Erratic idle after coming off gas

Discussion in 'Workshop Forum' started by Regan Burba, Aug 18, 2019.

  1. Regan Burba

    Regan Burba Daily Driver

    Hi, I am having an idling issue with my 87, with FI of course. When I let off the gas when coming to a stop the idle dives way down, almost to the point of stalling, then comes back up. Sometimes but not often it will stall. The engine seems to run and accelerate well otherwise. It is strange but I have noticed that it will idle appropriately and more consistently when it is first started and driven, (even when at 190 temp) but if I make a stop and get back in and restart it the idle issue will tend to crop up. The idle also seems to be worse after a harder deceleration and stop. I have the fuel injection guide, which I have studied and learned about various aspects of the system, but have admittingly not completely immersed myself into testing everything. I have checked several times for vacuum leaks as it sometimes sounds like there is one but when I check for one nothing stands out. I did some work to engine previously (plugs, distributor, set idle) but not a lot, I was fortunate in that it did run when I first got it. I have spent most of my time on this car welding and welding some more, along with a long list of other mechanicals such as suspension,steering, pedals, exhaust etc. My point being that I will eventually get heavy into the FI and motor but would rather not right now if I don’t have to as it is a (rolling restoration) and I have other aspects of the car I’m focusing on. Do any of you have any thoughts on what could be causing this issue?
  2. beezee

    beezee True Classic

    I had a similar problem, the car would sometimes die after I let off the throttle, the idle was erratic , and it would sometimes be hard to start when warm. After replacing the air flow meter the problem was resolved. I think the resistive element eventually wears out, especially in the the area of the element where the car idles.

  3. Mxgrds

    Mxgrds True Classic

    Check the throttle position sensor. You should hear a contact clicking from idle stand to giving driving. In idle the contact tells the ecu to not let the rpm go to low.
    kmead likes this.
  4. Regan Burba

    Regan Burba Daily Driver

    The throttle position sensor is something I did
    Adjust. Per the manual I used an ohm meter and adjusted it I do remember hearing the clicking when turning it.
  5. kmead

    kmead Old enough to know better

    The TPS can move so I would go back and check for the click as it closes and opens just off the throttle stop.

    I would also want to see if you are getting vacuum at the vacuum port under the throttle plate. This often clogs with the blow by schmutz. Cleaning it out to ensure good vacuum at idle may help
    Last edited: Aug 19, 2019
  6. Regan Burba

    Regan Burba Daily Driver

    Beezee mentioned his X had trouble starting when warm before replacing the AFM, mine does this also. I’m also wondering if the air/sucking noise I hear is the vacuum port Kmead mentioned trying to get air, I’ll check that first. Fortunately if it ends up being the AFM I have a few extra to try (hopefully they function) Thanks for the
  7. Dan Sarandrea (Phila)

    Dan Sarandrea (Phila) Waitin' On Parts...

    Philadelphia, PA
    Just another question to gather info: what is the idle speed when at operating temp but not under the conditions of a fast closing of the throttle?

    For example, let's say you were parked, engine at operating temp, engine cover open, and using a rag to keep from getting burned, you opened the throttle manually to say 2500 rpm, held it for say 3-5 seconds, and then gradually closed the throttle over 5-7 seconds.....

    Compared to the same conditions, but instead of gradually closing, you let go and let is snap shut...

    Same almost-die, or different?
  8. Regan Burba

    Regan Burba Daily Driver

    I will try to answer this in 2 ways, I have tried to replicate the idle by hand as you described but it doesn’t do it then. It happens more so when I am driving and decelerating from an rpm of lets say approximately 3500 or higher.
    To keep it from happening as much, I tend to keep the rpms down when decelerating or take it out of gear and role to a stop.
    When it is “acting up” the idle would be about 900, but dip down to almost zero and come back up. If as I described in the first post, start the car and drive without stopping (turning the key off) it will sometimes run very well, without the idle dipping and at a slightly higher rpm at rest.
    It sounds crazy but it was running really bad yesterday, which was a very humid day it CT.
  9. Regan Burba

    Regan Burba Daily Driver

    Hello, I’m revisiting this post as I am back at trying to get my X running correctly again. I’ve had it off the road for awhile working on rear suspension and rebuilding calipers. I had a chance to try a few things but have not had success. It is still stalling after warming up. When it is first started it runs well, if I drive for awhile and stop (at a store) and start driving again it has a tendency to stall when coming off gas, when coming to a stop. When first started and driven it idles a little higher (but very well) but will eventually idle lower (about 900). When it starts to stall the rpms kind of waver and then just drops out, it starts right up after stalling. It occurs generally after it warms up, car is turned off, sits for a little while, and then driven again.

