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  • Stumble stall upon accelerator depress after warm

    Ok Im no noob & have been chasing this for a bit, but am now running out of patience.
    Backstory: picked up a 93 shortly ago for a couple hundred bucks. good but not great shape.
    Brought home performed resuscitation including tune up and oil change and knowing it had sat for some time, before starting drained tank, cleaned Pump PU sock & replaced filter. Started right up aside from a sticky lifter that eventually quieted down ran great shifted well & with only 80K on the OD made good power!
    Took it for it maiden voyage a 15 min drive to my dads place 20 miles away got almost there when it started missing or stumbling on 1 & 2nd gear small input throttle.
    Eventually so bad unable to drive.
    Idled fine but gag'ed at any throttle tip in, but never died always returned to perfect idle.
    if I could keep it up there running & in 3rd or higher it would drive but as soon as you would slow & down shift ..no go.
    Got it back. No chk. eng, light. Pulled the pump & cleaned the sock again which showed a lot of dirt/rust. Pulled the AFM & intake hoses & cleaned it and the TB.
    Put it all together again and it ran great for a few days then it all started over again.
    When cold it does not display symptoms only after driving normally for a 10 or more mins does it begin to display behavior.
    At this point Im guessing this is probably the reason the car was parked.
    With no codes Im guessing that the pump or regulator are the issue?
    Ive search for topics that would help but typically they all tend to be scattershot suggestions & rarely report a resolution.
    Anyone have a BTDT?
    Last edited by needragr; 03-17-2020, 06:42 PM.

  • #2
    "Pulled the pump & cleaned the sock again which showed a lot of dirt/rust." - dirt in tank?
    Original owner of silver grey carburetted 1989 Festiva. 105k km as of June 2006.

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    • #3
      Probably not. Scale / rust. When I pulled the cover to look inside it showed some rust but not all encompassing. Ive now been through a couple tanks of gas + some line dryer & an application of MMO

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      • #4
        Anyone?

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        • #5
          May not be your problem but it appears that this might be engine temperature related. Check the connection and wiring to your engine temperature sensor that screws into the back intake manifold. If wiring looks OK, pull of the connector and clean the contacts. You might want to remove the sensor and do a thorough cleaning of it. If the ECU does not detect that the engine is hot when it is hot, the efficiency of the engine goes down and pollution goes up. The sensor could be bad but in my experience, failure of the sensor itself is rare.
          You check engine bulb could be burned out thus it would not be indicating codes are present.

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          • #6
            thanks for the reply. no chk eng light is working. Id expect if that sensor had failed It would trip. Im continuing along the the pump / regulator may be the problem ?

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            • #7
              Yes, the engine coolant sensor illuminates the check engine light. It can be intermittent. The light goes out again when the coollant sensor is withing range but the code is stored in the computer. I had to replace the sensor on my car last year.
              Original owner of silver grey carburetted 1989 Festiva. 105k km as of June 2006.

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              • #8
                Originally posted by WmWatt View Post
                I had to replace the sensor on my car last year.
                & were the symptoms that led to that the same/similar to mine?

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                • #9
                  Here is a little article that explains a lot. It talks more about engines with carburetors but is also true for efi engines when when some of the things that control the amount of fuel getting to the engine are out of whack. https://www.badasscars.com/index.cfm...rod/prd289.htm

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                  • #10
                    "were the symptoms that led to that the same/similar to mine?"

                    no actual symptoms. all it does is reduce fuel efficiency (on my carburetted model). I drove with the light comming on and off intermittently all winter (low milage) until it was wam enough in spring to get under the car and replace the sensor.
                    Last edited by WmWatt; 03-20-2020, 08:45 AM.
                    Original owner of silver grey carburetted 1989 Festiva. 105k km as of June 2006.

