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1999 Pontiac Grand Prix Misfire under load

This is a discussion on 1999 Pontiac Grand Prix Misfire under load within the Domestics and Foreigns forums, part of the Vehicle Specific category; I can't figure out what's causing a random misfire under load in my GP. It's the worst when going up ...

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    LSX whore allbaugh_04's Avatar
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    1999 Pontiac Grand Prix Misfire under load

    I can't figure out what's causing a random misfire under load in my GP.

    It's the worst when going up a hill, I have to make it shift out of OD and hit about 3k rpms so it doesn't misfire. So any time it hits OD and the load is high, it'll miss HARD. Often times it'll miss with the A/C on at a light, or even idling as well.

    Things I've changed/tried:

    -TB cleaned
    -MAF cleaned
    -Air filter changed
    -Fuel filter changed
    -EGR valve cleaned, then replaced with a different used one
    -Plugs/wires changed (off-brand) might try ACDelco
    -all 3 ignition coils, changed, new

    Perhaps a faulty ignition control module?
    Fuel? I know nothing about fuel systems, so i'd need a pretty lengthy explanation on what to check.

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    Senior Member Whamhammer's Avatar
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    Quote Originally Posted by allbaugh_04 View Post
    I can't figure out what's causing a random misfire under load in my GP.

    It's the worst when going up a hill, I have to make it shift out of OD and hit about 3k rpms so it doesn't misfire. So any time it hits OD and the load is high, it'll miss HARD. Often times it'll miss with the A/C on at a light, or even idling as well.

    Things I've changed/tried:

    -TB cleaned
    -MAF cleaned
    -Air filter changed
    -Fuel filter changed
    -EGR valve cleaned, then replaced with a different used one
    -Plugs/wires changed (off-brand) might try ACDelco
    -all 3 ignition coils, changed, new

    Perhaps a faulty ignition control module?
    Fuel? I know nothing about fuel systems, so i'd need a pretty lengthy explanation on what to check.
    I presume this is a 3.8 v6 code K engine? I remember selling alot of intake gaskets for those when I worked for a Pontiac parts dept. I would check for a vacuum leak at the manifold, but I dont think it will fix you problem all together. Have you ruled out the IAC motor or the TPS? If they are faulty, that could throw off the ECM.

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    Mike The Guz's Avatar
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    Common issues with a 97-03 grand prix are the

    Lower Intake Manifold Gaskets
    The Upper Intake if it is not supercharged can warp and leak coolant.
    Front Wheel Bearings
    Ignition Switch

    Do you have a service engine light? Did you clean the IAC valve?

    Check for any vacuum leaks

    Have you ever done a trans filter and fluid change?
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    I would check fuel pressure - not sure what the spec is for your car though. Absent a fueling issue, I have to agree with what has already been posted. Are you simply getting a misfire code through the check engine light, or is something else popping up too?

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    Senior Member kingls1's Avatar
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    I had a 99 that was doing the same thing and I couldn't figure it out after changing the same stuff. So I sent it down the road. Really liked it but it was time to go.
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    LSX whore allbaugh_04's Avatar
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    Quote Originally Posted by Whamhammer View Post
    I presume this is a 3.8 v6 code K engine? I remember selling alot of intake gaskets for those when I worked for a Pontiac parts dept. I would check for a vacuum leak at the manifold, but I dont think it will fix you problem all together. Have you ruled out the IAC motor or the TPS? If they are faulty, that could throw off the ECM.
    Yes K engine. I don't hear a vacuum leak, but I was looking online and u just use carb cleaner and spray around the intake? Is that about the only way you can check? I have no codes for the IAC or TPS, no I haven't ruled them out then.

    Quote Originally Posted by The Guz View Post
    Common issues with a 97-03 grand prix are the

    Lower Intake Manifold Gaskets
    The Upper Intake if it is not supercharged can warp and leak coolant.
    Front Wheel Bearings
    Ignition Switch

    Do you have a service engine light? Did you clean the IAC valve?

    Check for any vacuum leaks

    Have you ever done a trans filter and fluid change?
    I just had a few codes for the map sensor come up, so I replaced that with a used one from the dump. I'll take it for a drive today; been busy.

    I have not cleaned the IAC valve, I will try that if it persists.

    But I did have a P1811 code for Maximum Adaptive & Long Term Shift. I replaced the trans fluid and filter probably 40-45k ago. I would have to check my maintenance log is in the car, but I'm not sure if that needs done again, what you think?

    Quote Originally Posted by pajeff02 View Post
    I would check fuel pressure - not sure what the spec is for your car though. Absent a fueling issue, I have to agree with what has already been posted. Are you simply getting a misfire code through the check engine light, or is something else popping up too?
    How do I go about checking fuel pressure, I've never dealt with a fuel system on a car.

    Quote Originally Posted by kingls1 View Post
    I had a 99 that was doing the same thing and I couldn't figure it out after changing the same stuff. So I sent it down the road. Really liked it but it was time to go.
    I feel the exact same way, it's been pretty frustrating. I really like it as my point A to B car though. :/

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    You can rent a fuel pressure gauge from most parts stores... or just buy one. Find the rail that supplies fuel to the engine and there should be a plastic cap screwed onto a schrader valve (pretty much looks like a metal valve stem on a wheel). Remove the cap, attach the gauge and energize the ignition to see what you have for fuel pressure. Then start the engine and see what happens - once the engine is warmed up a bit rev on it to see if it holds pressure or if you see a significant drop. You will have to find the spec for fuel pressure as I am not certain what it should be for your car.