    I tried a different Air Flow Meter, but it did not appear to help with the stalling. What I did start to notice though is that the fuel pump was running non stop when key was on and was very loud. I assumed it may have been clogged so I switched it with an extra from a parts car and added a filter before pump. I then noticed that it was still constantly running as before. I then plugged in the previous AFM and the fuel pump only went on when I opened the AFM, so I reinstalled the other AFM. I then also installed a different coolant sensor. I started it, ran good, drove for awhile, went to store, started fine, (rpms a little lower) but stalled several times on the way home. I can keep it from stalling if I really ease off the gas but that’s difficult to do. The fuel pump is much quieter now though. I have tried several times to locate vacuum leaks but no success.

    Any thoughts? Should I suspect the auxiliary air valve? What would make the fuel pump run constantly with the other AFM that I had installed?
  10. Dan Sarandrea (Phila)

    Dan Sarandrea (Phila) Waitin' On Parts...

    Philadelphia, PA
    There is a set of contact points inside the AFM that provides a circuit breaker for fuel pump power. The idea is that if the engine is not running (that is, there is no air flow thru the air flow meter as determined by the relative position of the flap inside the AFM), then the fuel pump should be shut off as a safety measure. It's likely that the points in the AFM that allowed constant running of the pump needed attention.

    If you think the issue is within the FI system, then I would suggest using the FI troubleshooter in the Wiki section of the forum and methodically working thru it. Of course, step 1 is to confirm that your issue is not actually an ignition issue.

    There is a physical stop for the throttle plate. It is a small headless slotted screw secured by a nut on the pass side of the plenum next to where the short throttle cable connects to the throttle arm. I'm thinking maybe this needs to be turned down a bit more (1/4-1/2 turn) to keep the throttle plate open a tad more. Have you assessed the integrity of your throttle plate bushings and seals? Worn bushings could cause the erratic behavior you're describing, and worn bushings could in turn create problems with the throttle shaft seals which could cause a "false-air" or erratic idle situation.
  11. Jefco

    Jefco Daily Driver

    Portland OR
    Is your idle bypass passage clean? I found mine was clogged with carbon, like the distributor fitting on the bottom can be.
  12. Regan Burba

    Regan Burba Daily Driver

    I had cleaned the idle bypass previously but after cleaning it again this morning I’m thinking it may not have been clean enough.
    I have been trying to follow the FI guide, I have tested components as recommended.
    However, there really isn’t a problem solving situation described in it that is specific to what I’m experiencing.

    The bushings in the throttle seem ok, but I will check them again. I drove 10 miles to work this morning after cleaning the idle bypass, it ran well, idle stayed on the higher side, I wanted to let it sit and then drive it to see if idle went back down and the stalling issue continued but I didn’t have time. I’m wondering if it is feasible to think that maybe it’s a combination of a few things, the Auxillary Air Valve being stubborn to close, and when it does (after it is driven and then sits for a few to warm the engine compartment up) the idle bypass was clogged so it struggled to stay running at idle. Hope to drive it longer after work and see what happens. Driving season almost over in CT, hope to get this straight before the salt flys.
    Thanks for responses, I give an update soon.
  13. Regan Burba

    Regan Burba Daily Driver

    I think I’ve had some success with the stalling.
    I cleaned the idle bypass again and adjusted the stop for the throttle plate just far enough that the idle didn’t get to high. It seems that the rpms want to dive down at times but it is not stalling.
    I drove 10 miles home before making the adjustments, the idle was high. I left it running and clamped the hose from auxiliary air valve and the idle went down. I turned it off, let it sit for awhile and when I started it the idle was lower and the idle didn’t change when I clamped the hose again. So it does appear that the AAV does seem to be operating on the slower side.
    Should it be opening and closing faster than this or is this the norm? I’ve read that some people have been successful in freeing them up which I may eventually try.
  14. Dan Sarandrea (Phila)

    Dan Sarandrea (Phila) Waitin' On Parts...

    Philadelphia, PA
    The AAV opens and closes in response to temperature, of course, opening when cold and closing when hot. Being mounted directly on the cyl head assures that it will stay plenty hot once it gets hot, but just being mounted on the head does not get it hot fast enough, so the Thermo-Time switch's (TTS) job is to send electrical current to the bimetallic strip inside the AAV to get it to close more quickly. If it seems the AAV is not closing quickly enough, follow the checks in the Troubleshooter to make sure that the AAV is getting juice from the TTS for that specific time period after cold startup.

    If the AAV is still allowing air past when it is hot, then it has to be visually inspected for function. Could be it needs adjustment (there is a small adjustment nut on it) or a good cleaning.
  15. myronx19

    myronx19 True Classic

    Toronto, ON Canada
    I also have fuel injection issues (I have no AC on mine). My injectors are the issue.

    If I were you, try cleaning the injectors with this. I used it this past season, and wow.... my car runs fantastic! I used one bottle for a full tank (add it when the tank is near empty, fill it to full - don't refill the tank until you're nearly empty again so that you don't dilute it).

    If this doesn't help, then you can continue digging deeper. This stuff isn't magic, but it does work for dirty injectors! You may have a bad spray pattern.


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