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                    • #11
                      On EFI engines there is also a Vane Air Temperature Sensor inside the VAF which sends signals to the ECU in coordination with the Engine coolant temperature sensor.
                      Under normal conditions the added volume of fuel injected to enable easier starting and smoother warm up should not be enough to cause the engine to bog down on acceleration once the engine warms up...usually it's just a matter of the efficiency of the running of the engine so it saves fuel and reduces emissions as WmWatt says.
                      I'm just trying to think of anything that could be involved that relates to the engine running good when cold vs bad when warm.

                      Other thing to check are the Idle air control and the Idle air adjust screw.

                      There is also a thing called FMEM (Failure Mode Effects Management) FMEM allows the car to operate safely in case of a failure in the electronic engine control system.
                      The ECA will initiate FMEC if a failure is detected in one of the following circuits: Vane air flow (VAF) meter, Vane air temperature (VAT) sensor, Engine coolant temperature (ECT) sensor
                      Barometric pressure (BP) sensor, Exhaust gas oxygen (EGO) sensor. There is a chart in the manual which shows driveability conditions based on the value of the specific sensor.
                      That chart indicates Poor driveability after warm-up for a failed ECT sensor. A failed VAT or EGO indicates driveability Not Noticeably Affected, Failed VAF indicates Poor driveability and a failed BP indicates Poor driveablility at high altitudes.

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                      • #12
                        I suspect the FMEM devices 1990new wrote about would trun on the check engine light if they failed so not likely to be the issue here?
                        Many people who have EFI vehicles have reported problems with the air flow regulator (MAF?) and the little door which lets air in getting dirty and sticking.
                        Another issue not so common is the EGR valve. You'd have to look that one up. Try the search feature.
                        Original owner of silver grey carburetted 1989 Festiva. 105k km as of June 2006.

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                        • #13
                          Im aware of the issue with the AFM that is why I cleaned it (mentioned prior), If the EGR had failed Im pretty sure that would trip a code. what are failure symptoms of a bad regulator ? The Pump is putting out good pressure

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                          • #14
                            Quoted from the manual:
                            The fuel pressure regulator is mounted to the return line end of the fuel rail. It is controlled by manifold vacuum and will always maintain fuel pressure at constant 250 kPa (36.3 psi) above intake manifold pressure. When intake manifold vacuum is low A(fuel demand high) spring pressure inside the regulator causes the valve to partially close, which increase fuel pressure in the fuel rail. When intake manifold vacuum is high (fuel demand low), vacuum acting on the diaphragm compresses the spring, opening the valve further. Return fuel flow increases, resulting in lower fuel pressure in the fuel rail.
                            The regulator has no sensors attached. Its actions are purely mechanical and completely dependent on the intake manifold vacuum pressure.
                            From your description of the problem which says the engine accelerates normally when the engine is cold, the only thing I can think of relating to the fuel pressure regulator would be to check the vacuum hose going to it and make sure it is not collapsing when it gets hot... this would cause the fuel pressure to remain too high when the manifold vacuum is high.


                            BTW... unplug you oxygen sensor and see if your check engine light comes on.
                            Last edited by 1990new; 03-23-2020, 02:33 PM.

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                            • #15
                              So update. I have a 91 that is a running driving car with just a tired engine. So I started by pulling the afm cleaning it thoroughly & swapping to the 93. On the possibility that it was ignition (even though it feels like fueling to me) I swapped the ICM..no go. Back to my seat of pant fueling theory I pulled & swapped the pump. took it for a drive & It seemed to have solved the problem (based on past time diving before it starts acting up). So I decide to go all in on a freeway loop...big mistake! As soon as I launch down the on ramp ...stumble I get it back to the house & by that time its wont launch from 1st but while its lurching to get under way if I quickly row up the gears it will get under way & not stumble.
                              Never stalls always returns to perfect Idle.
                              Sitting in the running car in the driveway Im listening to it & I can hear something, an ...interruption.
                              I climb into the back seat listening intently & I hear a slight rumble then a pause then a resumption of the rumble then intermittent pauses...
                              Im listening to the fuel return pumping gas back into a 1/4 full tank and it should be consistent but its not!!
                              The plot thickens ~
                              Suggestions?

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