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    LSX whore allbaugh_04's Avatar
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    Quote Originally Posted by pajeff02 View Post
    I would check fuel pressure - not sure what the spec is for your car though. Absent a fueling issue, I have to agree with what has already been posted. Are you simply getting a misfire code through the check engine light, or is something else popping up too?
    Quote Originally Posted by pajeff02 View Post
    You can rent a fuel pressure gauge from most parts stores... or just buy one. Find the rail that supplies fuel to the engine and there should be a plastic cap screwed onto a schrader valve (pretty much looks like a metal valve stem on a wheel). Remove the cap, attach the gauge and energize the ignition to see what you have for fuel pressure. Then start the engine and see what happens - once the engine is warmed up a bit rev on it to see if it holds pressure or if you see a significant drop. You will have to find the spec for fuel pressure as I am not certain what it should be for your car.
    Perfect, thanks a lot, I will give it a shot.

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    You can check for an intake leak with either WD-40 or even a propane torch (not lit, of course ) With the engine idling spray around the base of the intake (or run the torch around it) and if the engine suddenly idles up on its own then you found your leak.

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    Mike The Guz's Avatar
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    I change my trans fluid every 30-50K miles. You may want to install a trans go shift kit the next time you do it.

    I did find this

    Harsh Shift, P1811

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    LSX whore allbaugh_04's Avatar
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    Ok i changed that map sensor, drove nice for a day.

    Tested ignition control module since that is free. Its good, they didn't have fuel pressure tester, going to try that tomorrow then check prices for iac and tps sensors.

    Could this be injectors or a plugged cat? O2 sensor?

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    LSX whore allbaugh_04's Avatar
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    I posted this^ when I was at advance auto. I cleaned the contacts of the ICM and the grounding points when I got home and it hasn't misfired since. I won't hold my breath though, as soon as I unhook something and hook it back up, the misfire usually comes back a few days later, but we shall see if it persists or not.

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    Is this a 3800 engine?


    My 96FB had an issue that turned out to be the plate the coil packs sat on that was similar in what your having problems. It got so bad it blew plug wires apart.

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    LSX whore allbaugh_04's Avatar
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    Quote Originally Posted by SMWS6TA View Post
    Is this a 3800 engine?


    My 96FB had an issue that turned out to be the plate the coil packs sat on that was similar in what your having problems. It got so bad it blew plug wires apart.
    Yes, the 3.8 series II. And yes, that plate is what I cleaned up. It's been 3 days and it's misfiring, but it's hardly even noticeable. Most people would never notice it. I might take it apart and clean it some more. I hope that's the issue, because I am so sick of this issue

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    Spaz is My Mentor SMWS6TA's Avatar
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    I know there was a notice put out that said those plates needed replaced every 70,000 miles. I'm not saying that's the issue just that it seems to be the same problems I had. It took me 6 months chasing that bitch down. Then I came across the notice and replaced it, night and day difference.


    Have you pulled the plugs? You might have an injector going bad.

    Have you check that fuel pressure yet?

    May be time for a compression test. This will help rule out any gasket leaks.
    Last edited by SMWS6TA; 08-07-2015 at 08:34 AM.

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    LSX whore allbaugh_04's Avatar
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    Quote Originally Posted by SMWS6TA View Post
    I know there was a notice put out that said those plates needed replaced every 70,000 miles. I'm not saying that's the issue just that it seems to be the same problems I had. It took me 6 months chasing that bitch down. Then I came across the notice and replaced it, night and day difference.


    Have you pulled the plugs? You might have an injector going bad.

    Have you check that fuel pressure yet?

    May be time for a compression test. This will help rule out any gasket leaks.
    I haven't ruled out an injector for sure. Spark plugs have been changed twice since this started. I have not had time to check fuel pressure, hopefully I find time this weekend, the fuel pressure tester has been hard to track down without buying one.

    If there was a decent wrecked grand prix at the dump, I would go pull one of those plates off, but they are all over 200k miles. I'll probably have to buy a new one.

  17. #17
    Spaz is My Mentor SMWS6TA's Avatar
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    If you do get a plate from another car at the very lease you can see if it fixes it temporary till you can. Those damn things are expensive, at lease they were back in 2002.

    I just look they've come way down ($123 on rock auto DELPHI DS1004)

    I ask about the plugs because looking at the tips will tell you a lot.

    Wet & smells like gas - you have an injector issue
    Black and oil - head gasket or rings
    Smells like coolant - head gasket
    White - lean on fuel


    Compression test will also help narrow down. They should be with 10% of each other.


    Fuel Pressure will let us know if it's charging the system and keeping it at constant pressure. One more thing to add to what Jeff said to test the fuel pressure. Turn key to on, no start, pressure should come up, it should hold and not bleed down, if it does then the pickup tube for the pump to the lines has a crack in it. It happens with age.

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    LSX whore allbaugh_04's Avatar
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    Thanks for all the help guys. Thanks Scott.

    I took some sandpaper after that plate a month ago. I haven't had it misfire since. Weird issue, but I think it's over now
    SMWS6TA likes this.

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