Question about 1991 Chevrolet Caprice

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I have a well kept 91' Chevy Caprice V8 5.0L TBI. I replaced the sparkplug wires, distributer cap and rotor because it was missing when accelerating at driving speed, but there still an issue left, that is when I come to a complete stop the car stalls, after that the car starts right up again, another example of the same is when the car is in idle and shift it into gear it cuts off again. so to compensate that I have to drive with my to feet, one on the brake pedal and the other on the gas pedal to keep it running when I have to come to a complete stop.. I checked the EGR valve, I checked for vacuums leak, check the M.A.P sensor, what else can it be? maybe the "idle air control valve" or the " throttle position sensor".. I my opinion would be the TPS (Throttle position sensor) because as long I keep my foot on the gas it stays running fine. If the IAC (idle air control valve) is malfunctioning the rpm will be surging ... I want to know what are your thoughts on this, I will appreciate any help ... thanks.

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  • Mvp4eva Jun 21, 2010

    Its an automatic,..kick down cable? if you mean throttle cable its fine and no slack to it, checked that also.

    its when the car is under load at a complete stop or shifting into drive after being in idle. in some cases when I rev up the engine up too 3.5k high enough while in idle and let go of the throttle the rpm drops suddenly and cuts off.

    I think thats when the TPS should take control and compensate and not let the rpm go under 1.200 rpm.

    Another example for the TPS to compensate the idle speed is, when you steer, the engine gets under load rpm drop and the TPS keeps the rpm at a level.

    Any second opinion would be a plus.

  • Mvp4eva Jun 22, 2010

    This is my latest update: I checked and cleaned the EGR,PCV,IAC valve, M.A.P sensor and Throttle body(bore/butterfly). with carb cleaner.

    Replaced: plugwires, cap/rotor and put in a new TPS.

    checked sparkplugs, they are in good cond. I clean them and adjusted gaps to specs.

    checked vacuum lines for leaks: no leaks.

    After doing all that the car feels smooth and quiet when in idle. but there is a mild miss fire that lingers and still makes my car stall out at low RPMs.

    When the car is in park and in idle the rpm reads between at steady 1200 or 1300 rpm, the miss fire appears but it doesn't stall the car at does rpm.

    Now when I step on the brakes and put in gear the rpm drop at steady 1000 rpm.. when the miss fire appears it stall my car because it makes the rpm drop below 1000 to suddenly.

    Same while driving, when slowing down or coming to a stop rpm are dropping and the miss fire makes it go lower too suddenly and then stalls.

    I haven't checked for a dirty or clog gas filter, I haven't replaced it in years or may there's some impurities in the gas tank.

    I always use 94 octane gas.

    whats your thoughts. thanks

  • Mvp4eva Jun 22, 2010

    I haven't sprayed the carb cleaner at the base of the TBI yet, but I will.........

    I had to change the TPS because it was sticking and shooting up the rpm.....

    After cleaning and replacing most parts the car feels smooth and quiet when in idle......but there is a mild missfire thats lingering... while in idle the missfire appears but it doesnt stall the car at idle......but when I press on the brakes than shift into gear and hold it there the rpm hovers steady around 800/950 now when the missfire appears it makes the rpm drop too sudden and the car stalls.

    To describe the missfiring, it sounds like it sputter rapidly and goes away and like a hiccup.

    I cleaned the IAC thoroughly and inside its housing with carb cleaner......... the rpm moves when I steer and also with the A/C and I can see it compensates so its working properly.

    So while tasking the IAC by streering or shifting into gear, A/C cant compensate because while doing so, theres that mild missfiring that drops the rpm too sudden before the IAC can level the rpm..

    I have two task to do......I'll check for leaks from under the -TBI and forgot to tell you, I think it could be the fuel filter, I have to see if its dirty or clogged since I havent changed that filter for over 8 years..also maybe there is some impurities inside the gas tank?.......

    thanks for hanging there with me you have giving me lots of good tips...I'll update as soon as i change the filter and check under the TBI for any leaks.

  • Mvp4eva Jun 23, 2010

    Latest update: I sprayed carb cleaner at the base of the TBI and everything seems fine, no leaks..

    I sprayed at it while the car was running and didnt see anything.. I was looking for any signs of a leak, I put soap and water on there to check for foams and nothing.

    I just got the new gas filter, I will do that next..

  • Mvp4eva Jun 24, 2010

    . My latest update: I went through it again and checked for any vacuum hose leak, everything seems fine, I did check under the TBI gasket by spraying carb cleaner and didnt hear any diff. if there was any leak.......Replaced the TPS (throttle position senor) with a new one..... I cleaned and checked the PVC,EGR, M.A.P sensor..... I cleaned the IAC (idle air control valve) and its housing ... I cleaned the inside of TBI with carb cleaner has new cap/rotor n plugwires.. and a new fuel filter.. But it still missfiring a little and at low rpms it stalls for that reason..... I'm running out of ideas.. I think it could be the IAC (idle air control valve) as I never had replaced it yet. ...... A question how can you tell when the IAC is bad or worn out by looking at it?.............

    it stalls even when you turn or move around the steering wheel at Idle speed.. is like the IAC is not compensating for that because the missfiring don't let him do it ........ I want to know what is causing the missfiring like a hiccup is not a constant missfiring, it comes and go .....its also like a cough that makes the rpm drop suddenly and only at idle speed or at low rpm when driving it makes it stall ......I'm trying my best to describe it, but its hard to tell......So, should I replace the IAC also?

