, at your expense. If your going to attempt the repairs, I have no problem in instructing you with testing given that you have access to some equipment, but otherwise it's best to ask the dealer for what they're test measurments were, so that we can make educated guess on what might be the solution. First, can you find how they determined the failure of the fuel pump? I'm curious as too the pressure readings. Too high of readings could be the result of a plugged fuel retun system, which will richen the mixture. Or too low of pressure reading could entail a restriction in the fuel system ahead of the fuel pump and filter. Also, ask how much they measured the fuel flow at. What was the exhaust backpressure? Ask if they took engine compression readings and what were they?? Ask if the IAC (Idle Air Control Valve) passed the scan test, and to what degree. Find how they verified there are no engine vacumm leaks, and what the actual engine vacuum stays at on the guage??? Also, what was the MAP voltage at idle?Let me know the outcome of these questions and I can let you know if you should be looking into a new ECU (computer), or possible failure in the fuel system, or computer control. Thanks,

-Rip

RIP
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Category: Chevy
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Experience: A.S.E Certified Master Technician, AC Delco service training, Ca State Smog License, L1 level
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