  • Mvp4eva Jun 25, 2010

    I cleaned the M.A.P sensor with a dry tooth brush just to take the dust off of it.. sorry for not being specific......thats exactly the one in the picture......

    This is and hopefully my last update: I finally fixed it, I decided today to go through everything again to see if I missed anything, 1) checked vacuum hoses, they are fine.....2) srayed crab cleaner at the base of the TBI, no leaks........ 3)took out again the IAC cleaned it and tried to adjust the valve by pushing it in....4) I didn't no there was a hole inside the IAC housing, I sprayed carb cleaner in that hole and gunk started pouring out .....5) I sprayed carb cleaner into all the vacuum inlets of the TBI .........6) I sprayed thoroughly again carb cleaner inside the TBI (butterfly/bore).........7) I tighten the bolts that secure the TBI to the intake manifold.......8) I took apart and dry cleaned the ignition system (plugwires, cap, rotor, coil and Control Module).........9) I put everything back together but left the distributer shaft loose and started the car, I adjusted the timing what I thought was best.......Now the car idles as smooth as you don't even know or tell is .......You've been a good sport, I thank you so much for your help.... I do mechanics as a hobby, but never had a troubleshoot like this that made me have sleepless nights.....I mean its hard to do a job that requires a scan tool when you don't have none........
    Thanks again you don't know how happy I am .....My name is Felix and hope your there again when I need your help and opinions..



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It's very common for these engines to develop a vacuum leak around the base of the throttle body. You can check for this with a stethoscope or a can of aerosol carb cleaner. Also, IAC valves are a common problem, but easy to clean before you replace it. Remove it and use the carb cleaner and a brush to remove the build up, also the crud inside the IAC orifice. Next clean the throttle butterfly valve and throttle bore thoroughly with carb cleaner. If this doesn't help, remove the egr valve and try and poor a little paint thinner through it when you're holding it upside down, it may weep a little through the pintle but it should not obviously allow the paint thinner to flow through. If all of this fails to help, you'll have to get your hands on a scan tool and read IAC counts at idle to see what the computer is trying to do. If the counts are high...above 50, the computer is trying harder to open the IAC to keep the engine running because of some kind of problem, usually mechanical. If the counts are very low...below 5, the the computer may be trying to counter a vacuum leak that is providing most of the idle air, which is unstable and can cause the stall.

Hope this helps.

Posted on Jun 21, 2010

Testimonial: "I added more comments for the first poster but maybe you can put your second though also. I checked the EGR and took it off for a closer inspection, also took out the IAC cleaned it with rag with some solution and brushed it. My question is, how far out should the needle be or how can I tell when the IAC is warn out. thanks ."

  • 1 more comment 
  • Dave  C
    Dave C Jun 22, 2010

    I think you may have confused the TPS with the IAC. The IAC (idle air control valve) compensates for added loads, like steering or A/C kicking in, on the engine by varying the amount of idle air that is allowed to bypass the throttle plate.
    The TPS really has very little to do with the idle quality and idle speed. It is really just to tell the computer when enrichment is required during rapid acceleration or when a downshift is required.
    Without some special tools, it is difficult to test either of these. The simplest method is to use a scan tool, so if you don't have one that reads live data, it may be worth it to have a professional check your car. I can tell you that your car should never idle at 1200 rpm when it is warmed up. If it does, this really suggests a vacuum leak at the base of the throttle body. Have you tried spraying some of the carb cleaner only at the base gasket area with the straw that came with the carb cleaner?

  • Dave  C
    Dave C Jun 24, 2010

    When you spray the carb cleaner, you need to LISTEN for a change in rpm, as the cleaner is drawn into the intake the engine will speed up. There will be no visible bubbles or anything like that. The fuel filter is certainly good maintenance but it won't be causing this problem.

  • Dave  C
    Dave C Jun 24, 2010

    When you say you "cleaned the map sensor", I'm wondering what you mean? The map sensor has vacuum port on it but you should never ever clean it. If carb cleaner was sprayed into it, it may have been ruined. I'll try and attach a picture of the map sensor, if this what you cleaned, you may want to consider replacing this. It is possible that you have a bad IAC, but they are not cheap, and niether is a MAP sensor. You may want to try picking up these items at a scrap yard and trying both. Without a scan tool, you are shooting in the dark.


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Like you said it could be the TPS, but is it an auto or manual,, If its a manual I would be suggesting the TPS,, If its an auto I would look at the kick down cable

Posted on Jun 21, 2010


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